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Further Advances In Quantum Computing
Porfiry writes: "Scientists at the U.S. Department of Energy's Los Alamos National Laboratory have taken another step forward in the quest for a quantum-based computer by demonstrating the existence of a physical state immune to certain types of information-corrupting "noise," which could otherwise disrupt computations based on quantum states. The essential phenomenon that the Los Alamos team demonstrated is a state in what is called a "decoherence-free subspace." The researchers showed this state's existence using entangled photons, paired particles of light whose conditions are intimately linked."
This looks like an ideal holiday destination to me. It could even beat Hawaii if it would accept being part of the Union.
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He responded "well, yes and no...".
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If someone came up with an equasion you could plug an NP complete problem into, you could plug any NP complete problem into it, but quantum computer isn't a 'mathmatical breakthough' in the sense people talk about solving NP complete things. It's the same math, just doing a bunch of it at once. You probably could put in any NP complete problem, but it's not 'solving' them in anything other then the standard way, it's just doing it really really parallelized.
-David T. C.
If corporations are people, aren't stockholders guilty of slavery?
Actually, many things in computers work of probability. For example, when finding if a large number is prime, that's basically a crap shoot. The odds are only 99.9999999999% it's prime, using the standard calculation methods. However, the odds that your computer will get his with a cosmic ray or have some sort of internal failure are actually lower then this. It will be the same with quantume computers. You just repeat the calculation ten times or so, and everything's fine. Hey, they should be running at the speed of light anyway. And anyone using a quantum computer to add two number is crazy. If we ever get them, they will not be 'quantum computers', they will be 'normal computers with a quantum co-processor'.
-David T. C.
If corporations are people, aren't stockholders guilty of slavery?
I might agree with you and the moderators that this post was funny, but I unfortunately actually have a clue about quantum computing.
Sorry, it's just that moderators here go bananas over +:Funny. Maybe it's me, but it seems like they're taking the attitude "it's a joke, so it needs to be moderated as funny" instead of "that's really funny, I should mod it up." But then I'm not fond of most of the moderation here. I'd love to see Slashdot with about 1/100th of the mod points given out. But whatever, any +mod is better than a -mod.
According to quantum theory, particles such as those in the "spooky" experiment do not have a defined state, until some event causes their wave function - in which all possible states are simultaneously superimposed - to collapse. An observation of the state of one of the particles would be such a collapsing event.
One widely-accepted current understanding of what can cause quantum wave function collapse, is interaction with the "environment", meaning all the other objects with which it interacts. This phenomenon is known as decoherence, which is where the term "decoherence-free subspaces" comes from. For quantum computing, you want to remain decoherence-free, to be able to take advantage of state superposition.
Regarding the spooky particles, it's not a question of us just not knowing what their state is; the particles don't have a defined state, and exist in a superposition of all possible states, until something forces their state to be "chosen".
Assuming for the moment that this postulate somehow represents a form of reality that is meaningful to talk about, if you wait for the entangled particles to separate a bit - or a lot - and then measure the state of one of them, the model requires that the other particle instantaneously assumes the appropriate state dictated by the state "chosen" by the first particle, more or less. This appears to require "spooky" faster than light communication (or, according to string theory, requires 11 dimensions or so.)
This all goes back to Heisenberg's famous/notorious Uncertainty Principle, which not only puts limits on our ability to measure the states of particles, but puts limits on a particle's ability to be in defined states under certain conditions - for example, if we measure one aspect of a particle's state very accurately, we force other aspects of its state to become undefined, or put another way, force those aspects of its state to exist in a superposition of all possible states for that aspect.
Spooky enough for you yet?
Can anyone explain how quantum physics researchers create entangled photons and can track the members of the pair?
"In quantum physics, individual particles have no precise location and can coexist in more than one place at a time. Even distantly separated particles can share nonlocal correlations in a relationship known as "quantum entanglement." The entangled particles in the Los Alamos study are photons the basic particles of light engineered in such a way that they always have correlated polarizations. Polarization is the direction in which the photon's electric field vibrates."
:)
anyone else think this sounds a lot like philotics and ansibles from Enders Game?
-- My Sig is a P228.
""News for Nerds" in this case apparently means a bunch of network admins spouting off about a subject they have no real understanding of. "decoherence-free subspace"? C'mon, explain what that means if you're all so goddamn smart!" It's Star Trekese. I'm sure they'd find that if they reinitialize the subspace manifolds to meet the resonance of hot spaghetti with a nice wine-based sauce with fresh mushrooms that they'd get more computing power. FREE BEOWULF! OPEN NATELIE PORTMAN! GRITS PUBLIC LICENSE!
Steve's Computer Service, Hobbs, NM
Bell's inequality has been tested with entangled ions with nearly 99% detection efficiency (from the same group at Rice who demonstarted 4 particle entaglemnet in Science). Bell violations are real. Deal with it.
It is only the correlations that are non-local in measurements made on entangled states. As long as you cannot control what result you get when you make measurement (e.g. photon polarization), which of course you can't according to quantum mechanics, there is no way to send a message using effects like this. Which is not to say they are useless. You *can* do things like establish a random list of measurements on one of an entangled pair of particles that you know the your partner with the other particle will also measure. This is the basis of quantum cryptography and has been well-demonstrated experimentally.
And I did not make up the numbers (though I did get the reference wrong). See: C.A. Sackett, et. al, "Experimental Entanglement of Four Particles" Nature 404 (2000).
It is true that these entanglement measurements are not a perfect test of the Bell inequalities since, as Sackett pointed out when I heard him speak recently, they do not close the "locality" loophole. They do, however, close the important "uniform sampling" loophole that you are (rightly) critical of.
Many other experiments (with photon entanglement) have closed the locality loophole. It is true, as far as I know, that no single experiment has gotten sufficient sensitivity using spacelike separated measurements... yet. It seems somehow perverse though to hang a defense of local realism on this fact. When the definitive test of Bell's inequalities arrives, I have no doubt that they will be violated. Again. Mike
Of course, they cannot produce entangled states with 100% efficiency. But, according to Sackett's data (which I have no reason to doubt), they see Bell violations when averaging over all the data, not just those they have deemed to be entangled states. Thus (barring explanations based on hidden, subluminal communication between the ions) they see violations in the true Bell sense (i.e. taking unbiased expectation values over all data points).
I agree that we have not yet seen a perfect Bell inequality test, but the position you seem to be taking is that quantum mechanical non-locality cannot exist and that there must be a flaw in any experiment that purports to detect it. This is no more reasonable than insisting that quantum non-locality is real in the absence of any evidence.
Mike
Having said that, I don't think that the meaning of measurement and how it occurs is central to the Bell tests at all. Of course, it is central to the quantum mechanical interpretation of the tests, but (if you believe the results I mentioned earlier) Bell's inequality is violated experimentally. This does not prove quantum mechanics correct, it just proves that classical-like theories cannot be correct.
Wavefunction collapse is an ad-hoc postulate, and it is right, in my opinion, to be very critical of it. Nevertheless, the collpase postulate is central to quantum mechanics (or something very like the collapse postulate). There are countless experiments that do not involve entanglement at all that the collapse postulate correctly predicts the results of (Stern-Gerlach experiment with multiple magnets in series, quantum "watched pot" effect, etc...). While the collapse postulate is unpalatable, quantum mechanics in its current form cannot survive without it. The solution is not just to toss out the collapse postulate, but to understand the physical nature of measurement and reformulate quantum mechanics so that measurement is no longer a "deus ex machina" kind of thing.
But I honestly believe the universe is non-classical. Maybe someone will actually be able to factor a small number in 10 years using Shor's algorithm. Would you believe then?
While the quantum mechanical explanation of Bell violations relies of the collapse postulate, of course, this has nothing to do with the empirical question of whether Bell violations occur in nature. You seem unconvinced that they have been measured, and I can respect this position to the extent that there are genuine flaws in the experiments. My point is that an empirical measurement of Bell violations has nothing to do the collapse postualte or any other theoretical construction. It is simply a fact about the behavior of nature.
Secondly, while I have no doubt that the physics orthodoxy has a lot at stake in protecting certain theories, I think you misrepresent the physics community. John Bell himself hated the Copenhagen interpretation and non-determinism in general. He was a strong supporter of Bohm's theory. It seems weird to me that you prevent Bohm's theory as a realist challenge to the more "mystical" aspects of quantum theory since Bohm's theory is explicitly non-local (the pilot wave can change instantaneously throughout space), and Bohm himself was a pretty "mystical" character with his belief in the "implicate order" and whatnot.
I used to think that Bohm's theory made a lot more sense than orthodox QM, and I thought that the only reason that people held to the orthodox view was inertia or a kind of philosophical malaise left over from Bohr's ideas. Bohm's theory has its own disturbing anti-commonsensical features, however. Look up articles on "surreal trajectories".
The bottom line is that nature is non-classical, and we have not done very well at understanding that non-classical behavior. Every theory that I know of is somehow deeply unsatisfying, and yet quantum mechanics is able to predict things with incredible accuracy that classical physics completely failed to deal with.
Bell's theorem sauys (and I'm not being precise here): take a certain class of theories which we can call local and real in the sense that there can be no superluminal influences and a particle locally carries all the information necessary to determine the outcome of any measurement. (Notice that this doesn't say anything about which theories are "reasonable" or not). It can be shown that certain inequalities must be satisfied by any physical theories of this type.
I have not said anything about quantum mechanics or state vector collapse or anything. Nor do I have to. It is true that the predictions of quantum mechanics violate Bell's inequality (that's why Bell's inequality is interesting).
It is a question open to experiment whether nature violates Bell's inequalities. We seem unable to agree on whether Bell violations have been measured. If they have been then locally real theories are wrong (not that quantum mechanics is right). If they weren't violated when they should have been, then quantum mechanics is wrong. There aren't any other choices.
Do you still keep your slide-rule around, because these pesky calculators of today just don't cut it?
I took the SAT for years in high school, from before they allowed calculators to after. The first year they allowed calculators I brought a slide rule instead, just to make it interesting. I got a better score with it than I did with the calculator.
Slide rules were laid out for calculation, not for arithmetic: you performed one operation, then flipped the rule over and performed the next, then flipped it over again and performed the next. Math tends to follow certain patterns in calculations, and the slide rule's design took advantage of it. Thus one was able to get the answer faster and more easily. Not to mention that the rule familiarised one with logarithms. Or that it was a Really Cool Thing.
Still have it somewhere in its leather scabbard.
I thought that it was proven that if there is a polynomial time solution for one problem in NP then there is one for *all* problems in NP? Why should this be different for quantum computing based solutions?
Towards the end of the article, it mentioned how quantum cryptography generates unbreakable keys which can be used to unlock quantum encrypted data. Anyone who knows more than I on this subject care to comment on that? Is that possible, a non-crackable encryption? (Obviously, since this is all questions, I don't have any answers... ;-)
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everyone was born right-handed, only the greatest overcome it.
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everyone was born right-handed, only the greatest overcome it.
http://leftorium.net
Extremely challenging, like in "it can't work and it won't ever work..."
...which makes for nice sounding rhetoric despite its being false. (Normally I hate being baited by trolls, but it's morning and I haven't finished my coffee...).
A quick search of the Physical Review Letters web site shows 20+ letters in the last five years alone deomonstrating the preparation of entangled quantum states in the laboratory. Furthermore, quantum computation (an application of Grover's algoritm--see, e.g., "Experimental Implementation of Fast Quantum Searching" by Chuang et al., Physical Review Letters Volume 80, Issue 15, pp. 3408-3411) has been demonstrated in the laboratory, so your claims of quantum computation being a mere "mathematical abstraction" do not appear to be valid.
I'm curious what motivates your objection to quantum mechanics. Do you reject the mathematical theory of quantum mechanics (in all of its various guises) which has held up rather well to experimental validation, or is it instead that the heuristic, post-Copenhagen interpretation of the theory (i.e. "spooky action at a distance") rubs you the wrong way? If the latter, then I think your objections are more semantic than substance.
> physical state immune to certain types of information-corrupting "noise,"
>In the corporate world, we call this "management"
Actually, I was under the impression that management was the information-corrupting noise.
Or, alternatively, that management works in a state immune to information, which they consider noise.
Semiconductors were developed based upon quantum mechanics. If quantum mechanics doesn't work then neither do semiconducting devices such as transistors, diodes, etc.
Best Slashdot Co
This is a far cry from understanding the basic principles of EPR.
It's been a long time since I saw the ellipsis used in place of the period, comma, or semicolon. After all, most people stop doing that a few weeks into their email experience.
Thanks for the flashback!
Nobidy has said anything anout Michael Chricton's Timeline and quantum compuitng? cool book. I would love a realistic application like that
Fine, use a closed, observerless environment for both the photons and the cameras. Again, I ask, how are these two physical scenarios different?
I find the above description similar to the following scenario:
Throw a rapidly spinning coin up in the air. Have two cameras facing it from opposite directions each take a photo of it using high-speed film. Each camera has a 50-50 chance of taking a picture of a heads or a tails. The cameras now contain a pair of "entangled exposures", so to speak. Send the cameras off in opposite directions by a couple of light years or so, and develop the film. As soon as one picture is developed and shows "heads", we know that the other camera's film will show "tails". Before we developed either picture, each one had a 50-50 shot at showing either heads or tails. But now, as soon as we develop one, the other one can be determined, as if the first camera sent an instantaneous message (ooooo! spoooky!) light years away to the other camera.
How is this different? If it's not different, then why is either case spooky?
first i would just like to say that whatever 3rd grader posted the first post on that site should be drug into the street and beaten with frozen maceral.
/. article it was refering to, but i crtl c, crtl v'd this into my quote.txt file when i saw it in the comments, whoever it is, makes sense to me
now, maybe quantum computing is a swindle, I'm not smart enough to know, so I'll leave that judgement to you. what i do know is that there are numerous examples of 'science fiction' ridiculousness becoming real science. and i think there is a real need to research into every conceivable area of science, because what your looking for is not always what you find. there are also many examples of research in one area leading to advances in completly different subjects. your rant makes me think your one of those people who always say it can't be done, while others are working on ways to make it happen.
I think this quote is relevant here, and imho it's very interesting. I wish i had a link back to the
On a similar note, it is implied by quantum physics, that quantum particles appear to be aware of eachother, and that this action at a distance has no time delay, such as the limit of the speed of light. I wonder if SETI is listining to the wrong thing. Imagine this technology coupled with the action at a distance principle, so that you could choose coupled quantum particles to communicate over vast distances with no time delay. Couple these technologies with virtual reality, and it could be possible for communities, seperated by vast distances of space, to communicate and interact in real time. Wouldn't it be surprising to find a universal (literally) quantum network that was in use, and that we have been looking in the wrong place all this time. I would imagine that any species entry in to the universal forum would be predicated by their discovery of the technologies, and their ability to apply them to interact with this universal network. Instead of physically traveling in a space ship to distant worlds, we instead project our consciousness over vast distances with the help of virtual reality and quantum communication. Perhaps ET isn't going to show up in a galactic cruiser, but instead is patiently waiting for us to pick up the damn quantum telephone. We just haven't heard it ring yet.
-Master Switch, one more element in the machine
i don't want my tax money to pay for silliness that could win an ig nobel, but then again, how do you know what will lead to the next major advancement?
you're all figments of my deranged imagination
The idea behind the "quantum leap" phrase is that it is discreet. A quantum transition from eg. An N=1 to N=2 state never passes through "N=1.5" (which doesn't even exist, or make sense at all). So people adopted the term in a colloquial fashion to refer to an instantaneous jump, as opposed to a gradual "classical" advancment.
Of course, like every other colloquialism, it quickly became grossly over- and mis- used.
Would this work on two pieces of equipment that WEREN'T attached by optical fibers?
I don't think it has to be fiber.. That's mainly a photonic medium. Theoretically I believe it could be a super-conducting wire, or even a virtual super-conducting channel through space. Essentially it's anything that will not disturb the messenger quantum particle(s). Light is the easiest thing to deal with as far as I know. As an EE undergrad, we've studied electrons out the wazoo, and we know of their relationship to photons, but I never fully got comfortable with them.. They're discrete transmitter energy packets for charged particles. When an electron slows down, it emits it's momentum energy in the form of a photon.. When an electron speeds up, it's because it was hit by a photon. (though it's also possible for physical collisions, which carry momenum in the normal macro-scopic scale.. They're called phonons I believe). The amount of energy released is the frequency of either the photon or phonon. In both cases (I believe), the medium dictacts the ratio of wave-length to frequency (e.g the speed-limit).
From what I understand, a photon travelling through space is affected by forces such as gravity (though I don't think any others), but otherwise travel uni-directionally through space until they collide with another charged particle (quark or lepton / sub-nucleid or electron), where it transfers the energy. It's path however works like a wave.. If you consider the medium to be water, and the wave-front itself to be the photon, then it makes more sence, except that the wave's amplitude is so low that only one thing in the entire ocean will ultimately feel it. If enough discrete particles emite photons (even at random frequencies), then the effect will be more like a river wave-front. I don't fully understand how the wave-particle chooses it's path. It's not really attracted to a charged particle, yet at the same time, it's collision rate is substantially higher than say a neutrino (a nearly mass-less, chargless lepton (a brother of the electron)), who can pass through entire galaxies w/o incident.
The thing that bugs me about quantum physics is the quote, "if it doesnt' completely confound you, then you don't understand it". Well, I've always thought it seemed intuitive, so I must be missing something.. The intuition is that it seems to act very similarly to macro-scopic physics, so long as you consider invisible forces to be virtual springs. For example, the "Einstein Podolsky Rosen Paradox", where nothing can be known about either photon until one hits it's final polarized destination, and that somehow they're transmitting information back and forth instantaneously.. The idea that I read suggests that they are independant somehow in all ways except that they must be orthoginally poloraized (I guess I'd have to learn more about how you can garuntee their orthogniality). That when they collide, they pull from some localized "random information database" and know what the state of the other one was.. It's supported, I believe because you can not monitor their states without messing up the experiment. But it seems to me that there was prior knowledge between the two states.. At the time of their creation, and that they simply carried their information in seperate directions.
The schrodenger cat experiement (that I recall anyway) said that if you had a cat in a box, and cut the box in half, the cat would be in one of the two boxes (though it was unknown). and that if you seperated the boxes by 100 light years, then opened one, you've immediately transmitted the information to the other box.. The other cat will either be or not be. It's a high level analogy for various quantum properties, but it seems asinine, to suggest that the info was being transmitted only at the time of measurement.. The darn cat _knew_ which box he was in from the beginning. Radomness dictated which box the quantum particle was closer to when they were seperated and they just altered their quantum orbital path to the new confines and inertial frame. The whole abstraction of quantum physics that us lay people get makes it difficult for us to get what's really going on.. All the analogies I've been told do not express the "quantum-wierdness" that everybody always talks about, they're logically founded.
-Michael
but continuing the problem, doesn't it all boil down to random distribution of quantum particles? You had a particle that was created through an interaction of some kind - the electron radiated a photon, or two particles collided/interacted and formed 1 or more resultant particles. Everything about those particles is known to those particles, just not the observer. I understand that you can't "measure" the particle without disturbing it, but the particles themselves aren't magical.. They just move through space and time (and what-ever other curled up dimensions) with the potential to react to one of a discrete set of events.
If two particles are known to have orthogonal properties (such as polarity), then each particle knew all along what they were, they were just created or selected by some special process right? (I don't really know why Einstein calls it a paradox).
-Michael
-Michael
Hmm, this is right. sounds just as silly as storing a bunch of electrons on an array of millions of small capacitors, and then refreshing them dozens of times per second, to store data bits. And if extra charge comes along, or it's not refreshed long enough, the fragile data gets wiped out. Oh wait a second, that's how DRAM memory works.
Okay, but it's certainly as ridiculous (sp) as making little tiny EXTREMELY static-sensitive transistors wired in a feedback path with current maintaining them in 2 possible states so they can flip and flop between the two, and thus store data bits. Yeah, and if someone rubs their sock on said transistor substrate, they'll send the data into total chaos. Hey wait a minute, that's how SRAM works.
So you can see these seemingly delicate scenarios are both in use today, almost certainly both being made to use in the computer you've used to post your sarcastic little statement. BTW, Dr. Kool, since you're at Harvard University, you're probably very aware of some of your fellow Harvard faculty research with quantum structures, and, I believe, quantum bits.
Good day.
make world, not war
Manipulating individual electrons is a fool's errand.
Are you the same AC from the beginning of this thread? If it's a fool's errand, then it's a bigger fool who stays with yesterday's 'comfortable' technology instead of pushing forward. You'd probably still be using vacuum tubes instead of solid-state with your forward-thinking ideals. Do you still keep your slide-rule around, because these pesky calculators of today just don't cut it?
You can't even imagine how small a single electron is.
Well, if you think of a single isolated electron as a spin 1/2 realization of Poincare algebra, you may be right, but such cases are pure thought experiments. It's the actions of the electron in the presence of other particles that matters. So, look at the solutions for spherical harmonics of solutions for Schrodinger's equation for an electron-proton pair (ie, the hydrogen atom) to get a feel for the size of an electron cloud in the smallest of one of it's common configurations. Probably on the order of Angstroms.
The electron probability cloud probably get much bigger, fuzzier, and far more complex when you think of electrons in metals and other matter (organics maybe?). In terms of low-dimensional systems (quantum dots / 2-D electron gases) there may be some interesting numbers for the 'size' of the electron, but I don't these cases offhand.
In terms of not imagining the scale, here you are perhaps right. It's difficult to visually comprehend 10 or so orders of magnitude in distance scale. But I can get some idea of it.
make world, not war
But it's not as simple as that, it's still a very confusing philosophical confusing notion. Feynman supposedly said anyone that's not bothered by this has rocks in their head. To top it all off, the professor suggested we all to go home tonight, take a bong hit, and think about it some more.
make world, not war
I was just using the slide rule as an example back to antiquity, but of course you probably realized that :-)
I like one of the scenes in Apollo 13, where they're checking the gimbal coordinates, and you see the NASA engineers with their slide rules hacking away. it's great.
make world, not war
I just found this page with some descriptions, and a taste of funky math :-) I haven't really checked it out fully, but it looks like it's probably a good place to get a basic idea of some of these principles (and hopefully they have some decent movies too).
enjoy.
make world, not war
This means that if the spin of one particle is measured in any direction (say out of X or Y or Z for cartesian coordinates), then the spin for the other particle is going to be opposite that measured for the first particle, BUT ONLY IF IT'S MEASURED IN THE SAME DIRECTION. So if you measure the z component of particle 1, you get either h-bar/2 or -h-bar/2, and you know that particle 2, if measured in the z direction, gives the opposite one. This will work if both measurements are in the x, or y, or any other combination of directions. But they must be the same direction.
One fundamental aspect of spin is that spin operators in different directions don't commute. that is, if one measures the spin in one direction, say Z, then another direction, say X, and then measures the Z direction spin again, it won't necessarily be the same. That is, measuring the X direction between the two Z measurements changed the state of the system.
So the part of this thought experiment that bothered Einstein and company is that if one can see that if both particles are entangled such that any spin measurement made will be opposite the other particle's measurement, providing the spin direction being measured is the same, then this implies that there are some sort of hidden variables in nature to account for this. Namely, the particles are entangled in seemingly all directions, until that first measurement is made. Surely, then, nature must possess some knowledge about all three orthogonal directions simultaneously.
But what Bohr and Heisenberg maintained is that one cannot simultaneously measure the X,Y,Z spins. That is, we CANNOT ask about measurements that could be made but were not made, we can only talk about those measurements that were made.
So it's a bit different than the analogy the article gives about two pennies, one being heads up and one being heads down, because if your penny is heads up, it'll always be heads up, as that is not a fundamental spin-1/2 particle.
sorry if this post makes ZERO sense, i'm just blabbering about what was pretty cool in quantum class. hopefully tomorrow we'll learn s'more to make it make more sense.
make world, not war
damn...things should work now. Pair networks sucks!!
Look at this .
Yeah... but it doesnt have much... Just a few quotes, a funny dilbert cartoon, and some amusing quotes from 1/2 centry ago.... (oh.. and the pps who are working on the projects)--- But nothing of any hard content as to progress/work/etc that I could find.
A brief introduction to Quantum Cryptography Looks interesting... I wish I could find more info on the Los Alamos site about what they've done with crypto.
(Sorry about the double-post; Didnt think about what when I completed the subject line)
And therefore be illegal under the DMCA.
I'm trying to teach myself to set people on fire with my mind... Is it hot in here?
Quantum cryptography is the only encryption method that eliminates person in the middle attacks. Naturally there has been a lot of interest from the security industry. Photons are a good medium for communications since they don't give off much of an electronic or magnetic "wake" that could be detected.
Slashdot has covered the implications of increased speed in factoring and how that might effect the PGP key space in past articles. The only thing new that I found in the article was how superposition in quantum computing could speed database searching.
(insert Natalie Portman joke here)
You can't effect the outcome of a random event. The article does a good job of explaining this in simple terms using the example of two quarters. Say one quarter is left in an unopened box on earth, and the other quarter is sent to mars to be flipped. The box on Earth can only be opened AFTER the martian flips his quarter on Mars or it voids the experiment. If the Martian quarter comes up heads, they will find the quarter in the box on earth heads up as well. The problem with using this phenomena for communication is that the Martian cannot effect the outcome of the coin toss..
I imagine a chess program written for a quantum computer would always play black and you would never win.
The goverment will classify a working model so they can keep thier secrets.
Do you have a reference to the actual paper describing decoherence free subspaces and one describing what was actually performed in this experiment? I've always been sceptical of quantum computing papers because it has seemed pretty obvious to me that decoherence effects grow exponentially so I'd love to see a good paper contradicting me!
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-- SIGFPE
If you encrypt a message and send it away while keeping a copy for yourself and someone makes a copy of your message and trys to crack it you would know immediatly because their actions would alter your original copy and you could trash their copy by altering your copy? eh?
All you need in this life is ignorance and confidence -- and then success is sure. Mark Twain
A few years ago you had to have a license to use a dec alpha as it was classified as a supercomputer
Free Techno/Jazz/DNB/MI Music by guys obsessed with monkeys!
"Well, this guy in overalls showed up and said he needed to take the drive out to be cleaned. I'm a scientist, for Christ's sake, not a technician. How was I supposed to know," one scientist, who spoke to us on condition of anonymity told us.
Officials at Los Alamos are confident that, by following the lead of the State Department and offering a $25,000 reward, they will soon recover their lost data.
"Otherwise," said the scientist, "we are, like, so totally fucked. I mean, this project has been hell! Sixty-hour weeks for six months is harsh!"
sig not found
Look, Quantum Theory is a theory just like any theory except that it tends to explain a few things that more rudimentary theories cannot. It is not the ultimate reality, though. All of these theories are just casting our eyes further away from the shadow we perceive as reality and towards what is casting that shadow.
I did research for a paper on quantum computing a couple of years ago. There have been demonstrated uses of quantum effects in this field, as well as the dozens of other fields where quantum theory is applied.
In fact, they have been able to use NMR to glean the bulk spin of the composing atoms of a liquid and perform simple operations. In another angle of quantum computing, they've been able to use lasers to super-cool cesium atoms and manipulate their quantum states to the same effect.
All of this is pointing to the fact that quantum theory correctly predicted the ability of such quantum computing. It is enabling the theories from long ago, as well as newer ones, to finally be applied. It has the potential to disrupt (and even re-invent, but that's another story) encryption as we know it.
Furthermore, quantum entanglement is not a requirement for these processes.
You seem to believe that reality is what you see. You cannot see quantum states from our macroscopic world. Nor can you see relativity in action. Hundreds of years ago, people couldn't "see" gravity, either. And long before that, people couldn't even "see" air. Please, see the light. Quantum theory, like all theory, is a mathematical abstraction of how the world works. And each successive model is getting closer and closer to what can actually be experimentally observed.
"The question of whether a computer can think is no more interesting than that of whether a submarine can swim" -EWD
Does anybody else think that the term "coherent" could be substituted for "de-coherence free"? Aren't we making things a little bit more complicated than they should be or is this already so complicated that a little bit more can't hurt?
sarchasm: The gulf between the author of sarcastic wit and the person who doesn't get it.
That spookyness, I think, comes from our inability to know the polarisation before the event of taking te measurement. As you said, but I think that you can also say that since you don't know the orientation then the polarisation can be in ANY arbitrary orientation. If you think about it, there will then be a 50% chance of being correctly oriented to pass through the filter. If this is the case, then the other photon will always have a 100% chance of passing through if the first of blocked, or a 100% chance of being blocked if the first passes through.
If you concentrate on the orientation of the first photon as being a probability question and then second photon being of opposite polarisation then you don't see it as a case of many possibilities occupying the same space simultaneously, merely 1 value that you don't happen to know at this time. This point of view seems to give me fewer headaches than others. Hope it does the same to you.
dnnrly
the Los Alamos team demonstrated is a state in what is called a "decoherence-free subspace." subspace
and even better: (although I admit it's a bit of a stretch) "Decoherence in Kwiat's system is intentionally created by passing the entangled photons through a roughly 10 millimeter piece of quartz." Ummmm - di-litium, anyone?Sorry, these just seemed ironic direct corelations...
mas cerveza, por favor politically incorrect stu
In other news today, the Internet has been declared a "mathematical abstraction with no bearing on how the real world works." All servers are to be shut down at midnight EST tonight.
I watched C-beams glitter in the dark near the Tannhauser gate.
Guess NSA can't crack RSA yet, or they wouldn't be interested in this technology :)
m
So do I, I'm a physicist who has been working with computers long enough to remember using a soldering iron to 'write' a program.
Lighten up. It's just a stupid goof, and to be fair it was only ONE moderator who thought it was funny and I never even expected that.
Ok, I was wrong, it was two. My mistake, but someone had the good sense to knock it down one.
I can't be held personally responsible for the moderators.
" We got increadable speeds out of this puppy, but we arn't certain just what the data actually is" tester says.
"When we nailed the data down the thing slowed to a crawl" he continued.
"To make things worse, sometimes we weren't even sure where the damn thing WAS."
We'll keep you posted on continuing development, but it looks like it has a ways to go.
Silicon ships are vulnerable to thingsl ike EM interference and other things. Thats why we have ECC (error correcting code) memory. It tries to cut down on the number of mistakes for situations like 24/7 servers that can't afford to corrupt over time. Even so, these chips still aren't _perfectly_ error proof. So, as long as the probability of failure for the QC is about the same as the probability of failure for a cilicon ship then its all good.
Rules of Conduct:
#1 - The DM is always right.
#2 - If the DM is wrong, see rule #1
In 1935, Einstein met with Boris Podolsky and Nathan Rosen to formulate a theory which basically said that particles intrinsically possess certain properties before these properties are measured. a "side effect" of the theory is known as the Einstein Podolsky Rosen Paradox (EPR Paradox).
Suppose you entangled a pair of photons polarised at 90 degrees to each other. You can't know what the polarisations are until you measure them; they could be vertical, horizontal or any angle in between. All you do know for sure is that they are perpendicular to each other. You send these photons off in different directions. At some point as they shoot off into the distance the photons will run into polarising filters you've cunningly put in their path.
Suppose one photon passes straight through a vertically aligned filter. It must be vertically polarised, so its partner must be horizontally polarised. The second photon would therefore pass through any horizontal filter in its way, but not through a vertical filter. So far so good. One photon is vertically polarised, the other is horizontally polarised, so they are at right angles as they should be, and all's well with the world.
Not quite. Until the first photon hits the filter, you have no idea whether it will go through or not. And for that matter, the photon doesn't know, what sort of filter it is going to hit until it gets there. Since you know nothing about either photon's individual polarisation until you make a measurement, you only know that the odds of it going through are fifty-fifty, no matter what angle the filter is set at. So the second photon can't know what the first photon will do until it actually does it. Yet the actions of the first photon determine the actions of the second. The second photon has to get some sort of tip-off from the first, even though they are physically a long way from each other.
What's more, this tip-off has to be instantaneous, because it has to work even if the two photons hit their filters at exactly the same time. It's impossible to predict what either photon will do, and yet the two of them must act in concert so that their polarisations have the correct relationship to each other. This is the "spookiness" that Einstein, Podolsky, and Rosen took such exception to.
- "Hear that?! The percolations are imminent! Cease your ingress!"
Good point!
That's what scientists called 'hidden parameters'. (hidden, because unlike the colour of the ball, you can't simply look at the photon and see it's polarisation)
I'm sorry I can't simply explain you why this assumption is wrong, it's a couple of pages in a quantum physics book and even that explanation is too short and a little but inaccurate.
The point is that you can never measure all of your hidden parameters, because one measurement destroys the other parameters. But still the hidden parameters, if they exist, do change the results of some statistical calculations. Therefore its possible to do an experiment to decide if the hidden parameters had some certain value even before you measured them.
These experiments showed that, in fact, the parameters you don't measure don't have a certain value at all. It's not hidden, it's simply non-existent.
(There is a difference in quantum physics between 'you don't know some value' and 'it doesn't have a certain value')
According to many other physicists quantum entanglement does occur, and there is supporting experimental evidence. This evidence is disputed by some people, however the majority accept it as valid.
I note that Caroline Thompson on her page that she suspects that the experimenters have produced [possibly without realising] detectors that mimic quantum theory. A non-specialist in the field who is skeptic of the results is not the same thing as a definite disproof by a trained experimentalist. Please note that Caroline Thompson offers no experimental results, only some suggested experiments that have not been tested, which could disprove QM as it currently stands.
Only two things are infinite, the universe and human stupidity, and I'm not sure about the former. (Einstein)
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as that's the sign that appears on the screen, anyway back to the point)
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Entanglement has a firm experimental footing as well as an fantastically strong theoretical basis. There've been some experiments in Italy (so no ref) that have been running over the past couple of years (not "the 1980's") that have "proved" entanglement over very large distances (kilometres)
requiring no belief in mystical instant action-at-a-distance
It was shown quite beautifully and clearly by one of my Philosophy of Physics lecturers, why EPR doesn't actually break causality, it's not mystical and requires no "belief" (taking you to mean the religious version rather than the scientific one).
Anyway, now might be good time to remember when the phrase "action-at-a-distance" was first used. It was Newton, trying to explain something he didn't quite understand, which puts you in lofty company indeed.
No dictionaries were harmed, or indeed used, during the production of this post
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I was expecting to have to come in here and "kick some ass" as my American neighbour would say, but so far all I've found is valid well backed up comment (apart from this one, of course, and that chap further up who doesn't believe in QM).
If you want to read real numpty science take a look at this baby
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I stand corrected.
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That's an interesting way to reformulate the Schrodinger's Cat "Paradox" (and one that connects it to the EPR "paradox"), but the classical version usually involves killing the cat. Basically, you put a cat in a box with something that could kill it (eg. a device which releases poison gas). Connect the device to a purely random quantum system, like a particle which has a 50% chance of decaying in a given time period. Then, after the given time period, you could say that the cat is in a superposition of live and dead states. Then you open the box and "the waveform collapses" and the cat is either fully alive or fully dead.
Note that the paradox doesn't really say anything new. It just takes a strange microscopic process and bumps it up to the level of an equally strange macroscopic process. I guess this might serve the purpose of convincing some people of it's strangeness, but beyond that it's pretty pointless.
When people aren't confusing quantum computing with quantum cryptography, they're busy confusing the purposes of the two. Quantum cryptography (of which the above is a reasonably accurate description) is handy method of realtime authentication between two parties. Despite it's name, however, it is not in the most general sense encryption. In real life, people need to encrypt files. They need to be able to leave them on their hard drive in a secure form. They need to send them multiple times to various people. They need to have the capability to use public and private keys and an existing infrastructure (the internet). Quantum cryptography will be useful for a very small handful of organizations who can afford dedicated connections (and who will need other means to secure data once it's been sent). It is in no way a replacement for encryption.
I'm not an expert, but if and when quantum computing becomes feasable, I see no way that encryption could exist in the way it does today. At best, individuals will have to rely on security through obscurity, creating complicated algorithms using proprietary (quantum) hardware that would be prohibitively expensive for another party to reverse engineer. Or maybe someone will come up with something really new that doesn't rely on quantum-computable math. In any case, the age of PGP and the like will be over.
The story hasn't been up for 10 minutes, and look what we have:
/usr/www/users/davew/b2tb/geeklog/public_html/comm on.php on line 79
:)
Warning: MySQL Connection Failed: Can't connect to MySQL server on db11.pair.com (61) in
Oh well... I'll just have to keep on hitting "reload".
Even though we cannot instantly transmit information using quantum entanglement, transmitting randomness is still very useful.
[snip]
Note that in quantum encryption, we are transmitting the code instantly. The actual message will arrive much more slowly -- at only the speed of light.
Why can't we transmit information this way? If by observing or altering the particle at my end, it causes the particle at the other end to be altered, I can send a message easily. I believe the experiment you refer to did just that. A change was made on the one particle which was recognized in the other particle. I could be wrong here- feel free to point that out. But I believe in essense the idea is that by touching one particle in some way, it nescessitates that the other particle was always conjoined to the first and so a message could in fact be passed.
If this is true, one could decide on a predefined syncronization series of changed particles within the background radiation of the universe (like a kind of modem sync). I could change a whole buncha particles and you could look at a whole buncha particles for the signal. Hopefully some of the particles you look at would be paired with some I changed. Once syncronized, we could send messages.
If this was possible you would even be able to send messages across vast interstellar distances (once you had the sync scheme). Maybe alien races already communicate this way :-) Nah, there's probably not enough paired particles from the big bang in sufficient quantities in the two seperate places. No free lunch and that kind of thing- but it might be theoritically possible.
Again, I'm way outa my league here, so feel free to point out if I misunderstood the experiment. I just think its a fun speculation.
You could be saying the same thing about aeroplanes.
Research and experimentation are rarely a waste of effort, whether to prove or disprove, because we have to find out. The skeptics said we would never build aircraft, split the atom, travel safely in railway carriages, go to the moon, etc etc etc.
I think it is far better to err on the side of curiousity, than to just sit around never asking any questions. History proves this irrefutably. We learn from our successes. We sometimes learn from our mistakes too. It is worth the effort.
So let these guys go and discover what they can! Let the engineers make it work! Just like the early experiments with aircraft, there will be hundreds of failures before someone gets it right, and when and if it happens, all those who made the effort will be vindicated.
--
NO TOUCH MONKEY!
physical state immune to certain types of information-corrupting "noise,"
In the corporate world, we call this "management"
Dirty Pirate Hooker
People who didn't understand Newton said the same thing about rockets.
They were wrong too.
Which is not to say this will work.
Just to say don't trust "common sense" over pure maths.
'There is a Light that never goes out.'
A very good book for anyone who's interested in this 'spooky' stuff is 'speakable and unspeakable in quantum mechanics' by J Bell - the man that invented Bells inequalities (the statistical quanties which were alluded to early).
...gets punched.
Honorary Member of Jackie Chan's Kung Fu Process Servers
More on Quantum Cryptography at:
http://qso.lanl.gov/qc/
For those of us who are Paranoically Inclined (tm)
I want the future now!
http://www.lanl. gov
This one works.
Sure, quantum computing can factor enormous numbers really fast, but its been pointed out a number of times that as Quantum Computing Taketh Away, it also Giveth:
Encryption Destroyed and Resurrected
As mentioned above, the classic problem that a quantum computer is ideally suited for is cracking encryption codes, which relies on factoring large numbers. The strength of an encryption code is measured by the number of bits that needs to be factored. For example, it is illegal in the United States to export encryption technology using more than 40 bits (56 bits if you give a key to law-enforcement authorities). A 40-bit encryption method is not very secure. In September 1997, Ian Goldberg, a University of California at Berkeley graduate student, was able to crack a 40-bit code in three and a half hours using a network of 250 small computers.15 A 56-bit code is a bit better (16 bits better, actually). Ten months later, John Gilmore, a computer privacy activist, and Paul Kocher, an encryption expert, were able to break the 56-bit code in 56 hours using a specially designed computer that cost them $250,000 to build. But a quantum computer can easily factor any sized number (within its capacity). Quantum computing technology would essentially destroy digital encryption.
But as technology takes away, it also gives. A related quantum effect can provide a new method of encryption that can never be broken. Again, keep in mind that, in view of the Law of Accelerating Returns, "never" is not as long as it used to be.
This effect is called quantum entanglement. Einstein, who was not a fan of quantum mechanics, had a different name for it, calling it "spooky action at a distance." The phenomenon was recently demonstrated by Dr. Nicolas Gisin of the University of Geneva in a recent experiment across the city of Geneva.16 Dr. Gisin sent twin photons in opposite directions through optical fibers. Once the photons were about seven miles apart, they each encountered a glass plate from which they could either bounce off or pass through. Thus, they were each forced to make a decision to choose among two equally probable pathways. Since there was no possible communication link between the two photons, classical physics would predict that their decisions would be independent. But they both made the same decision. And they did so at the same instant in time, so even if there were an unknown communication path between them, there was not enough time for a message to travel from one photon to the other at the speed of light. The two particles were quantum entangled and communicated instantly with each other regardless of their separation. The effect was reliably repeated over many such photon pairs.
The apparent communication between the two photons takes place at a speed far greater than the speed of light. In theory, the speed is infinite in that the decoherence of the two photon travel decisions, according to quantum theory, takes place at exactly the same instant. Dr. Gisin's experiment was sufficiently sensitive to demonstrate the communication was at least ten thousand times faster than the speed of light.
So, does this violate Einstein's Special Theory of Relativity, which postulates the speed of light as the fastest speed at which we can transmit information? The answer is no -- there is no information being communicated by the entangled photons. The decision of the photons is random -- a profound quantum randomness -- and randomness is precisely not information. Both the sender and the receiver of the message simultaneously access the identical random decisions of the entangled photons, which are used to encode and decode, respectively, the message. So we are communicating randomness -- not information -- at speeds far greater than the speed of light. The only way we could convert the random decisions of the photons into information is if we edited the random sequence of photon decisions. But editing this random sequence would require observing the photon decisions, which in turn would cause quantum decoherence, which would destroy the quantum entanglement. So Einstein's theory is preserved.
Even though we cannot instantly transmit information using quantum entanglement, transmitting randomness is still very useful. It allows us to resurrect the process of encryption that quantum computing would destroy. If the sender and receiver of a message are at the two ends of an optical fiber, they can use the precisely matched random decisions of a stream of quantum entangled photons to respectively encode and decode a message. Since the encryption is fundamentally random and nonrepeating, it cannot be broken. Eavesdropping would also be impossible, as this would cause quantum decoherence that could be detected at both ends. So privacy is preserved.
Note that in quantum encryption, we are transmitting the code instantly. The actual message will arrive much more slowly -- at only the speed of light.
-Ray Kurzweil, The Age of Spiritual Machines, pg. 115
Oh and I suppose you'd have to include www.entangled-photons-suck.com
Please stop APK.. you're only hurting yourself.
. . . but after reading all of the recent insane patent/copyright/censorship articles and listening to the 2600 radio show tonight where I FINALLY heard the audio of the reasoning behind the DeCSS decision, I just cant rejoice about this. In 15 years Ill have a shiny new quantum machine that I can play freecell on and write papers that automatically get mailed to m$. Hooray! were all doomed it seems.
Define practical in your context.
I have witnessed controlled fusion experiments both in Real Time via a camera into the core of a reactor and through post experiment film.
Controlled fusion reactions are possible. I believe that fusion will make it into usage during my lifetime (I'm 24).
Don't be so quick to poo-poo fusion just because the field is in it's infancy. We have a long way to go in Materials Science before fusion becomes a mainstream energy source. Were the first bicycles practical? Absolutely not, they were fixed gear and had iron wheels. Prior to that the velocipedes had no gears of any kind, or pedals, and they weighed over 100 lbs.
All technology goes through a refining process.
it "will be accessible in 24-72 hours"
now, that I've opened myself up to some lawsuits, someone go register www.entangled-photons-suck.com, so I can make up for the legal fees by suing someone else.
Don't you just love the web??
-MSD.dyndns.org
"Sucks to your ass-mar"
Does this mean that the result of a quantum computer's calculation will never be 100% definitely the correct answer? With conventional electronic computers, we assume that (for simple calculations at least) debugging can be performed which will result in a program which has effectively a 100% probability of getting the right answer - if we can calculate 100+200 and 300+400 in our C program we assume we can find 200+300.
But if there's some factor involved in the function of quantum computing that means we may obtain different results for 200+300, even if the probability of getting 500 is 99%, then such computers would surely have limited usefulness.
Is this an inherent limitation, or can it be solved by engineering?
If you're a jock, inflict some pain / If you're a nerd then use your brain - DAPHNE AND CELESTE
You must realize that the decoherence mechanisms which occur at the event horizon probably does not have a symmetry that can be exploited. I mean the only people who write if you're so smart are the people who already know the answer, right? lmao
dabacon
The paper which has this experiment was just published in Science. You probably need to be connecting from somewhere that has subscribed to Science.
A good intro reference to DFSs is here.
dabacon
OK, being as I am a reasearcher who has done work on decoherence-free subspaces (DFSs...they are also known as quantum error avoiding codes or noiseless subspaces...damn nomenclaturese) I thought I'd give all you netadmins a real simple explanation of what a DFS is. Of course, being a simple explanation, it will fuzz over a bit. But I thought I'd at least try!
Suppose you are trying to send some bits down a noisy communication channel (sending an email from Timbucktoo to Weed, CA). Now the noise will cause the bits that you send on one end of the line to sometimes come out different on the other end of the line. Many of you know how we get around this in real world situations: we use error correction. The basic idea of error correction is to use redundancy to transmit information. Thus, for example, instead of sending the bit 0 you might send ten 0's and instead of sending the bit 1 you might send ten 1's. If the channel isn't too noisy then the reciever can figure out what bit you ment to send by looking at the ten bits he recieves and deducing if of those ten more are 0's or 1's. Basically you can reduce the noise rate of information transmission at the cost of increasing the number of bits you need to send in order to transmit one bit of information. (Sorry for those of you who know this shit like the back of your hand).
Decoherence-free subspaces work on a similar "encode the information" (i.e. 0->ten 0's, 1->ten 1's), but they "use symmetry" to protect the information.
Suppose that after extensive testing of the phone line you are using you notice that if you send two bits down the line in rapid sucession the line either does nothing to these two bits or flips both of them. Thus for example, if you send 00, the reciever always gets either 00 (no error) or 11 (error!) and if you send 01, the reciever always gets either 01 (no error) or 10 (error!). Your phone line has a symmetry! How do exploit this symmetry?
Well, what you do is simply encode the information you want to send into the parity of the two bits. This simply means that if you want to send 0 down the line, you send 00 (or 11) and if you want to send 1 down the line, you send 01 or (10). Now the noise can flip 00 to 11 (or vice versa) but it cannot change 00 to 01. Thus the you can perfectly recover the information you sent down the line regardless of an error occuring. What is neat about this is that it doesn't depend on the strength of the noise (the probability that an error occurs, for example). By using the symmetry of the noise you can avoid the noise completely! Symmetry=>protection.
What I've explained to you is an example of a decoherence-free subsystem (a generalization of decoherence-free subspaces, but the same basic idea) in the real "classical" world. To build a quantum computer we need to deal with similar problems but in the "quantum" world.
When Peter Shor (quantum computing god) invented a quantum computing algorithm for factoring (the one that breaks RSA), one of the main problems in actually implementing such a computer was quickly understood to be noise. Noise in quantum system is called decoherence (at least by me) and is much more nasty than the classical noise you get when (say) you are talking on your cell phone. The problem with quantum systems is that if they interact with external systems they completely lose their quantum nature. And making this problem even harder, whenever you observe a quantum system it also loses its quantum nature.
But following his work in discovering the factoring algorithm, Peter Shor noticed that he could do error correction on quantum systems to avoid this decoherence problem (hence Peter Shor=quantum computing god). A huge host of people then developed the theory of quantum error correction which showed that the decoherence problem could be overcome. This is probably one of the most amazing new ideas of the past decade: that quantum information can be in principle be sheilded from its environement by suitable error correction.
Anyway, decoherence-free subspaces are like quantum error correction in that you encode quantum information, but they, like the example above, use the fact that often noise has some sort of symmetry. Think about it this way: decoherence of a quantum system is like you looking at the system (you are interacting with the system!). But say you have two atoms which are so close together to each other that you cannot distinguish atom A from atom B. Then there is a symmetry in the way in which observe the system: you cannot distiguish that atom A is on the left or if it is on the right. Such a symmetry can then be shown the produce encodings of information which are protected from your observation!
Ah well, I had to try. Thanks to anyone who made it this far without "man I want to kill this dork" thoughts.
dave bacon
Staff are vigorously checking underneath every photocopy machine to see if they are with the hard drives containing information about disarming nuclear warheads.
Can you imagine a beowulf cluster of these?!?
I can't... I somehow doubt that it would do any good... Besides, how does one fit a network card into the subatomic level?
"Titanic was 3hr and 17min long. They could have lost 3hr and 17min from that."
IBM had PL/1, with syntax worse than JOSS,
And everywhere the language went, it was a total loss...
They said that they would never break the sound barrier.... They said that they would never walk on the Moon. As a matter of fact there were a lot of people watching the Lunar Landings in 1969 with tears as they recalled a youth where thoughts about walking on the moon were up there with the fairies. So the moral is, never say never as who knows who is working on what. And who knows when the next big thing arrives. In fact, there is SOOOO much happening now its hard to keep up with it all. StarTux
This guy is absolutely correct. I mean, he is a DOCTOR from HARVARD for cryin' out loud....
I myself believe that vacuum tubes are the way to go: small, cheap and reliable. As a benefit, lots of heat as a byproduct to keep the lab toasty.
I believe that there are many other technologies that are similar to quantum computing in that they are a waste of time because I do not understand them. Radio, for instance. It will never work!!! And combustion engines? WTF? Maybe in major industrial uses but.... Airplanes? What is this "lift" phenomenon everyone is giving so much attention to? If man was meant to fly, we'd all have turboprops in our asses.
Clearly, if I do not understand the technology, it does not exist and cannot be valuable in any future applications. The "Cold-fusion on the desktop" hoax taught us never to try anything new, or at least it should have. Tell all the scientists to go home, there is nothing new to know.
Pay no attention to the man behind the curtain with all your metadata.
Does anyone remeber Harry Selden from Asimov's Foundation Trilogy? He had a hand held device that given the right input could give the probability of any major future event. It's the same concept as the butterfly flapping his wings in Tahiti and 2 months later, that little wind grows to a hurricane.
With enough computing power, we can do real world simulations on weather patterns, economics, crash testing, materials fabrication, heck, we can even fuck around with genetics on HUGE computational scales.
With the right applications and a sufficient amount of data, I can predict the future. The problem is that even with a Pentium 4, it would take until the future was ancient history. [not to mention the archetectural flaws in the chip...]
If I had a sufficiently fast platform with practically unlimited memory: would that help me do the Harry Seldon bit? I think it gets me lots closer.
Cool.
Pay no attention to the man behind the curtain with all your metadata.
how'd that get a 1?
back to the topic... this could be an interesting advance, however, it doesn't mean that quantum computers are very much closer to our desks. Anyone have any specific time-related information?
So what? If the government wants to find out I'm playing Unreal Tournament 26, the Comeback at 8192X6000 or something like that, let them go right ahead.
,doesn't it?
Besides, if both computers increase in power, encrypting will become equally more powerful just as decrypting
People replying to my sig annoy me. That's why I change it all the time.
As far as I know, a Quantum computer should in theory be able to use all 32 states of an electron. This means that instead of 0 and 1 you have 0,1,2,3,well you get the point up to 31.
People replying to my sig annoy me. That's why I change it all the time.
Yeah, but aspell will compile really really fast, though.
Patent entangled photons.
Has anyone done any research on using QC to play chess?
size doesnt matter, the stream of electrons already in your computer, better known as electrics is already manipulatable, just think of this as the next step. By the way do YOU know the size of a electron????????
Make a man a fire and he will be warm for a day, set a man on fire and he will be warm for the rest of his life
At every advent of new technology, there are those who wish to keep Pandora's Box closed. So sad, really.
Run back inside, Chicken Little, and don't come back out until you have the actual formula that tells us what we should and shouldn't know.
Why are you letting these clowns ruin our country?
I used a similar example to debunk a well-known physicist
Wow, you debunked a well-known physicist - and maybe even quantum mechanics itself just by not understanding it! You're really smart!
I'd be willing to bet that you've also debunked evolution and all those non-Christian religions.
Skepticism is great, but pride in ignorance is another thing altogether. After you've debunked your VCR for thinking it's 12:00, visit the link below.
Why are you letting these clowns ruin our country?
With enough computing power, we can do real world simulations on weather patterns
Well, our simulations will improve with improved computing power, but computing power is only one half of the equation. Accurate measurement of existing conditions is the other half. If you figure out a way to strap some sensors on all of the butterflies causing all of our weather problems, then you're talking!
Why are you letting these clowns ruin our country?
How can I refute a one-sided anecdote where an AC claims to have debunked a well-known physicist? Maybe you should use bold HTML tags to point out where you actually formed an argument worthy of refutation.
All that's left is for me is to ridicule your post, your technique of citing your brilliance of debunking a well-known physicist, and your obvious pride despite being ignorant of the subject at hand.
You continue your folly by characterizing the physicist as "trying to mislead a group of students". What, we should just take your word on it?
You still haven't managed to form an argument. What exactly did the physicist say? What theory was he trying to support? What were your brilliant debunking questions? Where can we see his side of the "argument"?
Without any real argument on your part, all I can do is to make sure to voice my criticism of your piss-poor attitude. QM physicists have been producing valuable mathematics and experimental confirmation of their theories for the last fifty years. Yes, it's hard to get your mind around those theories. Yes, application of those theories in the macroscopic world is extraordinarily difficult. Yes, the effort thus far has taken a great amount of time, money, and effort.
That doesn't mean that I'm going to let you anonymously malign those physicists' motives with impugnity.
Why are you letting these clowns ruin our country?
please can i have a new box?
--
Primitive civilization with a rudimentary understanding of the laws of physics.
OK, the definition of a decoherence-free subspace:
Quantum mechanical wavefunctions are described in terms of their projection onto a set of basis functions. This is exactly analogous to a Fourier transform of a function (i.e. the projection of the function onto a set of sine and cosine function).
As the wavefunction of say an electron evolves with time, the weights of the various basis functions will typically change. If the wavefunction is coupled to other systems (i.e. other electrons, surrounding atoms, molecules, etc.), then the wavefunction becomes very complicated as the pieces that describe the single electron mix up with the pieces describing the other parts of the system. This is termed decoherence.
The subspace referred to in the posting is a subset of the full set of basis functions (like taking a finite bandpass of the Fourier space). For wavefunctions that can be described completely in terms of sums of the basis functions in this subspace, the wavefunctions will maintain their coherence, which means that as they evolve in time, they don't get mixed up with the wavefunctions describing the surrounding environment
This isolation of a subset of the degrees of freedom describing a system (i.e. the decoherence-free subspace) is essential for quantum computing, as a quantum computer uses the subtle correlations within a wavefunction to perform what are essentially massively parallel computations. Should the system decohere, the subtle structures in the wavefunction are lost.
Curtains for windows?
everyone will run faster follow the power of new computing
----- I have not programmed any program before I programmed my first program .
Okay, this will send a shockwave and everyone will say that i have no understanding of the subject matter... polarity/pairs, etc. okay i buy that - but, what if - what if our bi-polarity thinking is wrong? what if it's not '0' or '1' but '0', '1', 'or' ? what if there is a third player on the field? what if the actual 'state of existence'/'matter'/'on'/'horizontal' and 'nonexitence'/'off'/'vertical' are not the answer to all questions? what if both need a third defining factor? what if that factor is either 'when' or 'where'? when we talk about 'intantaneous communication between tow photons' we still are bound by the thingking pattern of three dimensions and are spooked by the appearance. but if we assumed that 'where' plays a role beyond the mere location in relation to one another, then this raises the interesting thought that if you eliminate the point of reference altogether, then there is nothing spooky about it anymore. furthermore, if you then bring it the 'when' without point of refernce, then it only becomes natural that all actions and reaction happen at the time/space or zeitgleich. of course, this theory would burn every scientific theory and idea that has ever been published. but then, imagine if you can, that any distance is only relative to the point of reference and that the time to cover that distance is only relevant if you confin yourself to three dimensions. now, if the speed of light is breached and time begins to slow, relative to a point of reference, of course, and if you then keep accelarating, what happens? can time slow indefinitely? wouldn't time stand still after a given [as yet undefined] speed is achived? would then time start spinning backwards if that speed was again breached? those assumptions are of course very unlikely [despite the good reading they make in sci-fi]. so, if that is not the case, then our three dimensional approach to the subject matter is very questionable to say the least. therefore, i now claim that the two photons do not communicate at all - instead, they both 'knew' what they would decide before they even started. they also had passed through the filters before at the exact same instant that they were sent on their respective paths, as they had no distance at all to cover and where in essence never apart from each other. think wormwhole and folding space...
There is a classical entanglement between regular EM waves, which is formally identical to the quantum state entanglement for photons (as appearing in Bell's inequalities). There is nothing mysterious about that meaning of "entanglement," everything is perfectly local, computers made using them have no special powers (beyond what an analog computer would have, if one could build a usable one). Dutch physicist Robert Spreeuw (well known name in the optics of atoms) had worked out quite far the mathematical correspondence between the two formally identical phenomena, including the extensions to the so-called "quantum computing." So, in that sense, the "entangled" state has been produced many times, in quantum or classical systems.
What separates the "quantum entanglement" is the presumed ability of quantum entangled state to collapse instantly the remote subsystem state when a measurement is done on the other subsystem. To test that key property, on which the Quantum Computing contact with reality vitally rests, Bell inequality (or its more recent variations) is used. And that is where no test to date has come even close to being "loophole free." While papers and preprints may not emphasize or even mention the loophole, the fatal loophole is present in every single experiment. Merely showing that the state is consistent with the entangled state (which is how far the actual, raw, non-adjusted data ever goes) doesn't prove that it has the key property of remote subsystem collapse.
To prove that, the raw data would have to be used, before any assumptions about the missing data (inferred to exist from the triggers of some, but not all necessary photo-detectors, and which is routinely pressumed to obey the "fair sampling" hypothesis) are used to adjust the raw data (raw detector counts). Additional data adjustments are also done, specific to the experimental setup (especially common are subtractions of background counts, again under unproven and unprovable assumptions about unmeasurables).
Only after all the adjustments, the wishfully massaged data does violate (amazingly enough) the Bell inequalities (which is the objective if one wishes to prove the existence of quantum entanglement).
Having been once a theoretical physicist (and having written masters thesis in this area), I had corresponded over years, via mail and email with most experimenters who had published the "almost" proofs of the quantum entaglement. And when pushed, yes every one will tell you that indeed, there is a loophole of one kind or another, the hand put assumption about the missing data which makes the adjusted data violate the Bell inequalities. Without it, on the raw data, none violates the Bell inequalities.
To put it in plain terms for the non-physicists here, one could for example, having no data on where you were 1 minute ago, assume that you were 1000 miles away from your current place. And, say, now we can all find you on your current place reading this message. We're missing the data on where you were 1 minute ago (and there are no outside witnesses to say one way or another) but with our 1000 mile assumption, that seems pretty amazing how you managed to travel that far in so short time. Now we take for granted that you did travel 1000 miles in 1 munute, and then we start constructing ever more marvelous hypothetical technology based on that enormous speed for an earthly vehicle. Sure, you could come up with some pretty nifty transportation systems if you build upon that 1000 miles/minute vehicle you supposedly must have used. And if someone is going to pay us to do research on these possibilities, we're not going to pursue seriously the issue whether our key unverified assumption (your location 1 minute ago) might be wrong.
That is what the issue in quantum entanglement comes down to. Without making the unverifiable assumptions of certain particular kind (like that you were 1000 miles away 1 minute ago) about the missing data, nothing unusual or amazing can be deduced, the entanglement measured on actual data is no different than the classical one (which of course, can't collapse instantly the remote subsystem state).
There've been some experiments in Italy (so no ref) that have been running over the past couple of years (not "the 1980's") that have "proved" entanglement over very large distances (kilometres)
You probably mean Geneva experiments by Tittel et al from 1998-99; check on the Los Alamos preprint archive for their 10 km exeperiments. There is the group's email address at the top of the abstract, go ask them if the experiments were loophole free (they were not). Now, of course, they'll tell you stories and do the usual handwaving dance to convince you (if you appear in-the-know enough to be worthy their reply) why their unprovable assumptions were "natural" and why the "loophole" isn't likely to change their conclusions, should it ever be possible to do experiment without it. Yeah, sure.
And that is the bottom line. Just like with my "assumption" about your location 1 minute ago, I could dance and handwave all I want, why it is "natural" to assume, provided there are no witnesses to contradict me (i.e. data missing for good), that you were 1000 miles away, unless one can show without such assumption existence of the 1000 miles a minute vehicle, why should anyone believe it. Or fund it with their tax dollars.
When you peel off the layers of technical jargon and euphemisms (such as "loophole") protecting the dirty little secret and its priesthood's well being from the outsiders, that's the vital presumed "fact" on which the whole marvelous technology of Quantum Computing rests on -- the wishful assumption about the unmeasured, unmeasurable and irretrievably lost data points.
Quantum mechanics, yes. But there is nothing ever measured explained by the quantum entanglement hypothesis. It has never even been confirmed experimentally (without loophole), much less that it has any use in explaining any phenomenon actually observed.
That hypothesis is based on an independent (from the rest of QM) postulate on how the composite system state transforms in the measurement. The actual postulate in fact contradicts the normal unfolding of a quantum state (as prescribed by the evolution operators, or Schrodinger equations), saying in effect that when the "quantum measurement" (never really defined with usable precision) is done, the regular dynamical equation somehow cease to be valid, for unspecified time interval, in an undefined way, the state "collapses," then somehow, at some, again undefined, point in time, the normal dynamical equations take over again.
It takes years of conditioning in physics to be able to submit ones common sense to such mind twisting "logic." And even then it never tastes quite right, there is always some fishy odor around it. It takes then more years to be able to snap out of it. But it can be done.
Eventually, of course, the peddlers of quantum magic will have to face the question -- where is the beef, show me the "Quantum Computer." Let's benchmark it. And when the proverbial rubber finally meets the road (which they've been skillfully avoiding for some years; read Kwiat's hedging with "extremely difficult"), the only thing which will collapse will be that whole house of cards.
Anyway, I know a number of folks who are working on various bits of QC research.
The actual Quantum Computing, if understood as a pure mathematical discipline, unrelated to physical reality or actual computing can be quite interesting (to those who enjoy that kind of abstract mind challenge). And it is not impossible that it may end up having some applications in analog computing (the plain classical style) if that can be made to work. Or inspire some neat devices based on quantum dots. One can always stumble into the right thing for the wrong reason. After all, Columbus thought he was going to India, and believed he found it.
These are not shady scientists looking to push alternative theories under the rug and secure government grants on false promises.
Well, that's true of most large scale swindles. Take various large scale superstitions (the religions) -- most priests will sincerely believe dogmas they preach. But try touching the core of the dogma, then you find out how it really works. The peer review in this field, for all major journals and commissions for research grants, is stacked extremely one sided way against anyone pointing to the fundamental flaw of the theory -- the vital experimental connection with reality of the whole QC superstructure is based purely on assumptions about the (iretrievably) missing data. If one makes different assumptions, as one is free to do about something which is unmeasurable, the whole relation of the QC with real world fizzles away.
Now, the old boy network running the peer review in all major physics journals, will let anyone challenge any detail of the superstructure, argue how fast QC will be, will some QC algorithm really work in n*log(n) time or some such. But try touching the vital "loophole," and you might have as well spit at the editors face or kicked the african killer-bee nest -- your manuscripts will be turned down with the lamest of excuses and run-arounds, your grant proposals flatly rejected. Just ask Trevor Marshall, who is quite a competent physicst, and has published many dozens of well cited papers (in other sub-fields) -- once he seriously poked into the "loophole" he couldn't get published in any of the major journals, or get funding for experimental refutation of the quantum entaglement, and ended up taking early retirment from his tenured position out of frustration with, what he came to call, the "priesthood."
See my earlier comment on difference between general entanglement (which is a well established process) and quantum entanglement.
The element necessary in any form of quantum computing, which provides the ability to read out the "computation" result, is the quantum measurement postulate (also referred to as projection postulate or state collapse postulate). For any nontrivial "computation" QC uses its special case, the assumption that a measurement on one subsystem will collapse the state of the remaining subsystem (this is supposed to happen even without any interaction with the remaining subsystem, which could be miles away and physical field/force is needed to carry out this remote collapse).
That element is an independent postulate of the theory. Nothing in actual explanatory power of QM requires it, no great successes of QM (which include much of todays semiconductor technology, where quantum solid state theory, developed in 1950s, helped point the way and provide theoretical tools for the more practical experimental research later) over decades need it. All detector count results are computed using correlation functions (such as Glauber's multipoint correlations used in quantum optics to predict correlations among multiple photodector counts) which don't require any collapse. The collapse is a gratuitous and by itself useless in any explaining (of any observed phenomenon), a parasitic add-on to the theory.
It has no operational use other than in "explaining" the experiments trying to confirm it (and none of them did, unless you're willing to believe several additional quantitative assumptions, about the state of unmeasured and unmeasurable quantities, assumptions which are outside of the postualtes themselves). And of course, they're used in "predicting" the existence of quantum computing, quantum teleportation and other assorted miracles.
Any claim by experimenters to have demonstrated QC, if they're fully honest, will disclaim it the same way Bell inequality tests are disclaimed -- yes it was shown, provided we accept additional assumptions (which are outside of postulates themselves) about the missing (unmeasured and unmeasurable) data. Only the so adjusted "data" exhibit QC or violate Bell inequalites. The raw data (unadjusted detector counts) show no such phenomena. These adjustements are sometimes expressed in terms of extrapolating the results to the sufficiently decoherence free state.
You seem to believe that reality is what you see.
Of course, not. I have been theoretical physicist long enough to know better (and I have been doing lots of other the after the academia to know even better than that). Reality, as far as physics is concerned, is the what the best underlying models say it is, which means it is subject to revision as the models are revised.
What that means is that there is "model," postulates, mathematical formulas and algorithms used to extract the behavior of the models. Model by itself is thus a pure construct. To make it mean anything there is also a set of "correspondence rules," prescribing how the properties of the model map into the observation. This is where the model makes the contact with conventional empirical reality. Without such contact it is not a scientific model, but a pure speculation (the artsy-fartsy stuff).
In modern physics the core components of "reality" of the model (strings, quarks, electrons) are never observable in any direct way. What is actually done is to compute some long chain of (mathematical) consequences of the model, then at the end of chain there is a set of numbers which can be associated, via the corrsespondence rules, with the experimental facts, e.g. with counts in some detectors. If the counts match the numbers produced by the model, and if one can vary the input assumptions of the model while still maintaining this match, well, the model is said to be "reality" behind the phenomenon. Now that doesn't exclude possibility of alternative. The present models of physics don't exclude alternative models. What experiments show is at best that a proposed model is consistent with the experimental facts, not that no other model can account for those facts.
The problem with QC model is that its vital contact with reality (without which the result of the "quantum computation" could not be read off) is that it relies on the state collapse postulate, which so far does not make sufficient difference by itself to be confirmed empirically, in the sense of exluding the common sense alternatives (such as allowing that QM is incomplete, i.e. there are variables missed by the quantum description, which only covers the statistical properties of such variables, but misses their detaled individual properties).
Despite 7 decades of experiments attempting to establish absence of those "hidden variables", including the 3 decades of Bell inequalities tests, no "loophole free" experiment exists as yet. The odd wording, "loophole free" experiment, is a customary euphemism trying to say in a nice way that no actual data in the experiments show what the experimenters wished to show (ultimately, the non-existence of local hidden variables, or specifically, the Bell inequalities violations). Only the fictitious "data," obtained via a very "special" kind of adjustments from the real data does violate the inequalities.
The justification for these adjustments is not the postulates of quantum theory themselves, but are ad hoc one shot assumptions about the unverifiable missing data, belonging to no theory, and useful for nothing else in physics but getting the otherwise "unwilling" data to fit the desired conclusion (violation of inequalities). If you don't accept them (as you don't have to, unless your funding proposal vitally depends on being able to bedazzle the fools with money), no violation follows, no QC (can't read the result of QC, it is still there, allegedly, but it can't be gotten to), no teleportation (and no funding).
To separate that postulate from more plausible alternatives (such as "incompleteness of QM," i.e. the existence of additional quantities for which the rest of QM gives only statistical, but individual, predictions), Bell inequalities were devised (by late John Bell), which give a cutoff point on statistical correlations in a special type of multipoint photon/particle detections. If the counts violate the inequality, the "local [hidden, unknown] variables" are exluded as a possible explanation of these experiments. If the counts don't violate the inequality, some underlying [unknown] quantities are consistent with the rest of QM.
It turns out that actual data doesn't come even close to violating the Bell inequalities. Only after great deal of handwaving, aiming to convince that certain conjecture about the statistical properties of the missing data (the instances when the photons failed to trigger detectors) ought to be accepted as plausible, only then they can "adjust" (change to a more desirable form) the data, and lo and behold, the new and imrpoved data miraculously violates the Bell inequalities.
The fact that very specilized experiments are needed to find out whether such subsystem collapse occurs at all (consequently whether QC can ever work, even in principle), ought to tell even non-specialists that the successes of QM have nothing to do with that postulate. It is a parasitic add-on, with no real use (other than parting the cash from the fools who fund the swindle).
Caroline is an outsider to the field, but otherwise an intelligent person and specialist in her own field (statitstics), which has great deal of bearing on the interpretation of Bell inequality tests. While I certainly don't buy all of her skepticism, and even less her conjectures about the alternatives, she does have a very sharp eye to spot a swindle, a loophole in setup and the argument. Also, as an outsider, she does present the key issues in a much more accessible way to non-physicists, without skipping over or hiding the tricky points (as popular books by physicsts tend to do).
Any experimenter who did such experiments (and there were dozens of experiments over the last 3 decades) will acknowledge that the experiment was not "loophole free," which is a euphemism for "the actual data didn't confirm it, but if I assume certain statistical properties of the photons that escaped detection (and which, luckily, no one could now show to have been one way or the other), and then remove the data points which don't fit such assumption, and also fill in the gaps for the missing points (which just happen to make up 995 out of 1000 final data points) with the points having the assumed statistics, then the statistics of these new and improved data points does violate Bell inequalities, proving thus the quantum entanglement." That's what it comes down to, when you peel of the jargon, the handwaving and the euphemisms.
Go to the Los Alamos preprint archive (I gave the link earlier), pull the emails of the authors claiming such verifications and ask them whether their experiment was "loophole free" (none was) and when is the "loophole free" version scheduled to start (not scheduled, not designed, not within present detector technology).
Research and experimentation are rarely a waste of effort, whether to prove or disprove, because we have to find out. The skeptics said we would never build aircraft, split the atom, travel safely in railway carriages, go to the moon, etc etc etc.
The key difference from your analogy is that in your examples, the skeptics were the establishment physics, the folks who decide what is published and what gets funded, while the people claiming something is possible were the outsiders to the establishment.
In the case of QC and quantum entanglement, it is the establishment which claims something is possible while the heretics claim it isn't. So, the much closer analogy is the case of medieval church, an establishment of that era, claiming various superstitions and miracles are possible, while skeptical heretics (such as Galileo or Bruno), who were outside the establishment, claimed that was bunch of bunk.
Sure, and I got a nice bridge I could sell you, real cheap.
The detector efficiency alone on the photon wavelenghts normally used won't get you beyond 60%, if you wish to take huge noise, 30% and up depending on wavelengths (which in turn requires background subtractions, invalidating the experiment as a "loophole free" Bell inequality test, ie. it fails to exclude the local hidden parameters).
But if you reduce the detector trigger thresholds (such as using ultra-low noise low temperature photo-multipliers), to reduce the noise down to 1-2 percent, the sensitivity, hence the detection efficiency, drops down to 20% or below. It is a no win tradeoff, you either get noise or you get low sensitivity, invalidating in both cases the "loophole free" status for the experiment.
The parametric down conversion (PDC) sources used nowdays also give outright 50% loss, making them entirely unsuitable for the loophole free test. Of course, if you accept additional hand-put assumptions (which are unverifiable and which are outside of the QM postulates), you can handwave it as unimportant.
One try to improve detection efficiency, without noise, by going to higher frequency photons, but that has a downside in decreasing the polarizer effectiveness (depolarization increases). Thus for the ultra-high frequency photons, such gamma photons, which can be detected at near perfect efficiency, the optical polarizers don't work at all, and one has to use Compton effect to obtain very low power effective polarizer (these tests have been done, too, but are of much worse quality than those with the optical photons).
Then you get aperture and geometric losses, which often exceed all other (small aperture is often chosen to avoid noise, reflected photons and accidental coincidences from the multiple sources). Again, these can be adjusted away via yet another hand-put assumption.
To test Bell inequalities in a loophole free experiment, none of these adjustments on the raw data may be done, and the net efficiency on the raw data has to be over 82%.
The grant savvy experimenters have in recent years gotten into habit performing several adjustments implicitly (usually embedded into the software operating the setup) and then quoting as their setup "efficiency" the figures of some minor hand adjustments done at the end (such as polarizer losses), ignoring to mention the extensive data "cleanup" done by the software. That kind of gimmick may be fine for engineering aplication, but has no bearing on the Bell inequality tests.
The only thing which could remotely come close to 99% (if you haven't made the figure up, as it seems likely) would be if someone had "assumed away" all other adjustments, except for the depolarization, which may be brought to 1-2% in losses (for optical photons and if the aperture is made very small; of course, then the aperture losses become quite large, well over 80% losses just from that).
If anyone tells you they have made a "loophole free" Bell inequality test, they're trying to be humorous or are flatly lying. Since nothing is even remotely close to it and anyone even casually informed in the field knows it perfectly well. It would be like someone telling you in confidence to run and buy Intel stock, since they have a new Pentium prototype running at 500 GHz. You would know right off it ain't so. With or without adding the beowulf cluster.
The issue with QC is that it is advertising itself as a physical theory, and even being promoted as an applied physics or enineering based on a dubious and unverified link with the reality.
Of course, there is nothing wrong in scientific conjectures, as long as they're being labeled as such, with clear and honestly acknowledged distinctions between the facts and the hypotheses.
In case of QC, the weak hypothesis at the root of that theory is labeled as a fact (the subsystem state collapse, necessary to read off the results of the "computation").
Even worse, the legitimate critics, including some reputable physicsts with many years of publishing well received and cited works, end up getting gagged by the peer review "priesthood" (as Trevor Marshall, one of such critics, calls them), their research grants somehow dry out, as soon as they point out that emperor has no clothes, that there is a rot in the core this hyped-up discipline.
The ion based tests open their own set of loopholes, while closing the common photon tests loopholes.
Unlike photons, ions can be detected with high efficiency (theoretically 100% since they're massive particles). But since the spin coupling energy (with Stern-Gerlach magnets, the analogue of polarizers in optical experiments) is much lower than their kinteic energy, the spin measurements (selection) are much less reliable than those of optical photon polarizers. This results in large background counts (due to depolarization) and this subtraction is a well known loophole for LHV models (also occuring with photon tests when the sensitivity of the detectors is increased).
Additional problem is in reliable production of the entangled pair state, again due to the low energy of spin-spin coupling (between the spins of the ion pair) relative to other energies involved in the process of pair production and collimation. The result is again a large number of "accidental coincidence" detections, i.e. another contribution to the background to be subtracted.
Hence the background subtractions make ionic tests very similar to the case of photon tests when the photo-detection is increased to near 100%. This can be achieved by using very high energy photons (e.g. gamma photons from the electron-positron annihilation), in which case one can have near perfect detection, but polarization measurement doesn't work too well (via Compton scattering), producing very large background which needs to be subtracted (exactly as with ions, and for essentially the same reason).
An alternative way to increase the photo-detection efficiency is to use very sensitive (low threshold) detector, but that produces large dark current, the background noise, which again has to be subtracted. The resulting unsubtracted data in such case are almost exactly what a classically entangled EM wave packet would produce (i.e. what Maxwell equations would predict). You can, for example, see the actual raw data from Asepect's PhD thesis, where he did his famous cascade experiments, on Caroline Thompson's web site. For more modern PDC based experiments, see the similar Stochastic Electrodynamics models (which are again the Maxwell equations based models, but with stochastic initial & boundary conditions) which reproduce the raw data for these experiments, on the Trevor Marshall's site.
It is true, as far as I know, that no single experiment has gotten sufficient sensitivity using spacelike separated measurements... yet. It seems somehow perverse though to hang a defense of local realism on this fact.
This is the oldest handwaving argument for dismissing the loopholes in Bell tests, i.e. why would some future increased sensitivity (for detection or polarizer efficiency) suddenly switch from the good agreement with QM (modulo loopholes) and give preference to the local realism.
First one should note that the non-adjusted experimental data is already consistent with the local realism (and there are numerous local models for variety of the setups reproducingt he experimental non-adjusted data). So nothing here has to change for more sensitive experiments. Only the wishfully adjusted data (when the loopholes are dismissed via ad hoc unverifiable assumptions) exclude local realism.
Second, it is perfectly natural for any local realistic model to make the efficiency of the detector or polarizer dependent on the values of the local (hidden) variables. Such dependency is outright excluded by the fair sampling (be it sampling by the detectors or the polarizers; in the ion case the critical fair sampling problem is at the Stern Gerlach magnets, the "polarizer," not the detector, which is here near-perfectly efficient; while in photon case it is at the photo-detectors, and not at the near-perfect polarizers). The ion tests only shift the fair sampling (in the hidden variable space) problem closer to the source, at the Stern-Gerlach and the pair source production depolarizations (which results in the background counts, which is presumed to be a fair sample in the LHV space, thus it is flatly subtracted subtracted from the total counts).
To illustrate this point in a form more accessible to non-physicists here, consider a national poll using email. Such poll will fail to detect people who do not have computers with internet connection and email account. Suppose now the poll analysis expert wasn't informed about the method of communication used to obtain the poll data. This is analogous to the Bell test experimenter looking at the counts of particles detected, but not being able to measure or know anything about the so-called local hidden variables (analog to the email, which is here invisible variable/value to the poll analyst).
The analyst, not knowing anything about the underlying means of communication, proclaims now that he will assume that the chance of being detected by the pollster doesn't depend on the (invisible to him) method of communication, hence the sampling is proclaimed to be fair, by declaring it to be so. This is exactly how the experimenters in the Bell tests establish that their sampling is fair (at detectors or polarizers), independent of the hidden variables -- by proclaiming it to be so.
Now suppose in our poll, a question asked "what was your income?" Clearly, the sample is biased here, and the income discovered this way will be higher than the national average. In other words, the poll is more sensitive in "detecting" higher income than lower income people. In the extreme, if the question is "are you homeless," the sample will completely miss any homeless person, i.e. this "detector" is made completely insensitive to the "homelessness" by virtue of the particluar value of the hidden variable (the means of communication = email). OTOH, if the means of communication were walking through the parks at 10PM, and asking anyone encountered the same questions, the results would be biased the other way. Here the hidden variable "means of communication" = "asking people in the parks at 10PM" -- the variable has different value, and the detection profile is quite different.
In other words, the hidden variables will perfectly naturally bias the sample, almost by necessity, since they do affect (or are correlated with) what is detected and what is missed. Ruling out the sample bias by fiat (as is done in every Bell test), is effectively excluding the hidden variables upfront, by declaring it so.
The common interpretation of Bell inequlity tests is merely another manifestation of the problems which were part of QM since the projection postulate (or wave function collapse) was introduced in 1920s. The EPR phenomenon was application of the projection postulate to the subsystem, in order to show how absurt it is. Bell's inequality came in 1960s (and tests in 1970s), when QM was already four decades old.
Einstein and Schrodinger who were among the handful key founders of QM did not accept the wave function collapse from the start. It was absurd, an ugly kludge trying to hastily patch up the holes in, what at at the time was, immature new theory, to make it appear complete.
The core absurdity of the projection postulate is that it switches the rules of how the system evolves midstream -- suspends them at an undefined moment in time, based on no objective criteria, for undefined length of time, then changes the state of the system in a way cotradicting the original equations (which were "luckily" now suspended, otherwise there is a contradiction), then the new rules suddenly go away, at some undefined time and for no rime and reason, and the old rules are said to hold again.
It is one ugly kludge devised to patch up the contradiction between dynamical equations and the presumed effect of measurement -- since you can't have two contradictory rules in a theory, the "solution" was to effectively inject divine will into the game and have it willfully switch the rules midstream back and forth, as it pleases.
The Bell's inequalities were devised (by late John Bell) to serve an experimental test which would decide whether this switching of the rules is really necessary -- whether there may exist a single set of "reasonable" rules (i.e. without instant action at a distance), which always hold, which need no divine intervention to suspend them and then let them work.
The way I see the experimental results so far is that indeed the test does show that the existence of a single set of rules is consistent with the experimental data. Only when the proponents of the divine intervention (the collapse kludge), sticking with that style, I guess, inject yet more divine intervention, the arbitrary hand-put assumptions (which are outside of the postulates of QM agreed upon upfront), which conveniently let them weed out (or inject new ones) the data points after they have them already at hand -- only then these hand-tweaked "improved" data excludes the possibility of the single set of rules. Well, who would have guessed that.
So, ignoring the euphemisms and other verbal gimmickry (e.g. talking about "loophole free proof" instead of "proof that actually works" or "proof that proves" or just "proof") and silly little games with data after the "bad news" are in -- the nature, as far as it has revealed itself until now, seems to be perfectly happy in using single set of reasonable rules. The rules we don't know as yet, but which are not excluded by any facts known to date.
Well, that's a different topic. All they did is create an entangled state (which is the type of state serving as an input into the Bell inequality tests). Even the classical Maxwell equations allow for entangled EM wave packets of the same form. From theoretical viewpoint they're just one form of initial & boundary conditions, there is no controversy about it at all. The classical entangled EM wave packets do not violate Bell inequality. (And neither do the quantum ones, as far as anyone could measure to date.)
A computer built on such 'classical entanglement' would be merely an analog computer, having no special powers. And neither would QC, unless one can establish the additional very special properties of that state, the subsystem collapse which the Bell inequality experiments were devised to test.
The paper doesn't even try to prove that those states exhibit the subsystem state collapse property, which is the point of Bell inequality tests. It only deals with preparing particular initial & boundary conditions, which is fine (and probably useful), but not relevant for the subsystem collapse hypothesis or the Bell tests of it (which in turn is the only way the QC could read off the results of the "computation").
The two are inseparably entangled, as it were. If one falls the other goes, too.
First, it is precisely the subsystem state collapse, as prescribed by the orthodox interpretation, how the Bell's (or EPR-Bohm) theorem deduces the quantum prediction side of the Bell's inequality. (And this is the same collapse that quantum "computers" are supposed to use to extract their "computation" results out of the entangled states; which is what brought us here.)
Second, as explained in the previous message, the Bell tests were the culmination of a long debate whether the hidden variables are excluded by QM (or its predictions). First there was von Nemann's proof (from 1928), which claimed to prove HVs are inconsistent with QM predictions. That was accepted by the mainstream as the final word, which in turn gave rise to the present form of the measurement problem -- if HVs are impossible, then the statistical nature of QM cannot be due to unknown or uncontrolled values of some hidden variables, being merely revealed by a measurement.
The linearity of the evolution equations of QM precluded the combined system, the "measured system" + the "measuring aparatus" from ever being able to chose or settle into one of possible results. Von Neumann's "solutions" was to declare that it was human mind, the mind of the observer which finally made such decision (this view was also promoted by Wigner).
That was a clear acknowledgment that there is no way the rules of the evolution along with his interpretation of them, especially his no-go theorem on HVs, can reproduce what happens in a measurement -- the system, in real world, does seem to pick somehow one among the alternatives as the measurement result, i.e. it appears as if the state got projected onto the one definite aparatus state (eigenvector).
Others, who didn't like the idea of "human mind" becoming a part of a physical theory, but who bought into the von Nemann's no-go theorem for HVs, kludged from the two contradictory pieces, what nowdays is called the orthodox interpretation. Since the two ways of evolution contradict each other, we will give each a role but never at the same time. How this is to be decided, which one is in charge and when and where it is supposed to switch the rules, that was never really explained. But too keep the physics students from asking too many questions, they wrapped the kludge into the term "measurement" and wrapped all that into the generous layers of warm and fuzzy talk about "irreversibility" "macroscopic aparatus" etc.
Not everyone bought it, of course. One who didn't buy either, the von Nemann theorem or the "measurement" fog, David Bohm went out and constructed an explicit HV theory reproducing the QM prediciton -- in effect showing that von Neumann theorem was wrong.
Well, that meant there was no need any more for all the "measurement" fairy tales, for the two sets of rules and a divine intervention (or a mind), to switch between them. The orthodox school didn't give in though -- all those multitudes of leared tomes and deep papers they produced to "splain" it all, to the simple-minded physics students, who stubbornly clung to their common sense -- all that work down the tubes. No way. All those deep thoughts on complementarity and marvelous connections with the hindu worldview, to dump it all. We can't let that happen.
In the scramble that followed Bohm's theory, the orthodox school kludged quckly another proof, Kochen-Specker theorem. It backed off a bit from the von Nemann's grand claim that no hidden variables of any kind are possible, to a lesser claim that "no non-contextual hidden variables are possible." And since the non-contextual kind was declared good and the contextual kind was declared to be a no-no, everything seemed to be now back where it was before Bohm. The Bohm's HVs were "contextual" hence no good, hence ignore them, shut the Bohm up. Bohm squeeled for a while, but what could he do, everyone's got to eat at the end.
No major objections to Kochen-Specker were allowed into the open for several years, the wavering orthodox line was held, until John Bell published his paper showing (what Bohm and everyone else who cared about the whole mess, already knew) that the Kochen Specker theorem suffered essentially the same flaw as von Neumann's, so it is no good. But since Bell had another proof in his paper, the Kochen Specker fell, the Bell inequalites arrived. They did retrench a bit from Kochen-Specker, but better something than nothing, since Kochen-Specker could not have been held for much longer, anyway.
The core claim of Bell inequalities is that if QM predictions are correct any hidden variables which reproduce these predictions must be non-local. And since non-local is "bad," then no "good" hidden variable theory can exist. And thus the whole mass of learned production from the best and sharpest minds of the orthodox school on the mysteries of the quantum "measurement" is still a valuable contribution to science and a required reading for the poor and stubborn physics students.
So the Bell's inequalities are the last defense line upholding the whole orthodox interpetation -- they provide a need for its existence. Namely, if a local hidden variable theory could reproduce the QM predictions (which Bell's theorem prohibits), then as with the fall of von Neumann's therem, there would be no need for the measurement theory mistique, collapse and the rest. Everything would revert back to common sense, measurement simply reveals the values some variables had at the time of measurement. No need to write "learned" tracts on reconciling and understanding the contradictory or for invoking hindu deities to arbitrate between the two contradictory set of rules.
At the same time, as pointed out at the top, Bell's theorem uses, none other, but orthodox interpretation of collapse to deduce what the QM prediction ought to be. So, it is a mutually supporting arrangement, between the Bell's theorem and the measurement theory of the orthodox interpretation -- if one falls, the other has to go. Well, one must admit it was cleverly crafted. But the nature (un)fortunately is the last judge what ought to happen. The actual data from the experiments showed no violation of Bell's inequalities. What to do now?
Well, the clever folks who fought off valiantly the great Einstein himself, then the scary Bohm's assualt, surely won't fold their wares just for some little experimental data. So the Quantum side of the Bell's inequality (the supposed QM prediction based on the orthodox interpretation of subsystem collapse, which violates inequality, but doesn't match the actual measured data) was relabeled "ideal QM prediction" and the experimental data, by implication, as non-ideal, thus in need of fixin', requiring the massive hand adjustments of the obtained data points, rationalized by a variety of unverifiable assumptions pulled with a lots of handwaving out of a magicians hat ("massive" as in changing or injecting 990+ points on of every 1000 presented as the final all nice and fixed-up result).
Ain't that so convenient. Instead of, as anyone with an ounce of common sense would suggest, producing the correct QM prediction and comparing that to the experimental data, the alleged QM prediction of the Bell inequality is kept as is (since otherwise, of course, there would be no inequality for the correct prediction, the one that matches the actual data -- hence no Bell theorem, hence the last defense line of the orthodox dogma would fall) and the experimental data is wishfully twisted around to bring it into compliance with this alleged QM prediction.
Unfortunately, a poor physics student taking courses in QM has no chance of disentangling this ball of tangled up nonsense which has come to resemble talmudic law or some such endless web of legalistic word games.
The new line of defense (to become effective as soon as a confusing enough theorem can be put together), is going to be "decoherence" (an euphemism for incoherence, I guess). That is when the "quantum entanglement" (another very appropriate name) will be quietly dropped.
NOTE: I am not suggesting above that someone actually set down and plotted out all the twists and turns of the developments described. No need for that. It is the phenomenon of the same kind as the Adam Smith's "invisible hand" guiding the economic developments as if they were cleverly planned, even though there is no explicit plan and every player is merely pursuing their own narrow self-interest. Science does operate as a marketplace of ideas and models.
You are objecting to the part one of my message. The connection you are talking about is the essence of the part two of my response. In brief:
- a) If the empirical tests cannot show (loophole free) the violation of Bell inequalities, that implies there is no justification or need for the orthodox version of collapse and the rest of its "measurement theory." The collapse, to the degree it occurs, would then be a consequence of some underlying (local, nonlinear) dynamics, like some type of phase transition.
Thus the empirical (loophole free) confirmation of the Bell inequality violation is the only thing which would uphold (and not very convincingly, at that) the orthodox version of the collapse.b) If the empirical tests do show (loophole free) the violation of Bell inequalities, then any underlying local dynamics (local realism) is excluded. That still doesn't mean the orthodox interpreation or its version of collapse is valid, it merely prolongs its viability since no good alternative is available.
The Bell's theorem as a theoretical reuslt is what had placed the same question [i.e. is there any need for orthodox "mesurement theory" at all (including its state collapse)] in a testable form, giving thus the orthodox interpretation the lease on life pending the definite experimental results. Following the fall of von Neumann's and Kochen-Specker theorems (as proofs of impossibility of any hidden variables consistent with QM predictions), the orthodox interpretation had no reason to exist, no reason for its version of wave function collapse, since everything could, at least in principle, understood as a result of some, presently unknown, underlying dynamics.
The above shows how the Bell's inequalities and their empirical tests (if case (b) occurs) support the orthodox interpretation and its version of collapse.
The other side of this coin is that the theoretical Bell's inequalities rely on the orthodox interpretation of collapse to deduce its QM predicion (which sets LHV boundary in the inequality). So the boundary in the inequalities depends on the Orthodox form of collapse.
Now, it turns out the experimental data don't violate the inequality. Normal scientific method would in that case be to work out a more accurate QM model (to replace the QM side of Bell's inequality) which can replicate the actual data as it was measured. After all, the detector's efficiency (or the background "noise") isn't some god given untouchable and unspeakable decree, but it is one of the physical/QM properties of the experimenal system.
Instead, the orthodox school had merely renamed the Bell's QM prediction as "ideal QM prediction" and then gone out their way to fix the "non-ideal" experimental data, based on several unverifiable ad hoc assumptions, which are outside of the postulates and which almost by definition exclude LHVs all by themselves (with no tests needed, e.g. the "fair sampling" is directly contradictory to what one would expect from almost any LHV - that the particle detection does depend on LHVs [what else could it depend on but HVs, anyway] - the "fair sampling" imposes upfront the condition which excludes any LHV theory with such dependence.)
You seem unconvinced that they have been measured, and I can respect this position to the extent that there are genuine flaws in the experiments.
What I am saying about the tests is not a matter of convictions or tastes at all. What the test have excluded is not all LHVs but the LHVs which satisfy not only the normal requirement to be able to replicate the experimental data (or equivalently, to replicate the exact, not just "ideal," QM predicitons, which of course ought to agree with the actual data, as well) the additional assumptions applied to the experimental data to bring them into agreement with the "ideal QM predictions" of Bell's inequalities.
The only thing which is a matter of tastes or convictions is whether these additional assumptions are "plausible" or "reasonable" constraints on LHVs. But there is no dispute at all whether these additional assumpitions are being made -- anyone with any knowledge in the subject knows they are being made.
Now, to see whether these additional constraints on LHV are actually plausible and reasonable, one has too look at a variety of proposed LHVs which can replicate the experimental data of those tests and check whether they satisfy these additional constraints.
If you check the literature (especially those from the Trevor Marshall, Emilio Santos and their school) you will see that those additional, innocently sounding, contraints are preposterous -- they exclude reasonable theories (such as Sotochastic Electrodynamics [SED] and Stochastic Optics [SO]) upfront, with no experiment needed. The SED/SO does in fact replicate the data from all the PDC photon experiments for Bell inequalities and for the other so called non-classical effects in quantum optics. To exclude such broad types of theories upfront based solely on what someone wishfully labels "fair sampling" or "non-enhancement" hypothesis, is absurd. At the very least such theories demonstrate plainly that those euphemistically labeled assumptions are throwing out, all by themselves (with no experiment needed to support them) much more than what is usually claimed to be excluded a priori (i.e. by the added assumptions alone).
That is a correct -- the inequality doesn't exclude any LHV theory a priori. But when the experimental results are being adjusted, to match the so-called "ideal QM prediction" ("ideal" is an euphemism meaning that it doesn't actually predict the data correctly), the additional assumptions, claimed by their advocates to be "reasonable," are introduced which all by themselves exclude the whole classes of HLV theories (for not complying with these "resonable" additional requirements). So that is how the "resonable" HLV constraint comes into the play here.
It is a question open to experiment whether nature violates Bell's inequalities. We seem unable to agree on whether Bell violations have been measured.
It is not a question at all, for anyone with some knowledge of the subject, whether any actual experimental data so far had violated Bell inequalites. None ever did. Not a single experiment had such data.
What violated the inequalities in all experiments making such claims are the adjusted data, the data filtered out and modified based on some arbitrary and unprovable assumptions. Consequently what the experiments exclude is not an arbitrary LHV theory, but only a subset of LHV theories which, not only reproduces the measured results (and some do so quite well), but also complies with those ad hoc assumptions (which come from no basic postulate or experimental results; after all, they are put in by hand precisely because they don't follow from anything else).
If they have been then locally real theories are wrong (not that quantum mechanics is right). If they weren't violated when they should have been, then quantum mechanics is wrong. There aren't any other choices.
The sticky part is "when they should have been." Namely, if you have a theory which supposedly corresponds to reality (as far as experiments can establish), say QM, then the results of Bell inequality tests should match this QM prediction. They don't. But that doesn't mean QM is wrong.
Specifically, the Bell's QM "predicition" showing that QM ought to violate his inequality is very sketchy, back of the envelope style, which ignores many additional effects in any real experiments. So it is no wonder that such "prediction" doesn't match the data. It is not a real prediction but a sketch, a hint for someone to do the full calculation, taking into account all the effects involved in a real phenomenon (i.e. it should be a full Quantum Electrodynamics [QED] calculation, not the little skeletal calculation of the Bell's paper).
If anyone had ever made the full QED calculation of any Bell inequality experiments, the prediction of such full calculation would match the obtained data, no one on either side doubts that. (The non-relativistic QM is not suitable for such prediction, except as a rough indicator of qualitative behaviours.)
In other words, the full and exact quantum electrodynamics prediction, capable of predicting the actual data (the average photo-detector counts obtained), does not violate Bell inequalities in any of the set-ups tried so far.
The only thing which has ever violated the inequalities is the "ideal QM prediction" which is the prediction Bell sketched in his papers (deduced using orthodox QM collapse interpretation), and which doesn't match any actual experimental data.
And, obviously, any synthetic "data" (the data obtained from the actual data via ad hoc adjustments and filtering to match the "ideal QM prediction") will violate the inequality.
Therefore there is not a single known experimental setup to date for which the full quantum theory (QED) prediction would violate the Bell inequalities. Namely, if there were such set-up, since the full QED prediction would predict the actual data correctly (it would have computed detector efficiency and noise levels), we would have had already a loophole free test.
It is unfortunate how the ideological zealotry in this field, the desire to uphold the orthodox QM interpretation with its collapse at all costs, has distorted the normal scientifc procedure. Normally, when you get data that don't match the theoretical prediciton based on the initial model (the Bell's sketchy QM calculation), one would try to refine the model, include additional effects which may have been overlooked in the initial model, and see if that agrees with the experiment better. Additionally, an experimental setup would be modified to control better any unforseen and undesired effects.
But in this unique case, contrary to all common scientific practice, instead of refining the model, the data already obtained gets subjected to additional substantial alterations, rationalized by ad hoc, unfounded and unverifiable, conjectures, to bring them into agreement with the sketchy, incomplete model (the so-called "ideal QM prediction" as sketched by Bell). And all that trouble only to avoid disturbing, or even casting doubts on, the orthodox version of collapse used in the essential way to obtain that so-called "QM prediction."
If somehow the Bell inequality tests are not so charged with emotions and high stakes, if they were just some result in an obscure paper that nobody cared much about, and that a professor had assigned to a graduate student to test experimentally, and if the student, having obtained the results and having noticed they don't match the predictions of initial crude model, had decided to keep the model anyway (figuring it is too much trouble to work out a more realistic one), and instead chose to modify the obtained data to fit the crude model, rationalizing that he is only making "reasonable" (at least to his and his family's personal satisfaction) assumptions and adjustements --- well, that student would likely get thrown out of the school for research fraud (these things have happened).
Alternatively, say you took a group of physics students, educated them for several year in such a way that they never had any access to Bell inequality, hidden variables or the related controversies. At the end you give them the setup used in Bell inequality tests and ask them to obtain results of coincidences and come up with QM/QED model describing the situation. Not in a million years would any of them just come up with the kind of convoluted reasoning and rationalizations to tweak the data you see in the actual papers on these tests. They would obtain the same data everyone else obtains (but they wouldn't know it would be desirable if the data could violate the Bell's inequality), they would work out the QED model which, if you had picked the batch of the brightest, would match to a reasonable degree the data obtained. And neither data nor the QM/QED prediction would violate inequalities, and they would be perfectly happy.
It takes years of conditioning into the particular absurd twists and turns and leaps of logic at the just right places of these kind of experiments and their analysis, to ease someone into "cheating" on the Bell tests and not feeling any guilt or even being aware they're doing it. All the dubious leaps, which would normally raise red flags in a mind of any rational person, have been drilled into the fully automated operation, into habits devoid of any thought or doubt.
It is true that the predictions of quantum mechanics violate Bell's inequality (that's why Bell's inequality is interesting).
Well, as explained above, that is not true. A real quantitive QM prediction, the kind you can compare its figures with the experimental data and obtain a good match, requires a massive calculation specific to a given experimental setup (including the full initial and boundary conditions, to solve the partial differential equations and inclusion of effects of all dynamical laws involved). And if the QM is correct and the model of the experimental setup is properly constructed, the QM prediction ought to match the data obtained. And since data obtained in any set-up to date had never violated the Bell inequalities, not even remotely, that means that the so-called "QM prediction" which violates the inequalities, isn't a real prediction at all, but a glorified thought experiment supplanted by a bit of "back of the envelope" sketch of QM-like calculation, useful at best for qualitative or heuristic purposes, but of no much use for the quantitative predictions of this phenomenon.
I agree, Bohm theory isn't the real thing. As Einstein said, it was too cheap. Its main virtue was that it had served as a direct counterexample to the von Neumann no-go theorem for any hidden variables.
The origin of its inadequacy and the main source of its difficulties was in retaining the QM's 3N dimensional phase space for the N particles. That made it easier to map into the QM, to show the equivalence of their predictions, but at the cost of turning the theory into conceptual dead-end.
There are several much better alternatives, in particular the nonlinear field theories of A.O. Barut and E.T. Jaynes. They went back to the Schrodingers original idea when he came up with his equation: he intended to take the classical Maxwell equations coupling charged particles and the EM fields, then replace the Newtonian point particle currents with his Psi field (the wave function) currents. Unfortunately, the resulting system is a set of nonlinear partial differential equation, which were far beyond the mathematical techniques of his day. Additionally, an unfortunate computational error he made in one of his initial tries with this approach (on a simpler example), convinced him incorrectly that it wouldn't work even if he could carry the fuller calculations out. So he gave up on it.
It was only in early 1970s that Jaynes noticed the error and carried out more complicated calculations, finding that it actually works. He then expanded the idea into, what he called "neoclassical theory" which was supposed to be a conceptually clean alternative to quantum electrodynamics. But then, as with Schrodinger, a computational mistake in attempts to replicate QED predictions (after severa successful examples) sidelined the "neoclassical theory."
In mid 1980s, A. O. Barut found and fixed that error, and developed it into "self-field theory" with an impressive volume of work (helped along by several younger collaborators) and replicated many QED results, including the higher order radiative corrections, as far as they could compute them and beyond any experimental accuracy at the time. Thus they did demonstrate that as far as the agreement with the major experiments, QED had no advantages over the self-fields.
Conceptually, of course, the self-fields were far cleaner theory -- just the coupled interacting (and self-interacting) classical matter fields and EM fields, with no orthodox QM absurdities or QED divergencies. In principle the model was capable producing the mass of electron (or the fine structure constant) from the first principles, and they did manage to get some relatively crude soliton-like solutions and electron mass approximations (the nonlinear partial differential equations involved were very difficult, even by todays standards).
Barut's group also failed to demonstrate the equivalence of the self-fields to the predictions of QED to all perturbative orders. Then A. O. Barut died in early 1990s, and the group lost its track, each of the younger guys went their own way.
There are still some Russian and Ukrainian physicsts pecking at the variations of the Barut's self-fields (such as Oleinik, who claims to have obtained good approximation for electron mass from it), but it hasn't overall moved very much since the Barut's death. Jaynes, who inspired by Barut's results, went back to rework his "neoclassical theory," also died recently, so that work is left orphanned, too.
Another class of theories was spawned from the Nelson's stochastic formulation of QM from 1960s, which from stochastic mechanics grew into electrodynamics, named Stochastic Electrodynamics (SED). Marshall and Santos group has in recent years developed furhter a subfield of SED dealing exclusively with quantum optics problems, they call their model Stochastic Optics [SO]. The SO has reproduced the experimental results of all the major PDC based experiments of recent years for testing Bell inequalities or demonstrating other "non-classical" properties of photons.
The SED/SO use classical EM theory supplanted by the random background EM field (Zero Point Field) and classical point particles for the charged particles (or, in some variants, the Schrodinger eqation, with the external EM field approximation, i.e. without the self-interaction). Although more effective in producing the final results than Barut's self-fields, the two theories can be seen as one fundamental theory (the self-fields) and its computationally more practical (external EM field type) approximation, the SED, where SED simulates the complicated self-fields via the stochastic EM background and the soliton solutions of the self-field with point particles in this stochastic EM field. In the last couple years, Marshall has moved toward expanding SED toward extended particles and their stochastic fields, while retaining the stochastic EM background. Whether that will improve SED so that it can replicate more of QED predictions (beyond the quantum optics, where SO works quite well), it is not clear at present.
There have been several other more exotic variations among the alternatives to QM/QED in recent couple decades, but I think we are reaching the practical limits of the slashdot's message board scheme, so I'll leave it for some other occasion. Thanks for the nice discussion. You can have the last word in this thread, if you wish.
Extremely challenging, like in "it can't work and it won't ever work, but I hope the government and the industry sponsors won't find that out, at least until I retire, preferably after I am dead."
The whole field of Quantum Computing is a mathematical abstraction (fine, as any pure math is, as long as you don't try to claim that's how the real world works). Its vital connection with the real world is based on a highly dubious (even outright absurd, according to some physicists, including Einstein) conjecture about entangled quantum states (roughly, a special kind of "mystical" non-local correlation among events) which was actually never confirmed experimentally. And without that quantum entanglement the whole field is an excercise in pure abstract math with no bearing on reality.
While there were number of claims of an "almost" confirmation of this kind of quantum correlations (the so-called Bell inequality tests), there is always a disclaimer (explicit or, in recent years, between the lines as the swindle got harder to sell), such as "provided the combined setup and detection efficiency in this situation can be made above 82%" (even though it is typically well below 1% overall in the actual experiment; the most famous of its kind, Aspect experiment from early 1980s had only 0.2% combined efficiency, while 82% is needed for actual, "loophole free" proof) or provided we assume that the undetected events follow such and such statistics, etc. The alternative explanations of those experiments (requiring no belief in mystical instant action-at-a-distance), which naturally violate those wishfull assumptions, are ignored, or ridiculed as unimportant loopholes when forced to debate the opposition, by the "mystical" faction. After all, without believing their conjecture all the magic of quantum computing, quantum cryptography, quantum teleportation, along with funding, would vanish.
For those interested in the other side of these kinds of claims, why it doesn't work and why it will never work, check the site by a reputable British physicist Trevor Marshall, who has been fighting, along with a small group of allies, the "quantum magic" school for years:
Quantum Mechanics is not a Science"
Unfortunately, the vast bulk of the research funding in this area goes to the mystical faction. As long as there are fools with money, there will always be swindlers who will part the two.
For a more popular account, accessible to non-physicists, of the opposing view, you can check a site by a practical statistician (and general sceptic) Caroline Thompson:
Caroline Thompson's Physics
So don't try to confuse us by making it sound like a fancy particle physics problem when its just a bit of common sense, cause and effect stuff.
You can repeat the experiment a few times to pump up the probability that you get the right answer. As long as the number of repetitions to get the answer within an arbitrary epsilon of 100% is only polynomially related to epsilon (well, 1/epsilon), then you still win from a computational viewpoint where constants don't matter because exponents dominate.
Even in classical computers, there is a nonzero error rate. It just so happens that those errors can be bounded -- Shannon's coding theorem can be applied to a computation as a communications channel and shows that a computation with arbitrarily small errors can be achieved in the face of noise if you trade off computation bandwidth (read, MIPS) to do error correction. That's what this article is about -- finding ways of error correcting quantum states to allow sustained, error-free computation...
CG rendering, eg, is a problem that's in P that we burn a lot of time trying to improve computing efficiency on. A quantum computer won't help with that because there's no exponential explosion of the search space to control. CG is dominated by the constants in its polynomial-time solutions. Many other tasks are similar. Maybe there's a way to cast these problems so as to take advantage of the QC's exponential search space, but maybe not. QC won't help with the constants.
While Shor's factoring algorithm (which permits polynomial time rather than exponential time integer factoring, and therefore could undermine the security of RSA) may well be a "killer app" for quantum computing, it's worth pointing out that it's not yet been shown that QC can help us with general computing problems.
...
The big win in QC comes from the superposability of states -- it is possible for the system to be in all of its states at the same time. For n quantum bits (qubits), this is 2**n (two to the n) states. Operations on a system that is in such a superposed state are performed on every possible state at once. Great, neat, cool. But there's a catch -- the information you want can't come from a single measurement of the resulting system. The exponentially large amount of data you've computed is stored in the probability distribution (in some sense). In order to read this out, you need to repeat the experiment again and again to measure out the distribution instead of a single instance of the random variable.
Guess what, in order to get out the exponentially large amount of information from the probability distribution, you need to make an exponentially large number of measurements. So you're no better off, right?
Well, in general, maybe not. But there may be special cases. In the cases we've found so far, something funny happens in the quantum mechanical phase space that lets us actually read out the correct answer. Grover's search algorithm is a particularly clear example of what happens. In this case, the "right answer" can be read out because there is a computation that can cause a particular state to be selected with near 100% probability -- this state is the "winning" state that is being searched for (see L.K. Grover, Phys Rev Lett, 79, 325 (1997)).
Anyway, QC is only useful for those problems that can be computed in such a way that the answers can be read out of the QC in polynomial time. Right now, that's factoring (admittedly a biggie, but not likely something that'll, eg, get you 200 fps at quake, which you don't need anyway... oh wait, wrong thread), Grover's search, and a few other examples. Right now, though, QCs look like they'll be special-purpose code breakers. Hmm. Collossus?