Breakthrough for Quantum Measurement

said_captain_said_wo writes to tell us that PhysicsWeb is reporting that two teams of physicists have developed a new method for measuring the state of quantum bits in a quantum computer without disturbing the state. From the article: "In the future, the Josephson capacitance could be used for operations in a large-scale quantum computer," says Mika Sillanpaa of Helsinki University. "The Josephson inductance and Josephson capacitance together would also allow us to build new types of quantum 'band engineered' electronic devices, such as low-noise parametric amplifiers."

Comments

Heisenberg Uncertainty Principle?

[SRA8]

Wouldnt this violate the Heisenberg Uncertainty Principle?

Re:Heisenberg Uncertainty Principle?

[Anonymous Coward]

I'm not sure

Re:Heisenberg Uncertainty Principle?

[Centurix]

Maybe. Maybe not, who can tell?

Re:Heisenberg Uncertainty Principle?

[arrrrg]

> Wouldnt this violate the Heisenberg Uncertainty Principle?

Reading a qubit doesn't violate the Uncertainty Principle by itself; if qubits couldn't be read or written, they'd be worthless. The issue you are probably thinking of is that entanglements between qubits will be destroyed by the reading process (and there is no way to "read" such entanglements without destroying the individual qubit values).

Re:Heisenberg Uncertainty Principle?

[maxzilla]

Not neccisarily, by one quantum bit changing can be measured because it influences a number of other bits in a uniform way. I think you could imagine one gear spinning that drives a set of other gears, as the quantum bit changes it changes the whole setup of the capacitor so it is evident which way the bit was moving without changing its spin by measuring it. you change the spin of one of the bits in the capacitor, not all of the bits or the original bits

Re:Heisenberg Uncertainty Principle?

[BlkItlStl]

I don't think so. I'm pretty sure that has to do with limits on measuring momentum and location. http://en.wikipedia.org/wiki/Uncertainty_principle

Re:Heisenberg Uncertainty Principle?

[Obvius]

Heisenburg's Uncertainty Principle - DxDp>=hbar/2 Or as my old prof used to say "When you've got energy, you don't have the time. And when you've got the time, you don't have the energy."

Re:Heisenberg Uncertainty Principle?

[arrrrg]

I should clarify: reading the qubit will destroy all quantum effects (superposition as well as entanglement), effectively making the qubits look like ordinary bits (when you open the box, the cat's either dead or alive, not both). However, quantum computers are designed with this in mind; reading the output destroys any quantum properties it may have, but a computation can be repeated many times to get an idea of what uncertainty was present in the output.

Re:Heisenberg Uncertainty Principle?

[sanman2]

I still don't get what the basis is for carrying out entanglement, even if we observe its existence. What is actually connecting the 2 disparate qubits/particles that is the mechanism for communication between the 2? Rather than merely settling for "spook action at a distance", there has got to be some conjecture/speculation on an underlying mechanism, even if we lack the means to measure/verify it at the moment. I would just feel more secure if we could use more rational descriptors than "spooky". It just sounds very mentally lazy, and even patronizing, like when mom told you to make sure to come home before dark, or the "boogeyman" would get you.

Re:Heisenberg Uncertainty Principle?

[Anonymous Coward]

The uncertainty principle just states that you cannot know both
the position and momentum of a particle at the same time ... in
other words you can know both but not precisely, if you know one
precisely you do not know the other ... because you have interacted
with it.

If you know that the particle is in a certain band, you do not need
to know its location ... that IS its location ... or it is essentially
trapped .... you do not care.

If you have your cow in a pasture, you do not care where it is, as
long as it is eating grass or hay in the pasture and how not escaped. ... best I can do.

Re:Heisenberg Uncertainty Principle?

[proudhawk]

are we even sure that Heizenberg exists now?

"what state is Heizenberg in now?...
I don't know, that ^%^%(*&^ cat hasn't reappeared yet!".

Re:Heisenberg Uncertainty Principle?

[pontifier]

my theory:

In the beginning, all the matter/energy in the universe was lumped together in a singularity. At that time we know the speed was exactly zero because there was only one reference frame to measure from. We also know the location exactly because everything was in the same spot. With both known to infinite precision, the universe explodes in a cloud of infinite uncertainty.

Re:Heisenberg Uncertainty Principle?

[Jamu]

Heisenberg's Uncertainty Principle places no limit on measuring just the position, or just the momentum; it only limits measuring both with precision. Measuring the states of quantum bits won't violate similar principles provided you're measuring commutating properties, and not non-commutating properties like position and momentum.

Re:Heisenberg Uncertainty Principle?

[cablepokerface]

So it's still intact then?

Re:Heisenberg Uncertainty Principle?

[Punkrokkr]

Although, from my understanding, I don't think the principle is restricted to position and momentum. The definition states: "One cannot assign with full precision values for certain pairs of observable variables, including the position and momentum, of a single particle at the same time." Position and momentum (or velocity) are the most common used examples because it is easy to understand why you cannot know this. To accurately measure velocity you need to measure over time and distance to get a precise measurement. Therefore you cannot know a particle's velocity (and therefore momentum) and position at the same exact time (with precision of course). However, like I stated in the beginning, the principle does not restrict you to these two measurements.

Re:Heisenberg Uncertainty Principle?

[Young Master Ploppy]

" Wouldnt this violate the Heisenberg Uncertainty Principle?"

You'll have to try it to find out...

Re:Heisenberg Uncertainty Principle?

[Haydn Fenton]

Netcraft confirms it, Schrödingers cat is dead.

Re:Heisenberg Uncertainty Principle?

[laboratorymonkey]

I'm uncertain and I think that would be improbable, so therefore it may be ok.

Re:Heisenberg Uncertainty Principle?

[mumrah]

The Heisenberg Uncertainty Principle states that the more precisly you measure momentum, the less precisly you can measure position. This is a specific case of a more general uncertainty principle that states if two observables (momentum, position, speed, energy) do not commute, then they cannot be measured to close percision.

The reason that x (position) and p (momentum) do not commute comes from their operators. In x-space (what you're used to), the operator for x is just x, the operator for p is -i*hbar*d/dt. Long story short, x and p do not commute because of the time derivative in the p operator. This physically means that these two particular observables cannot both be measured with high precision, and as i turns out the product of the uncertainty in x with the uncertainty in p must be less than or equal to hbar/2 (which is the Heisenberg Uncertainty Principle). Not sure how relavent all of this is.

Re:Heisenberg Uncertainty Principle?

[cciechad]

String therory and multiple dimensions attempts to explain this. It is very interesting and elegant stuff, but currently untestable.

Re:Heisenberg Uncertainty Principle?

[Y2]

Wouldnt this violate the Heisenberg Uncertainty Principle?

"You keep using that word. I do not think it means what you think it means."

There are certain pairs of quantities ("conjugate variables") which cannot both be measured to arbitrary precision. If you measure, for example, the position of a particle along the x-axis (in your choice of coordinate system) very precisely, you cannot measure the momentum along that same axis very well. The product of the two imprecisions (or uncertainties) is a small but finite lower bound. You can work this out for yourself by making up a wave function localized in space and doing a Fourier transform to get the momentum spread.

You can measure x-position and y-momentum both very precisely, or various other quantities which are not conjugate to each other.

Re:Heisenberg Uncertainty Principle?

[EntropyEngine]

Would this be the Heisenberg Compensator, like in the Transporters in StarTrek?

Cool!

Re:Heisenberg Uncertainty Principle?

[tod_miller]

But don't worry, they are not certain it is working yet.

They are still uncertain, until they are measured. Does measuring one make the other certain?

Quantum cryptography? Wassup with that now?

o.0

Re:Heisenberg Uncertainty Principle?

[tolkienfan]

Actually, some algorithms depend on this behavior. For example, the Quantum Factorization algorithm uses this behavior to resolve one of the registers in the calculation, the other register contains only probability waveforms that are consistent with the value read in from the first register.

Re:Heisenberg Uncertainty Principle?

[marcosdumay]

If I understod it right, you just described quantum entanglement. So, looking for that single gear will change the spin of all the other gears.

The GP had a very good point, because what the submiter says is known to be impossible. That is also why I'm reading the comments, to discover what the researches really did. But it seems that I'll need to RTFA.

Re:Heisenberg Uncertainty Principle?

[icarusfall]

It's more general than that. The Heisenberg Uncertainty Principle says that any two non-commuting operators can't be simultaneously measured with arbitrary accuracy. As it happens, position and momentum don't commute, but there are plenty of others, x axis spin and y axis spin for instance.

Re:Heisenberg Uncertainty Principle?

[sickofthisshit]

The Heisenberg uncertainty principle can be generalized to "conjugate" quantities.

For instance, in condensed matter physics, the wave functions that have to do with superfluidity have an uncertainty relationship between the number of particles participating in the condensate and the quantum phase of the condensate.

Re:Heisenberg Uncertainty Principle?

[Spooon69]

Yes it does, but as long as Heisenberg Compensators are working, I think it's OK.

Re:Heisenberg Uncertainty Principle?

[joabj]

>http://en.wikipedia.org/wiki/Uncertainty_principl e [wikipedia.org]

It should be noted that the uncertainity principle also applies to Wikipedia itself, insofar that you can never be entirely sure if the information posted there is legit ;-)

Re:Heisenberg Uncertainty Principle?

[Anonymous Coward]

Not "because you have interacted with it". This is an extremely common explanation for the effect, but it's wrong.

A particle simply does not have a precise position and momentum, and this is true all the time, whether you measure it or not. Look at the equation (delta-x * delta-p = h-bar). There's nothing in there at all about interactions. It's just a flat statement of the bounds on the condition.

Other such conjugate pairs exist, such as energy and time. This is the property that permits "virtual pairs" to come into existence, or tunnelling in diodes to occur. Such happens whether anyone is "measuring" or not.

Re:Heisenberg Uncertainty Principle?

[JohnFluxx]

You've just described why an electron doesn't fall into an atom.

Re:Heisenberg Uncertainty Principle?

[Antisquark]

Nah, not if their Heisenberg Compensators are online.

Re:Heisenberg Uncertainty Principle?

[TexVex]

Take two coins. Orient them so both are heads up. Tape them together. Flip the combined coins onto a table and cover them with your hand without looking. Now, separate them, still without looking and without turning either coin over. Move the coins to separate locations. Now, uncover one. It will either be heads or tails with 50% probability. Now, what about the other coin? Yes, when you uncover it and look at it, its heads/tails state will be the same as the other coin with 100% certainty.

The only confusing thing here is the rule where when you check to see if a coin is heads or tails, you have to immediately flip and cover it again.

What connects entangled properties between two quantum objects? The same thing that connects the coins. "Spooky action at a distance" isn't so spooky when you know that quantum things can't become entangled unless they interact with each other at some point.

Re:Heisenberg Uncertainty Principle?

[sanman2]

Alright, fair enough, now that you mention it, I had once heard something like that before, but using a piece of paper torn into 2 portions with each kept in separate envelopes. Mail them off to different places, and opening one will tell you how much paper is inside the other. Alright. Fine, but you can't make a computer out of that. I thought entanglement means that you need some dynamic transmission going on between things. What good's a computer if it's not dynamic? I still find the mysteries of the Quantum Vacuum to be more exciting.

Re:Heisenberg Uncertainty Principle?

[gauge boson]

"Erwin, what did you do to the cat? The poor thing is half-dead!"

Shroedinger's cat?

[mcrbids]

Does this mean that we can find out if the cat is dead without opening the box? Sure sounds like it.

IANASPP (I Am Not A Sub-atomic Particle Physicist) but this seems to be quite a breakthrough that might save millions of subatomic cats from untimely deaths...

Anybody with some actual knowledge care to elucidate?

Re:Shroedinger's cat?

[Anonymous Coward]

hey guys I am not anything to do with this article or the science involved, but aren't I funny by making the same retarded physics joke that people would make in any article whatsoever including ones about the XBox 360's release? hurr hurr guys comedy gold!

Re:Shroedinger's cat?

[Solra Bizna]

If that's true, I wish I'd known about it before I tried that experiment.

-:sigma.SB

Re:Shroedinger's cat?

[maxzilla]

maybe its seeing if shrodinger's cat is dead, its knowing that when it does die it affects others with its smell... I get the sense that this whole method has to do with the one bit actually moving a set of other particles that influences the superconductor so we can check the magnetic spin of the particle...

Re:Shroedinger's cat?

[Anonymous Coward]

Catode display monitor. All those cats being accelerated to slam into phopshor. But how many states can you look up with an infinite state bit?

Re:Shroedinger's cat?

[Anonymous Coward]

Are you serious? Jesus, buddy, get a grip.

Re:Shroedinger's cat?

[Anonymous Coward]

Does this mean that we can find out if the cat is dead without opening the box? Sure sounds like it.

That's exatcly what it means, the way the headline presents it, which would mean that QP is disproved, which again leads me to beleive that the poster has misunderstood something.

Re:Shroedinger's cat?

[Anonymous Coward]

According to the Decoherence program the cat is either alive xor dead because it is such a large macroscopic object that it decoheres far to fast for any superposition to occur.

A Qbit being microscopic would be somewhat differnt.

Re:Shroedinger's cat?

[maxwell demon]

Given that TFA makes exactly the same statement, it's not really the poster who's misunderstanding you're seeing.
I guess what they managed is to do a measurement which behaves like an ideal quantum measurement: After you measured the system to be in state x, it really is in state x (unlike e.g. a typical photon measurement where after measuring the state of the photon, it isn't actually in any state because it doesn't exist any more).
Which means that if the system is already in an eigenstate of the measurement, it isn't changed by the measurement. Which is as close to non-disturbance as you can get in quantum mechanics.

Re:Shroedinger's cat?

[bernywork]

It's proven. Shroedinger's cat is dead.

Re:Shroedinger's cat?

[bhiestand]

It's proven. Shroedinger's cat is dead.

Actually, it's a dead horse now.

Re:Shroedinger's cat?

[Tony Hoyle]

Lots of ways... it stops moving about, for example. Also the cat itself knows whether it's dead or not.

I always had a problem with that experiment - it implies that human intellegence has some link with the state of the universe. I don't buy that for one minute. What happens if an ant crawls into the box for example? Because it's not 'really' an observer the cat is still half alive?? Plus since it's by definition unobservable it's also unprovable - so the whole experiment relies on philosophy rather than science.

Re:Shroedinger's cat?

[tendays]

What happens if an ant crawls into the box for example? Because it's not 'really' an observer the cat is still half alive??

Yes the cat is still half alive, and the ant is half seeing a dead cat and half seeing a cat alive. What happens when the ant walks in the box is that its state gets correlated to that of the cat.

Note that the either state of the ant is unaware of the other.

When you open the box you will either see a dead cat and an ant that has been seeing a dead cat all along, or a living cat and an ant that has been seeing a cat alive all along

Oh, and finally, there are many indirect but observable consequences of these superposition states, and they are precisely what quantum computing takes advantage of.

by the way, sorry I know this is off topic, but is anyone else having problems logging in in slashdot.org subdomains? That problem started for me a couple of weeks ago : I'm logged in slashdot.org but not in science.slashdot.org or it.slashdot.org etc so I can't mod in these domains unless I comment first

Re:Shroedinger's cat?

[slavemowgli]

I don't think you understand. It's not about someone being *aware* of whether the metaphorical cat is dead or alive - it's about the quantum state being disturbed. Measuring it can disturb it; whether the measurement is presented to a human mind afterwards or whether it's thrown away, for example, is irrelevant. So, yes, if an ant crawls into the box, the cat will either be dead or alive.

As for QM not making any testable hypotheses - that's also not true. Quite the opposite, in fact; QM works exceptionally well so far, and our findings continue to match its predictions. Do read up on it a bit - it's a very fascinating topic.

Re:Shroedinger's cat?

[Y2]

What happens if an ant crawls into the box for example? Because it's not 'really' an observer the cat is still half alive??

Yes the cat is still half alive, and the ant is half seeing a dead cat and half seeing a cat alive. What happens when the ant walks in the box is that its state gets correlated to that of the cat.

Note that the either state of the ant is unaware of the other.

When you open the box you will either see a dead cat and an ant that has been seeing a dead cat all along, or a living cat and an ant that has been seeing a cat alive all along

Why do you think the human observer is more special than the ant? Why don't you believe that when you open the box you become correlated with both the ant and the cat, and enter a state which is a mixture of you seeing a dead cat and seeing a live cat - with your own two states each, as you say, "unaware of the other?"

That's what the many-worlds interpretation is all about, not some sci-fi multiple universes schtick we always run across.

Re:Shroedinger's cat?

[Al Dimond]

Because this whole problem is about what the human observes. You could choose some other human standing outside of the room in which this takes place, who would walk into this room eventually and either: a human that has opened the box and seen a dead cat and an ant that has seen a dead cat; or a human that has opened the box and seen a live cat and an ant that has seen a live cat.

And that room could be inside a much larger house, and that house inside one of those pretentious "gated communities", which is inside a country which will be observed by some foreigner passing through customs as either (a) mourning or (b) not mourning the death of its most beloved catizen.

woooooooo!

Re:Shroedinger's cat?

[Anonymous Coward]

"I always had a problem with that experiment - it implies that human intellegence has some link with the state of the universe."

Comedy Princeton Engineering Anomalies Research reference.

Re:Shroedinger's cat?

[patio11]

Its closer to "we can find out if the cat is dead without accidentally killing him in the process" (or, even freakier, "we can find out if the cat is dead without accidentally bringing him back to life in the process").

Re:Shroedinger's cat?

[StressedEd]

..it's not 'really' an observer..

I think this is a common misunderstanding. The whole buisiness with the "Cat" is a rather over-stretched metaphor. When you read "observation" think "interaction". In this sense anything else is an observer if it interacts with the "Cat" (probes its state). This gets more complex of course when one realises that "everything is connected to everything else" [*]

...it's also unprovable..

I realise the following is probably more detailed than necessary, however it's worth keeping in mind.

"Proof" in science is another common misconception. One cannot ever prove a scientific theory with an experiment. One can only falsify a scientific theory with experiment (assuming of course the experiment is valid). If one takes the view that the aim of the gaim is to model and predict nature a manifestation within nature that does not agree with the theory clearly indicates that the theory is incorrect. The reverse is not true of course, nature cannot be incorrect! ;-)

In this way experimental physics is far more powerful than theoretical physics. For example, it takes just one experiment to demonstrate that Newtonian mechanics is not the "truth" (e.g. measure how far neutrons can fly before they decay). Instead one must realise that Newtonian mechanics is an approximation. Within the appropriate approximations, speeds much less than that of light, inertial reference frame, flat space-time etc., Newtonian mechanics is still an excellent theory. It is also a perfectly useful theory, after all one wouldn't analyse a bridge structure or aircraft with quantum mechanics or relativistic kinematics!

For all practical purposes use the most straightforward approach you can!

The use of "proof" is sadly a luxury of mathematicians and alcohol distillers! ------------- [*] Lenin - Though I don't mean it in the same context!

Re:Shroedinger's cat?

[Anonymous Coward]

Disclaimer: IANAP.

It sounds like there's a confusion between these two phenomena:

1. Disturbing the state in the process of measuring it.
2. Collapsing superimposed states by taking a measurement.

The first one is not all that strange and doesn't really require any weird quantum mechanics. Say I want to measure the speed of an object by shooting particles at it and observing their speed when they bounce back. In doing so, I have affected (ever so slightly) the speed of the object that I was observing.

The second one is the weird quantum effect. There's an object that could be travelling at two different speeds; by measuring it, I force it to "pick one", so that all subsequent measurements will be consistent with that first measurement.

Here's an example. Let's say that an object that is in some superposition of two states, one where it is travelling at speed 10 and another where it is travelling at speed 20. Let's also say that my technique for measuring the speed of the object slows it down by 1 unit. Now, when I measure the speed of this object, I will get one of two answers: 10 or 20. However, because my measurement affects the speed of the object, the object will now be moving at speed 9 or 19. Since the initial measurement collapsed the quantum state to either one speed or the other, all future measurements will be consistent with this first measurement. So, if I saw that it was moving at speed 10 and I measure again, I will see speeds 9, 8, 7, 6, and so on, since each measurement slows it down one more unit. If the initial measurement was 20, then I will see speeds 19, 18, 17, 16, and so on for future measurements.

So, this new technique for reading a qubit still collapses the state of that qubit, but it doesn't then change that state to something else in the process of measuring it.

Again, though, IANAP, so take this with a grain of salt.

Re:Shroedinger's cat?

[Anonymous Coward]

I always had a problem with that experiment - it implies that human intellegence has some link with the state of the universe.

It does not! It just means that when you observe something, you yourself become entangled with the state of the thing you observed!

For completeness's sake, let's repeat the Schrodinger's Cat argument. Let's say we have an atom that has a 50% chance to emit an alpha particle in the next unit of time. Quantum mechanics says that until we observe whether a particle has been emitted, the world is in a superposition of two states, one where the particle was emitted and one where it wasn't.

Now let's show how these effects could be "magnified" to real-world scale. We're going to put a cat into a box, and along with the cat we place a device that will measure whether this atom emits an alpha particle. If it does, then the device will release a poison that kills the cat. If it doesn't, then the device will not kill the cat. We close up the box. After one unit of time, there's a 50% chance that this particle has been released. We don't know whether it has been released, so as far as we're concerned, it's still in a superposition of states. Since the cat is either alive or dead depending on what happened, the cat itself is also in a superposition of states. Essentially, the inside of the box has two superimposed states: one where the particle was emitted and the cat is dead, and one where the particle was not emitted and the cat is alive. When we open the box, we collapse the state to one or the other.

Now let's say that this experiment is taking place inside a windowless university laboratory--another box of sorts. There's a scientist inside who just opened up the box and found out whether the cat was dead or alive (and whether the particle was emitted). But to an observer outside the lab, the inside of the lab is still in a superposition of two states: one where the particle was emitted, the cat died, and the scientist saw the dead cat, and one where the particle was not emitted, the cat is alive, and the scientist saw the cat leap out of the box and beg for food. When this outside observer walks into the lab, they will collapse the state (from their perspective) to one or the other of these options.

So, quantum mechanics does not privilege human intelligence over anything else. It just says that to observe an event is to become entangled with that outcome. When you observe something that is in a superposition of states, you yourself end up in a superposition of states, one where you saw one outcome, and one where you saw the other.

Need a drink yet?

Re:Shroedinger's cat?

[tomhudson]

I always had a problem with that experiment - it implies that human intellegence has some link with the state of the universe.

Sounds weird, but its already been proven to be the case - look for two-slit diffraction experiments if yo really want to warp your brain. And no, it doesn't mean that humans are special - its just one case where takeing a measurement alters the state of an object.

Think on a macro scale. You take a cold thermometer and put it in a big bucket - the bucket's temperature doesn't change much, the thermometer does, and registers the change.

Now, substitute a drop of water for the bucket. Doesn't matter how hot that water is, sticking it on the cold thermometer is going to change its temperature by a significant amount.

Now go to the sub-atomic level. There is no way that ANY measurement can't help but affect the thing being measured. Its like the drop of water - you tried to measure its temperature, and in doing so, changed its temperature.

Objects affect each other when they interact - or, as the saying goes, "shit happens"

Re:Shroedinger's cat?

[blincoln]

Plus since it's by definition unobservable it's also unprovable - so the whole experiment relies on philosophy rather than science.

The cat is a thought experiment that's useful for getting across the general idea. As far as I know, it's never been argued as something that would actually be performed, because cats are macroscopic.

You can use the concept on the quantum scale to predict lab-testable results like the two-slit experiment. If you put detectors (e.g. observers) in the slits, the effect disappears. Eerie, eh?

Re:Shroedinger's cat?

[CheshireCatCO]

"IANASPP (I Am Not A Sub-atomic Particle Physicist)"
Probably just as well. If you were, you would be too small to use a keyboard or to set up your experiments.

Sorry. Well, half-sorry. I just couldn't resist.

Re:Shroedinger's cat?

[millennial]

Does Netcraft confirm it?

Re:Shroedinger's cat?

[bernywork]

No not yet, the state has been confirmed by scientists, not the rest of the world.

Re:Shroedinger's cat?

[Phronesis]

Does this mean that we can find out if the cat is dead without opening the box?

No, it means that you can find out that the cat is alive without killing it in the process.

Most quantum measurements require you to kill the cat in order to determine that it was alive when the measurement started. For instance, drop a large weight on the box and listen for screams coming out. Think of measuring photons: the usual way to measure a photon is to allow it to be absorbed by something such as a photomultiplier tube. This tells you about the photon, but destroys the photon in the process.

Often it would be nice to measure the state of a photon now, then manipulate the photon and measure the state at a later time. This is clearly impossible with photomultiplier tubes. Serge Haroche and others have developed nice ways to measure the number of photons in a superconducting cavity without destroying the photons (killing the cat). They used these techniques to perform the first serious analogues of Schrödinger's cat experiment, except using superpositions of macroscopic photon Fock states instead of actual cats.

What the article describes is a similar type of quantum nondemolition measurement of the state of a qbit that lets you read the qbit out of a Josephson junction without destroying it.

Is quantum computing useful beyond decryption?

[jimmyhat3939]

The problem with quantum computing, as I understand it, is there are very very few applications.

Essentially, it's only useful in a situation where you need to repeatedly run the same computation over and over again with different input values to see which of those values produces a valid output.

I have a friend who has suggested repeatedly that eventually computers will contain some sort of quantum processor that helps with such tasks as gaming. I don't think this is realistic because of the serialness of the tasks that quantum computing tackles. In particular, something like rendering an environment in real-time won't be helped because there's an unpredictable input (the human).
--
Free 411! 1-800-411-SAVE

Re:Is quantum computing useful beyond decryption?

[smeek]

Quantum computing is also good for solving problems in quantum mechanics. No, really.

Re:Is quantum computing useful beyond decryption?

[Jace of Fuse!]

In particular, something like rendering an environment in real-time won't be helped because there's an unpredictable input (the human).

Durring the 1/60th (or less) of a second that your system is rendering a single frame in that game, the state of the scene and all objects (as well as light positions, textures, and overlays) is very static. It just doesn't seem like it to you, because you are very slow compared to your computer.

There could be hundreds of applications of a Quantum Co-Processor in a game, from testing for occlusion in a 3D scene, to making AI decisions in computer controlled characters.

Quantum Computing may very well not be immediately useful in many traditional computation tasks ("While this value is true then do that") but it will open up whole new ways of tackling processes that are time consuming with today's methods ("do any of these things give us this, that, or something in between?").

Just thinking about it gives me that Fuzzy Logic Feeling...

Re:Is quantum computing useful beyond decryption?

[ericcantona]

quantum computers have an enormous serial calculation potential; the clock speed would be limited by the plank time, which I am sure you know is 10^-43.
So... a quantum computer would process at 10^43Hz. This is fairly impressive isn't it? (remember a 1Ghz pentium ==10^9Hz).
Given this incredible computing power you could, I suppose, use it to game, rather than try to model the weather of the planet at 1cm scale, or solve untold protein folding medical problems, or develop fusion reactors...etc

Re:Is quantum computing useful beyond decryption?

[Anonymous Coward]

Um, no. And if you had any idea what you were talking about, instead of just parroting things you've read elsewhere, you'd know that it's "Planck".

Re:Is quantum computing useful beyond decryption?

[quoll]

There are very few problems in classical computing that work efficiently with Quantum Computing. That's because Quantum Computing is very good at problems that are NP complete, while classical computing avoids NP complete problems (because they can't be done is a reasonable amount of time).

So computer scientists rarely come up with applications that are appropriate for Quantum Computing. But that doesn't mean that these applications do not exist! Calculating protein folding and factoring large numbers are just two appropriate examples.

The best general purpose Quantum Computing algorithm is Lov Grover's database search algorithm. This can be applied to most problems, and is provably the most efficient implementation for many situations.

Re:Is quantum computing useful beyond decryption?

[Anonymous Coward]

I skimmed through

A fast quantum mechanical algorithm for database search

and

Grover's quantum search algorithm

And I still don't get how it works. Do you know of any quantum programming languages and code that implements this algorithm, because that would be infinitely better than the crap in those papers.

Re:Is quantum computing useful beyond decryption?

[mattpalmer1086]

Are you mods on something? Why is this modded funny? Parent is correct.

Re:Is quantum computing useful beyond decryption?

[Sigg3.net]

In particular, something like rendering an environment in real-time won't be helped because there's an unpredictable input (the human).

1. Remove the unpredictable input.
2. Problem solved.
3. Profit!

Re:Is quantum computing useful beyond decryption?

[werewolf1031]

Are you mods on something? Why is this modded funny? Parent is correct.

It's funny because it's absurdly obvious, in that "no s#it!" kinda way. C'mon, lighten up a little. :)

Re:Is quantum computing useful beyond decryption?

[mattpalmer1086]

I shall consider myself lightened-up :) But seriously, it's not obvious to me that it would be useful for calculating results in quantum physics, just because it is based on that theory. Sounds likely, but IANAQP. "Obvious" and "quantum physics" aren't words I normally encounter close to one another ;)

Re:Is quantum computing useful beyond decryption?

[Anonymous Coward]

Wouldn't quantum computing be the ultimate in SIMD?

Re:Is quantum computing useful beyond decryption?

[werewolf1031]

"Obvious" and "quantum physics" aren't words I normally encounter close to one another

Erm... yeah, good point. :)

Re:Is quantum computing useful beyond decryption?

[Anonymous Coward]

well, "only useful" is misleading I think. Theoretically speaking, the set of problems
that can be solved efficiently (polynomial time) on a quantum computer (with bounded
error) is a superset of the problems that can be solved efficiently on a classical machine,
and there is some evidence that it may be a strict superset, eg shors algorithm.
I'd say the situation you descibe is, very roughly, where a quantum computer appears
to have an advantage. Problems that fit this description or can be phrased as a question
in this form are not rare in the least.

Re:Is quantum computing useful beyond decryption?

[PermanentMarker]

ehm yes as quantum computing is verry powerfull.
i mean realy realy realy verry powerfull.
Given the idea that any PC is basicly a calculator.
We now what slows down calculations:
- long caclulation
- complexity with multiple unsure varibales
- speed of the device itself.

Cryptograhy is an example of this.
But also DRUG design, against deseases.
>> imagine to calculate dhemical bounds of milions of chemics under any 3d aprouch to the target molecule...
Or think of car crash tests

In the end even simple computers would benefit from it. Altough it would require some time and some math ideas to create a new language for programming i gues.
Based on it's power i believe such device should allready exist. I cann't imagine that inteligence offices wouldn't like to have to have a decrypter. Some articles talk about 2 bit or 4 bit quantum computers, if it is like modern transistors then it's likely that someone might have allready coupled larger bit devices for 'secured' usage ;O

(a few bits more increase much more rapidly their power then the chips we use to day, a 8 bits quantum computer might be like a todays supercomputer).

Re:Is quantum computing useful beyond decryption?

[Anonymous Coward]

It's not necessarily obvious. Is Babbage's analytical engine good at solving quantum mechanics problems just because the atoms it's made of obey quantum mechanics? Sure, a quantum computer uses QM in a more obvious way, but that doesn't make it necessarily good at performing tasks related to QM that people find useful.

Re:Is quantum computing useful beyond decryption?

[Do You Smell That]

How about for AI? The methods I've used so far involve figuring out all possible moves that could be made (classical computer), then using a heuristic to determine which, of all the possible moves (different input values), is the best. Even a simple game of Tic-Tac-Toe has 362880 different outcomes. Imagine if all computer-controlled AI characters were able to make the optimal moves without having to sit there for 5 seconds trying to figure them out. Difficult mode could consist of more than enemies with stronger armor and more powerful bullets for once.

Quantum Cryptography??

[laggist]

juz wondering.. would this result mean anything to the already available systems whereby quantum properties are used to securely send data from point to point??

Re:Quantum Cryptography??

[fitten]

I was thinking the same thing. Since the whole premise of quantum security is to be able to tell if someone had looked at the data sometime between when it was sent and when you got it, it seems like this new breakthrough has just invalidated an entire school of thought in quantum computing.

Re:Quantum Cryptography??

[Anonymous Coward]

Quantum Entanglement is the "crypto" of the quantum world. Actaully, nothing is encrypted but you know when someone is trying to look at your data. That is MUCH more useful than encrption. And the thing is, we can't break it. Not with current physics anyway :)

Re:Quantum Cryptography??

[waxigloo]

No, because if someone were to perform a QND on the photon carrying the information, it would would introduce noise in the conjugate variable which the receiver would be able to detect as an eavesdropper.

QND is essentially defined as introducing no noise into the variable you measure, which doesn't violate Heisenberg because all of the noise is introduced into the conjugate variable.

no fair

[FidelCatsro]

You changed the state of the quantum bits by measuring them

Re:no fair

[Anonymous Coward]

That joke isn't quite right. It's not so much that you "change" the state of a quantum bit by measuring it but more like you force it to choose between two possible states when you measure it.

Re:no fair

[Anonymous Coward]

It's Futurama paraphrased

Re:no fair

[Anonymous Coward]

Yeah I got that.

Implications in reverse order

[Flying pig]

Talk about looking for grant funding! Problem is, scientific illiterates in Government etc. think they understand what a quantum computer can do (application a long way in the future if at all) but not what you can do with very low noise parametric amplifiers (which might be relatively near term applications.) In terms of exciting progress in studies of brain function, small scale biochemistry, remote sensing and signal processing, very low noise amplifiers are critical components, whereas quantum computers don't yet exist, and by the time they do conventional computers should be adequate to deal with a lot of the data processing.

Not to knock the discovery, which is very interesting, but it's a pity quantum computers have to be dragged into everything to justify research. I doubt that Tom's Hardware will be reviewing millikelvin coolers for your qubit box any time in the next 20 years (though I'd like to be proved wrong)

Re:Implications in reverse order

[insignificant1]

I concur. NIST Boulder, as an example that I am familiar with, is developing certain techniques that can be used for quantum computing. (http://tf.nist.gov/ion/index.htm)

But the reason why the Time and Frequency division at NIST cares is because these techniques may yield better clocks in the future. (In fact, many breakthroughs in fundamental theoretical/experimental physics are applicable to clocks.) Meantime, however, the project gets mainstream-media publicity for quantum computing implications, gets funding from NSA, QuIST/DARPA, etc.

I'm sure it's a windfall for the physicists to do fundamental research, though, so "Hoorah Quantum Computers!!! (and cut me that check...)"

At least one other group (http://qdev.boulder.nist.gov/) is working on research similar to that published on PhysicsWeb, specifically using Josephson junctions in creating quantum bits & logic. That group was just recently setting up an honest phase measurement system, though, so are probably a bit behind in the research mentioned.

The reason that group plays with Josephson junction devices (how they justify it under the NIST banner)? Voltage standards. The reason they tell other funding agencies? Quantum computing, communication, and good-old-Alice-and-Bob.

Re:Implications in reverse order

[Lifewish]

I thought the main point of quantum computing is that it could solve problems that can't be tackled by digital computers. So, for example, whereas factoring a 64-bit prime number might take a fairly hefty digital computer a decent chunk of time, all a quantum computer would need is 64 qbits and the problem would be effectively solved.

As a mathematician in training, my biggest worry is that all the interesting cryptography jobs will have been obsoleted by the time I get that far.

Re:Implications in reverse order

[Kagura]

So, for example, whereas factoring a 64-bit prime number might take a fairly hefty digital computer a decent chunk of time

I can factor even the largest Mersenne primes in under two seconds in my head. Maybe I can help these scientists out a little bit with factoring primes... :)

spooky action at a distance

[ericcantona]

presumably, given entanglement, measurement of qbit state allows potentially for instant communication ? (which would be really spooky!).

Re:spooky action at a distance

[eclectro]

resumably, given entanglement [wikipedia.org], measurement of qbit state allows potentially for instant communication ?

No, nothing can travel faster than light. Which keeps us from developing meaningful space transportation and long distance communication with aliens, which would really be spooky.

I wish physicists would get beyond it myself, but you know how they are.

Re:spooky action at a distance

[ericcantona]

The current consensus, it may surprise you, is that the question is undecided.
It may be possible to communicate (via entanglement) faster than light in a 'many worlds model'

Re:spooky action at a distance

[Anonymous Coward]

Consider an infinitely long, yet light, rigid bar, with a switch at one end and a solenoid at the other.

At the _same_time_ as the solenoid pushes one end of the bar, the switch at the other end moves. The "information" that the solenoid at one end of the bar has moved has been instantaneously transferred to the other end.

I could go into why this analgy might work for quantum entanglement if you want...?

Re:spooky action at a distance

[Anonymous Coward]

On a molecular level, the information is not instantaneously transferred, although it is very fast (but of course, so is the light). While the bar is a rigid mass, the intramolecular forces that actually cause the motion is kind of like a long string of train cars... they bump up against each other.

I debate the widely accepted notion that no information can be transferred faster than light - I agree with the "maybe" assertion... Heisenberg's uncertainty took forever to be accepted in itself, so ideas about faster-than-light communication would have slow uptake too.

Re:spooky action at a distance

[patonw]

I'm a little rusty (maybe a lotta bit rusty), but no. this is a common misconception. Regardless of the mechanism for "spooky actions at a distance" to communicate any actual data you would need to send the results of the measurement through classical means which are limited by the speed of light.

Re:spooky action at a distance

[gauge boson]

presumably, given entanglement [wikipedia.org], measurement of qbit state allows potentially for instant communication ?

No, it doesn't. The closest you can come is instant synchronization of states, but you don't get to choose what state that is. For example, you can have two particles entangled to have the same (or opposite, as in the EPR thought experiment) spin orientation, but you can't send a signal from one to the other by choosing the orientation. Instead, it's random whether each one is spin up or spin down - the only guarantee is the relationship between the measurements. This would be great for things like cryptographic key exchange, since you can't have a man-in-the-middle attack if there is no middle, but it's useless for sending information. See: The No-Communication Theorem (warning: requires crazy math skills to avoid the MEGO effect)

nothing can travel faster than light.

I call bullshit. Relativity prohibits* local superluminal motion; non-locally, it's fair game. See, for example, the Alcubierre Warp Drive - the only question of whether it's possible or not (aside from new physics) rests on whether there's any local superluminal energy propagation at the edge of the spacetime bubble. Plus, QM allows for lots more in the way of non-local effects (even if you assume hidden variables, since Bell's Theorem rules out local hidden variables based on current experimental results), though, as I noted above, it still doesn't allow for superluminal communication (or teleportation, for that matter).

* Minor caveat: this is not counting tachyons, since nobody knows if they exist.

Re:spooky action at a distance

[maxwell demon]

Sorry, but the link you gave doesn't hint anything bout that FTL communication should be possible. And I can assure you the Many Worlds Interpretation (MWI) by itself doesn't allow you to do FTL. Indeed, one of the strengthes of the MWI is that it can explain entanglement and quantum teleportation completely without FTL effects.

Re:spooky action at a distance

[ericcantona]

yes, but have I got this wrong or not:

signal transmission via entanglement would appear to the observer to be instant (i.e., 'spooky'), the MWI model explaining this without need to recourse to FLT nonsense ?.

which, going back to my orig Q. implies instant comm is possible ? doesn't it ?
(if the quoted study is correct/replicable etc)

Re:spooky action at a distance

[maxwell demon]

You cannot use entanglement to do FTL communication because there's no way you can control the outcome of the measurement.

If Alice and Bob share an entangled pair, and both do a measurement, they both get a completely random result. It's just that both get the same result. But there's no way for Alice to influence the result which Bob gets. Think of it as a one-time pad which materializes only on observation.

Now, with quantum teleportation, Alice doesn't just measure her pair particle, but a combination of a pair particle and the particle the (possibly unknown) state of which she wants to transfer to bob. That measurement gives an (unpredictable) result, which describes, which transformation Bob has to do on his particle to get the state. However, Bob cannot know which transformation he has to do before he gets the measured (classical) information from Alice. Therefore Alice still cannot send FTL messages, unless she already knows another way to send classical information with FTL speed.

Or to say it another way: Even if the message appears to be transmitted with FTL, at the same time it gets perfectly encrypted, and Bob will not be able to read it until he gets the key from Alice. And that will have to happen another way, since otherwise it would get encrypted as well. But even this is not really accurate, because until Bob gets the message from Alice, he even cannot determine if Alice has sent anything, because any measurement he might do will get random results. Therefore FTL communication is not possible this way.

Re:spooky action at a distance

[bill_mcgonigle]

it still doesn't allow for superluminal communication (or teleportation, for that matter)

Didn't the IBM guys already demonstrate information teleportation? IIRC they figured out some way to get additional particles to synchronize with the Bell pair, then read the state of the additional particles, leaving the Bell pair un-read.

Re:spooky action at a distance

[gauge boson]

Indeed they did, but that still required subluminal separation of the entangled particles. Basically (very basically - this is only a rough summary), for quantum teleportation, you need to entangle the particles and move them apart normally in order to create the 'transmit' and 'receive' points. That still allows superluminal teleportation later, but there's a major catch: the Bell state can be rotated by the event, and it requires sending classical information about the initial Bell state to account for this. To use the traditional example, if Alice sent a particle to Bob with a known initial Bell state (the entanglement relationship between the sent particle and her 'transmission' particle), Bob has to know the result of Alice's Bell state measurement in order to get any information about the sent particle. Otherwise, the no-communication theorem would be circumvented. IOW, you can teleport something, but its information and usefulness still travel no faster than light.

Of course, the big problem with quantum teleportation is that it requires coherent quantum states for entanglement - good luck doing that with a human (even a dead one). IIRC, C70 fullerenes (buckyballs) are around the current experimental upper cutoff for coherence.

Unchanged State

[squoozer]

I thought the state had to be changed to measure it or am I confusing a technique used in quantum crytography with this technique in quantum computing. As an ex-chemist my understanding of things quantum was never that good anyway but I seem to remember someone saying that in order to measure something you had to change it. Any physicists in the house?

Re:Unchanged State

[quoll]

The article seems misleading in its wording. It says "read the value of a qubit without changing its value." This can't mean that it doesn't change the original quantum value, as this makes no sense. Reading a quantum value (a qubit) collapses the probability to the value read, by definition. This means that the value is no longer quantum. The original probability function cannot be read (though it can be calculated).

The statement without changing its value must refer to reading the value reliably. When reading the state of an individual subatomic particle it is extremely easy to have the result perturbed by noise. Given that there is a probability of reading an alternative value, then it is not normally possible to tell when the wrong value was read. It appears that this makes the process much more reliable.

IAAQP (I Am A Quantum Physicist). Though I could still learn a thing or two about subatomic physics.

Re:Unchanged State

[oldwarrior]

But is it not possible that the probability function only exists to paper over the part of the equation that our current theories fail to fully explain? I remember reading about an experimental law of probabilistic incompleteness that states whenever an unknown relies on a probability then it is incomplete - i.e. there is unaccounted for functionality deeper than the theory can handle perhaps due to experimental limits. This would be similar to luminiferous ether explanations before space flight and better ideas about gravitational forces. Reading the above discussions makes me feel like a victorian wondering what the 21st century scientists will think of our apparently complete explanations of 19th century scientific observations...

Re:Unchanged State

[WryCoder]

Reading a quantum value (a qubit) collapses the probability to the value read, by definition.

Yeah, my eyes popped a bit, too. However, one of the abstracts referenced on the article web page says,

"In addition, we show in practice that the method offers an efficient way to do nondemolition readout of the CPB quantum state."

I'd have to read the referenced article to see how he supports that seemingly provocative statement.

Re:Unchanged State

[Anonymous Coward]

A 2-level quantum system (hence the qubit) can be in either its excited state (1), ground state (0) or in a quantum mechanical superposition of the two. The thing is when reading the qubit out and if it is in a quantum superposition, you need to dephase the qubit which means you need to slowly put it out of its superposition(collapse of the wavefunction). After doing that you will end up with either 0 or 1.

The thing with the "breakthrough" is that you during measurement dephase the qubit, but you dont relax or excite it. Relaxation is if the qubit is in its 1 state and release energy, while exciattion is when the qubit is in the groundstate (0) and is put in the 1 state. This is what can happen when you measure your qubit, the detector is backacting on the qubit during measurement and either relax or excite it. This report shows that it might be possible to probe the qubit without this relaxation and excitation.

Physicist in the House

[anomalousman]

They can do what they say, but it's a lot more trivial than measuring the entire quantum state of the system, which is, as others have suggested above, impossible.

The Heisenberg Unccertainty principle implies that measuring a quantity must add noise in the conjugate quantity. For example, measuring the momentum of an object spreads out the wavefunction. Another example, measuring the state of a qubit (whether it is a zero or a one) destroys the relative phase between the zero and the one.

So the "non-destructive" measurement they are talking about means that they aren't changing it from a zero to a one or vice-versa. But they are (and must) destroy the information about the phase of the qubit state during the measurement. For a more in-depth discussion, look up "quantum nondemolition measurements".

Re:Physicist in the House

[booch]

How much experimental evidence do we have to prove the Heisenberg Uncertainty Principle? I couldn't find anything in the Wikipedia article providing any verification of the principle. If Einstein was right in his arguments, then it's possible that there are missing variables underlying quantum states that could be measured, correct?

Re:Physicist in the House

[DustMagnet]

How much experimental evidence do we have to prove the Heisenberg Uncertainty Principle?

Prove? Physics/science can't prove anything. You can disprove a theory. You can show a theory predicts things, but you can never prove it true. The Uncertainty Principle has been tested many times and no one has managed to disprove it. I'm sorry, but that's as good as we're going to get.

If you are disputing that the Heisenberg Uncertainty Priciple is predictive, you clearly haven't read any basic quantum mechanics texts. It's directly tied to wave-particle duality.

Re:Physicist in the House

[booch]

Um, my question was pretty clear in asking for experimental evidence supporting the theory. I've no idea how you could have mis-construed it as asking for 100% provability, a la the anti-Evolution crowd. And thanks for not answering my question.

Re:Physicist in the House

[JohnFluxx]

What authority was he appealing to?

Inifinite storage.

[bloodbob]

If this really works and you can read states then we have just found the ultimate information storage system. 256 qubit disk would store way more bytes of data then there are atoms in the universe.

Re:Inifinite storage.

[maxwell demon]

Sorry to disappoint you, but as long as you want to be able to reliably retrieve the information previously stored, a 256 qubit disk will be able to store exactly 256 classical bits, and no single bit more.
It could store something which classical disks cannot, which is quantum states. Which would make a quantum hard disk quite interesting, but not for storing more classical information, but for storing quantum information which you simply couldn't store on a classical disk at all.

Credit where it's due.

[kimmop]

The article isn't totally clear about it but the Finnish university in question is the Helsinki University of Technology (in the city of Espoo) and not the University of Helsinki. These are the largest two universities in Finland and both have Physics departments so the distinction is important.

Re:Credit where it's due.

[grimJester]

I just realized I've studied at both but graduated from neither. How did you collapse those wavefunctions again?

Crap!

[werewolf1031]

I changed the article by reading it! Someone tell me what it says now...?

Re:Crap!

[physman_wiu]

Well, since the article was written thus therefore must have been read while being written, the state of the measurement of the article didn't change from the original measurement state. It's just data that has arrived from the original measurement. The article could not change since the original autor must have "measured" the article while it was being written. So don't worry It still says the same thing.

Re:Crap!

[$RANDOMLUSER]

"I changed the article by reading it" is a terrific joke.

"The author changed the article by writing it" may be the best analogy to quantum computing I've run into. At the moment he finished the article, the author caused the article to collapse from all the articles it might have been the the article it actually was. As he was writing it, it simultaneously passed through all the possible articles (states) it might have been, to become the final article (state). Becoming is infinite, being is finite.

Re:Crap!

[maxwell demon]

Well, it depends on how he was reading it. If he was reading a complementary observable, the original information got completely destroyed.

Re:Crap!

[physman_wiu]

"The author changed the article by writing it" may be the best analogy to quantum computing I've run into." hmm, you got a three for restating what me and the other guy said, how is that? Oh well, lol.

Re:Crap!

[TwoScoopsOfPig]

*Drops jaw*

That was beautiful... and I'm stealing the last bit for my sig.

Re:Crap!

[$RANDOMLUSER]

> ...I'm stealing the last bit...

I'm honored. :-)

mis-statement (I think)

[sdedeo]

It's been awhile; I do GR now, not QM (much simpler.) But any measurement will change the state; this is the famous "collapse of the wavefunction" (in the Copenhagen interpretation.) What they mean is that the measurement will collapse the wavefunction as usual, but that it will not then alter the system being measured so that the state changes. i.e., if the amplitude is 0.1 A and 0.9 B, and the measurement collapses the wf to B 90% of the time as it will, then when the measurement is done the system will be B 90% of the time as expected, and it will be B "the right" times.

Re:mis-statement (I think)

[corellon13]

So, if I am 0.1 A, and 0.3 S then I can conclude that I am 1.0 AS^2.

Re:mis-statement (I think)

[Artifakt]

You've just done a very good job of reducing several paragraphs from the article to a nice, simple form without oversimplifying them. Your last clause, clarifying that the results have to be right, not just in the aggregate, but seperately, for this research to be significant, is an especially good example of how not to oversimplify a scientific subject.
        One more pedantic point though - I've never liked the use of the word 'collapse' in the Copenhagen interpretation. It has a certain negative connotation re. the post quantum state, like things fall down into existence from some superior state above existence. It seems to prejudice the arguement, by implying that probabilities become discrete things by comeing forth from a sort of Numenon, or Platonic Ideal, or maybe the literal Mind of God. While personally, I actually prefer that interpretation, it's only fair to recognize the alternatives for all the Non-theists out there. Since you show a very good grasp of the subject, I urge you to look at more neutral words such as 'Reification', which is admittedly an uncommon word, but which I personally think is a much better word for most QM discussions, because it doesn't shift the area of discourse into abstract philosophy nearly so much. Please don't take this as critical of your post, which is overall, one of a mere dozen or so responses in this thread that have focussed mostly on the actual science.

Re:mis-statement (I think)

[Phronesis]

any measurement will change the state

Not quite. If the system is originally in an eigenstate of the operator being measured, then it should be possible to perform a measurement without changing the state. For instance, if a photon is in a pure momentum eigenstate, then it should be possible to measure the photon's momentum without changing its state.

The problem is that most quantum measurements destroy the state, even in such a case. For instance, with the photon, we usually absorb the photon in order to measure it, so afterward there is no photon. A quantum nondemolition measurement is one that will leave the system in the measured eigenstate. If you measure the photon's momentum using QND techniques, you will leave the photon in the appropriate momentum eigenstate. If the photon was in that eigenstate to begin with, the measurement does not change its state.

The article in question reports a technique for performing quantum nondemolition measurements on qubit states in coupled Josephson junctions. If you initially prepare a qubit to be "true," then repeated measurements of its truth state should leave it in a "true" state.

Re:mis-statement (I think)

[Anonymous Coward]

Wow, I have studied quantum theory for a while, and I've never throught that "collapse" had that connotation. And after reading your post, well...

I still don't. While I can see, kind of, how you may come to that conclusion, I think you are seeing something that isn't there.

Quantum cryptographic links?

[evilviper]

Sounds to me like the security of quantum fiber-optic links are now in question. This isn't directly applicable to taping one, but it's a start.

(Not a quantum physicist, but I can play one on slashdot can't I?)

Re:Quantum cryptographic links?

[Anonymous Coward]

This is a technological breakthrough, not a theoretical one. It allows a practical measurement to be closer to an ideal quantum measurement. In quantum cryptography it is always supposed that the eavesdropper may do any perfect quantum measurement, even though this is not technologically feasable today.

For a measurement of a totally unknown state, the state is always changed by the measurement. But if you know that the qubit is in one of the basis states it is not changed by an ideal measurement. Now we're closer to doing this in practice.

Re:Quantum cryptographic links?

[Anonymous Coward]

Sounds to me like the security of quantum fiber-optic links are now in question. This isn't directly applicable to taping one, but it's a start.

The security of quantum key distribution (QKD) does not depend on the technology of the eavesdropper: it is assumed she can do any attack allowed by quantum mechanics. The security only depends on Alice and Bob's (the legitimate users) ability to actually produce and measure the quantum states used by the protocol. Finally, there are *proofs* of the security of QKD. The only way it becomes insecure is if we learn that quantum mechanics is an incorrect theory.

(Not a quantum physicist, but I can play one on slashdot can't I?)

Sure. Just try to play a better one next time! ;)

Re:Quantum cryptographic links?

[Anonymous Coward]

>(Not a quantum physicist, but I can play one on slashdot can't I?)

Only if you slept at a Holiday Inn Express last night.

Does this mean...

[NcF]

That we're one step closer to prooving 1+1=3?

Re:Does this mean...

[Dorceon]

For extremely large values of 1?

Re:Does this mean...

[mrselfdestrukt]

It's been proven with sex.

Whoooow... Quantum Amplfier!

[Anonymous Coward]

K3wl! Quantum Amplifier! I want one for my car!

So what does Linus have to say about it.

[Anonymous Coward]

Coz, you know...he's Finnish too. Come on, people, it's a law or something!

Re:So what does Linus have to say about it.

[maxwell demon]

Will there be a Linux port for the quantum computer? :-)
However I imagine debugging on a quantum computer will be no fun: After all, quantum programs will behave different when you look at them with a debugger!

Re:So what does Linus have to say about it.

[grimJester]

That's the distinction the grandparent was talking about. Linus studied at the University of Helsinki, while this discovery was made at the Helsinki University of Technology.

Which makes it even less likely Linus will have any strong opinions on this.

Re:So what does Linus have to say about it.

[kimmop]

However I imagine debugging on a quantum computer will be no fun: After all, quantum programs will behave different when you look at them with a debugger!

And how is that different from debugging C pointers? ;)

Yes, but..

[Anonymous Coward]

It'll take 2.1 gigawatts to power the Josephson capacitor.

But is it a flux capacitor?

[unitron]

Let's see, a "Josephson junction consists of two superconducting layers separated by a thin insulating layer." Two conductors separated by an insulator. Can anybody tell me why they wouldn't have predicted that it would behave like a capacitor seeing as how two conductors separated by an insulator *is* the definition of a capacitor?

Re:But is it a flux capacitor?

[Anonymous Coward]

I'm guessing here, but it's probably something like this:

Consider if you had a single cooper pair in the system. From a non-quantum point of view, the cooper pair must either be in the top or bottom superconducting loop. However, the cooper pair can tunnel through the insulator to the other loop so the system exists as a superposition of the two states.

Of course if you start with the cooper pair on one side, the chance of it being found on the other side should increase with time. I'm guessing that they're cooling down the system to the point where the state of the system changes slowly enough that it can serve as a relatively stable qubit. Then if you want to prepare a particular state, you can start with the cooper pair on one side and then lower the temperature to "freeze" it at the right moment so that it's in the desired state.

So can any physicists tell me if I got this right? I've studied a little quantum computing but we didn't cover implementation that much.

Re:But is it a flux capacitor?

[Magada]

My understanding of TFA is that someone has, in fact predicted this. The experimenters in question proved that the prediction is true.

Re:But is it a flux capacitor?

[unitron]

And my point is, isn't this like predicting that rain will be wet since it's made of water. A capacitor is two conductors separated by an insulator. This thing is two (super)conductors separated by an insulator. It would be a big deal if they had predicted that it wouldn't act like a capacitor.

Re:But is it a flux capacitor?

[Anonymous Coward]

If you read the article you might see the difference betweeen the Josephson capacitance and the geometric capacitance. Where your capacitor is the geometric part and the Josephson capacitance is due to the energy band of the Josephson Junction. The nice thing with the Josephson capacitance is that you can change it by modifying the energy potential of the system with for example a gate coupled to the device. The fact that the Josephson Junction circuit have discrete energy levels which have opposite sign to a change in potential even results in negative Josephson Capacitance.

Re:But is it a flux capacitor?

[unitron]

Somebody mod this up. Interesting, perhaps.

Some corrections

[Catullus]

Quantum computers are not known to be very good at solving NP-complete problems, and in fact it is considered very unlikely that they will be able to solve such problems efficiently. Grover's algorithm provides a square-root speed-up in solving any problem in NP; however, this is not enough to make an unfeasible problem feasible, and for any given NP-complete problem, there is likely to be a classical algorithm that outperforms this "brute force" approach.

Grover's algorithm is only the provably best implementation in a "black box" setting, which is unrealistic for many problems.

Finally, quantum computers are not known to be able to do anything useful for protein folding - this would be an application of an efficient quantum algorithm for the graph isomorphism problem, which nobody has come up with yet...

Re:Some corrections

[Anonymous Coward]

Not having come up with a formula means nothing, general abilities for each system do exist. It is like not haing come up with a formula for a certain normal calculation problem yet either. Next to that, your argument about it not being good in NP theory is contradictory. It is good in NP problems compared to normal systems, and the brute force methodology has more often useful application then allowed for you as well. Oftently more clever routines are used, yes, but these are not guranteed to get the most optimal solution. And as long as the computational burden is not to extreme, the small advantages to be gained by finding the optimal solution would likely be well worth it.

Helsinki U. of Tech., not Helsinki U.

[msmikkol]

Just a minor correction to the linked article: Mika Sillanpää worked at the Helsinki University of Technology, not at the Helsinki University when he wrote the paper in question.

Cool,

[robiurl.biz]

now we will be able to measure the size of george w. bush's member!

Re:Of course not, you non-scifi-watching dweeb!

[Lord Bitman]

It measures Quantum States without effecting them, it's obviously a Heisenberg compensator.

Holographs vs. qbits

[TubeSteak]

Which is going to be more important for us?

Holographic storage or quantum computing?

Because it isn't an insulator, of course

[Flying pig]

The description of the Josephson Junction is aimed at all the non-physicists out there. The "insulating layer" is a bandgap layer. The point is that cooper paired electrons can tunnel through it, i.e. it acts as a superconductor itself. It is an insulator for ordinary electrons only. And the definition of capacitor is nothing at all to do with physical conductors or insulators. It is a region of space where a potential gradient can be created, and the capacitance is the measure of how much energy has to be pumped into the region in order to create a given potential gradient. "Empty space" requires the lowest energy and has the lowest capacitance per unit volume, while certain ceramics with relatively mobile but limited electrons have very high values. If you cannot create a potential difference across your region of space, you have no capacitance - and at first sight, if that region is superconducting you cannot have a potential difference.

Re:Because it isn't an insulator, of course

[unitron]

Gradients are what you get with resistors. As you "move" through the resistor body you gradually (see root word of gradient) move from one value in the difference of potential to the other. A capacitor with a gradient is a leaky capacitor.

Re:Man in his quest to be God ... it applies here

[The Cornishman]

> Somewhere... Not here, pal, at any rate.

Sigh

[Anonymous Coward]

So if I write an application that uses a quantum computing, does that mean I have to make it both thread-safe and universe-safe???

Re:Sigh

[Archimboldo]

So if I write an application that uses a quantum computing, does that mean I have to make it both thread-safe and universe-safe???

Not thread-safe, but superstring safe.

Yea, its possible

[TubeSteak]

Does this mean that we can find out if the cat is dead without opening the box?

Sure, just ask the cat.

Shor's Algorithm

[stelmach]

I'm trying to grasp what the implications are of this. Let's take Shor's algorithm as an example. It is my understanding that the Quantum Fourier Transform (QFT) is applied to the result of the algorithm to peak the probability amplitudes, which will help the result to collapse into the correct state when measured. So does this mean that the QFT will not need to be applied, and the result of Shor's algorithm can be read with 100% accuracy?

Does this break quantum cryptography ?

[cryptoguy]

If I can meausure the state of a quantum bit without altering the bit, I can evesdrop on a quantum key exchange without being detected. Or am I missing something?

A Golden Age is Coming

[TheZorch]

Quantum Computers will usher in a golden age in computing. There are all sorts of applications that they could be used for. For a time they'll serve a role that most super-computers today serve and that's for engineering computations and scientific experiments that require massive number crunching.

For long term space travel like the proposed mission to Mars a quantum computer would be invaluable. It would be able to monitor the crew and spacecraft faster than today's computers and will be able to react to any kind of critical issue 100x faster. Please, no "2001/Hal" comments. I'm being serious.

Also, quantum computers could be used for gene sequencing that can be done in minutes rather than hours, months, or years for the creation of new drugs or gene therapy. A single quantum computer could be used to replace dozens of computers in a corporation's server room. Just one machine could do the work of 20 or more so you don't need a separate database server, email server, web server, web proxy server, or any other kind of server a large company would need. These computers would benefit businesses like Ford, GM, and all the other car makers allowing them to make better engineered cars.

I can also imagine the graphics industry would benefit. Imagine if Pixar had one of these machines. Imagine being able to render a movie at final-draft production quality in "realtime". We'd also finally have a computer that could make Virtual Reality better than it has been in the past. The applications for this technology aren't as limited as you might think.

Re:A Golden Age is Coming

[theurge14]

Well, while they're not compiling Gentoo in 15 minutes, they could be in clusters to run the Matrix.

So much for Quantum encryption

[Shadez666]

What was all the fuss about.....

thank goodness

[revery]

The Josephson inductance and Josephson capacitance together would also allow us to build new types of quantum 'band engineered' electronic devices, such as low-noise parametric amplifiers.

I'm very glad, as I have a current-model parametric amplifier and man is it LOUD....
I should have figured as much, seeing as how it goes up to 11.

Useful?

[necro81]

From RTFA, I am wondering if this new discovery will actually be of much use to anyone. The apparatus involves cooling down to a few millikelvin. I am guessing that this is so that the thermal noise in the circuit is greatly reduced, and also because the superconducting threshold of whatever their Johnson capacitor is made from might in fact be that cold. Pure copper becomes a superconductor, but not until several degrees Kelvin, I believe.

In any event, cooling down to such temperatures implies a couple of things: lots and lots of very expensive equipment to cool down a tiny tiny volume of space. Even the first transistors didn't require such great lengths.

The article also makes reference to the capacitance of the Johnson capacitor changing signs depending on the state of the qubit, which is part of how the whole thing works. Does this mean that someone has discovered negative capacitance? Whoa! What would that mean?

Re:Useful?

[grcumb]

"... [T]he superconducting threshold of whatever their Johnson capacitor is made from might in fact be that cold."

As a Canadian, I can vouch for this. In extremely cold temperatures, my Johnson shrinks significantly and becomes much easier to measure.

... But what exactly is a 'Johnson capacitor'? Is that like Viagra or something?

No threat to Q-crypto

[DMiax]

Quantum crypto is untouched by this.
It (at least in E97 protocol) relies on quantum entanglement, which cannot be "read". It is a property of a composite system, which cannot be measured if you own just one.
You won't even know whether your qubit is entangled or not.
(even if you own two, you cannot know, you need lots of pairs to observe entanglement)

Non destructive measuring is not a problem too: measuring a system means not projecting it in the measured state (as many say, nor it is just adding "noise"): that is the "easy" way of thinking.
You can couple your system to an "amplifier" which measures and leaves almost the same state. You will simply not know where it was before.

It was theorically possible, it is now practically.

Don't confuse measurement with knowing

[snowwrestler]

If we send a robot into the forest to cut down a tree, the tree falls even if a human intelligence does not witness or perceive it.

Likewise if we use a machine to measure the position of a particle, the superposition collapses whether or not a human intelligence ever reads the results. In QM measurement is an act with consequences, just like swinging an axe at a tree. But it doesn't require a human intelligence.

Just Remember What Mr Josephson Tells Us...

[JohnPerkins]

Capacitance counts.

They're so silly

[jav1231]

Hey, it could be worse. Quantum Theorists could be using their imagination to sit around and dream about getting laid.

Re:They're so silly

[Anonymous Coward]

Actually, I do............ What's wrong with you?

Danger of thought experiments

[snowwrestler]

Lots of things are possible when you posit things that cannot exist in real life, like, for instance, an infinitely long yet light and completely rigid bar. You might as well start out with "consider a super-fast method of communication that tranmits information faster than light." It's as close to (or far from) reality as your bar.

Applications for quantum computing

[Anonymous Coward]

As many other posters have noted, the actual effect that the physicists found is a reliable way to measure individual qubits, not a way to do so without collapsing the state which is, of course, impossible. The article is still interesting since it appears to be a means of implementing quantum memory in some kind of reliable way. This in itself is not going to produce a quantum computer, but could be used with other techniques.

Several other posters have asked about the impact that quantum computers would have. The most ofter cited example is Peter Shor's algorithm for factoring. This algorithm is fundamentaly faster than the best known algorithms on classical computers (polynomial vs. subexponential). This can be used to break RSA and other factoring based public key systems. Other techniques can be used to break DSA, elliptic curve systems and just about every public key system in use today. This would mean that the entire security infrastructure of the internet and other networks would have to be replaced, possible with quantum key distribution. There are, however, public key systems that seem to be resistant to quantum computation. These include the McElice codes, which are not widely used because they are quite slow.

Besides cryptography there are numerous applications for quantum computers. The mosty widely applicable algorithm is probably Grover's search algorithm. This algorithm searches a list of items for a marked item (like finding a particular entry in a database). The advantage is that it can do so in the square root of the time it take for a classical algorithm (\sqrt(n) instead of n for a list of n items). Besides the obvious application of database searching, this can be used to speed up any procedure which requires searching through a list. For example, it can be used to speed up a brute force key search on a symmetric key cipher.

Of course there are no quantum computers of significant power yet. The largest quantum computer I know of can process 12 qubits, which can be used to factor some double digit numbers (eg 32). It cost something like $6 million. Real quantum computing is estimated to be about 40 years in the future.

Probabilistic computing?

[Urusai]

Or aleatory computing? I realize there are certain problems that are deterministically intractable but with feasible probabilistic "solutions", but statistics based computing is just...dirty. I don't think a lot of people understand that quantum computing doesn't actually provide hard answers, that you have to run the same "algorithm" a lot of times to get an approximation.

Re:Probabilistic computing?

[Anonymous Coward]

But if you think abuot it, all our algorithms are probabilistic. At any moment, e.g., a stray cosmic ray could flip a bit in your computer causing a calculation to give an incorrect answer. There's no way to be 100% sure that the result of a computer's calculation is accurate. It may _seem_ less reliable to depend on a quantum calculation, but you can always run the calculation enough times that the probability that you have the wrong answer is no worse than the probability that a standard computer malfunctioned.

Misleading Title/Intro?

[Anonymous Coward]

I'm still unsure of the exact implication of this discovery. Does this mean that they can now actually *measure* the entire superposition of states, without actually disturbing the qubit via the measurement process, hence not making it collapse to one of it's eigenstates? Or is it just that they are measuring via a compatible operator so that they will only either get a 0 or a 1. If it is the latter then I don't exactly see why this is something new and exciting, NMR (Nuclear Magnetic Resonance) has been used for doing the same for a long time now. I would greatly appreciate if somebody could explain the exact implication of this finding.

Schrödingers cat

[Morosoph]

Alas, poor Yorick! I knew him, Horatio: a fellow of infinite jest, of most excellent fancy...

Brian Josephson

[SiliconEntity]

It's funny to read Josephson this and Josephson that in the summary. Brian Josephson is of course a very famous Nobel prize winner, but he has become a pariah in the scientific community.

Look at his web page! He's pushing cold fusion, ESP and other paranormal powers, and all kinds of bizarre theories. He's gotten into fights with the highly respected archiv.org physics publication site over their habit of removing crackpot papers. In short Josephson is an embarrassment to the scientific community, someone who refuses to go along with the conventional wisdom and insists on using his reputation to attack conventional scientific beliefs.

I know what you really want to know: who's right? Josephson or the scientists? Here's a tip. Any time it's one guy against the scientific community, bet on the scientific community. You'll be right 99+% of the time. The fact that Josephson did good work back in the 1960s with his junction doesn't make him an expert on ESP and cold fusion. If there were any substance to those fields, the normal scientific process would have found it. That's the safe way to bet.

Re:Brian Josephson

[jabelar]

Mainstream science is good for improving on previous bodies of work, "wierd science" is good for finding entirely new ways of understanding. Mainstream science only finds revolutionary things by accident. The best thing is let all hypotheses come forward, then use science to test them -- don't discount them before that!

Re:Brian Josephson

[Ariane 6]

Agreed that he's rather nutty. I got to see him speak in Lindau a little over a year ago, and even got to ask him some questions later on. An interesting guy, to say the least, but he's a nice person and pretty much harmless.

Besides, you never know, he might even be right about one of those things. If you throw enough darts you might just hit something.

Re:Brian Josephson

[Anonymous Coward]

From someone too busy doing science to have the time to create an account: rumour has it that Josephson's graduate student has just been passed by his examiners for a Ph.D. thesis on Quantum Coherence, so he must be doing something right. And anyway, shouldn't his earlier expertise in superconductivity make him at least as well qualified to judge subjects such as ESP and cold fusion as people who have spent less time than he has studying the literature relating to these topics? And can his vocal critics identify a genuine problem with his paper "Biological utilisation of quantum nonlocality", at

http://www.tcm.phy.cam.ac.uk/~bdj10/papers/bell.ht ml

which he claims is the same idea as one discovered independently by another physicist, Anthony Valentini, suggesting that it is not completely crazy?

QC gaming accelerators...

[PaulBu]

Maybe not on your PC, but a box made by these guys: http://hardware.slashdot.org/article.pl?sid=05/06/ 22/0610220&tid=126&tid=137
attached to an MMORG server...

Full disclosure: I work for them...

Paul B.

Nooo

[Anonymous Coward]

You don't back anyone, regardless if whether they're right 99 % of the time. That 1 % could make all the differnce - the odds are not always with us. And as for the controversial scientific papers - omitting them from archives is potentially more damaging to Science than leaving them there..

Even Einstein himself was very relucatant to accept quantum theory and got left behind by allot of sub atomis physics (even if he was getting old). Reputation isn't everything

Deja vu

[w.timmeh]

People said the same thing about lasers when they were invented in the 1960s: "A solution without a problem".
But look at us now - where would we be without them?

Can you elaborate a bit? NEWB WARNING

[aztektum]

I'm a newb when it comes to physics. I like to read about it, but I don't quite get how measuring something creates such a change.

Is it our processes for trying to detect and measure states that create a change or do we not know why?

Wrong

[MarkusQ]

I always had a problem with that experiment - it implies that human intellegence has some link with the state of the universe.

Sounds weird, but its already been proven to be the case

Actually, it has been found not to be the case. The experiment essentially uses a mirror instead of a cat, and either breaks or doesn't break the mirror. A mirror is used because (using an other aspect of QM) you can distinguish a mirror that is in a superposition of states (broken | unbroken) from one that is definitely in one or the other without gaining any information about which state it's actually in.

It turns out, the cat is always either alive or dead, the mirror either broken or unbroken. Macro scale objects do not enter into a superposition of states in the real world. The assumed reason for this is that the larger the object, the more fragile (speaking very loosely here) a superposition of states is. By the time you get to a macro scale object at normal temperatures, you will never get a superposition--just moving an electron somewhere in another galaxy would be enough to collapse it.

On the other hand, the idea of macroscale objects in superposition is not nearly so fragile. It seems to be entrenched (paradoxically) because it's so counter intuitive.

--MarkusQ

P.S. As for the rest of your explanation, it's right in spirit but wrong in fact. Operating at the same level of analysis, you could just keep making smaller and smaller thermometers, or playing clever tricks with them to measure the temperature of the water indirectly, or without affecting it as much, so the argument as you present it begs the question.

The whole point of QM is that there is a bottom to all this, a point where you can't build a smaller thermometer, and no combination of clever tricks will help you. Your argument only seems to explain this because you silently assume it from the very beginning.

Re:Wrong

[tomhudson]

Um, no. You're wrong. The 2-slit experiments I'm referring to neither require nor use a mirror. You're referring to something else.

As for the temperature, there is no way that a thermometer of any size can't affect the sample its measuring unless it already is at the same temperature as the sample.

Every measurement, even in the macro universe, affects the thing measured. That this would NOT be the case at the quantum level, while counterintuitive at first, would upon reflection be surprising.

Also, there is no need to resort to a superposition of states (which has always been a bit of a brainfuck imho) when there are better models. Superposition is only required if the time scale is unidirectional and can't be "rotated out" of the question and replaced by another vector, which has never been shown to be the case. The copenhagen gang lacked sufficient imagination to see the obvious.

But that's just my opinion, and this is slashdot, and its not Tuesday :-)

Re:Wrong

[MarkusQ]

The 2-slit experiments I'm referring to neither require nor use a mirror. You're referring to something else.

The two-slit experiments you are referring to do not put (or claim to put) macroscopic objects in a superposition of states (which is what Schroedinger's cat is all about). So they don't prove (as you claimed) that this actually happened. The only experiments to date that I am aware of to actually test the issue did use mirrors (actually, a metal grid that served as a mirror at the wavelengths in question), for the reasons I outlined.

As for the temperature, there is no way that a thermometer of any size can't affect the sample its measuring unless it already is at the same temperature as the sample.

This really has nothing to do with QM (except in the popularized explanations), but in any case it both misses the point and is wrong.

For a clear example of how it is wrong, consider a thermometer that works by passively measuring the emission spectrum of the sample; in that case, it needs to be at the temperature of the background it is obscuring not at the temperature of the sample. But if it is far enough away even that won't matter, because the speed of light will prevent any influence from the thermometer on the sample.

To see why it misses the point, consider that, if you really meant it, you would in effect be claiming that all measurement was impossible. The real question is, does the thermometer affect the temperature enough to in turn affect the measurement it is making? This is a real question, of practical significance in many situations, and is the only part of all this that touches on QM. In the classical realm, you can always contrive a finer, more devious way to make the measurement to the desired accuracy while keeping the disturbance below the threshold. The key point of QM is that you can't do this indefinitely, that there is a quantifiable, universal limit to what is knowable without regard to any such "disturbance" by the measurement and thus not thwartable by clever design of instruments.

Every measurement, even in the macro universe, affects the thing measured. That this would NOT be the case at the quantum level, while counterintuitive at first, would upon reflection be surprising.

Not strictly true (for another example, I can attempt by various means to measure the distance Alexander the Great traveled during his life, but I cannot in any way affect it), but in any case this isn't QM, it's just an analogy used (misleadingly, IMHO) to explain QM (specifically the Copenhagen interpretation) without getting into the math.

Also, there is no need to resort to a superposition of states (which has always been a bit of a brainfuck imho) when there are better models. Superposition is only required if the time scale is unidirectional and can't be "rotated out" of the question and replaced by another vector, which has never been shown to be the case. The copenhagen gang lacked sufficient imagination to see the obvious.

There may be no need to resort to superposition in general, but the specific point in question here, "Do macroscopic objects, such as cats, enter into a superposition of states as claimed?" can not be dodged in this way. You said that of Schroedinger's cat (which asserts that macroscopic objects enter into a superposition of states) "Sounds weird, but its already been proven to be the case"; to now claim that this is "a bit of a brainfuck" while still maintaining that it is a proven, real effect doesn't wash.

-- MarkusQ

Re:Wrong

[tomhudson]

you would in effect be claiming that all measurement was impossible

an accurate measurement of anything IS impossible. WE already know that.

because the speed of light will prevent any influence from the thermometer on the sample.

actually, you're making an assumption that strict causality exists. It has never been proven, and is an unworkable assumption because it then requires artificial constructs such as superpositions of states.

BTW, you misquote me when you say:

You said that of Schroedinger's cat (which asserts that macroscopic objects enter into a superposition of states) "Sounds weird, but its already been proven to be the case"; to now claim that this is "a bit of a brainfuck" while still maintaining that it is a proven, real effect doesn't wash.

What I said - cut-n-pasted from my original quote:

Every measurement, even in the macro universe, affects the thing measured. That this would NOT be the case at the quantum level, while counterintuitive at first, would upon reflection be surprising.

... has nothing to do with Schrodinger's pussy, as I made clear that I don't buy into the "superposition of states" construct. You assumed that my statement implied it. You assumed wrong. Remove strict causality, and there is no need for any "superposition of states" bullshit. The whole copenhagen position sounds good at first, but after a few decades of consideration, it stinks. Better to have non-strict causality, which also explains strange actions at a distance, as well as fully resolving any ambiguities in the 2-slit experiments.

It also doesn't require (unlike the superposition of states crap) that matter or energy be created from nothing, then disappear back to nothing when the wave function collapses. So, unlike schrodingers' cat, non-strict causality doesn't violate known thermodynamic laws.

Re:Wrong

[MarkusQ]

A few things we seem to agree on

• If Schroedinger's Cat were conducted, the cat would not "be both alive and dead" in any meaningful sense.
• In your words "The whole copenhagen position sounds good at first, but after a few decades of consideration, it stinks."

Now back to our regularly scheduled debate

you would in effect be claiming that all measurement was impossible

an accurate measurement of anything IS impossible. WE already know that.

You are misquoting me right after quoting me verbatim. My point was (and is) not that you would in effect be claiming that infinitely accurate measurement was impossible, but that all measurement was impossible. The position is logically untenable precisely because it reduces to an absurdity.

because the speed of light will prevent any influence from the thermometer on the sample.

actually, you're making an assumption that strict causality exists. It has never been proven, and is an unworkable assumption because it then requires artificial constructs such as superpositions of states.

Say what? Please explain how, even in the absence of strict causality, when I measure (via spectral analysis) the temperature of a star that no longer exists (because, unbeknownst to me, it went nova millions of years ago) the temperature of my instruments can affect the temperature of the star, That doesn't sound like QM to me, it sounds like voodoo.

BTW, you misquote me when you say:

You said that of Schroedinger's cat (which asserts that macroscopic objects enter into a superposition of states) "Sounds weird, but its already been proven to be the case"; to now claim that this is "a bit of a brainfuck" while still maintaining that it is a proven, real effect doesn't wash.

No, I am quiting you verbatim from here.

--MarkusQ

Interesting discussion

[tomhudson]

I've changed the topic heading b/c this is not a discussion about right or wrng - its damn interesting. :-)

the temperature of my instruments can affect the temperature of the star, That doesn't sound like QM to me, it sounds like voodoo.

According to some theories, that actually happens. We can already see that here on earth, where the number of receivers can affect the load of a proximate transmitter, due to coupling. Think of the 2 coils of a transformer. Now separate the coils by a foot ... now a yard ... now a kilometer. There is still some minute affect on the current-carrying coil due to the mere existence of the other coil, and if the other coil is connected to a load, the transmitter will be affected very minutely. The NSA is into that sort of shit - trying to exploit it to determine where a receiver is based on the loads on transmitters, etc.

Now on to your spectral detector. It would seem that C means your receiver can't possible affect the output of the star millions of years in the past ... except that some particle theories require a symmetry along the time axis - either a 1-to-1 correspondence (1 particle going back for every particle going forward) or the same particle reversing direction at one end of the time scale. The beauty of these models is that they actually explain how particles can interact, something we take for granted ("well, sure, particles can interact"), but we don't really understand the mechanics of it.

Here's an example. Your body is composed of particles that are affected by every other particle in the cone of light (the distance light has travelled since the big bang) around you. How is that possible? Just the amount of information that represents is greater than the total of all the particles in the universe. And its constantly being updated.

Simple answer is, according to information theory, that its impossible.

So, if, rather than reject the theory, which seems to have worked for everything else, why not question the assumption that "of course particles can interact" and ask why they even should in the first place.

One proposal is that its all just the same single proton, electron, and neutron shuttling back and forth between the beginning and end of time. Now, since they are in all possible places at all times, they can certainly have "experienced" everything there is to experience, forming the background, the "matrix" (that fucking movie ruined the term!!!) against which the universe seems to be unfolding.

In other words, its not that they can interact - they already have, in every scenario that will ever be experienced. So, no information is needed to be preserved, because its already complete, just as you can throw out the intermediate calculations when you've got the solution to a complex problem.

Its a lot cleaner as a theory than the supposition of states ever could be. Its elegant. Its simple. Its ridiculous - which is why its so damn intriguing. Because, as ridiculous as it appears at first glance, it sticks with you because it succeeds in explaining a lot of things that other theories fail miserably at. It also gives a bit of insight into why time is different on our scale than the other dimensions, when there is no reason for it to be.

Re:Interesting discussion

[MarkusQ]

Two more points of agreement:

• I've changed the topic heading b/c this is not a discussion about right or wrng - its damn interesting. :-)
• ...the "matrix" (that fucking movie ruined the term!!!)...

Back to the discussion:

There are a few problem with the "bouncing back and forth in time" explanation:

• We should see equal quantities of matter and anti-matter
• The amount of "memory" the magic particle would need would be astronomical (not a killer, since there's only one of them, but it seems inelegant)
• It conflates thermodynamic time, casual time, and time-as-a-dimension in a way that seems less than helpful here

The last point is the real kicker. If you allow the backward flow of information along world lines it seems to me that you loose the ability to compute intervals, and your whole space-time metric falls apart.

(I'm aware that there are some "interactionist" models that accept this, but I don't recall much about them other than the assumption that the information flow across a unit of area is strictly limited to preserve the gross properties of the macroscopic world (e.g. we can't drive to the last century for the weekend) which would again give you the ability to measure something without affecting it. And thus even in them, there is unidirectional causuality of a sort, even if it isn't tied to a well-defined space time.)

If you don't allow backward flow of information, then being "strictly after" an event (delta_t > 0 and delta_t^2 > delta_x^2) should make it possible to measure it without affecting it.

--MarkusQ

Re:Interesting discussion

[tomhudson]

Well, the antimatter part is easy:

1. we don't know what most of the universe is made of ... (funny if it turned out that we were in a little bubble of matter and the rest of the universe was left-handed :-) .. I know, current theories don't show it, and haven't for 2 decades.. Oh, well, on to the second point ...
2. going back in time does not an antiparticle make. Think of spin as an example. An wheel going back in time would also be revolving in the opposite direction than when it was going forward, so to the everyday observer, it would still seem to be rotating in the same direction as the one that's traveling forward in time (I know, that sounds SO fucked up, you get these pictures of wheels doing the moon walk ... but it works, and its one reason why the single-particle theory can't be discarded out of hand).

The idea of the single particle bouncing all over the place is that there is now no need whatsoever for it to retain any state information. It's already been there, and done that (not for every possible solution, just for those that have/will happen). The intermediate info is not retained, and not needed. Solves a lot of problems (sure it makes some new ones. wouldn't be too useful if it didn't :-)

If you allow the backward flow of information along world lines it seems to me that you loose the ability to compute intervals, and your whole space-time metric falls apart.

How do you know your time measurements are continuous? Or that 2 intervals that you consider equal are in fact equal? For example, if time is "spongy", squizhed up in some parts, stretched out in others, you, having no external frame of reference, would have no way of knowing. Besides, you can't compute intervals exactly - that would lead to a violation of the unertainty principle, because you are now able to state with certainty 2 exact points in time and 2 positions. Remember - the more exact the timing, the less certainty of the position, and vice versa. If you take even 1 "snapshot" that you are dead certain of the time, then the position basically becomes the whole universe. Would make a pretty neat warp drive, though.

Think again to the basic assumption that we've all made - that any object even has the ability to interact with another object. If it can interact with one, it has to interact with all - simultaneously. We accept that as a daily fact of life, but the consequences just don't compute. We're doing something wrong, in that we aren't looking at it right, but that may be an artifact of our existence and form, and something we can't fix. We may not be "built right" to understand what's relly going on, any more than a cat that is raised in an environment with no horizontal surfaces can jump on a table.

Who knows, maybe we'll solve the problem by gengineering our descendants so they can see things differently, from a mental perspective.