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Quantum Computing Explained! (Well, Sorta)
An anonymous reader writes "Valiant effort to 'explain' quantum computing over on silicon.com — covering the difference between classical computers and quantum machines."
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You can't possibly know if the article explains quantum computing until you actually read the article.
Interesting but the chatty narrative is really annoying and is getting in the way of the actual useful information
One more thing, there is a minority of scientists who believe that building a quantum computer will turn out to be out-and-out impossible.
However, if those scientists are right, the implication of not being able to build such a machine is that quantum mechanics itself, as a description of nature, is wrong. Either way, the stakes could not be higher.
One possible failure mode is the theoretical power required could exceed the light fluxs of the visible universe, that would be a bummer. Maybe in true supercomputer style, a formerly computational problem is merely converted into an I/O problem, the interface to the classical world might be too slow/imprecise/analog/noisy/random to pull useful results out of it. Nothing wrong with quantum theory at all, just not possible to interface usefully with the greater classical world.
Or the more practical engineering/accounting failure mode where it would simply be cheaper / faster / more efficient to use mass produced classical processor, possibly for any problem.
"Science flies us to the moon. Religion flies us into buildings." - Victor Stenger
...for the leap in the right direction!
Tired of my customary (Score:1)
From the geniuses in H.R.
Wanted:
Quantum Computer Developer.
Qualifications:
Five years in depth Quantum computing experience. Certification in Quantum Computing highly desired.
In depth knowledge of Quantum Computing principals and a proven track record of creating Quantum Computing applications.
Principals only.
I miss Q*bert.
.... ...
[Phone recording] You reached the secure communications quantum cryptography help desk. We detected that you pressed "Reset my password". This option requires a transfer to teh 8th dimension. We are now connecting you to Lord John Whorfin
[Lord John Whorfin] May I pass along my congratulations for your great interdimensional breakthrough. I am sure, in the miserable annals of the Earth, you will be duly enshrined. How may I help you?
L'esperienza de questa dolce vita (The experience of this sweet life) - Dante Alighieri, The Divine Comedy
Everyone always talks about the differences between a standard computer and a Quantum computer. Graphics cards are good for floating point numbers, why can't we have a Quantum Card to handle quantum operations? Does it really have to be one or the other?
The more precisely the article explains it to him, the less precisely it will explain it to you.
Maybe I could read the whole thing at once, if I had a quantum computer?
Murphey's fighting Occam, and we're in the stands.
... on one page and continue on the next.
Do you remember the Google Quantum Powered Image Search
http://www.newscientist.com/article/dn18272-google-demonstrates-quantum-computer-image-search.html
Some folks have questions about D-Waves technology, but there are people at Google who have been writing applications for Quantium computers.
Sorry to be so negative but in my opinion the article is horrible. It doesn't explain anything unless you think bad analogies and jovial metaphors help you understand things better. After having read it, I don't know a single qubit more about quantum computers than before.
As obvious as it may be to include a "picture" of an atom -- a Rutherford model -- it seems terribly incorrect to use it as the primary image to be associated with a quantum-mechanical phenomenon. Though I guess it's good enough to make the article feel "science-y".
I can't help but recall Wyoh Knott, the heroine of Heinlein's "The Moon is a Harsh Mistress", who conceived of an electron as "about the size and shape of a small pea".
Stressed? Me? Of course not. Stress is what a rubber band feels before it breaks, silly.
I tried to RTFA, but the author wants to be too cute and buddy-buddy with me. When you're trying to learn something new, little jabs and crappy jokes prevent me from getting into the learning zone. Stick with the wikipedia article.
... the article is completely right and absolutely wrong at the very same time.
Shouldn't the possibility of instant communications be an explored issue??????
Quantum mechanics is NOT a description of nature. It is a probability theory which predicts the outcome of future observations. You do not need to describe nature in order to predict it: you are predicting the distribution of outcomes of a lot of observations. This is exactly the same as not needing to know the physics of dice throwing in order to predict the outcome of a large number of throws.
Have to agree with the comments above, that article is pretty useless.
Coincidentally, though, at a university book sale a few weeks ago, I picked up a copy of N. David Mermin's Quantum Computer Science: An Introduction, for just $5 (seems to be about $30 on Amazon) and I can't recommend it highly enough. It's an intro to quantum computing textbook, about 200 pages, written specifically for people who have CS or math (as opposed to physics) backgrounds, and while it's almost impossible to get into the nitty-gritty of why quantum computing works without a lot of quantum mechanics esoterica, this book does a great job of explaining how it works (which is plenty complicated on it's own).
It's not a light read (it's a textbook, after all), and contains some serious math, but it's nothing someone with a college education can't handle and it really helped me understand this whole mess better than any popular news article.
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...my brain processes became quantumly entangled.
Since quantum computing itself is partially inexplicable, and building a physical machine is currently impossible, we probably won't be seeing this in the near future unless it is on an episode of Star Trek OR if they will use it to make Wall Street trading machines faster.
He who knows best knows how little he knows. - Thomas Jefferson
This is a newbie question I always had. Regarding computation alone (not those "entanglement" based communication properties etc.) will quantum computers be able to do something an ideal Turing machine can't? Can one assumes they are like our existing computers, just a lot faster for some kinds of operation?
Unfortunately, the author does not seem to have understood the concept of entanglement. Correlations between particles (even perfect ones) are also found in classical physics as the example with the socks implies. Quantum entanglement, however, is much more subtle, and distinguishing between useful entanglement and useless classical correlations is typically a highly nontrivial tasks.
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Entanglement for secure channels of communication I believe. Quantum "computing" in the sense we usually think of computing looks phony.
Sure, such a quantum computer, if built, could process, say, 10^50 quantum inputs simultaneously. But where does one get the 10^50 inputs? Each input is a delicate quantum-entangled state. Do I pull my 10^50 helium-cooled quantum state composition machines out of my closet? It is a promise which is can only be employed by leprechauns and unicorns.
The bottom line is, as stated by the article, quantum computers would useless for most everyday computations. Which things a quantum computer could actually be used for is clear as mud.
I would welcome a link to a technical article that is grounded in reality, proving me wrong.
All theories are really just models of the universe, some work better and tell us more about how the universe does work. Quantum mechanics does tell us many very important things about our universe. Most importantly, and confusingly for everyone first learning it, our universe is not deterministic. Unlike dice, where we could predict with absolute certainty what the outcome would be if we collected enough data about the throw, we cannot do that in the quantum world. There are no "hidden variables" that we could use to increase the accuracy of our probabilistic predictions.
See bells theorem for more mind bending details.
Well.. maybe. Or Maybe not. But Definitely not sort of.
So Ive always read that quantum computers excel in parallel calculations like code-breaking and such, but if its probability calculations that it are it's bread and butter, would that extend to weather prediction modeling? I'd love for the Weather service to get as good as they were in Back to the Future II, weather predictions accurate to the second.
Common Sense isn't as Common as people think...
This shared state means that a change applied to one entangled object is instantly reflected by its correlated fellows - hence the massive parallel potential of a quantum computer.
Unless I missed some major recent development, modifying an entangled particle and "instantly" observing the effect on its correlated partner is precisely what you cannot do with an entangled pair. Gets into that whole pesky faster than light communication thing that makes causality not work.
Is he just conflating entanglement and quantum teleportation?
sic transit gloria mundi
Maybe Schrödinger's cat is hiding in there too? And if she's dead, will she be brought back to life by observing her?
This is a rather strong assertion with essentially no backup. You probably don't actually understand quantum mechanics either, too, but you haven't said enough to belie your lack of understanding.
There is a theory, probably via Roger Penrose (or not, or both), that biological brains are so curiously different from standard computers and good at diffuse problems like pattern matching exactly because they are tapping into quantum entanglement as a material for decision making. So... build ye a quantum computer and see it stare back from the (quantum) abyss at you... (or not)
From the article:
"This shared state means that a change applied to one entangled object is instantly reflected by its correlated fellows"
Why, oh why, is this nonsense repeated again and again. If you change one entangled particle, you do not change the other. For example, if you have two spins entangled in a way so that if one is measured "up" the other is measured "down" and vice versa, and you turn the one spin around (without measuring it) then you'll have an entangled state where if you measure the first spin "up" the other one is also measured "up", just as you'd expect. As long as you don't measure, there's no "spooky action at a distance" but only local changes. The "spooky action at a distance" happens at measurement (which BTW destroys the entanglement), and it's all but a given that there's indeed an action at a distance (you only need it if you want a certain type of interpretation, where basically "under the hood" the system behaves completely classical, but we don't see it because there are so-called hidden variables which we cannot determine). The point is that in an entangled state the correlation is all which is defined, and the result of local measurements are completely undefined (OK, strictly speaking this is only true for maximally entangled states, for others there's less correlation and more local information; it's basically a trade-off between the two). Now when you measure the spin of one of the particles, the value of the spin gets a defined (but random) value (up or down, in the direction you measure), and also the value of the spin of the other particle gets a defined value, which is determined by the entangled state and the result you got for the first particle, i.e. if the entangled state was "both particles have opposite, but otherwise undefined spin" then after measurement, the particles will have opposite, well-defined spin. However, since the result is random, if you have the other particle, you cannot see any difference whether the first particle has been measured or not; he will get a random result either way. Only if he gets told the measurement result of the first particle, he can predict (or, if he already measured, compare) his measurement result.
Oh, and yes, I'm working in the field of entanglement, so I know what I'm speaking about.
But I absolutely like the following statement from the article:
"Hang on, what's quantum entanglement when it's at home?
I was afraid you were going to ask."
I hope the above explanation is understandable ...
The Tao of math: The numbers you can count are not the real numbers.
No, no, no, no, no! It's exactly this kind of statement that has people thinking QM means faster-than-light communication or other voodoo. But the statement itself is just flat-out not true.
Properties of particles can either be in known states, or a unknown states. Two properties in unknown states on different particles can be entangled. Any attempt to read the property or set it to a desired value will destroy any existing information already there. This includes entangled states.
With entanglement, however, you can gain information about the other particle without disturbing it.
Imagine having a pair of magical coins. When you touch them together, then the next time you flip them they will each come up on the opposite facing. But it's only good for one flip. Until either coin is flipped, both have a 50% chance of coming up either heads or tails -- it's truly random. But once one coin is flipped, then the other will be guaranteed to come up the other way. This holds true even if the coins are flipped simultaneously!
Entanglement is indeed strange, but it does not allow for the violation of causality like the quoted statement implies!
The only way this could condescend further to its audience is if the author had put "Tee hee! Math is hard; let's go shopping!" in one of the questions.
Note that not everyone rejects hidden variables. Claiming that QM implies a non-deterministic universe because of the absence of hidden variables is in fact subtly wrong. The dominant interpretation of QM (which claims the nonexistence of hidden variables) *assumes* nondeterminism, it doesn't conclude it. You can find a complete quote on the Wikipedia page for Superdeterminism, but there was an assumption in the design of the EPR experiment that assumed non-determinism as a means of preserving the free will of the experimenters. In other words they mixed philosophy into science then got freaky weird results, and now most of their followers fail to consider the possibility of a causal link between the two.
Or does it mean that none of this will come to pass? To quote the article
the average computer user probably won't have much use for a quantum computer
I'd say it's quite important to know which way our future is headed - anybody know?
politicians are like babies' nappies: they should both be changed regularly and for the same reasons
What Bell's theorem disproves is local hidden variables. Non-local hidden variables are perfectly possible, as Bohmian mechanics proves.
The Tao of math: The numbers you can count are not the real numbers.
We're nerds; we already understand how quantum computing works. Else, what would be the use of every single article that's already been posted about it?
Funny, but such candidates exist.
Right here at my employer (NIST), we've had groups working on quantum computers for at least the six years that I've been around. And back in grad school I knew of candidates who spent their ~5 year graduate careers studying quantum computing. So while you can't buy a quantum computer at Best Buy yet, there are at least dozens of people who actually do meet the qualifications (aside perhaps from the certification).
...and I read most of the fine article. It puts too much emphasis on quantum entanglement, which is useful for quantum cryptography, but not as important as quantum interference. It's the weird quantum states of the individual qubits that interfere with each other, that make a quantum computer. If you can figure out how to encode information into the quantum state of a qubit, and get a bunch of them to interact in a given way, you get the quantum interference to cancel out the information you don't want, and leave the information that you do want (probabilistically speaking). Unfortunately, the math and creativity needed to encode problems as such requires some truly stellar mathematical/physical thinking, and we wouldn't be having any Quantum code-monkeys like we do with classical computers.
Yes, but "there's a signal that propogates at infinite speed and yet can't be used for communication" is a road you really don't want to go down.
Bell's theorem forces you to choose between rejecting locality and rejecting counterfactual definiteness (i.e. the idea that there is a pre-existing property that is "waiting" for our measuring to find out what it "really was" the whole time).
Information theory is life. The rest is just the KL divergence.
NSA Ion Trap on a Semiconductor Chip
As others have noted, the author's explanation of entanglement is faulty. IMO the key fault (although there are others) is the implication that entanglement can be used for (perhaps super-luminal) communication.
Perhaps only a minority of scientists think building large scale quantum computers is impossible, but I think a majority of physicists think it is impossible, I certainly do. I strongly disagree with the idea that the only way building large scale quantum computers would be impossible is if quantum mechanics does not describe nature. The key stumbling block is a phenomenon known as quantum decoherence. Wojciech H. Zurek recently wrote a fairly long review paper summarizing what is known about decoherence. IMO, just as the computing power of a quantum computer increases exponentially with the number of qubits, so does the interference from quantum decoherence.
It is like the old story of how chess was invented to please a king. The king was so pleased he offered the inventor his weight in gold. The inventor asked instead for some rice. He said put a single grain of rice on the first square of the chess board, then two grains of rice on the 2nd square, four grains of rice on the 3rd square, eight grains of rice on the 4th square, etc. At first the king thought this was a paltry reward for such a great game but after trying to fulfill the request, the king got an inkling of exponential growth and had the inventor beheaded.
Finally, the author got the connection between security and factoring large numbers backwards. Banks and internet security depend on the difficulty of factoring large numbers. Large scale quantum computers could easily factor large numbers thus rendering perhaps all of our current encryption systems obsolete. I have no idea what they would be replaced with but it seems at least possible that anyone who wanted to communicate securely would need to use a quantum computer.
We don't see the world as it is, we see it as we are.
-- Anais Nin
The review paper I was thinking of was by Maximilian Schlosshauer. It was called "Decoherence, the measurement problem, and interpretations of quantum mechanics". He has also written a book about quantum decoherence.
We don't see the world as it is, we see it as we are.
-- Anais Nin
You're right in that I don't. Others don't mind, however, or at least they consider that the lesser evil compared to giving up a classical reality.
Also it could in principle still be an extremely high, but finite speed (it just has to be high enough that we don't hit the "non-correlation window" with significant probability in our experiments). That would, of course, imply a change to quantum mechanics (and also to relativity, because it would mean a detectable absolute frame of reference). Again, it's nothing I consider attractive, but it's not ruled out by our experiments (provided that finite speed is large enough).
The Tao of math: The numbers you can count are not the real numbers.
I find superdeterminisim to be even stranger than quantum mechanics, and too close to creationism/intelligent design to me. The answer to every question simply becomes " because that's the way God did it" Or "because that's the way the initial conditions set it up". It seems downright psuedo scientific and even more of a philosophical intrusion. I also find the lack of free will to be more mind boggling, with more bad implications in real life than quantum mechanics.
Well.. maybe. Or Maybe not. But Definitely not sort of.
I'm not a professional scientist. I'm a CS geek. I've also only made it through page one so far.
This struck me as very underwhelming, even disappointing:
Instead of having bits, as a classical computer does, which represent either a one or a zero, a quantum computer has quantum bits - qubits - which can represent zero, one, or a superposition of both - that is, any amount of either zero and one
simultaneously. As a result, unlike a traditional computer which can only store one number in a single register at any one time, a quantum computer can store more than one.
Doesn't that mean going from bits that hold either one or zero, to bits that hold floating point values of anywhere between zero and one, containing "percentages" of the present value between 0 and 1? What's so Earth shaking about that? Those are bytes, yes?
WTF?!?
"Tongue tied and twisted, just an Earth bound misfit
If you build 2 8 qbit systems you could use them as basically a wireless NIC that'd teleport the data between the cards with no wires or range limits? You could get real time data from a probe orbiting Jupiter or even further out? That is a bit mind twisting.
Quantum computing - very cool, but I'm more interested in the possibility of transmitting data instantly. Running synchronous replication half-way around the globe without latency problems. Nobody seems to talk much about it, but entanglement would make this possible?
I knew you were going to say that.
bla
You don't really need a free will of the experimenters. All you need is a deterministic cause in the past light cone of each measurement which is not also in the past light cone of the other measurement. It's easy to see that such events always exist.
The Tao of math: The numbers you can count are not the real numbers.
I find non-determinism to be stranger. In a deterministic world, you can always ask what caused anything that you observe. In a non-deterministic world, you observe something, and apparently it happened without a cause, it could have just as well been one of several other outcomes. If you accept non-determinism, it's not free will that results, it's that your will is governed by random chance and not by cause and effect.
Well I for one welcome our new Qbert overlords!
If our brains are quantum mechanical devices, that doesn't mean that all of our thoughts are random. Just that they are not 100% reliable. That seems to be my experience. Plus, you can force quantum mechanics non determinism to work for you to get some sweet results: like, uhmm, quantum computers. I'm okay my thoughts not being completely logical, and I'll admit sometimes my effects are not very closely related to any external causes, but that doesn't mean I have a random number generator in there.
Well.. maybe. Or Maybe not. But Definitely not sort of.
I think the free will issue in superdeterminism is frankly bullshit. Do we reject relativity because it is deterministic? Aren't all our sciences outside quantum physics deterministic? Just because we haven't generated free will in a simulation yet such as a 'true' AI doesn't mean that free will can't exist inside a completley deterministic system.
An unthrown dice "exists" in a superposition of all possible outcomes. A thrown dice exists in only one such outcome. Nothing in the present is ever in a superposition, but when an unknown state effects the future we use superpositions to deal with our lack of Knowledge of the exact states, to calculate probability distributions of possible outcomes.
Entanglement is simply when you know with absolute certainty a connection between the possible outcomes. For example we know exactly that if the dice has 6 on top, then 1 is on the bottom even though we don't know which roll comes out.
The article fails to mention an interesting point that David Deutch of Oxford university, the formost authority of QC, has made; that a correctly functioning quantum computer would proove the existence of a multi-verse. Its easy to envisage. Imagine a QC in a lab with a bunch of white coats around it. Now imagine several "exact" copies of the same scenario (one for each quibit) - a bit like when you look at yourself in a mirrow with another mirror behind you. When the QC calculation starts, each universe splits, but after the calculation, they reconverge and EACH qc in each universe has the correct answer. Thats why its referred to as parallel computing. One computer in each paralel universe. Its also been calculated that qc may be build to factor numbers up to 10 raised to the power 500, which is the maximun number of universes in the multi-verse. Have a nice day!
which means I can tell that I wouldn't have understood it from reading that article.
It's not one of those nonsense articles; the author clearly has some idea what she's talking about, but don't feel like you should be able to get some basic understanding of quantum computing from reading this. The information really isn't there. It starts with 'what is quantum physics' and very quickly moves on to 'what are quantum computers used for'. How they actually work is I think glossed over in the sentence "This shared state means that a change applied to one entangled object is instantly reflected by its correlated fellows - hence the massive parallel potential of a quantum computer. ", and if that was enough explanation for you, you're psychic.
David Mermin's lecture notes in an earlier comment though look great! Thanks for the link.
Also if anyone can explain to me what the article means by:
let me know. I'm guessing that this is a simplistic reference to something real, but I have no idea what, and I can't understand how it's consistent with the fact mentioned earlier in the article that 'toy' (i.e. few-bit) quantum computers have been demonstrated to work in the lab.
Disclaimer: i am from the field.
a) Its more than a small minority of scientists who do not believe in quantum computation, even if being a minority would make a difference in science. Making it sound like these people are a kind of weirdos does not give enough respect to a lot of great minds. There are practical reasons we will collide with and mother nature may hold more more us than we expect. It seems that Quantum mechanics holds for small systems and for massively uniform systems. I, as many others expects that it also holds in the range between, but thats a Hypothesis we are out to test.
b) Entanglement is not created in a distance, but it can be transferred there. This transfer of information and action is governed by the normal limitations (speed of light etc.). There is and own research direction in the field busy about how to use local interactions to distribute entanglement over wide distances. I can *definitely not* transmit information by and entangled state alone. The point is that i A measures and B measures, even if they measure perfectly correlated states , e.g A always measures 1 if B measures 0 and vice versa, they *DO NOT* transmit information.
c) The way in which superconducting systems are mentioned is outright scary. Most superconducting quantum circuits have nothing to do with the way of using superconductors described in the article, which is new and unexplored.
d) I find impossible to talk about quantum computation and only mention decoherence in two lines of 4 pages.
[q]Aren't all our sciences outside quantum physics deterministic?[/q]
Nope. They are not, if quantum mechanics is correct. Science is built upon science. If the lowest level of interaction of particles is deterministic then they all are. If it is not, then they are not(but for all practical purposes they certainly can be considered to be deterministic, but we know there is always a minisucle probability > 0 that something odd will happen.
Free will is in opposition to determinism. Either we can choose what we do, or it was predetermined in the initial conditions of the universe.
Plus that, that was the whole superdetermininisim argument in the first place: If Superdeterminisim is correct, there isn't such a thing as free will and thus nothing too strange about bells experimental results.
Well.. maybe. Or Maybe not. But Definitely not sort of.
I didn't like the article. I recall reading a better one a while back that explained quantum computing in terms of the Many Worlds (or parallel universe) hypothesis.