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Scientists Dubious of Quantum Computing Claims
Dollaz wrote with a link to the International Business Times, which questions the authenticity of D-Wave's Quantum computing. We discussed the 'Sudoku playing' computer yesterday, but scientists in the field have expressed a lot of distrust of the company's findings. The machine was not available for inspection during or after the demo, and even if the technology was working as intended there is some doubt that it can be scaled. The article points out that "notwithstanding lofty claims in the company's press release about creating the world's first commercial quantum computer, D-Wave Chief Executive Herb Martin emphasized that the machine is not a true quantum computer and is instead a kind of special-purpose machine that uses some quantum mechanics to solve problems." Good to see people in the field questioning 'breakthroughs'.
How would you?
I'm legitimately curious. Such a device has never been built, how do these guys prove they have one? They say themselves they aren't certain if it's quantum-ing up the sudoku.
I don't need no instructions to know how to rock!!!!
Quantum computing researcher Scott Aaronson wrote some good anti-hype pieces about the D-Wave PR here and here, focusing on their incorrect marketing claims to be able to solve NP-complete problems in polynomial time. The first link also has an update with comments from Lawrence Ip of Google, who clarifies what the D-Wave people are really claiming.
Let me guess: It is a regular computer that solves a regular problem the regular way. One function needed is a number generator.
You could pick any device that returns different numbers at different times. It could be a microphone, a geiger counter, a clock og a quantum device
Now pick the quantum device, and call the whole device a "Quantum computer"
This is normal in marketing departments. The only unusual thing here is that they got the engineering department to go with them.
"Fix it"
So the first part of the scam is this: even if this device wasn't a quantum device at all it would still work to some extent because when you allow systems to cool they fall into lower energy states. If the 'quantum' aspect of things works then it might find that state faster, but without careful monitoring there's no way of telling if the 'quantumness' had anything to do with what it did. In fact, for large systems we know that it won't be very 'quantum' at all because it will interact with its environment and decohere. But it's a perfect strategy for designing a machine that you can claim is quantum, when it isn't. It stinks of scam.
Secondly: suppose you want to solve a challenging problem with this device. For example you want to search some space for a miniumum of some sort. For this machine to be effective the state space must be pretty large or else you could use a regular classical computer. Consider a billion state problem (quite small really for combinatorial problems). You have to be able to get a system to settle into the minimum energy state despite the fact that there are a billion states nearby all of which have almost the same energy. Just the tiniest input of energy and it'll jump up from that minimum. There is absolutely no way that they can search a large enough state space and still have the minimum energy state sufficiently far from other states.
BTW This device is quite different from what is conventionally meant by a 'quantum computer', it's more like a quantum, analog computer.
Real and useful 'digital' quantum computers are a long way off. I expect that the size of quantum computers will grow by a bit or so per year at the most. (When I say 'bit' I mean total memory, not the size of the bus.)
Doesn't it make you feel good to know that our freedoms are protected by politicans, lawyers and journalists.
Scientists are skeptical because it hasn't been submitted for peer review. Yes, but that's true for any new scientific discovery. It's not entirely fair to spin that into "this quantum computer might not really work".
Also, while the article claims it might not be a "true quantum computer", it never really says how that's different from a "computer that uses quantum mechanics to solve certain problems", and given its audience, can't possibly expect its readers to know. To me, this just sounds like journalists looking for something to hype about.
Direct test for a quantum computer:i ty_problem)
Solve any problem polynomially reducable to SAT/3-SAT (http://en.wikipedia.org/wiki/Boolean_satisfiabil
without the use of heuristic algorithms. Further, show it being done in polynomial-time with respect to the problem size.
Naturally, the machine and program would also have to be subject to inspection to show that it wasn't just spitting out a canned response to a problem already worked on and answered by a team of supercomputers elsewhere....
Fortunately, checking the result won't take too long. The check should be calculable on a conventional computer in polynomial-time.
This is an odd statement, because that's generally what people "in the field" do. The author says this as though it's unusual to see anybody questioning lofty claims. In fact, it's very common. The first slashdot article about this was met mostly with skepticism.
Note: Replying to this post, because I am not getting a "reply" button for the story itself. Anybody else experiencing this bug?
... and then they built the supercollider.
Link 1 http://video.google.com/videoplay?docid=8033280380 582597891&hl=en/
Link 2 http://video.google.com/videoplay?docid=-687784917 8932916346&hl=en/
Depending on what you mean by "solve," I think you are a little mistaken on what it means for a problem to be NP-Complete. NP-Complete problems are actually relatively easy to solve. For example, take 3SAT, were you have a bunch of boolean variables, x[1] through x[n], and then a bunch of string of clauses ANDed together, as in (x[1] OR x[2] OR NOT x[3]) AND (x[24] OR NOT x[37] OR x[42]) ... To solve 3SAT you just have to tell me whether there exists some combination of variables such that the expression is true. You can just enumerate all 2^n possible combinations of variable assignments and see if any of them work out to be true. The problem is, it doesn't take very large values of n before it will take you longer than the time civilization has been around to try all the combinations. 3SAT is easy to solve, if you just want an answer. What's still an open question is whether we can come up with an algorithm that can solve it efficiently, where efficiently means in O(n^k) time for some k, rather than O(2^n).
For a problem that actually can't be solved, try the Halting Problem.
Now, the cool thing about NP-Complete problems is that any other problem that's known to be in NP (meaning we can solve them, just some instances will take a ridiculous amount of time to do so) can be efficiently transormed, meaning transformed in polynomial time, into an NP-Complete problem. This means if you can really solve general instances of Sudoku in polynomial time, you can take an instance of the 3SAT problem, efficiently transform it into an instance of Sudoku, then efficiently solve the Sudoku problem and then transform the answer into a solution to the 3SAT problem. If they have really built such a machine, this is a big deal.
I actually interviewed with these guys a few months back. I can tell you I was quite impressed with the facility and they came off as very bright. I also got a tour of the facility so I'll share what I know. The chip core is stored in a large tank roughly 2m tall and cooled to very very near absolute zero. That is then held inside what is in essence a very large faraday cage. All the refrigeration and electronic equipment is kept outside with only passive sensors allowed in the room wherever possible. Apparently electrical noise and stray heat has been a huge hurdle. From what I understood they saw themselves as building chips that would be housed on site and used remotely so it doesn't surprise me that they didn't have a setup that was available for public viewing. The company culture was essentially work till you drop and hope the stock options make you rich.
Solve any problem polynomially reducable to SAT/3-SAT
You have your reducibility direction mixed up: even really easy problems (like sorting, or outputting "2") are reducible to SAT. It's the hard problems that SAT reduces to.
Not that this matters, because quantum computing is very unlikely to be able to solve NP-complete problems. It does seem to help with very structured problems like factoring, though. No, factoring is (almost definitely) not NP-complete.
Further, show it being done in polynomial-time with respect to the problem size.
Polynomial-time is an asymptotic notion. It can't be verified for a particular problem size (or finite set of problem sizes). It is purely an analytical concept, not an experimental/testable one.