Friday, May 17, 2013

Quantum computer with no quantum speedup

MIT complexity theorist Scott Aaronson is back on the warpath against those claiming successful quantum computing:
“Look, Scott, let the investors, government bureaucrats, and gullible laypeople believe whatever they want — and let D-Wave keep telling them what’s necessary to stay in business.  It’s unsportsmanlike and uncollegial of you to hold D-Wave’s scientists accountable for whatever wild claims their company’s PR department might make.  After all, we’re in this game too!  Our universities put out all sorts of overhyped press releases, but we don’t complain because we know that it’s done for our benefit.  Besides, you’d doubtless be trumpeting the same misleading claims, if you were in D-Wave’s shoes and needed the cash infusions to survive.  Anyway, who really cares whether there’s a quantum speedup yet or no quantum speedup?  At least D-Wave is out there trying to build a scalable quantum computer, and getting millions of dollars from Jeff Bezos, Lockheed, Google, the CIA, etc. etc. to do so—resources more of which would be directed our way if we showed a more cooperative attitude!  If we care about scalable QCs ever getting built, then the wise course is to celebrate what D-Wave has done—they just demonstrated quantum annealing on 100 qubits, for crying out loud!  So let’s all be grownups here, focus on the science, and ignore the marketing buzz as so much meaningless noise — just like a tennis player might ignore his opponent’s trash-talking (‘your mother is a whore,’ etc.) and focus on the game.”

I get this argument: really, I do. I even concede that there’s something to be said for it. But let me now offer a contrary argument for the reader’s consideration. ... If that happens, then I predict that the very same people now hyping D-Wave will turn around and—without the slightest acknowledgment of error on their part—declare that the entire field of quantum computing has now been unmasked as a mirage, a scam, and a chimera.  The same pointy-haired bosses who now flock toward quantum computing, will flock away from it just as quickly and as uncomprehendingly.  Academic QC programs will be decimated, despite the slow but genuine progress that they’d been making the entire time in a “parallel universe” from D-Wave.  People’s contempt for academia is such that, while a D-Wave success would be trumpeted as its alone, a D-Wave failure would be blamed on the entire QC community.
I get the impression that there is some resentment of D-Wave's private funding. It is okay if university quantum computer researcher make wild claims, because they don't have to deliver a product. D-Wave promises a product that can be benchmarked, and subject to failure in the marketplace.

There is, unfortunately, no proof that quantum computing is possible. And it may never be. Peer-reviewed professors can live in an academic bubble, and pretend that it is possible.

The NY Times reports:
In tests last September, an independent researcher found that for some types of problems the quantum computer was 3,600 times faster than traditional supercomputers. According to a D-Wave official, the machine performed even better in Google’s tests, which involved 500 variables with different constraints.

“The tougher, more complex ones had better performance,” said Colin Williams, D-Wave’s director of business development. “For most problems, it was 11,000 times faster, but in the more difficult 50 percent, it was 33,000 times faster. In the top 25 percent, it was 50,000 times faster.” Google declined to comment, aside from the blog post.

The machine Google and NASA will use makes use of the interactions of 512 quantum bits, or qubits, to determine optimization. They plan to upgrade the machine to 2,048 qubits when this becomes available, probably within the next year or two. That machine could be exponentially more powerful.
It sounds great, but apparently there is some dispute about whether there is really some sort of magical quantum speedup.

Update: Peter Shor writes:
This [argument that an off-the-shelf classical can solve the D-Wave problem many times faster] is exactly like arguing that if you look at the Wright Brothers’ first flight at Kitty Hawk, they could have gone farther, faster, and much more cheaply if they had just used an automobile. It’s just not the right comparison. D-Wave’s money was not spent only to build this current device; you have to consider that from their viewpoint, it’s just one step on the pathway to a much more complicated and useful device.
SciAm explains:
I began by explaining the theory behind quantum computing and why they hold the promise of significantly faster processing. In essence, it relies upon the fact that whilst conventional “bits” can be 0 or 1, quantum bits (so called qubits) can be both 0 and 1 at the same time (known as superposition). If you can combine qubits (known as entanglement) you can have a system that can process values that expand exponentially with the number of qubits you entangle. As with conventional programming, these qubits are passed through various logic gates to achieve the desired results. Hence, this is known as the “gate theory” of quantum computing.
This is a convenient explanation of quantum computers, but is one that Aaronson denounces as incorrect:
I agree that thinking about the wavefunction “realistically” (as an exponentially-large classical object) seems to be a mistake that countless popular writers make, which then leads them to believe that quantum computers can solve black-box search problems instantaneously, store exponentially-many classical bits, and do other things that they’re known not to be able to do.

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