Update (May 25): See my Q&A about D-Wave’s new announcement at Forbes.com.
Update (May 26): See also this very helpful Quora post by Dave Bacon, who says mostly the same things I did (though it always sounds better when he says it!)
Clearly, there hasn’t been enough controversy on Shtetl-Optimized this past week. But I have just the thing to fix that: a new D-Wave post!
For three days, people have been sending me the news by land, sea, and air that D-Wave just published a paper in Nature, describing evidence for quantum annealing behavior in a system of eight superconducting flux qubits. The paper itself is behind a paywall, but the more detailed Electronic Supplementary Material is available for free (see also D-Wave’s blog post). As usual, my readers apparently expect me to render an instantaneous opinion.
But for the first time in the history of major D-Wave announcements, I’m unable to do so. For D-Wave is finally doing the very thing that I and others have been begging them to do for years: that is, directly addressing the question of whether their systems actually exploit quantum effects, or just perform classical simulated annealing. In the new work, they apply an annealing operation to eight coupled qubits arranged in a 1D chain, then plot the probability of a particular basis state as a function of time, by running the experiment over and over and stopping it at various intermediate points. They then look at the dependence of the probability-versus-time curve on a third parameter, the temperature, and claim that they can explain the curve’s temperature dependence by a numerical simulation that assumes quantum mechanics, but not by one that assumes classical simulated annealing.
To be clear, an eight-qubit spin chain with a quantum-mechanical temperature dependence is still a very long way from anything commercially useful (and it’s notable that, now that D-Wave has happily joined the ruling-out-the-null-hypothesis club, we’re down from 128 qubits back to 8). This paper also makes no claims to demonstrate entanglement, which is almost certainly necessary for any interesting quantum speedup, and which has been verified in other superconducting qubit experiments (e.g., the Schoelkopf Lab‘s at Yale), but as far as I know still not in D-Wave’s. Even so, after four years of the quantum computing community being told to review a restaurant based solely on its ice water and table settings, I’m delighted that D-Wave has finally brought an appetizer. Expert readers who’ve actually tasted the appetizer are urged, in the strongest terms, to share their analysis in the comments section. I’m looking forward to our least-uninteresting D-Wave discussion ever.
But first, let me anticipate the question that at least one commenter will ask (I mean you, rrtucci). No, I don’t have any regrets about pouring cold water on D-Wave’s previous announcements, because as far as I can tell, I was right! For years, D-Wave trumpeted “quantum computing demonstrations” that didn’t demonstrate anything of the kind; tried the research community’s patience with hype and irrelevant side claims; and persistently dodged the central question of how it knew it was doing quantum computing rather than classical simulated annealing. So when people asked me about it, that’s exactly what I told them. Now, whether because it took the skeptics’ criticisms to heart, or for whatever other reasons, D-Wave has done a real experiment that deserves the careful scrutiny it will receive. I just call the shots as they’re fired.
As some of you might be aware, I’m a theoretical computer scientist, not a physicist (much less an experimentalist). So in previous posts, the only reason I even presumed to comment on experimental matters is that D-Wave made it easy for me! My “expert analysis” mostly just consisted of pointing out, over and over, that D-Wave hadn’t yet brought the QEDB (Quantum-Effect-Demonstrating Beef)—and that, until they did so, there seemed to be little reason even to discuss the other issues that D-Wave’s marketing materials and the press were both spending 95% of their time on. Now that a slice of QEDB (or something that looks like one, anyway) is on the table, I think there’s at least as much need as ever for critical evaluation of D-Wave’s claims from the quantum computing research community, but I no longer see Shtetl-Optimized filling that need. So I hereby announce my retirement as Chief D-Wave Skeptic, a job that I never wanted in the first place. New applicants for this rewarding position are urged to apply in the comments section; background in experimental physics a must.