Desultory D-Wave

D-Wave, a startup in Burnaby, British Columbia, claims to have demonstrated a working prototype of what it says will become the world’s first “commercially viable” quantum computer. But as even its supporters admit, the company has a severe credibility problem in the scientific community (see “Riding D-Wave”). In looking at its claims, I think it’s helpful to ask three questions.

First, what has D-Wave actually built? Second, even if D-Wave has built a quantum computer, is there any realistic possibility of scaling it up to hundreds or thousands of quantum bits (or “qubits”) in a few years–as D-Wave has repeatedly promised it will do, and as would be necessary to solve any practical problems? Finally, if D-Wave has built a scalable device, what problems can it solve more quickly than existing computers can?

Physicists tend to doubt D-Wave’s claims because it has presented no convincing evidence that its current device is quantum-mechanical. As far as any outsider knows, all D-Wave has produced is an extremely expensive and inefficient classical computer with a grand total of 28 bits. By its own admission, D-Wave has not yet demonstrated quantum “entanglement” between even two qubits. (Entanglement is a quantum form of correlation between two or more qubits; all parties agree that it is a non-negotiable requirement for quantum computing.)

Among computer scientists, another source of skepticism is that D-Wave has misled the public about what quantum computers might be able to do if we had them. In particular, D-Wave’s publicity materials talk cavalierly about using its machine to solve what are known as NP-complete problems. These problems (of which the best-known example involves determining the shortest route for a traveling salesman visiting a number of cities) are important because they are common and are thought to be intractable for any computer today. However, almost all computer scientists also believe that these problems cannot be efficiently solved using a quantum computer. We have no good evidence that quantum computers–D-Wave’s or anyone else’s–could find even approximate solutions much faster than a classical computer in cases of practical interest.

Let me be clear: I think that quantum computers are possible in principle, and that D-Wave’s approach might even get us there. I’ve also met people from D‑Wave; I don’t think they’re frauds. But the human capacity for self-deception being what it is, scientists train themselves to look for red flags–and D-Wave is pretty much a red-flag factory.

Scott Aaronson is an assistant professor in MIT’s Department of Electrical Engineering and Computer Science.