The pioneers of superconducting quantum computation had been able to demonstrate the quantum nature of their devices by zapping them with fast microwave pulses and looking at their responses. But those devices weren’t adiabatic; they operated at speeds comparable to those of a conventional computer. The D-Wave device, by contrast, is purposefully slow: therefore, no zapping is possible. As a result, there are a limited number of experiments that can indicate whether the device is really doing quantum computation. One, however, is to vary the slowness with which the device oozes from its initial state to its final state. Halfway through the oozing process, the computer arrives at a point where it must start making the hard choices that lead to the problem’s solution. Here the computer is in a weird quantum state, in which every bit registers 0 and 1 at the same time. I urged the D-Wave researchers to explore this critical point and search for the telltale signs.
More recently, I spoke with Herb Martin, the CEO of D-Wave, and Geordie Rose, the company’s chief technology officer and cofounder, and emphasized the need for them to pursue these experiments if they are truly interested in explaining how their devices work. One experiment that I recommended to Rose is a specific protocol for creating and verifying the presence of a so-called Schrödinger’s-cat state, a specific instance of the state in which all the qubits register both 0 and 1 simultaneously. (The name comes from a thought experiment proposed by one of the founders of quantum mechanics, Erwin Schrödinger, who imagined a quantum cat that could be both dead and alive at the same time.) Both Martin and Rose seem enthusiastic: they are well aware that if they can’t prove that their device is really doing something quantum-mechanical, then their name within the scientific community will remain mud.
In November of last year, D-Wave demonstrated what it claimed was a 28-qubit adiabatic quantum computer. Now, the company’s scientists are attempting to demonstrate the fundamentally quantum-mechanical nature of their device. There is a strong motivation for doing the science and getting it right. Engineering is science so well established that even engineers like me can do it. If you can’t get the science of a 16-qubit quantum computer right, then your chances of building 512-qubit and 1,024-qubit devices (D-Wave’s next planned steps) are nil. On the other hand, if D-Wave can confirm that its current system enters the state where all its qubits are 0 and 1 at the same time, then it has a good shot at building quantum devices that are more complex.
And a 16-qubit superconducting Schrödinger’s cat would be pretty cool.
Seth Lloyd is a professor of mechanical engineering and the director of the Center for Extreme Quantum Information Theory at MIT.