Details about the D-Wave chip

Details about the D-Wave chip

Scientists from University College in London and the University of Southern California have weighed into the ongoing 'is it quantum' theory / D-Wave debate with an interesting approach, testing the device under a variety of noise conditions.

As their paper at Arxiv explains, the thermal environment of a D-Wave chip isn't directly accessible since the machine operates as a black box, in that respect.

But an energy model is part of how problems are coded for the computer as a whole, and that gave the researchers, led by USC's Daniel Lidar, a knob they were able to adjust in their tests.The 'control knob' the researchers accessed is that the behavior of the D-Wave device has a controllable overall energy scale, acting as an effective inverse temperature 'noise control knob.'

To be sure, reducing the energy scale amounts to increasing thermal excitations during the computation, and that's one of the main issues of the puzzle.

Why would this matter, you may ask? Well, the D-Wave chip is chilled to 20 millikelvin to prevent thermal noise from overwhelming the quantum effects the company says are the basis of its computations.

Therefore, the UCL / UC researchers reasoned, it should be possible from the input-output behavior of the device to predict the degree to which the chip's “quantumness” varied at different energy scales.

That, they say, is exactly what they observed. As they write-- “At the largest energy scale available, the annealing process appears to be dominated by coherent quantum effects, and thermal fluctuations are negligible. As the energy cale is decreased, thermal excitations become more relevant, and for a sufficiently small energy scale, the system behaves more like a classical annealer based on incoherent Ising spins.”

For this research, Lidar's group tested groups of 40 qubits against three classical models, and one quantum model: “The classical models are all found to disagree with the data, while the master equation agrees with the experiment without fine-tuning, and predicts mixed state entanglement at intermediate evolution times”.

Is this the end of the debate? Of course not-- it's not even a final proof that D-Wave is quantum, inside the black box. But Scott Aaronson says that this experiment does represent another addition to our knowledge of what's going on.

“I think the two sides are slowly converging on a real physical understanding of the current D-Wave devices – in particular, under what circumstances the devices can produce 'signatures' of various kinds of quantum behavior and under what circumstances those signatures go away,” Aaronson said.

He added that more evidence for quantum behavior still doesn't demonstrate that D-Wave is “faster” than classical computing even on its home turf.

While “clear evidence of global quantum behavior” is a prerequisite of ultimately achieving a quantum speed-up in computing, that doesn't yet guarantee that the speed up will ever be achieved.

“You can have global quantum behavior without a quantum speedup, but you can't have a quantum speedup without global quantum behavior,” Aaronson said, also noting that observing quantum-like behavior in special instances doesn't predict the scaling behavior of the D-Wave device.