Quantum optimizer manufacturer D-Wave Systems has been gaining a lot of traction recently. They've sold systems to Lockheed Martin and Google, and started producing results showing that their system can solve problems that are getting closer to having real-life applications. All in all, they have come a long way since the first hype-filled announcement.
According to a recent paper in Nature Communications, the D-Wave device is not doing classical simulated annealing. Which, unfortunately, means exactly that. It tells us what it isn't, but doesn't tell us what it is.
To go into this a little more deeply, the researchers analyzed how the coupling between the magnets created a ground state. The layout of the hardware consists of four inner magnets arranged in a diamond (so each magnet is coupled directly to two others). Each of these is coupled to one additional magnet, but those are not coupled to each other. This configuration appears to be set up such that the four inner magnets always have the same orientation, while the outer magnets are free to arrange themselves as they see fit.
This results in a rather strange set of 17 possible ground states, most of which can be reached in steps of single flips of magnets. Except for the last, which requires that all four inner magnets flip at the same time.
In a classical simulation, the set of magnets can sample many different states. But, if by chance it happens to flip into this last ground state, it becomes trapped there. Furthermore, once it is there, the outer magnets become trapped in a single state too, because all other configurations have higher energy. Of course, once in this isolated state, it can also get out by flipping all four inner magnets, but the isolation and lack of noise (the outer magnets can't flip either) mean that it is, in some sense, less likely to flip out of the state than into it.
In the quantum description of these events, this doesn't happen. After setting up the ground state, we start trying to move to the solution state (by varying the environment). As soon as we do that, the ground state splits up, and the isolated state where things get stuck raises up in energy, away from the ground state. Since everything is kept in the ground state, it is no surprise that we find that the probability of entering the isolated state reduces sharply.
But, notice that this is different from the classical case. In the classical case, there was no way to break up the ground state. In other words, the energetic descriptions of the classical and quantum ground states are not the same, and it is no surprise that they give two different results.
At heart, this difference was inevitable. When you get right down to it, we live in a quantum world, and if you are careful enough, that will shine through. In some ways, this shows how sloppy our thinking about the whole thing is. When we think of simulated annealing, or anything else like this, we imagine a purely classical or a purely quantum system. In reality, things are a lot more messy, with some aspects remaining classical and others showing their quantum nature.