Algorithms for the D-Wave – A Physics Perspective

The spin ½ chain is often used as an example problem that is particularly ill suited for the quantum annealing (QA) technique used by the D-Wave computer. Sometimes QA produces a correct classical ground state, but other times it produces a classical state which is only nearly correct. The thing that makes this spin chain problem ill suited for QA is degeneracy – the fact that the quantum ground state contains many classical states with the same or similar energy.

Dr. Olsen will discuss two problems in condensed matter physics. One is the well known damping of heat conduction by phonons in a disordered lattice. This damping occurs because disorder tends to spatially localize the phonons, equivalent to reducing the degeneracy in the spin chain problem. The second is an experimental system which is not yet fully understood, but which suggests that entangled particles can remain strongly correlated even in the presence of decoherence. Dr. Olsen will then return to the spin 1⁄2 chain. She will demonstrate the entirely expected result that addition of disorder reduces the degeneracy of the solution. But shealso shows that introducing correlations in the disorder is more effective at reducing the degeneracy, thus making the problem much better suited for QA.