The power of pausing: advancing understanding of thermalization in experimental quantum annealers

We investigate annealing schedules involving an intermediate pause, on the current generation of quantum annealing hardware: the D-Wave 2000Q. We show that a pause mid-way through the anneal can cause a dramatic change in the output distribution, and we provide evidence suggesting thermalization is occurring during such a pause. We demonstrate that upon pausing the system in a narrow region shortly after the minimum gap, the probability of successfully finding the ground state of the problem Hamiltonian can be increased by several orders of magnitude. We relate this effect to relaxation, after excitations occurring in the region near to the minimum gap. For a set of problems of size 500 qubits we demonstrate that the distribution returned from the annealer very closely matches a classical Boltzmann distribution of the problem Hamiltonian, albeit one with a noticeably higher temperature than that of the device.