Power of Pausing: Advancing Understanding of Thermalization in Experimental Quantum Annealers

TL;DR

This paper explores how pausing during quantum annealing on D-Wave hardware affects thermalization and solution success, revealing that strategic pauses can significantly improve ground state finding by promoting thermal relaxation.

Contribution

It demonstrates that a mid-anneal pause enhances ground state probability and provides evidence of thermalization, offering new insights into quantum annealer dynamics and thermal effects.

Findings

Pausing mid-anneal increases success probability by over an order of magnitude.

System distribution closely matches a classical Boltzmann distribution.

Larger problems tend to thermalize more effectively.

Abstract

We investigate alternative annealing schedules on the current generation of quantum annealing hardware (the D-Wave 2000Q), which includes the use of forward and reverse annealing with an intermediate pause. This work provides new insights into the inner workings of these devices (and quantum devices in general), particular into how thermal effects govern the system dynamics. 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 indeed 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 over an order of magnitude. We relate this effect to relaxation (i.e. thermalization) after diabatic and thermal excitations that occur in the region near to the minimum gap. For a set of large-scale problems of up to 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 temperature at least 1.5 times higher than the (effective) temperature of the annealer. Moreover, we show that larger problems are more likely to thermalize to a classical Boltzmann distribution.