Decoherence in adiabatic quantum computation

TL;DR

This paper investigates how decoherence affects adiabatic quantum computation and quantum annealing, showing that certain types of decoherence may not hinder success and that boundary cancellation methods remain effective.

Contribution

It demonstrates that decoherence in the energy eigenbasis does not necessarily impair adiabatic quantum algorithms and extends boundary cancellation techniques to open systems.

Findings

Decoherence in the energy eigenbasis can be non-detrimental to adiabatic quantum computation.

Boundary cancellation methods remain effective in open system settings.

A quantum Monte Carlo algorithm can interpolate between classical and quantum annealing.

Abstract

Recent experiments with increasingly larger numbers of qubits have sparked renewed interest in adiabatic quantum computation, and in particular quantum annealing. A central question that is repeatedly asked is whether quantum features of the evolution can survive over the long time-scales used for quantum annealing relative to standard measures of the decoherence time. We reconsider the role of decoherence in adiabatic quantum computation and quantum annealing using the adiabatic quantum master equation formalism. We restrict ourselves to the weak-coupling and singular-coupling limits, which correspond to decoherence in the energy eigenbasis and in the computational basis, respectively. We demonstrate that decoherence in the instantaneous energy eigenbasis does not necessarily detrimentally affect adiabatic quantum computation, and in particular that a short single-qubit $T_2$ time need not imply adverse consequences for the success of the quantum adiabatic algorithm. We further demonstrate that boundary cancellation methods, designed to improve the fidelity of adiabatic quantum computing in the closed system setting, remain beneficial in the open system setting. To address the high computational cost of master equation simulations, we also demonstrate that a quantum Monte Carlo algorithm that explicitly accounts for a thermal bosonic bath can be used to interpolate between classical and quantum annealing. Our study highlights and clarifies the significantly different role played by decoherence in the adiabatic and circuit models of quantum computing.