Defects in Quantum Computers

TL;DR

This paper proposes a practical test for assessing the adiabaticity and defect presence in quantum annealing hardware using the quantum Ising chain, implemented on D-Wave, to quantify imperfections in reaching the ground state.

Contribution

It introduces a simple, broadly applicable test based on an exactly solvable model to evaluate quantum hardware performance in adiabatic quantum computing.

Findings

The test quantifies defects via topological kinks in the Ising chain.

Implementation on D-Wave demonstrates the method's practicality.

Imperfections increase defect count, indicating deviations from ideal adiabatic evolution.

Abstract

The shift of interest from general purpose quantum computers to adiabatic quantum computing or quantum annealing calls for a broadly applicable and easy to implement test to assess how quantum or adiabatic is a specific hardware. Here we propose such a test based on an exactly solvable many body system -- the quantum Ising chain in transverse field -- and implement it on the D-Wave machine. An ideal adiabatic quench of the quantum Ising chain should lead to an ordered broken symmetry ground state with all spins aligned in the same direction. An actual quench can be imperfect due to decoherence, noise, flaws in the implemented Hamiltonian, or simply too fast to be adiabatic. Imperfections result in topological defects: Spins change orientation, kinks punctuating ordered sections of the chain. The number of such defects quantifies the extent by which the quantum computer misses the ground state, and is, therefore, imperfect.