Error-Mitigation-Aided Optimization of Parameterized Quantum Circuits: Convergence Analysis

Sharu Theresa Jose, Osvaldo Simeone

Research output: Contribution to journalArticlepeer-review

2 Scopus citations

Abstract

Variational quantum algorithms (VQAs) offer the most promising path to obtaining quantum advantages via noisy intermediate-scale quantum (NISQ) processors. Such systems leverage classical optimization to tune the parameters of a parameterized quantum circuit (PQC). The goal is minimizing a cost function that depends on measurement outputs obtained from the PQC. Optimization is typically implemented via stochastic gradient descent (SGD). On NISQ computers, gate noise due to imperfections and decoherence affects the stochastic gradient estimates by introducing a bias. Quantum error mitigation (QEM) techniques can reduce the estimation bias without requiring any increase in the number of qubits, but they in turn cause an increase in the variance of the gradient estimates. This work studies the impact of quantum gate noise on the convergence of SGD for the variational eigensolver (VQE), a fundamental instance of VQAs. The main goal is ascertaining conditions under which QEM can enhance the performance of SGD for VQEs. It is shown that quantum gate noise induces a nonzero error-floor on the convergence error of SGD (evaluated with respect to a reference noiseless PQC), which depends on the number of noisy gates, the strength of the noise, as well as the eigenspectrum of the observable being measured and minimized. In contrast, with QEM, any arbitrarily small error can be obtained. Furthermore, for error levels attainable with or without QEM, QEM can reduce the number of required iterations, but only as long as the quantum noise level is sufficiently large, and a sufficiently large number of measurements is allowed at each SGD iteration. Numerical examples for a max-cut problem corroborate the main theoretical findings.

Original languageEnglish (US)
Article number3103119
JournalIEEE Transactions on Quantum Engineering
Volume3
DOIs
StatePublished - 2022

All Science Journal Classification (ASJC) codes

  • Software
  • Computer Science (miscellaneous)
  • Condensed Matter Physics
  • Mechanical Engineering
  • Engineering (miscellaneous)
  • Electrical and Electronic Engineering
  • Computer Science Applications

Keywords

  • Quantum computing
  • quantum error mitigation
  • quantum gate noise
  • variational quantum algorithms

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