Extended flag gadgets for low-overhead circuit verification

TL;DR

This paper introduces extended flag gadgets for quantum circuit verification, enhancing error detection in near-term quantum algorithms with minimal overhead, leading to significant fidelity improvements under various noise models.

Contribution

It extends flag verification techniques by creating error-detection gadgets based on unitary transformations, especially for Clifford and near-Clifford circuits, requiring few additional gates.

Findings

Flags can double circuit fidelity after post-selection.

Effective against depolarizing noise, crosstalk, and overrotation.

Requires only a small number of additional Clifford gates.

Abstract

Flag verification techniques are useful in quantum error correction for detecting critical faults. Here we present an application of flag verification techniques to improving post-selected performance of near-term algorithms. We extend the definition of what constitutes a flag by creating error-detection gadgets based on known transformations of unitary operators. In the case of Clifford or near-Clifford circuits, these unitary operators can be chosen to be controlled Pauli gates, leading to gadgets which require only a small number of additional Clifford gates. We show that such flags can improve circuit fidelities by up to a factor of 2 after post selection, and demonstrate their effectiveness over error models featuring single-qubit depolarizing noise, crosstalk, and two-qubit coherent overrotation.