Introduction to Quantum Gate Set Tomography

TL;DR

Quantum gate set tomography (GST) offers a more accurate and self-consistent method than quantum process tomography for characterizing quantum gates, especially in the fault-tolerant regime, by effectively accounting for SPAM errors.

Contribution

This paper provides a comprehensive review and implementation guide for GST, highlighting its advantages over QPT in accurately estimating quantum gate errors.

Findings

GST outperforms QPT in estimating coherent errors near error correction thresholds.

GST accurately characterizes quantum gates in the fault-tolerant regime.

Simulation results demonstrate GST's effectiveness over QPT.

Abstract

Quantum gate set tomography (GST) has emerged as a promising method for the full characterization of quantum logic gates. In contrast to quantum process tomography (QPT), GST self-consistently and correctly accounts for state preparation and measurement (SPAM) errors. It therefore provides significantly more accurate estimates than QPT as gate fidelities increase into the fault-tolerant regime. We give a detailed review of GST and provide a self-contained guide to its implementation. The method is presented in a step-by-step fashion and relevant mathematical background material is included. Our goal is to demonstrate the utility of GST as both an accurate characterization technique and a simple and effective diagnostic tool. As an illustration, we compare the output of GST and QPT using simulated example data for a single qubit. In agreement with the original literature, we find that coherent errors are poorly estimated by QPT near quantum error correction thresholds, while GST is accurate in this regime.