Passive verification protocol for thermal graph states

TL;DR

This paper introduces a passive, efficient protocol to estimate the fidelity of thermal graph states in quantum computing, enabling better characterization of entangled matter qubits affected by thermal noise.

Contribution

It presents a passive fidelity estimation protocol for thermal graph states that requires only one measurement setting, unlike existing active protocols.

Findings

Efficient fidelity estimation for thermal graph states at any temperature.

The protocol is passive and does not require switching measurement settings.

Applicable to hypergraph states and quantum supremacy demonstrations.

Abstract

Graph states are entangled resource states for universal measurement-based quantum computation. Although matter qubits such as superconducting circuits and trapped ions are promising candidates to generate graph states, it is technologically hard to entangle a large number of them due to several types of noise. Since they must be sufficiently cooled to maintain their quantum properties, thermal noise is one of major ones. In this paper, we show that for any temperature $T$, the fidelity $\langle G|\rho_T|G\rangle$ between an ideal graph state $|G\rangle$ at zero temperature and a thermal graph state $\rho_T$, which is a graph state at temperature $T$, can be efficiently estimated by using only one measurement setting. A remarkable property of our protocol is that it is passive, while existing protocols are active, namely they switch between at least two measurement settings. Since thermal noise is equivalent to an independent phase-flip error, our estimation protocol also works for that error. By generalizing our protocol to hypergraph states, we apply our protocol to the quantum-computational-supremacy demonstration with instantaneous quantum polynomial time circuits. Our results should make the characterization of entangled matter qubits extremely feasible under thermal noise.