GHZ protocols enhance frequency metrology despite spontaneous decay

TL;DR

This paper demonstrates that GHZ states, combined with correlated measurements and nonlinear estimation, can significantly improve frequency metrology accuracy even with spontaneous decay, approaching fundamental bounds for up to 80 atoms.

Contribution

It introduces a GHZ-based protocol with a veto signal that mitigates spontaneous emission errors, achieving near-optimal gains under decoherence.

Findings

GHZ states with correlated measurements improve metrology accuracy.

The protocol achieves up to 2.25 dB gain with about 80 atoms.

Monte Carlo simulations confirm robustness against spontaneous decay.

Abstract

The use of correlated states and measurements promises improvements in the accuracy of frequency metrology and the stability of atomic clocks. However, developing strategies robust against dominant noise processes remains challenging. We address the issue of decoherence due to spontaneous decay and show that Greenberger-Horne-Zeilinger (GHZ) states, in conjunction with a correlated measurement and nonlinear estimation strategy, achieve gains of up to 2.25 dB, comparable to fundamental bounds for up to about 80 atoms in the presence of decoherence. This result is surprising since GHZ states do not provide any enhancement under dephasing due to white frequency noise compared to the standard quantum limit of uncorrelated states. The gain arises from a veto signal, which allows for the detection and mitigation of errors caused by spontaneous emission events. Through comprehensive Monte-Carlo simulations of atomic clocks, we demonstrate the robustness of the GHZ protocol.