Phase sensitivity approaching quantum Cramer-Rao bound in a modified SU(1,1) interferometer

TL;DR

This paper proposes a modified SU(1,1) interferometer with a beam splitter replacing the second nonlinear element, achieving near quantum Cramer-Rao bound phase sensitivity with robustness against noise, advancing practical quantum metrology.

Contribution

It introduces a new protocol for SU(1,1) interferometers that enhances phase sensitivity and robustness by replacing the second nonlinear element with a beam splitter.

Findings

Achieves sub-shot-noise-limited phase sensitivity.

Robust against photon loss and background noise.

Approaches quantum Cramer-Rao bound in phase estimation.

Abstract

SU(1,1) interferometers, based on the usage of nonlinear elements, are superior to passive interferometers in phase sensitivity. However, the SU(1,1) interferometer cannot make full use of photons carrying phase information as the second nonlinear element annihilates some of the photons inside. Here, we focus on improving phase sensitivity and propose a new protocol based on a modified SU(1,1) interferometer, where the second nonlinear element is replaced by a beam splitter. We utilize two coherent states as inputs and implement balanced homodyne measurement at the output. Our analysis suggests that the protocol we propose can achieve sub-shot-noise-limited phase sensitivity and is robust against photon loss and background noise. Our work is important for practical quantum metrology using SU(1,1) interferometers.