Improving broadband displacement detection with quantum correlations

TL;DR

This paper demonstrates how quantum correlations accessed through variational readout can surpass the standard quantum limit in broadband displacement measurements using an optical cavity with a silicon nitride membrane.

Contribution

It introduces a method to improve displacement sensitivity beyond the SQL by modifying the readout quadrature to utilize quantum correlations in broadband interferometry.

Findings

Sensitivity better than the SQL was observed at certain quantum efficiencies.

The experiment illustrates the effectiveness of variational readout in surpassing quantum limits.

Quantum correlations can be harnessed to enhance sensing performance in interferometers.

Abstract

Interferometers enable ultrasensitive measurement in a wide array of applications from gravitational wave searches to force microscopes. The role of quantum mechanics in the metrological limits of interferometers has a rich history, and a large number of techniques to surpass conventional limits have been proposed. In a typical measurement configuration, the tradeoff between the probe's shot noise (imprecision) and its quantum backaction results in what is known as the standard quantum limit (SQL). In this work we investigate how quantum correlations accessed by modifying the readout of the interferometer can access physics beyond the SQL and improve displacement sensitivity. Specifically, we use an optical cavity to probe the motion of a silicon nitride membrane off mechanical resonance, as one would do in a broadband displacement or force measurement, and observe sensitivity better than the SQL dictates for our quantum efficiency. Our measurement illustrates the core idea behind a technique known as \textit{variational readout}, in which the optical readout quadrature is changed as a function of frequency to improve broadband displacement detection. And more generally our result is a salient example of how correlations can aid sensing in the presence of backaction.