Continuous Force and Displacement Measurement Below the Standard Quantum Limit

TL;DR

This paper demonstrates a novel interferometric displacement measurement system that surpasses the standard quantum limit by utilizing strong quantum correlations in an ultracoherent optomechanical setup, achieving 1.5dB sensitivity improvement.

Contribution

The authors experimentally surpass the standard quantum limit in displacement measurement using quantum correlations in an ultracoherent optomechanical system.

Findings

Achieved 1.5dB sensitivity below the SQL.

Demonstrated measurement of force and displacement beyond previous limits.

Enhanced prospects for quantum sensing applications.

Abstract

Quantum mechanics dictates that the precision of physical measurements must be subject to certain constraints. In the case of inteferometric displacement measurements, these restrictions impose a 'standard quantum limit' (SQL), which optimally balances the precision of a measurement with its unwanted backaction. To go beyond this limit, one must devise more sophisticated measurement techniques, which either 'evade' the backaction of the measurement, or achieve clever cancellation of the unwanted noise at the detector. In the half-century since the SQL was established, systems ranging from LIGO to ultracold atoms and nanomechanical devices have pushed displacement measurements towards this limit, and a variety of sub-SQL techniques have been tested in proof-of-principle experiments. However, to-date, no experimental system has successfully demonstrated an interferometric displacement measurement with sensitivity (including all relevant noise sources: thermal, backaction, and imprecision) below the SQL. Here, we exploit strong quantum correlations in an ultracoherent optomechanical system to demonstrate off-resonant force and displacement sensitivity reaching 1.5dB below the SQL. This achieves an outstanding goal in mechanical quantum sensing, and further enhances the prospects of using such devices for state-of-the-art force sensing applications.