Back action evasion in optical lever detection

TL;DR

This paper demonstrates a simple optical method to evade quantum back action in optical lever detection, achieving noise levels below the standard quantum limit without using an optical cavity, which is crucial for quantum sensing advancements.

Contribution

It introduces a ray optics-based technique for back action evasion and experimentally verifies it in the classical regime, surpassing the SQL without an optical cavity.

Findings

Achieved a readout noise floor two orders of magnitude below SQL

Developed a lens system that cancels laser-pointing-noise-induced optical torques

Demonstrated back action evasion in a classical regime with high optomechanical cooperativity

Abstract

The optical lever is a centuries old and widely-used detection technique employed in applications ranging from consumer products, industrial sensors to precision force microscopes used in scientific research. However, despite the long history, its quantum limits have yet to be explored. In general, any precision optical measurement is accompanied by optical force induced disturbance to the measured object (termed as back action) leading to a standard quantum limit(SQL). Here we give a simple ray optics description of how such back action can be evaded in optical lever detection to beat SQL. We perform a proof-of-principle experiment demonstrating the mechanism of back action evasion in the classical regime, by developing a lens system that cancels extra tilting of the reflected light off a silicon nitride membrane mechanical resonator caused by laser-pointing-noise-induced optical torques. We achieve a readout noise floor two orders of magnitude lower than the SQL, corresponding to an effective optomechanical cooperativity of 100 without the need for an optical cavity. As the state-of-the-art ultra low dissipation optomechanical systems relevant for quantum sensing are rapidly approaching the level where quantum noise dominates, simple and widely applicable back action evading protocols will be crucial for pushing beyond quantum limits.