Quantum-limited optical lever measurement of a torsion oscillator

The optical lever is a precision displacement sensor with broad applications. In principle, it can track the motion of a mechanical oscillator with added noise at the standard quantum limit (SQL); however, demonstrating this performance requires an oscillator with exceptionally high torque sensitivity or, equivalently, zero-point angular displacement spectral density. Here, we describe optical lever measurements on Si3N4 nanoribbons possessing Q > 3 × 10⁷ torsion modes with torque sensitivities of 10⁻²⁰ Nm/ √ Hz and zero-point displacement spectral densities of 10⁻¹⁰ rad/ √ Hz. By compensating for aberrations and leveraging immunity to classical intensity noise, we realize angular displacement measurements with imprecisions 20 dB below the SQL and demonstrate feedback cooling, using a position-modulated laser beam as a torque actuator, from room temperature to ∼5000 phonons. Our study signals the potential for a new class of torsional quantum optomechanics.