Characterization of a double torsion pendulum used to detect spin-induced torque based on Beth's experiment

TL;DR

This study characterizes a double torsion pendulum to measure photon-spin-induced torque, achieving sensitivity near the thermal noise limit and confirming angular momentum transfer consistent with Beth's 1936 experiment.

Contribution

It demonstrates a highly sensitive torsion pendulum setup capable of detecting photon-spin-induced torque close to the thermal noise limit, building on Beth's classic experiment.

Findings

Achieved torque sensitivity of 2 x 10^{-17} N m in 10^4 s

Torque measurement is close to the thermal noise limit

Observed torque consistent with angular momentum transfer of hbar per photon

Abstract

We characterized a double torsion pendulum system, including measurements of the photon-spin-induced torque. Our experimental strategy was similar to that used in Beth's experiment, which was performed in 1936 to measure photon-spin-induced torque using forced oscillation caused by polarization modulation of light incident on a suspended object. Through simple passive isolation of the suspended object from external vibration noise, the achieved torque sensitivity was 2 x 10^{-17} N m in a measurement time of 10^4 s, which is close to the thermal noise limit and one order smaller than the minimum torque measured in Beth's experiment. The observed spin-induced torque exerted on the light-absorbing optics is consistent with the angular momentum transfer of hbar per photon.