Figure 2: Power dependence at 4.2 K.
From: Approaching the standard quantum limit of mechanical torque sensing
Optomechanically measured thermal noise spectra, calibrated in terms of linear and angular displacement, of the torsional mode in the presence of helium exchange gas. As the optical power injected into the device is increased, the photon shot noise is reduced and the measurement imprecision drops below the noise floor of 0.84 nrad/
(2.1 fm/
) corresponding to the SQL. Optical power input to the device (average intracavity photon number) corresponds to: orange 33 μW (6.0 × 103 photons), purple 86 μW (1.3 × 104 photons) and grey 156 μW (2.7 × 104 photons), resulting in angular (linear) displacement imprecision noise floors of 0.75 nrad/
(1.9 fm/
), 0.39 nrad/
(0.97 fm/
) and 0.25 nrad/
(0.62 fm/
). The zero-point noise spectrum at the SQL, calculated using measured device parameters, is shown as the blue dashed line.
