Laser control and readout of superfluid flow*

We report on progress in laser control and readout of excitations in two-dimensional
superfluid Helium. This system can potentially be used for a wide range of applications, including ground-state cooling, vortex-tracking and superfluid circuitry. Surface waves on a superfluid Helium thin film, so-called ”third-sound” waves, naturally form a mechanical resonator with low dissipation and very low mass. When such a film is condensed on an optical whispering-gallery mode resonator, the portion of the evanescent optical field outside the resonator body is sensitive to the third-sound-induced height fluctuations of the superfluid Helium film. This allows for unprecedented real-time readout and control of superfluid flow. Our group has recently demonstrated real time monitoring and control of third sound modes, as well as enhanced radiation pressure forces from superfluid flow, allowing for improved feedback on a mechanical resonator. Preliminary experimental results from a new generation of devices suggest real-time vortex tracking capabilities, improved opto-mechanical coupling and access to strong superfluid mechanical non-linearities.