Optical and mechanical properties of superfluid helium drops levitated in vacuum*

Many of the goals of quantum optomechanics require a combination of low optical and mechanical loss, low temperature, and high-precision measurement. As a material, superfluid helium offers a number of potential advantages in these regards: vanishing optical absorption and viscosity, high thermal conductivity, and the ability to cool itself efficiently via evaporation. Superfluid optomechanical devices have made considerable advances in recent years, but their performance is often limited by the materials used to contain the superfluid. To avoid these limits we have proposed the use of magnetic levitation to suspend a drop of liquid helium in vacuum[1] with the goal of using the drop's optical whispering gallery modes (WGMs) and its surface waves as an optomechanical system, while relying on the drop's evaporation to maintain low temperature. Each of these individual components (i.e., magnetic levitation, WGMs, and evaporative cooling) has been previously demonstrated by other groups, but combining them in a single device should offer several intriguing possibilities. We will describe some of these possibilities, and will also present recent measurements of mm-scale superfluid drops that are magnetically levitated in high vacuum. Specifically, we will describe the formation and trapping of the drops, and their evaporative cooling in the trap to ~ 330 mK. We will also present measurements of the drops’ mechanical resonances, and of their optical Mie resonances.

[1] L. Childress et al., Phys. Rev. A 96 063842 (2017)