Magnetically Levitated Drops of Superfluid Helium: Measuring Brownian Motion and Optical Backaction

We magnetically levitate mm-scale drops of Helium-4 in ultrahigh vacuum. Surface tension molds these drops into almost-perfect spheres that evaporatively cool to ~275 mK. The lowest evaporation rates we have measured (dR/dt ~100 fm/s) correspond to drop lifetimes of over 100 years. We use lasers (at five wavelengths) to control and probe the drop's mechanical surface modes and its optical whispering gallery modes.

For about half the drops we have measured, the surface mode spectrum matches theoretical predictions (for an elastic sphere) to within ~10 ppm for the first ~1000 surface modes. Their measured damping rates also agree well with first-principles calculations in the high-frequency limit. However, statistically significant deviations at lower frequencies suggest that the theory and our understanding of the underlying physics require further refinement. For about half the other drops we have measured, the surface mode spectrum deviates from the (same) theory predictions at the ~1% level! The specific form of these deviations suggests the presence of one or more vortex lines in these drops, but confirmation with minimally intrusive probes, such as lasers, remains outstanding.

We have measured optical whispering gallery modes with finesse values ranging from ~10 to ~20,000. The low evaporation rates allow us to lock a probe laser to a specific optical whispering-gallery mode for over a day. This has allowed us to measure the thermal (Brownian) motion of the surface modes, and also radiation backaction. Preliminary data also suggest mechanical "lasing" and mode competition (between surface modes) at ~10 nW of coupled optical power in the ~20,000-finesse WGMs.

I will describe these results in detail and discuss the challenges posed by the thermal motion of surface modes to couple light into WGMs that are theoretically expected to have finesse values > 1 billion.