He*

Liquid helium drops offer a combination of low temperature, isolation, superfluidity, and experimental access that is unique among condensed matter systems. These features can play an important role in a range of disciplines, including precision molecular spectroscopy, studies of cold chemical reactions, and fluid dynamics in classical and quantum regimes. In addition, such drops may be well-suited as nonlinear optical devices and for studies of macroscopic quantum phenomena.

In practice, the scientific questions that can be addressed by an isolated helium drop depend on its size, temperature, lifetime, isotopic composition, and on the experimental probes that can be applied to it.

In this talk, I will describe experiments with millimeter-scale drops of pure ⁴He and pure ³He that are trapped by diamagnetic levitation in high vacuum. We have measured the drops’ temperature, heat load, and evaporation rate, as well as their surface waves and center-of-mass motion. We have also measured the medium-finesse optical whispering gallery modes in the drops, and placed upper limits on their optical absorption. These results are in good agreement with theoretical predictions. We find that superfluid ⁴He drops can be trapped indefinitely with a temperature ~ 330 mK. Drops of ³He are expected to reach lower temperatures, but their levitation is complicated by the dynamics of their nuclear spins.