Exploring the effects of geometry, topology, dimensionality, and interactions on ultracold atomic ensembles has been a continually fruitful line of inquiry in the decades since the first observation of Bose-Einstein condensation (BEC). One heretofore unexplored configuration for ultracold ensembles is that of a bubble or shell, where trapped atoms are confined in the vicinity of a spherical or ellipsoidal surface. Such a system could offer new collective modes, topologically-sensitive behavior of quantized vortices, self-interference and shell collapse, as well as the exploration of trapped ultracold systems with mm-scale spatial extent. While techniques for the generation of bubble-shaped traps have been known since 2001, terrestrial gravity has thus far prevented the observation of ultracold bubbles.
With the construction of the NASA Cold Atom Lab (CAL) facility and its subsequent delivery in 2018 to the International Space Station (ISS) and commissioning as an orbital BEC user facility, experimental atomic physics schemes that require a sustained microgravity environment are now possible. I will present recent CAL observations of trapped bubbles of ultracold atoms, including a variety of bubble-trap configurations that are possible with this apparatus. I will also discuss the thermodynamics of ultracold bubbles and review open questions being explored in the ongoing second science run of CAL aboard ISS, which feature improved bubble aspect ratios and filling as well as improved imaging; I will also review upcoming changes to the CAL facility aimed at improved BEC quality, as well as recent progress made with shell structures in terrestrial laboratories.
