Substantial leaps in the understanding of quantum systems have been driven by exploring geometry, topology, dimensionality and interactions in ultracold atomic ensembles1,2,3,4,5,6. A system where atoms evolve while confined on an ellipsoidal surface represents a heretofore unexplored geometry and topology. Realizing an ultracold bubble—potentially Bose–Einstein condensed—relates to areas of interest including quantized-vortex flow constrained to a closed surface topology, collective modes and self-interference via bubble expansion7,8,9,10,11,12,13,14,15,16,17. Large ultracold bubbles, created by inflating smaller condensates, directly tie into Hubble-analogue expansion physics18,19,20. Here we report observations from the NASA Cold Atom Lab21 facility onboard the International Space Station of bubbles of ultracold atoms created using a radiofrequency-dressing protocol. We observe bubble configurations of varying size and initial temperature, and explore bubble thermodynamics, demonstrating substantial cooling associated with inflation. We achieve partial coverings of bubble traps greater than one millimetre in size with ultracold films of inferred few-micrometre thickness, and we observe the dynamics of shell structures projected into free-evolving harmonic confinement. The observations are among the first measurements made with ultracold atoms in space, using perpetual freefall to explore quantum systems that are prohibitively difficult to create on Earth. This work heralds future studies (in orbital microgravity) of the Bose–Einstein condensed bubble, the character of its excitations and the role of topology in its evolution.
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The datasets generated and analysed in Methods are available from the corresponding author upon reasonable request. All NASA CAL data are on a schedule for public availability through the NASA Physical Science Informatics (PSI) website (https://www.nasa.gov/PSI).
Calculation and analysis codes from the Methods are available upon reasonable request from the corresponding author.
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We thank the NASA/JPL Cold Atom Lab team for their support. Designed, managed and operated by Jet Propulsion Laboratory, the Cold Atom Lab is sponsored by the Biological and Physical Sciences Division of NASA’s Science Mission Directorate at the agency’s headquarters in Washington and the International Space Station Program at NASA’s Johnson Space Center in Houston. We also thank B. Garraway and E. Bentine for input.
The authors declare no competing interests.
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Extended data figures and tables
Cloud size versus time-of-flight fits for initial temperatures as set by rf evaporation, with frequency values given by a, 5.1 MHz, b, 5.0 MHz, c, 4.93 MHz, and d, 4.855 MHz, corresponding to the temperature data in Fig. 3a–d. Error bars represent standard errors.
Details of mechanism for rejecting |F = 2, m = 0⟩ halos originating in evaporative cooling, which otherwise would distort thermometry fits of shell structures relevant main text Fig. 3 in the main text. a, To proceed we first find (for a typical partially-condensed cloud) the approximate center of the halo marked by a vertical line. This location guides our nearby placement of a truncation region in the fits shown in b–d for three different use cases: b, a cold shell of moderate size and short TOF; c, a cold shell of moderate size and long TOF; d, a warmer, higher atom number shell of moderate size and short TOF. Truncation of a halo-dominant region improves fit capture of relevant shell features, with results shown in dashed red lines. More detail of the halo nature can be found in ref. 21.
a, Ramp time is varied 100–400 ms, with a 1,000-point frequency ramp extending 200 kHz upward from an initial frequency of 2.05 MHz + Δ, corresponding to variation in ramp speed 0.5–2.0 kHz/ms. Error bars (where visible) represent standard errors. b, Absorption imaging of Δ = +30 kHz clouds associated with marked ramp times (associated with red points above). For this dataset initial cloud temperature was set slightly below Tc, similar to that used in Fig. 3d.
a, Graining of the dressing ramp is varied, with resulting dressed-sample thermometry plotted as a function of the number of frequency steps. Error bars (where visible) represent standard errors. All dressing ramps extended 600 kHz upward from an initial frequency of 1.65 MHz + Δ, over 400 ms (ramp speed 1.5 kHz/ms), thus varying the step size from 300–1200 Hz. For this dataset initial cloud temperature was set significantly above Tc, similar to that used in Fig. 3b. b, Dressed (Δ = +550 kHz, i.e. a ramp 2.2–2.8 MHz) clouds at short (2.6 ms) TOF associated with each rf frequency step graining; note qualitative difference associated with 500-point graining.
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Carollo, R.A., Aveline, D.C., Rhyno, B. et al. Observation of ultracold atomic bubbles in orbital microgravity. Nature 606, 281–286 (2022). https://doi.org/10.1038/s41586-022-04639-8