Bose–Einstein condensation in space

Atoms cooled to subnanokelvin temperatures can be used to test quantum phenomena and make precision measurements of fundamental physical constants. (See the article by Markus Arndt, Physics Today, May 2014, page 30 .) The atoms are held first in magneto-optical traps and then in tight magnetic traps while they undergo the multiple cooling steps needed to reach such low temperatures. Experiments on ultracold gases are limited, however, by gravity. The final cooling stage involves weakening the trapping potential, but if it becomes too weak, gravity pulls the atoms out. Researchers also have just tens of milliseconds to make measurements on freely expanding atom clouds because without a trap, the atoms fall away.

Researchers have minimized gravitational effects by performing their cold-atom experiments in free fall generated by, for example, drop towers and zero-g aircraft. But those experiments were short-lived and not frequently repeated. To enable long-term, well-controlled cold-atom experiments in microgravity, NASA sent the Cold Atom Laboratory (CAL), shown in the figure, to the International Space Station in 2018. Remotely operating CAL from NASA’s Jet Propulsion Laboratory, David Aveline and colleagues have now produced rubidium Bose–Einstein condensates in orbit. Their measurements show the equipment’s successful operation and demonstrate the benefits of microgravity.

Without Earth’s gravity, the cloud of Rb atoms reached such a low temperature that once it was released, it persisted in the instrument’s observation region for more than a second. For comparison, when CAL was tested before launch, the cloud lasted only about 40 ms. The researchers also observed an unexpected difference: The cloud contained nearly three times as many Rb atoms as it did when CAL was tested before launch, and nearly half of those atoms were in a state that barely responds to magnetic fields. On Earth, gravity would have pulled those particles out of the trap during cooling. But in space, the trap held them in a weak orbit around the denser condensate.

The newly confined atoms’ insensitivity to magnetic fields may make them well suited for high-precision spectroscopy, one of the areas of research CAL is meant to advance. The researchers also predict that those atoms will have a uniquely flat density profile when confined in a three-dimensional trap, which would make them useful for studying phases of quantum matter under homogeneous conditions.

The experiments expand the scope of the research community’s plans for CAL. New trap geometries permitted by microgravity, such as bubble shells, are already being investigated, as are advanced cooling techniques for precision atom interferometry. Other planned experiments include studying quantum gas mixtures and probing yet-untested density and temperature regimes. (D. C. Aveline et al., Nature 582, 193, 2020 .)