States of matter known as Bose–Einstein condensates (BECs) were first observed 25 years ago1,2. Since then, these quantum objects have become a key tool in the study of quantum physics, and they are routinely produced in hundreds of laboratories around the world. Writing in Nature, Aveline et al.3 report the generation of rubidium BECs aboard the International Space Station, which is in orbit around Earth. The condition of perpetual free fall on the station offers new methods for probing BECs and for making a wide range of high-precision measurements.
A BEC is produced when a dense cloud of trapped bosonic atoms (atoms for which a quantum property known as spin is an integer) is cooled to temperatures near absolute zero4,5. In these ultracold ensembles, the atoms mainly populate the lowest energy state of the trap. A central tenet of quantum mechanics is wave–particle duality, whereby every particle can be described as a wave of matter. BECs are useful objects for testing quantum mechanics because the entire cloud of atoms can be regarded as a single matter wave. This property is called quantum degeneracy.
Bose–Einstein condensation is achieved by cooling the atomic cloud using several methods that involve combinations of light and magnetic fields. A commonly used final step is known as evaporative cooling6. In this approach, the atoms are confined in a magnetic trap, and those that have the highest kinetic energy (the ‘hottest’ atoms) are driven from the trap using radio-frequency radiation. The remaining atoms collide with each other and reach thermal equilibrium at a lower mean temperature than the initial temperature. This process is repeated until a BEC is formed.
As discussed, Bose–Einstein condensation requires low temperatures, at which atoms hardly move. However, when a BEC is released from a magnetic trap so that experiments can be carried out, repulsive interactions between the atoms cause the cloud to expand. Within a few seconds, the BEC becomes too dilute to be detected. The expansion rate can be reduced by decreasing the depth of the trap, and, thereby, the density of atoms in the trap.
On Earth, the planet’s gravitational pull restricts the shape of possible magnetic traps in such a way that a deep trap is needed to confine a BEC (Fig. 1a,b). By contrast, Aveline and colleagues found that the extremely weak gravity (microgravity) on the International Space Station allowed rubidium BECs to be created using shallow traps. As a result, the authors could study the BECs after about one second of expansion, without needing to manipulate the atoms further.
Before releasing a BEC, Aveline et al. observed that the tightly trapped condensate was surrounded by, and interacting with, a halo-shaped cloud of rubidium atoms. During evaporative cooling, these atoms had been transferred to a state that is insensitive to magnetic fields. The atoms then interacted only weakly with the trap through their quantum-mechanical properties, owing to a phenomenon called the second-order Zeeman effect7. On Earth, such atoms would be removed from the trap by the dominant force of gravity. However, in orbit, they remain in the trap and could be used, for example, to directly produce ultracold atomic samples that have an extremely low density.
The authors’ experiments mark just the beginning of many exciting studies on quantum-degenerate gases. For example, microgravity allows atoms to be confined or guided using trap shapes, such as that of a bubble8, that cannot be used properly on Earth (Fig. 1c). Future work on the evolution of such atoms will provide insight into few-body physics. Moreover, there are planned experiments on quantum-gas mixtures of potassium and rubidium9.
Earth-orbiting BECs could also advance atom interferometry — a measurement technique based on the interference between matter waves. The sensitivity of an atom interferometer to inertial forces is proportional to the square of the time that atoms spend in the interferometer10. On the ground, this time is restricted by the limited free-fall time. Microgravity facilities such as rockets10, aeroplanes11 and ‘drop towers’12 have been used previously to address this problem, but Earth-orbiting atom interferometers would enable many more experimental cycles.
For the future goal of high-precision measurements in space, a thorough analysis of all systematic effects and the implementation of techniques developed on the ground are essential. Such measurements could provide stringent tests of the universality of free fall (the principle that all objects accelerate identically in an external gravitational field) and theories of dark energy (the unknown energy that is thought to be causing the expansion of the Universe to accelerate). The expected sensitivities would also make BEC interferometry of interest for satellite navigation, exploration and Earth observation.
Aveline and colleagues’ technological achievement is remarkable. Their apparatus needed to satisfy the strict mass, volume and power-consumption requirements of the International Space Station, and be robust enough to operate for years without needing to be serviced. The authors’ Earth-orbiting BECs provide new opportunities for research on quantum gases, as well as for atom interferometry, and pave the way for missions that are even more ambitious.
Nature 582, 186-187 (2020)