China Space Station: optical-trap Bose-Einstein condensate cooled to 9.7 pK with 7 s free expansion

A Bose-Einstein condensate created in an optical dipole trap (ODT) aboard the China Space Station has reached an equivalent temperature of 9.7 pK (+3.2/−2.7 pK), according to a paper in Science. The condensate contains 2.0 × 10⁴ atoms, and the free expansion time was 7.0 s science.orgSciencescience.orgOpen source ↗. The work was done by physicists from Peking University and the Shanghai Institute of Optics and Fine Mechanics of the Chinese Academy of Sciences. The atoms were rubidium.

The authors note that the lowest equivalent temperature reported earlier in such a system was 38 pK with a free expansion time of 2 s. It was achieved in atom-chip-based magnetic traps in microgravity.

Optical trap instead of a magnetic one

ODT systems offer better optical access and can condense both magnetic and non-magnetic atoms. Until now they had not been used to prepare quantum degenerate gases in microgravity. A magnetic atom-chip trap holds only atoms in a particular quantum state, while a focused laser beam attracts atoms regardless of their state.

The equivalent temperature is not read from a thermometer. It is calculated from how slowly the cloud spreads during free expansion.

Weak trap and long observation

In microgravity the optical trap can be weakened almost to zero, because the atoms do not sink relative to the apparatus. The team first let the most energetic atoms escape from the trap, then sharply reduced the laser power, which narrowed the velocity spread in the cloud further.

A long free expansion time matters for atom interferometry, where a cloud is split into two parts that travel different paths and are then recombined. A tiny difference in gravity between the paths changes the result. According to the authors, the record-low temperature could stimulate a new generation of ODT-based experiments in quantum simulation and precision measurements in space science.orgSciencescience.orgOpen source ↗.

The authors also name tests of the equivalence principle among the possible uses of an orbital optical trap. Such a test would place atoms of two different elements in the trap, release both clouds at once and compare their fall with an interferometer. Measurements of this kind have not yet been performed on the station.