A record-low temperature for cooled atoms has been achieved aboard China’s Tiangong space station. A cloud of rubidium atoms was brought into a special quantum state and cooled to a lower temperature than ever before, either on Earth or in space. For the first time in orbit, atoms in this state were confined using lasers alone, without a magnetic field. The record was made possible by combining an optical trap with weightlessness.
The coldest cloud of atoms
A cloud of approximately 20,000 rubidium atoms was first brought into a Bose-Einstein condensate state, in which all particles occupy the same quantum state and behave like one large quantum wave. After the cloud expanded, its equivalent temperature dropped to 9.7 picokelvins, or 9.7 trillionths of a degree above absolute zero. Such a temperature is not measured with a thermometer but calculated from how slowly the cloud spreads out.
The colder the condensate, the longer it can be studied, which determines the accuracy of measurements. According to Live Science, the previous space record of 52 picokelvins was set by NASA’s Cold Atom Lab aboard the International Space Station. The new cloud was approximately 280 billion times colder than the cosmic microwave background, which fills the Universe and has a temperature of about 2.7 kelvins.
Lasers instead of magnets
The experiment was conducted by physicists from Peking University and the Shanghai Institute of Optics and Fine Mechanics of the Chinese Academy of Sciences. The results were published in the peer-reviewed journal Science. All previous condensates in orbit were confined using magnetic traps on atom chips, which can hold only atoms in certain quantum states. A focused laser beam attracts atoms regardless of their state.
In weightlessness, the optical trap can be weakened almost to zero because the atoms do not drift relative to the experimental setup. The team first allowed the most energetic atoms to escape from the trap and then sharply reduced the laser power, further narrowing the spread of velocities within the cloud.
Seven seconds of expansion
On Earth, atoms fall to the bottom of a vacuum chamber within a fraction of a second. In a free-fall tower, where a temperature of 38 picokelvins was achieved, the cloud could be observed for only two seconds, whereas aboard the space station this period increased to seven seconds.
Prolonged free expansion is necessary for atom interferometry, a technique that measures the strength of gravity using atoms. The cloud is split into two parts, which travel along different paths and are then brought back together. Even the smallest difference in gravity between the paths changes the result. The sensitivity of such an instrument increases with the square of the observation time, meaning that orbital interferometers could measure Earth’s gravitational field more accurately, as noted by the Chinese Academy of Sciences.
Testing the theory of relativity
Another area of research concerns the principle of equivalence. According to this principle, all objects in a gravitational field fall with the same acceleration, regardless of what they are made of. This principle underpins the general theory of relativity.
To test it, atoms of two different chemical elements are placed in a trap, both clouds are released simultaneously, and an atom interferometer is used to compare whether they fall at the same rate. The authors of the study list such measurements among the research applications for which an orbital laser trap is suitable. No such measurements have yet been conducted aboard the space station. However, if the two clouds ever fall even slightly differently, Einstein’s theory will have to be reconsidered.
