A new way of controlling the expansion of matter in a freely-falling Bose-Einstein condensate (BEC) has produced the coldest effective temperature ever measured: 38 pK (10-12 K) above absolute zero. The method, which allowed researchers in Germany and France to image the condensate’s evolution for more than two seconds, opens the door to enhanced measurements of the gravitational constant g and photon recoil, and could even offer an alternative means of detecting gravitational waves.
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At two billionths of a degree above absolute zero, however, this wasn’t quite cold enough. So the team ran an experiment at the Bremen Drop Tower research facility, dropping the BEC trap 120 m (393.7 ft). During the free fall, the team switched the magnetic field containing the gas off and on repeatedly.
To do so, the researchers started with a cloud of 100,000 rubidium atoms trapped in a magnetic field in a vacuum chamber. They then cooled this down to form a quantum gas called a Bose-Einstein Condensate (BEC), where the atoms begin acting essentially like one big atom, allowing strange quantum effects to become visible on the macro scale.
When the magnetic field is off, the gas begins to expand, and when it’s turned back on the gas is forced to contract again. This switching slows the expansion of the gas almost to a complete stop, and reducing this molecular speed effectively reduces the temperature. While the experiment only managed to achieve this record-breaking temperature for up to two seconds, simulations showed that it should be possible to maintain it for up to 17 seconds in a weightless environment, such as onboard a satellite.
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