A Cloudy Thermometer

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Science  23 Oct 2009:
Vol. 326, Issue 5952, pp. 503
DOI: 10.1126/science.326_503b

Some of the coldest places on our planet are created in physics laboratories, where dilute alkali gases are coaxed into showing their quantum character. To prepare the samples, experimenters employ irreversible methods such as evaporative and sympathetic cooling. It is often hard to measure the final temperature of the gases, however, particularly in contexts where they are loaded into optical lattices to simulate condensed-matter systems such as Mott insulators and superconductors. Catani et al. address this challenge by tracking entropy, rather than temperature (which, technically speaking, is a measure of how much a system's entropy changes on incremental energy input). Specifically, they load two distinguishable Bose gases into the same magnetic trap and then apply an additional optical trap that affects only one of them, the target gas. By slowly increasing the strength of the trap, they achieve an exact entropy transfer from the target gas (41K) to the auxiliary gas (87Rb), condensing the K and maintaining the temperature equilibrium between the two gases. Through cycles of compression and decompression, they observe oscillation of the K ensemble between condensed and uncondensed states. When the target gas is then loaded into a one-dimensional optical lattice, its temperature is directly reflected in the auxiliary gas cloud size after expansion.

Phys. Rev. Lett. 103, 140401 (2009).

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