Quantum Gases

Putting an old law to the test

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Science  01 Dec 2017:
Vol. 358, Issue 6367, pp. 1144
DOI: 10.1126/science.358.6367.1144-c

Most superfluids, liquids, and gases that flow without viscosity owe their exotic properties to Bose-Einstein condensation (BEC), the formation of a macroscopic wave function at low temperatures. However, not all particles in a superfluid condense; a fundamental limit is set by the interactions in the system. Lopes et al. studied the dependence of the superfluid fraction in a homogeneous BEC of potassium atoms on the (tunable) strength of interactions between them. The researchers used two laser beams to give a momentum “kick” only to the condensed atoms, causing them to physically separate from the uncondensed ones. The measured fraction closely followed the theoretical prediction made decades ago that had remained experimentally untested.

Phys. Rev. Lett. 119, 190404 (2017).

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