Negative Temperatures?

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Science  04 Jan 2013:
Vol. 339, Issue 6115, pp. 42-43
DOI: 10.1126/science.1232558

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Ultracold quantum gases present an exquisitely tunable quantum system. Applications include precision measurement (1), quantum simulations for advanced materials design (2), and new regimes of chemistry (3). Typically trapped in a combination of magnetic fields and laser beams, strongly isolated from the environment in an ultrahigh vacuum, and cooled to temperatures less than a microdegree above absolute zero, they are the coldest known material in the universe. The interactions between atoms in the gas can be tuned over seven orders of magnitude and from repulsive to attractive (4). The addition of standing waves made from interfering lasers at optical wavelengths gives rise to an optical lattice, a crystal of light, periodic just like the usual crystals made of matter. On page 52 of this issue, Braun et al. (5) use these special features of ultracold quantum gases to produce a thermodynamic oddity—negative temperature.