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optical molasses : ウィキペディア英語版 | optical molasses
Optical molasses is a laser cooling technique that can cool down neutral atoms to temperatures colder than a magneto-optical trap (MOT). An optical molasses consists of 3 pairs of counter-propagating circularly polarized laser beams intersecting in the region where the atoms are present. The main difference between optical molasses and a MOT is the absence of magnetic field in the former. While a typical Sodium MOT can cool atoms down to 300μK, optical molasses can cool the atoms down to 40μK, an order of magnitude colder. ==History==
When laser cooling was proposed in 1975, a theoretical limit on the lowest possible temperature was predicted. Known as the Doppler Limit, , this was given by the lowest possible temperature attainable considering the cooling of two-level atoms by Doppler cooling and the heating of atoms due to momentum diffusion from the scattering of laser photons. Here, , is the natural line-width of the atomic transition, , is the reduced Planck's constant and, , is Boltzmann's constant. Experiments at the National Institute of Standards and Technology, Gaithersburg, found the temperature of cooled atoms to be well below the theoretical limit (Lett et al, 1988). Initially, it was a surprise to theorists, until the full explanation came out.
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