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| [[Image:Molasses.gif|350px|thumb|Optical Molasses Schematic]]
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| '''Optical molasses''' is a [[laser cooling]] technique that can cool down neutral [[atom]]s 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.
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| ==History==
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| When laser cooling was proposed in 1975, a theoretical limit on the lowest possible temperature was predicted. Known as the '''Doppler Limit''', <math> T_d= \hbar \Gamma / {2 k_b} </math>, 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, <math> \Gamma </math>, is the natural line-width of the atomic transition, <math> \hbar </math>, is Planck's constant and, <math> k_b </math>, is Boltzmann's constant.
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| Experiments at the [[National Institute of Standards and Technology]], Gaithersburg, found the temperature of cooled atoms to be well below the theoretical limit. Initially, it was a surprise to theorists, until the full explanation came out.
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| ==Theory==
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| The best explanation of the phenomenon of optical molasses is based on the principle of polarization gradient cooling. Counterpropagating beams of circularly polarized light cause a standing wave, where the [[light polarization]] depends on the spatial location. The AC [[Stark Effect|Stark Shift]] of atoms in different [[Fine structure|magnetic sub-levels]] is also spatially dependent. The basic idea is that atoms moving with a velocity climb a polarization gradient hill, thereby losing their velocity. At the top of the hill, atoms are resonant with the other molasses beams, absorb a photon and decay into a lower energy magnetic sub-level, thereby having shed some of their velocity. {{Citation needed|reason=I believe that this is Sisyphus cooling. Optical Molasses is an extension of Doppler cooling|date=April 2011}}
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| ==References==
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| *''Cooling of gases by laser radiation'', T.W. Hänsch, and A.L. Schawlow, Optics Communications, 13, 68 (1975).
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| *''Laser cooling below the Doppler limit by polarization gradients: simple theoretical models'', [[Jean Dalibard]] and Claude [[Claude Cohen-Tannoudji|Cohen-Tannoudji]] JOSA B, Vol. 6, Issue 11, pp. 2023- (1989)
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| [[Category:Atomic, molecular, and optical physics]]
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