Optical molasses

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Optical Molasses Schematic

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[edit]

When laser cooling was proposed in 1975, a theoretical limit on the lowest possible temperature was predicted. Known as the Doppler Limit,  T_d=  \hbar \Gamma / {2 k_b} , 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,  \Gamma  , is the natural line-width of the atomic transition,  \hbar   , is Planck's constant and,  k_b  , 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.

Theory[edit]

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 Shift of atoms in different 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]

References[edit]

  • Cooling of gases by laser radiation, T.W. Hänsch, and A.L. Schawlow, Optics Communications, 13, 68 (1975).
  • Laser cooling below the Doppler limit by polarization gradients: simple theoretical models, Jean Dalibard and Claude Cohen-Tannoudji JOSA B, Vol. 6, Issue 11, pp. 2023- (1989)
  • Lett, P. D., R. N. Watts, C. I. Westbrook, W. D. Phillips, P. L. Gould, and H. J. Metcalf, 1988, ‘‘Observation of atoms laser cooled below the Doppler limit,’’ Phys. Rev. Lett. 61, 169.