Laser cooling
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Laser cooling is a technique that uses light to cool atoms to a very low temperature. The simplest form of laser cooling is referred to as optical molasses, since the dissipative optical force resembles the viscous drag on a body moving through molasses.
This technique works by tuning the frequency of light slightly below an electronic transition in the atom. Because the light is detuned to the "red" (i.e. at lower frequency) of the transition, the atoms will absorb more photons if they move towards the light source, due to the Doppler effect. Thus if one applies light from two opposite directions, the atoms will always scatter more photons from the laser beam pointing opposite to their direction of motion. In each scattering event the atom loses a momentum equal to the momentum of the photon. If the atom, which is now in the excited state, emits a photon spontaneously, it will be kicked by the same amount of momentum but in a random direction. The result of the absorption and remission process is to reduce the speed of the atom, provided its initial speed is larger than the recoil velocity from scattering a single photon. If the absorption and emission are repeated many times, the mean velocity, and therefore the kinetic energy of the atom will be reduced. Since the temperature of an ensemble of atoms is a measure of the random internal kinetic energy, this is equivalent to cooling the atoms.
The atom performs a random walk in momentum space with steps equal to the photon momentum due to the spontaneous emission. This constitutes a heating effect which counteracts the cooling process and imposes a limit on the amount by which the atom can be cooled. Moreover, the optical transition used for cooling in reality must have a finite frequency width, which limits the velocity discrimination (i.e. the likelihood that an atom will scatter light from the "correct" beam, as described above), and therefore the temperature. This temperature is called the Doppler temperature. Lower temperatures, down to the recoil temperature, may be obtained by sub-Doppler cooling. Counterpropagating sets of laser beams in all three Cartesian dimensions may be used to cool all the three degrees of freedom of the atom. Common laser-cooling configurations include optical molasses, the magneto-optical trap, and the Zeeman slower.