Raman cooling

In atomic physics, Raman cooling is a sub-recoil cooling technique that allows the cooling of atoms using optical methods below the limitations of Doppler cooling, Doppler cooling being limited by the recoil energy of a photon given to an atom. This scheme can be performed in simple optical molasses or in molasses where an optical lattice has been superimposed, which are called respectively free space Raman cooling and Raman side-band cooling. Both techniques make use of Raman scattering of laser light by the atoms. In atomic physics, Raman cooling is a sub-recoil cooling technique that allows the cooling of atoms using optical methods below the limitations of Doppler cooling, Doppler cooling being limited by the recoil energy of a photon given to an atom. This scheme can be performed in simple optical molasses or in molasses where an optical lattice has been superimposed, which are called respectively free space Raman cooling and Raman side-band cooling. Both techniques make use of Raman scattering of laser light by the atoms. The transition between two hyperfine states of the atom can be triggered by two laser beams: the first beam excites the atom to a virtual excited state (for example because its frequency is lower than the real transition frequency), and the second beam deexcites the atom to the other hyperfine level. The frequency difference of the two beams is exactly equal to the transition frequency between the two hyperfine levels. The illustration of this process is shown in the schematic illustration of a two-photon Raman process. It enables the transition between the two levels | g 1 ⟩ {displaystyle |g_{1} angle } and | g 2 ⟩ {displaystyle |g_{2} angle } . The intermediate, virtual level is represented by the dashed line, and is red-detuned with respect to the real excited level, | e ⟩ {displaystyle |e angle } . The frequency difference f 2 − f 1 {displaystyle f_{2}-f_{1}} here matches exactly the energy difference between | g 1 ⟩ {displaystyle |g_{1} angle } and | g 2 ⟩ {displaystyle |g_{2} angle } . In this scheme, a pre-cooled cloud of atoms (whose temperature is of a few tens of microkelvins) undergoes a series of pulses of Raman-like processes. The beams are counterpropagating, and their frequencies are just as what has been described above, except that the frequency f 2 {displaystyle f_{2}} is now slightly red-detuned (detuning Δ {displaystyle Delta } ) with respect to its normal value. Thus, atoms moving towards the source of the laser 2 with a sufficient velocity will be resonant with the Raman pulses, thanks to the Doppler effect. They will be excited to the | g 2 ⟩ {displaystyle |g_{2} angle } state, and get a momentum kick decreasing the modulus of their velocity. If the propagation directions of the two lasers are interchanged, then the atoms moving in the opposite direction will be excited and get the momentum kick that will decrease the modulus of their velocities. By regularly exchanging the lasers propagating directions and varying the detuning Δ {displaystyle Delta } , one can manage to have all atoms for which the initial velocity satisfies | v | > v m a x {displaystyle |v|>v_{max}} in the state | g 2 ⟩ {displaystyle |g_{2} angle } , while the atoms such that | v | < v m a x {displaystyle |v|<v_{max}} are still in the | g 1 ⟩ {displaystyle |g_{1} angle } state. A new beam is then switched on, whose frequency is exactly the transition frequency between | g 2 ⟩ {displaystyle |g_{2} angle } and | e ⟩ {displaystyle |e angle } . This will optically pump the atoms from the | g 2 ⟩ {displaystyle |g_{2} angle } state to the | g 1 ⟩ {displaystyle |g_{1} angle } state, and the velocities will be randomized by this process, such that a fraction of the atoms in | g 2 ⟩ {displaystyle |g_{2} angle } will acquire a velocity | v | < v m a x {displaystyle |v|<v_{max}} .