Raman cooling

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 [1] and Raman sideband cooling.

[2] 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 de-excites the atom to the other hyperfine level.

Raman transitions are good for cooling due to the extremely narrow line width of Raman transitions between levels that have long lifetimes, and to exploit the narrow line width the difference in frequency between the two laser beams must be controlled very precisely.

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.

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.

state, and get a momentum kick decreasing the modulus of their velocity.

By regularly exchanging the lasers propagating directions and varying the detuning

By repeating this process several times (eight in the original paper, see references), the temperature of the cloud can be lowered to less than a microkelvin.

It has been successfully applied to cooling ions, as well as atoms like caesium, potassium, and lithium, etc.

Since the atoms are not in their ground state, they will be trapped in one of the excited levels of the harmonic oscillator.

The aim of Raman sideband cooling is to put the atoms into the ground state of the harmonic potential.

For a general example of a scheme, Raman beams (red in the included diagram) are two different photons (

) that are linearly polarized differently such that we have a change in angular momentum, shifting from

Then, we utilize repumping with a single beam (blue in the included diagram) that does not change vibrational levels (i.e. keeping us in

[4] This more specific cooling scheme starts from atoms in a magneto-optical trap, using Raman transitions inside an optical lattice to bring the atoms to their vibrational ground states.

[5][6] An optical lattice is a spatially periodic potential formed by the interference of counter-propagating beams.

[7] An optical lattice is ramped up, such that an important fraction of the atoms are then trapped.

If the lasers of the lattice are powerful enough, each site can be modeled as a harmonic trap.

The optical lattice should provide a tight binding for the atoms, to prevent them from interacting with the scattered resonant photons and suppress the heating from them.

, which gives the ratio of the ground state wave-packet size to the wavelength of the interacting laser light.

[6] For specifically degenerate Raman sideband cooling, we can consider a two level atom, the ground state of which has a quantum number of

is equal to the spacing of two levels in the harmonic potential created by the lattice.

By means of Raman processes, an atom can be transferred to a state where the magnetic moment has decreased by one and the vibrational state has also decreased by one (red arrows on the above image).

In order to reach this efficient transfer to the lower vibrational state at each step, the parameters of the laser, i.e. power and timing, should be carefully tuned.

Additional complication to this naive picture arises from the recoil of photons, which drive this transition.

The last complication can be generally avoided by performing cooling in the previously mentioned Lamb-Dicke regime, where the atom is trapped so strongly in the optical lattice that it effectively does not change its momentum due to the photon recoils.

This cooling scheme allows one to obtain a rather high density of atoms at a low temperature using only optical techniques.

For instance, the Bose–Einstein condensation of caesium was achieved for the first time in an experiment that used Raman sideband cooling as its first step.

[8] Recent experiments have shown it is even sufficient to attain Bose–Einstein condensation directly.