<title>Cooling atoms in a far-detuned optical lattice</title>

Abstract

A new method of studying and cooling trapped atoms is discussed, with particular attention to atoms in far detuned, 3-D optical lattices. The technique, projection cooling, uses a combination of microwave and optical fields to cycle atoms between hyperfine sublevels. A single vibrational level will remain dark to both the light and the microwaves, so atoms will accumulate there. Cooling below the photon recoil limit is possible with this technique. As a diagnostic tool it promises to yield detailed information about atoms in the lattice, including vibrational spectra and the distribution of atoms among vibrational levels, even in the limit of relatively weak binding to lattice sites. Atoms cooled in this way and then allowed to adiabatically expand in their potentials could reach the Bose-Einstein condensation point in less than a second, or at least get close enough to reach it after only a modest amount of evaporative cooling in a larger volume trap. Atoms so cooled and trapped are also of interest for precision measurements.The discoveiy of the Bose-Einstein condensation (BEC) of atoms was in a sense the end game in atom cooling.1 By continuing to evaporatively cool beyond the BEC threshold, essentially all the atoms in a sample have been put into exactly the same state. That is as cold as cold gets. In all demonstrations of BEC to date, evaporative cooling has provided the final several orders of magnitude of cooling. It remains an interesting theoretical and practical question whether or not one can achieve such low temperatures and high densities using only laser cooling.Laser cooling has the practical advantages that it can work much faster than evaporative cooling and can be performed in regions with modest or zero magnetic fields, both desirable features in the atom optics experiments and precision measurements that can benefit from cold atoms. Laser cooling can work when collisions between atoms are not desired, as would be the case if the ratio of inelastic to elastic collision cross sections were too high, or if one wanted to cross the BEC phase space density threshold with atoms out ofthennal equilibrium. It can also work on atoms that are isolated from each other in a periodic trap, as in an optical lattice.We will discuss a new method for laser cooling trapped neutral atoms, projection cooling, that is currently being pursued at Berkeley. The aim is to cool almost all the atoms to the ground state of a trap. In the specific case of the 3-D far-detuned optical lattice which constitutes our trap, cooling will be to the lowest enery level of the lattice. Our method shares the essential feature of all other sub-recoil laser cooling schemes,2'3' that there is a motional class of atoms that does not absorb light. Atoms can enter this dark state by spontaneous emission, so they tend to collect there. As in the sideband cooling of ions, the dark state in projection cooling is the lowest vibrational energy level. In fact, projection cooling is broadly similar to Rainan sideband cooling, which has been demon...

Microwave control of atomic motional states in a spin-dependent optical lattice

“…This occurs at [23,24], which is λ L = 866 nm in our case. At this wavelength, for the spin |↑ state, equation (10) reads…”

“…, where λ 1 (λ 2 ) is the wavelength of the D 1 (D 2 ) line [23,24], which is λ L = 866 nm in our case. At this wavelength, for the spin |↑ state, equation (10) reads…”

“…With trapped ions or neutral atoms trapped in optical lattices, the resolved-sideband regime is typically realized by two-photon Raman transitions connecting two different hyperfine ground states [5][6][7]. Alternatively, in spin-dependent potentials it becomes also possible to use microwave transitions, which also offer sufficient spectral resolution [8][9][10][11][12].…”

Sideband cooling of molecules in optical traps

“…A potential advantage of state-dependent potentials is that they could enable projection cooling schemes [26,27]. For example, one could drive a rotational transition resonant only with molecules in the motional ground state followed by state-dependent detection to determine whether or not the molecule has made the transition.…”

3D Projection Sideband Cooling

“…Projection sideband cooling [26], a version of which was recently demonstrated in 1D [27], accomplishes coherent transfer to a lower vibrational level, n, without relying on the momentum of the transferring beam. It requires a state-dependent potential, which we create by rotating the linear polarization of one of a pair of optical lattice beams.…”