Nature http://dx.doi.org/10.1038/nature10668 (2011)

Like electrons trapped in the potential of a nucleus, bosonic atoms residing at the individual sites of an optical lattice — a periodic structure defined by interfering laser beams — can occupy discrete motional orbitals. Waseem Bakr and colleagues have now implemented a technique for deterministically controlling the distribution of atoms across these different energy states, and establish it as a new means of cooling quantum gases.

The key is a mechanism Bakr et al. call 'orbital excitation blockade'. They transfer ground-state atoms to a higher orbital by modulating the lattice depth at a suitable frequency; but once one atom is transferred, it shifts the energy levels to a degree that other atoms at the same site are pushed off-resonance — the first excitation blocks further ones.

Bakr and colleagues use this mechanism to reshuffle and selectively remove atoms from a lattice with random occupation numbers. As atom-number fluctuations across the lattice are the main source of entropy, levelling the number of atoms at each site amounts to cooling the gas. The authors expect that in the future their method can also serve in quantum computations on optical-lattice systems.