Science 337, 75–78 (2012)

Laser cooling can reduce atoms to a temperature lower than that achievable by any other known process, leading to fundamental insights in many fields. Now Matthias Wolke and researchers from the University of Hamburg, Germany, have demonstrated a cavity-based laser cooling scheme that makes it possible to prepare Bose–Einstein condensates with particle densities and temperatures inaccessible with conventional laser cooling. An optical cavity used for cooling can be characterized by two important figures: the Purcell factor (the rate at which free-space modes will scatter into a cavity mode) and the spectral width of the transmission resonances. The researchers operate their atom–cavity system in a previously unexplored quantum regime where the Purcell factor is greater than one (as high as 43) and the frequency shift associated with the scattering of a single photon exceeds the bandwidth of the cavity. Temperature control of the atoms is demonstrated on a subrecoil energy scale at densities of the order of 1014 cm−3. The authors predict that their scheme could cool a warmer-than-usual sample of atoms to the point at which quantum degeneracy occurs.