Demagnetization cooling of a gas - experimental demonstration

A. Kastler proposed to use spin cooling by optical pumping to cool gases and solids 1 . Prerequisite for this is a significant coupling between the spin and the motional degrees of freedom. This sofar prevented the realization of this idea in gases as the respective cross sections for one and two electron atoms for spin relaxation processes are not large enough. However in strongly dipolar atoms like chromium 2 the coupling between spin and motional degree of freedom is strong enough to realize this idea 3 . Here we report on a first demonstration of demagnetization cooling in a gas. Chromium atoms held in an optical dipole trap were spin polarized by optical pumping. Lowering the magnetic field to a value corresponding to Zeeman shift comparable to the thermal energy reduced the motional temperature on the expense of an increased spin temperature. Constant optical pumping in combination with an optimized magnetic field ramp reduced the temperature by a factor of two and increased the phase space density by a a factor of 8 to 1/200. The scheme is currently limited by our ability to control magnetic fields below 5 mG. The practical advantage as compared to evaporative cooling in an optical dipole trap are: no atom losses and constantly high curvature/confinement during the cooling process.