nt - p h / 04 03 03 3 v 1 3 M ar 2 00 4 Cavity cooling of a single atom

All conventional methods to laser-cool atoms rely on repeated cycles of optical pumping and spontaneous emission of a photon by the atom. Spontaneous emission in a random direction is the dissipative mechanism required to remove entropy from the atom. However, alternative cooling methods have been proposed 1, 2 for a single atom strongly coupled to a high-finesse cavity; the role of spontaneous emission is replaced by the escape of a pho-ton from the cavity. Application of such cooling schemes would improve the performance of atom cavity systems for quantum information processing 3, 4. Furthermore, as cavity cooling does not rely on spontaneous emission, it can be applied to systems that cannot be laser-cooled by conventional methods; these include molecules 2 (which do not have a closed transition) and collective excitations of Bose condensates 5 , which are destroyed by randomly directed recoil kicks. Here we demonstrate cavity cooling of single rubidium atoms stored in an intracavity dipole trap. The cooling mechanism results in extended storage times and improved localization of atoms. We estimate that the observed cooling rate is at least five times larger than that produced by free-space cooling methods, for comparable excitation of the atom. The basic idea behind cavity cooling can be understood from a simple classical picture based on the notion of a re-fractive index. Consider a standing-wave optical cavity resonantly excited by a weak probe laser blue detuned from the atomic resonance. For strong atom-cavity coupling, even one atom can significantly influence the optical path length between the cavity mirrors. Consequently, the intracavity intensity is strongly affected by the atom 6–8. For example, at a node of the standing wave the atom is not coupled to the cavity , thus the intracavity intensity is large. An atom at an antin-ode, in contrast, shifts the cavity to a higher frequency because the atom's refractive index is smaller than unity above its resonance. This tunes the cavity out of resonance from the probe laser and leads to a small intracavity intensity. However , in a high-finesse cavity the intensity cannot drop instantaneously when the atom moves away from a node. Instead, the blue-shift of the cavity frequency leads to an increase of the energy stored in the field. The photons finally escaping from the cavity are therefore blue-shifted from the photons of the probe laser. This occurs at the expense of the atom's kinetic energy. The reverse …