Ground State Laser Cooling Beyond the Lamb – Dicke Limit

– We propose a laser cooling scheme that allows to cool a single atom confined in a harmonic potential to the trap ground state |0〉. The scheme assumes strong confinement, where the oscillation frequency in the trap is larger than the effective spontaneous decay width, but is not restricted to the Lamb–Dicke limit, i.e. the size of the trap ground state can be larger than the optical wavelength. This cooling scheme may be useful in the context of quantum computations with ions and Bose–Einstein condensation. One of the major goals of atomic physics is to laser cool trapped atoms to the lowest energy state |0〉 of the confining potential. While this is certainly possible in the so–called Lamb–Dicke Limit (LDL) [1], whereby the spatial dimensions of the ground state |0〉, a0, are much smaller than the wavelength of the cooling laser λ (i.e. η = 2πa0/λ ≪ 1) [2, 3], there is not known mechanism that achieves this task in the opposite limit (η ≥ 1) [4]. Ground state cooling beyond the LDL is interesting from two perspectives. On the one hand, it is required to perform quantum computations in a linear ion trap [5, 6], as well as to produce some non–classical states of motion of a single ion or atom in scales of the order of the laser wavelength [3]. On the other hand, it may be a way to obtain Bose–Einstein condensation [7, 8] with all–optical means [4, 9]. So far, the most efficient laser cooling mechanism for trapped particles in the LDL is sideband cooling [10]. It allows to cool a single particle confined in a harmonic trap practically to the ground state, as first demonstrated experimentally by Diedrich et al. [1]. Beyond the LDL, the most effective techniques have been proved to be cooling schemes originally designed to achieve subrecoil temperatures of free atoms, namely, dark state [11] and Raman cooling [12]. The first scheme operates with an angular momentum Jg = 1 → Je = 1 internal transition, and it is specially suitable for flat–bottom traps [13], whereas the second one is based on timing laser pulses followed by a repumping process, creating and populating a dark state of zero velocity atoms. In this paper we propose a method which extends the sideband cooling mechanism beyond the LDL, combining ideas of these other schemes. It allows to cool a single Typeset using EURO-LTEX 2 EUROPHYSICS LETTERS