A continuous source of cold spin-polarized cold atoms

Abstract

Summary form only given. We propose a new method to produce a continuous source of spin-polarized cold atoms which are all-optically guided after their extraction from a magneto-optical trap (MOT), as depicted in Fig. 1 (a). The techniques combines several physical effects and relies on light-shift engineering [Fig. 1 (b)], implemented using two coaxially overlapped optical beams each one driving a given transition of a three-level atom in a ladder configuration (Ξ-system). In a well chosen configuration, the light-shift creates a state-dependent potential which implements the atom-diode [1] responsible for the continuous extraction of the atoms from a MOT into an all-optical guide.We performed a theoretical study by extending the Dalibard and Cohen-Tannoudji dressed-atom model [2] to the case of a doubly-driven Ξ-system. We analysed and quantified the dipole forces and the various sources of momentum diffusion in the resonant, non-perturbative, regime. In particular, from the Markovian evolution of the internal state, we obtain a general formula for the diffusion coefficient associated with the dipole force fluctuation. We proposed and studied in detail the implementation of the method for 87Rb atoms. We show that, using σ+polarized fields driving the transitions 52S1/2 → 52P3/2 and 52P3/2 → 42D5/2 at 780 nm and 1529 nm respectively, a closed Ξ-system can be isolated within the complicated structure of 87Rb. Moreover, with this choice of transitions, the atoms will be optically pumped in a given Zeeman sub-state thus polarizing the atomic sample. The theoretical results have then been used in a numerical simulation of Langevin-like equations [Fig. 1 (c)] to estimate the performance of the system. From the simulation of 10000 trajectories, we show that a large fraction of the atoms (~ 64%) is guided over at least 5 cm, the mean velocity at this distance being 5.6 m/s with a dispersion of 2.2 m/s. This guided distance allows to deliver the atoms inside a magnetic shield that cancels the continuously operated MOT magnetic field. The proposed method thus creates a continuous source of guided spin-polarized cold atoms. Such a source can be of particular interest for atom interferometry.