Laser Cooling below the One-Photon Recoil Energy by Velocity-Selective Coherent Population Trapping

Abstract

We present a new laser-cooling scheme based on velocity-selective optical pumping of atoms into a nonabsorbing coherent superposition of states. This method has allo~ed us to achieve transverse cooling of metastable He atoms to a temperature of 2 pK, lower than both the usual Doppler cooling limit (23 pK) and the one-photon recoil energy (4 pK). The corresponding de Broglie wavelength (1.4 pm) is larger than the atomic-transition optical wavelength. PACS numbers: 32.80.Pj, 42.50.Vk The lowest temperature T which can be achieved by the usual laser-Doppler-cooling method is given, for a two-level atom, by kaT/2 = hI/4, where I is the spontaneous-emission rate from the excited atomic state (for Na, T=240 pK). ' In order to reach lower temperatures, proposals based on Raman two-photon processes in a three-level atom have been presented, ' but the efficiency of Raman cooling has not yet been demonstrated. Recently, surprisingly low temperatures (around 40 pK) have been measured for sodium and tentatively interpreted in terms of a new friction mechanism.The recoil energy (hk) /2M for an atom with mass M emitting a photon with momentum hk represents another landmark in the energy scale for laser cooling. It has been suggested that optical pumping in translation space might be used to cool the translational degrees of freedom below this so-called recoil limit, by velocity-selective recycling in a trap. In this Letter, we present a mechanism of laser cooling below the onephoton recoil energy, based on optical pumping of both internal and translational atomic degrees of freedom.This velocity-selective process is based on coherent trapping of atomic populations and has allowed us to achieve a one-dimensional cooling of He atoms in the triplet metastable state down to a temperature of about 2 pK. This temperature is lower than both the Doppler cooling limit (23 pK for 1D cooling) and the one-photon recoil energy (4 pK).Our scheme involves a closed three-level A configuration where two degenerate ground Zeeman sublevels g( m = + 1) are coupled to an excited level eo (m =0) by two counterpropagating a~and o -laser beams with the same frequency coL and the same intensity (solid lines of Fig. 1). For an atom at rest, two-photon Raman processes give rise to a nonabsorbing coherent superposition of g+ and g-. If the atom is moving along Oz, the Raman resonance condition is no longer fulfilled as a consequence of opposite Doppler shifts on the two counterpropagating laser beams. This simple argument explains how the phenomenon of coherent population trapping (a) 4He e, e=2 P, 3 (b) g=2 3s, FIG. l. (a) Two counterpropagating rx+ and cr-polarized laser beams interact with He atoms on the 2 Sl-2 Pl transition. (b) The Zeeman sublevels, and some useful ClebschGordan coefficients. Since the eo go transition is forbidden, all atoms are pumped into g~and g -after a few fluorescence cycles. These two levels are coupled only to eo, and a closed three-level A configuration is realized (solid lines).can be velocity selective for appropriate l...

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