epl draft Gray molasses cooling of 39 K to a high phase - space density

We present new techniques in cooling K atoms using laser light close to the D1 transition. First, a new compressed-MOT configuration is taking advantage of gray molasses type cooling induced by blue-detuned D1 light. It yields an optimized density of atoms. Then, we use pure D1 gray molasses to further cool the atoms to an ultra-low temperature of 6μK. The resulting phase-space density is 2× 10 and will ease future experiments with ultracold potassium. As an example, we use it to directly load up to 3× 10 atoms in a far detuned optical trap, a result that opens the way to the all-optical production of potassium degenerate gases. Introduction. – Ultracold atomic gases are used for the study of quantum many-body physics in strongly interacting samples [1,2]. From this point of view potassium and lithium are two alkali of special interest because of the existence of both fermionic and bosonic isotopes and of broad Feshbach resonances at low magnetic fields that allow one to tune the inter-particle interactions [3–9]. These atoms however suffer from narrow hyperfine structures in their D2 excited state (see fig. 1) preventing from efficient subdoppler cooling with light far red-detuned from the cycling transition [10–12]. Several techniques have been used in order to overcome this limitation. Precise tuning of the lasers close to resonance on the D2 line has allowed one to reach subdoppler temperatures around 25μK in K [13, 14]. Narrow-line laser cooling was also demonstrated for both lithium [15] and potassium [16]. Recently, even lower temperatures were obtained in gray molasses cooling [17] on the D1 transition for both lithium [18] and potassium [19, 20] (12μK for K [20]). In this paper, we demonstrate a yet lower temperature of 6μK in D1 gray molasses and we also show that D1 light can be used to optimize previous steps of the optical cooling sequence. Gray molasses combine two effects, i.e. velocity selective coherent population trapping (VSCPT) [21] and Sisy(a)Present address: Institute of Opto-Electronics, Shanxi University, Taiyuan 030006, P.R.China (b)E-mail: [email protected] phus cooling [22]. The atoms are optically pumped in a dark state, whose departure rate varies with the square of the atom velocity, leading to less diffusion for slow atoms (VSCPT). This motional coupling to bright states happens so that the atoms repeatedly climb up the dipole potential hills in the bright states before being pumped back to the dark state (Sisyphus cooling). The paper is organized as follows. After a description of the experimental setup, we explain how D1 light can improve the phase-space density of optically captured atoms. First, a new type of hybrid D1-D2 compressed-MOT [23] combines blue-detuned D1 light, which induces gray molasses Sisyphus cooling on the F = 2 → F ′ = 2 transition, and red-detuned D2 repumping light, which gives the trapping force. It is characterized by a higher density and a much lower temperature as compared to standard compressed-MOT using only D2 light [13, 14]. We then show deep subdoppler cooling in pure D1 gray optical molasses. By ramping down the optical power, we cool all atoms to 6μK, far below the previously obtained temperatures in optically cooled potassium [19, 20]. We finally report the direct loading of an optical dipole trap at a phase-space density of 3 × 10 [24]. This result opens the route to an all optical production of a degenerate K quantum gas. Experimental setup. – Our experimental setup is composed of two chambers similar to those previously used for Bose-Einstein condensation of Rb [25]. A twop-1 ha l-0 08 70 07 4, v er si on 1 4 O ct 2 01 3