epl draft Sub - Doppler laser cooling of fermionic 40 K atoms in three - dimensional gray optical molasses

We demonstrate sub-Doppler cooling of K on the D1 atomic transition. Using a gray molasses scheme, we efficiently cool a compressed cloud of 6.5 × 10 atoms from ∼ 4 mK to 20μK in 8 ms. After transfer in a quadrupole magnetic trap, we measure a phase space density of ∼ 10−5. This technique offers a promising route for fast evaporation of fermionic K. Introduction. – Cooling of fermionic atomic species has played a fundamental role in the study of strongly correlated Fermi gases, notably through the experimental exploration of the BCS-BEC crossover, the observation of the Clogston-Chandrasekhar limit to superfluidity, the observation of the Mott-insulator transition in optical lattices, and the study of low dimensional systems (see for instance [1,2] for a review). When the temperature is further decreased, new exotic phases are predicted (p-wave superfluids for spin imbalanced gases, antiferromagnetic order..) and, as a consequence, intense experimental effort is currently under way to push the temperature limit achieved in ultracold fermionic samples in order to enter these novel regimes. Most experiments on quantum degenerate gases begin with a laser cooling phase that is followed by evaporative cooling in a non-dissipative trap. The final quantum degeneracy strongly depends on the collision rate at the end of the laser cooling phase and sub-Doppler cooling [3] is often a key ingredient for initiating efficient evaporation. In the case of fermionic lithium-6 and potassium-40, the narrow hyperfine structure of the P3/2 excited level does not allow for efficient Sisyphus sub-Doppler cooling to the red of a F → F ′ = F + 1 atomic transition [4, 5]. Experiments for producing quantum degenerate gases of K typically start with ∼ 10 atoms laser-cooled to the Doppler limit (145μK) [6]. More refined laser-cooling (a)These authors contributed equally to this work. (b)Email: [email protected] (c)Email: [email protected] schemes have produced K temperatures of ∼ 15μK, but with only reduced atom numbers (∼ 10) [5, 7, 8]. This relatively poor efficiency is due to the combination of the fairly narrow and inverted hyperfine level structure of the P3/2 excited state which results in the washing out of the capture velocity of the molasses when the laser detuning is increased [9]. To overcome these limitations, two groups recently realized Magneto-Optical Traps (MOT) in the near-UV and blue regions of the spectrum to cool Li [10] and K [11] respectively. The associated transitions, being narrower than their D2 counterparts, lead to a smaller Doppler temperature and were used to improve the final phase space density typically by one order of magnitude. In this Letter, we report efficient sub-Doppler cooling of K atoms using gray molasses on the D1 atomic transition at 770 nm. Thanks to the much reduced fluorescence rate compared to standard bright sub-Doppler molasses, we could produce cold and dense atomic samples. The temperature of a tightly compressed cloud of 6.5 × 10 atoms was decreased from ∼ 4 mK to 20μK in 8 ms without significant change of the density in the process. After transfer to a quadrupole magnetic trap, we achieved a phase space density of ∼ 2× 10−5. Gray molasses. – Sub-Doppler cooling using gray molasses was proposed in ref. [12] and realized in the mid ’90s on the D2 atomic transition of cesium and rubidium, allowing one to cool atomic samples close to 6 times the single photon recoil energy [13–15]. For an atomic ground state with angular momentum F , gray molasses operate p-1 ha l-0 07 38 26 0, v er si on 1 3 O ct 2 01 2 D. Rio Fernandes et al.