Loading and cooling in an optical trap via hyperfine dark states

Abstract

We present a novel optical cooling scheme that relies on hyperfine dark states to enhance loading and cooling atoms inside deep optical dipole traps. We demonstrate a seven-fold increase in the number of atoms loaded in the conservative potential with strongly shifted excited states. In addition, we use the energy selective dark-state to efficiently cool the atoms trapped inside the conservative potential rapidly and without losses. Our findings open the door to optically assisted cooling of trapped atoms and molecules which lack the closed cycling transitions normally needed to achieve low temperatures and the high initial densities required for evaporative cooling.Ultra-cold quantum gases have attracted much attention in recent decades as versatile platforms for investigating strongly correlated quantum systems [1] and as the basis for a new class of quantum technologies based on atomic interferometry [2,3]. Cooling of an atomic gas to the required temperatures requires a multi-stage process: laser cooling in a magneto-optical trap (MOT); sub-Doppler cooling; loading into a conservative magnetic or optical trap; evaporative cooling. Although quite efficient, this process is only possible for a small subset of alkali and alkali-earth-like atoms that can be initially cooled to low temperature by optical means.The sub-Doppler cooling phase typically uses light red detuned from the F → F = F + 1 cycling transition of a D 2 line nS 1/2 → nP 3/2 [4], and can be understood in terms of the "Sisyphus effect" [5][6][7][8]. Sub-Doppler cooling schemes involving dark states (DSs) [9] have emerged as a powerful alternative; they are known as gray molasses. Very recently they have been pivotal in obtaining an alloptical BEC in microgravity [10] and a degenerate Fermi gas of polar molecules [11]. The DSs are coherent superpositions of internal and external (momentum) states that are decoupled from the optical field; their creation does not require cycling F → F = F + 1 transitions, but can rely on any transitions of the F → F ≤ F form.To prevent the expansion of the cold atom cloud during cooling, it is tempting to combine DS sub-Doppler cooling with spatial confinement in a far-off-resonance optical dipole trap (FORT). However, the DSs are then strongly modified by the trap potential, which typically shortens their lifetime and eventually couples them to the light field.In this letter, we show that DS cooling can be used in combination with FORT when strong differential light * devang.naik@institutoptique.fr † andrea.bertoldi@institutoptique.fr shift are present. We use the effect of FORT trapping light close the 5P 3/2 → 4D 3/2;5/2 transitions of rubidium at 1529 nm [12,13] to maximize the cooling action on the surrounding of the trap and at its borders. We observe an order of magnitude improvement in the number of trapped atoms. Additionally, we explore the possibility of cooling the atomic ensemble in the FORT. We achieve a notable temperature reduction of the trapped sample in a few ms and without losing atoms and analyz...

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