We demonstrate an efficient scheme for continuous trap loading based upon spatially selective optical pumping. We discuss the case of 1 S0 calcium atoms in an optical dipole trap (ODT), however, similar strategies should be applicable to a wide range of atomic species. Our starting point is a reservoir of moderately cold (≈ 300 µK) metastable 3 P2-atoms prepared by means of a magneto-optic trap (triplet-MOT). A focused 532 nm laser beam produces a strongly elongated optical potential for 1 S0-atoms with up to 350 µK well depth. A weak focused laser beam at 430 nm, carefully superimposed upon the ODT beam, selectively pumps the 3 P2-atoms inside the capture volume to the singlet state, where they are confined by the ODT. The triplet-MOT perpetually refills the capture volume with 3 P2-atoms thus providing a continuous stream of cold atoms into the ODT at a rate of 10 7 s −1 . Limited by evaporation loss, in 200 ms we typically load 5 × 10 5 atoms with an initial radial temperature of 85 µK. After terminating the loading we observe evaporation during 50 ms leaving us with 10 5 atoms at radial temperatures close to 40 µK and a peak phase space density of 6.8 × 10 −5 . We point out that a comparable scheme could be employed to load a dipole trap with 3 P0-atoms.PACS numbers: 32.80. Pj, 82.20.Pm The unique spectroscopic features of two-electron systems and their usefulness for the fields of time metrology [1], cold collision physics [2, 3] and quantum gases [4] has led to extensive efforts to improve laser cooling and trapping techniques for alkaline earth (AE) atoms [5,6,7,8,9,10,11,12]. Calcium is a particularly interesting example, because, aside from its excellent performance in optical atomic clock scenarios [13,14], its singlet ground state (in contrast to the most abundant strontium isotope [15]) has a large positive scattering length with favorable prospects for reaching quantum degeneracy [16]. Optical trapping is a key technique in modern atomic physics, indispensible in numerous recent experiments with ultracold atoms and molecules [17]. In particular, if magnetic trapping techniques fail to work, as in the singlet manifold of the AE group, optical dipole traps (ODTs) practically have no alternative.ODTs typically provide good compression, but suffer from limited trap depths of several hundred µK, owing to limitations in available laser powers. Thus, efficient loading of ODTs typically requires a magneto-optic trap (MOT) permitting sufficiently low temperatures well below 100 µK, as available for alkaline atoms. Although magneto-optical trapping of AE-like atoms in the ground state is in fact possible using their principal fluorescence lines, the attainable temperatures of several mK are too high for efficient direct loading of an ODT. In some cases, e.g. for strontium or ytterbium, additional cooling by means of intercombination lines connecting to their ground states [5,10] have been used for ODT loading with large phase space densities. Unfortunately, this does not likewise apply to calcium, because o...

We present a route to Bose-Einstein condensation devised for two-electron atoms, which do not admit practicable cooling techniques based upon narrow intercombination lines. A dipole trap for 40 Ca atoms in the singlet ground state is loaded from a moderately cold source of metastable triplet atoms via spatially and energetically selective optical pumping permitting 4 orders of magnitude increase of the phase-space density. Further cooling to quantum degeneracy is achieved by forced evaporation optimized to minimize three-body losses. In a combined loading and evaporation cycle of less than 3 s we are able to condense 3000 atoms.

We study binary collisions of metastable calcium atoms ($^{40}$Ca) in an optical dipole trap. Collisions between $^{3}$P$_{0}$-atoms and between $^{3}$P$_{0}$ and $^{1}$S$_{0}$-atoms are considered. In the former case, the elastic and inelastic collision parameters are found to be $5.4\times 10^{-11}\,\mathrm{cm}^{3}\mathrm{s}^{-1}$ and $3.6\times 10^{-11}\,\mathrm{cm}^{3}\mathrm{s}^{-1}$, respectively. A fraction of the collisions between $^{3}$P$_{0}$-atoms is found to produce cold trapped atoms in the singlet $^{1}$S$_{0}$ state, suggesting that the internal energy for these collisions is dissipated by radiation. For collisions between $^{3}$P$_{0}$ and $^{1}$S$_{0}$-atoms we find a two-body loss parameter of $8.5\times 10^{-11}\,\mathrm{cm}^{3}\mathrm{s}^{-1}$. Our observations show that metastable calcium samples in the $^{3}$P$_{0}$-state are not stable at high densities, as for example required in quantum computing or many-body quantum simulation schemes.Comment: 5 pages, 7 Figure