Evaporative cooling at low trap depth

deCarvalho, Robert; Doyle, John M.

doi:10.1103/physreva.70.053409

Cited by 15 publications

(19 citation statements)

References 11 publications

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“…We include in our model the density dependent loss processes, assuming that the density profile of the atoms is, at all time, well described by the Boltzmann distribution at thermal equilibrium in the non-truncated trap. This relies on two main assumptions: first, the elastic collision rate is large enough to reach thermal equilibrium on the timescale of the experiment (defined by the dominant loss process); second, the trap depth is large enough compared to the magnetic trap temperature, so that one can neglect modifications in the thermal distri-bution linked to the finite depth of the potential ( [7], [8]). In our experiment, thermal equilibrium is most likely not reached at the beginning of the loading of the MT, when the number of atoms is still small.…”

Section: Theoretical Model and Interpretationmentioning

confidence: 99%

“…η is the ratio of the trap depth to the trapped atom temperature, and f (η) is given in equation (11) of ref [8] (which is valid for η ≥ 4).…”

Section: Theoretical Model and Interpretationmentioning

confidence: 99%

“…We also assume that the trap depth is large enough for the thermal distribution in the truncated trap to be identical to the thermal distribution in the non-truncated trap ( [7], [8]). The comparison between our experimental data and our simulation points towards a strong heating process due to collisions between the magnetically trapped atoms and the MOT atoms, which strongly limits the achieved phase-space density.…”

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confidence: 99%

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Accumulation and thermalization of cold atoms in a finite-depth magnetic trap

et al. 2007

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We experimentally and theoretically study the continuous accumulation of cold atoms from a magneto-optical trap (MOT) into a finite depth trap, consisting in a magnetic quadrupole trap dressed by a radiofrequency (RF) field. Chromium atoms ( 52 Cr) in a MOT are continuously optically pumped by the MOT lasers to metastable dark states. In presence of a RF field, the temperature of the metastable atoms that remain magnetically trapped can be as low as 25 µK, with a density of 10 17 atoms.m −3 , resulting in an increase of the phase-space density, still limited to 7.10 −6 by inelastic collisions. To investigate the thermalization issues in the truncated trap, we measure the free evaporation rate in the RF-truncated magnetic trap, and deduce the average elastic cross section for atoms in the 5 D4 metastable states, σ el = 7.0 × 10 −16 m 2 . We finally discuss the possibilities for using this scheme of continuous accumulation and RF evaporation to rapidly reach high phase-space densities from a MOT.PACS numbers: 32.80. Pj, 47.45.Ab, 32.80.Cy Magneto-optical trapping is one of the greatest recent advances in atomic and molecular physics, opening many new areas in physics, including the study of Bose-Einstein condensation in dilute systems. One of the advantages of a magneto-optical trap (MOT), is that cooling and trapping are simultaneous. However, inherent limitations of the cooling mechanism, for example light-assisted collisions or multiple scattering of light, limit typical phasespace densities to 10 −7 when strong fluorescence lines are involved in the cooling transition [1]. To reach higher phase-space densities, it is usually necessary to use sequential operations, such as cooling in molasses, loading in conservative traps, and forced evaporation, which greatly limits the rate at which a Bose-Einstein condensates (BEC) can be produced.New possibilities arise for atoms whose electronic level structure includes metastable dark states (such as Sr, Er, Yb, or Cr). Spontaneous emission from the excited state of the cooling transition depumps atoms into a dark metastable state, which can be trapped, either optically, or magnetically. This provides an interesting means to continuously accumulate atoms in a magnetic trap [2], or for a continuous loading of an optical or magnetic waveguide. Cold metastable atoms are directly produced inside the waveguide or the trap, which reduces heating associated with the sequential loading from a MOT. This may have promising applications in the prospect of continuously producing a coherent beam of atoms, by performing forced evaporation as the atoms propagate along a waveguide [3].In this paper, we study the accumulation of metastable chromium atoms in a magnetic trap (MT), dressed by a radiofrequency (RF) field (see Fig 1) . We observe different regimes as a function of the RF frequency. When the RF frequency is small, atoms in internal states adiabatically connected to high field seeking states at small magnetic fields are accumulated in a (3D) W-shaped potential. When the RF ...

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Section: Theoretical Model and Interpretationmentioning

confidence: 99%

“…η is the ratio of the trap depth to the trapped atom temperature, and f (η) is given in equation (11) of ref [8] (which is valid for η ≥ 4).…”

Section: Theoretical Model and Interpretationmentioning

confidence: 99%

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confidence: 99%

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Accumulation and thermalization of cold atoms in a finite-depth magnetic trap

et al. 2007

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“…Approximations are usually made to arrive at analytic results for required quantities (e.g., [19]) in order to facilitate solution of the differential equations. This is straightforward for high-η conditions [9] and also in situations of low-η and with sufficient trap symmetry [20]. For low-η conditions and traps that lack spatial symmetry, numerical evaluation of statistical mechanical quantities is the only option, and that is the approach we follow, with the exception of calculation of pot .…”

Section: Description Of the Numerical Proceduresmentioning

confidence: 99%

“…For example, this yields analytic expressions for thermodynamic quantities and allows approximation of optical dipole traps [18] as parabolic potentials [19]. By taking advantage of the high degree of spatial symmetry in a linear potential, analytic expressions for thermodynamic quantities were derived for the low-η situation (η < 4) in this particular geometry [20]. It is worth emphasizing that the model of Luiten et al [10] is, in principle, valid for low η as long as the assumptions of ergodicity and a truncated Boltzmann distribution are also valid.…”

Section: Introductionmentioning

confidence: 99%

Numerical modeling of collisional dynamics of Sr in an optical dipole trap

et al. 2011

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We describe a model of inelastic and elastic collisional dynamics of atoms in an optical dipole trap that utilizes numerical evaluation of statistical mechanical quantities and numerical solution of equations for the evolution of number and temperature of trapped atoms. It can be used for traps that possess little spatial symmetry and when the ratio of trap depth to sample temperature is relatively small. We compare simulation results with experiments on 88 Sr and 84 Sr, which have well-characterized collisional properties.

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Optimized production of large Bose-Einstein condensates

et al. 2006

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We describe several routes to quantum degenerate gases based on simple schemes to efficiently load atoms into and evaporate them from a "dimple" crossed dipolar trap. The dimple is loaded nonadiabatically by collisions between atoms which are trapped in a reservoir which can be provided either by a dark spontaneousforce magneto-optical trap ͑MOT͒, the ͑aberrated͒ laser beam itself, or by a quadrupolar or quadratic magnetic trap. Optimal loading parameters for the dimple, relatively high temperature, and tight optical trap are derived from thermodynamic equations including possible inelastic and Majorana losses. Evaporative cooling is described by a set of simple equations, taking into account gravity, the possible occurrence of the hydrodynamical regime, Feshbach resonances, and three body recombination. The solution implies that to have efficient evaporation the elastic collisional rate ͑in s −1 ͒ must be on the order of the trap frequency and lower than 100 times the temperature in microkelvins. Following this approach Bose-Einstein condensates with more than 10 7 atoms should be obtained in much less than 1 s starting from an ordinary MOT setup.

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Evaporative cooling at low trap depth

Cited by 15 publications

References 11 publications

Accumulation and thermalization of cold atoms in a finite-depth magnetic trap

Accumulation and thermalization of cold atoms in a finite-depth magnetic trap

Numerical modeling of collisional dynamics of Sr in an optical dipole trap

Optimized production of large Bose-Einstein condensates

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