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arXiv:1601.05103 · Physical Review A

Rapid cooling to quantum degeneracy in dynamically shaped atom traps

Richard Roy, Alaina Green, R. Bowler, Subhadeep Gupta

Published Apr 4, 2016• 67 citations
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Abstract

We report on a general method for the rapid production of quantum degenerate gases. Using $^{174}\mathrm{Yb}$, we achieve an experimental cycle time as low as 1.6--1.8 s for the production of Bose-Einstein condensates (BECs) of $(0.5--1)\ifmmode\times\else\texttimes\fi{}{10}^{5}$ atoms. While laser cooling to $30\phantom{\rule{0.16em}{0ex}}\ensuremath{\mu}\mathrm{K}$ proceeds in a standard way, evaporative cooling is highly optimized by performing it in an optical trap that is dynamically shaped by utilizing the time-averaged potential of a single laser beam moving rapidly in one dimension. We also produce large $(>{10}^{6})$ atom number BECs and successfully model the evaporation dynamics over more than three orders of magnitude in phase space density. Our method provides a simple and general approach to solving the problem of long production times of quantum degenerate gases.

DOI
10.1103/physreva.93.043403

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We report on a general method for the rapid production of quantum degenerate gases. Using 174Yb^{174}\mathrm{Yb}174Yb, we achieve an experimental cycle time as low as 1.6--1.8 s for the production of Bose-Einstein condensates (BECs) of (0.5−−1)\ifmmode×\else\texttimes\fi105(0.5--1)\ifmmode\times\else\texttimes\fi{}{10}^{5}(0.5−−1)\ifmmode×\else\texttimes\fi105 atoms. While laser cooling to 30\ensuremathμK30\phantom{\rule{0.16em}{0ex}}\ensuremath{\mu}\mathrm{K}30\ensuremathμK proceeds in a standard way, evaporative cooling is highly optimized by performing it in an optical trap that is dynamically shaped by utilizing the time-averaged potential of a single laser beam moving rapidly in one dimension. We also produce large (>106)(>{10}^{6})(>106) atom number BECs and successfully model the evaporation dynamics over more than three orders of magnitude in phase space density. Our method provides a simple and general approach to solving the problem of long production times of quantum degenerate gases.
We report on our progress towards realization of a toroidal trap for Bose-Einstein condensates (BECs) of 87Rb using a time-averaged optical dipole potential. To achieve BEC we utilize a 20 W laser operating at 1064 nm to form an optical dipole trap in which we evaporatively cool the gas. Loading of the dipole trap is achieved by overlapping the dipole beams with a standard magnetic optical trap. Evaporative cooling is first performed in a single beam trap, followed by compression and additional confinement with a second orthogonal beam. This prevents stagnation of the evaporative cooling cycle, and allows the gain of 105 in phase space density necessary to achieve quantum degeneracy.
We report on our progress towards realization of a toroidal trap for Bose-Einstein condensates (BECs) of 87Rb using a time-averaged optical dipole potential. To achieve BEC we utilize a 20 W laser operating at 1064 nm to form an optical dipole trap in which we evaporatively cool the gas . Loading of the dipole trap is achieved by overlapping the dipole beams with a standard magnetic optical trap. Evaporative cooling is first performed in a single beam trap, followed by compression and additional confinement with a second orthogonal beam. This prevents stagnation of the evaporative cooling cycle, and allows the gain of 105 in phase space density necessary to achieve quantum degeneracy.
A Bose–Einstein condensate with a large atom number is an important experimental platform for quantum simulation and quantum information research. An optical dipole trap is the a conventional way to hold the ultracold atoms, where an atomic cloud is evaporatively cooled down before reaching the Bose–Einstein condensate. A carefully designed trap depth controlling curve is typically required to realize the optimal evaporation cooling. We present and demonstrate a simple way to optimize the evaporation cooling in a crossed optical dipole trap. A polyline shape optical power control profile is easily obtained with our method, by which a pure Bose–Einstein condensate with atom number 1.73 × 105 is produced. Theoretically, we numerically simulate the optimal evaporation cooling using the parameters of our apparatus based on a kinetic theory. Compared to the simulation results, our evaporation cooling shows a good performance. We believe that our simple method can be used to quickly realize evaporation cooling in optical dipole traps.
We report a rapid evaporative cooling method using a hybrid trap which is composed of a quadrupole magnetic trap and a one-beam optical dipole trap. It contains two kinds of evaporative coolings to reach the quantum degeneracy: initial radio-frequency (RF) enforced evaporative cooling in the quadrupole magnetic trap and further runaway evaporative cooling in the optical dipole trap. The hybrid trap does not require a very high power laser such as that in the traditional pure optical trap, but still has a deep trap depth and a large trap volume, and has better optical access than the normal magnetic trap like the quadrupole-Ioffe-configuration (QUIC) cloverleaf trap. A high trap frequency can be easily realized in the hybrid trap to enhance the elastic collision rate and shorten the evaporative cooling time. In our experiment, pure Bose—Einstein condensates (BECs) with about 1 × 10 5 atoms can be realized in 6 s evaporative cooling in the optical dipole trap.
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 spontaneous-force magneto-optical trap (MOT), the (aberrated) 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 ${\mathrm{s}}^{\ensuremath{-}1}$) 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 107{10}^{7}107 atoms should be obtained in much less than 1s1\phantom{\rule{0.3em}{0ex}}\mathrm{s}1s starting from an ordinary MOT setup.