a r X i v :0804.0060v 1 [c o n d -m a t .o t h e r ] 1 A p r 2008Fast,Runaway Evaporative Cooling to Bose-Einstein Condensation in Optical TrapsChen-Lung Hung,Xibo Zhang,Nathan Gemelke,Cheng ChinJames Frank Institute and Physics Department,The University of Chicago,IL 60637(Dated:April 2,2008)We demonstrate a simple scheme to achieve fast,runaway evaporative cooling of optically trapped atoms by tilting the optical potential with a magnetic field gradient.Runaway evaporation is possible in this trap geometry due to the weak dependence of vibration frequencies on trap depth,which preserves atomic density during the evaporation process.Using this scheme,we show that Bose-Einstein condensation with ∼105cesium atoms can be realized in 2∼4s of forced evaporation.The evaporation speed and energetics are consistent with the three-dimensional evaporation picture,despite the fact that atoms can only leave the trap in the direction of tilt.PACS numbers:67.85.Hj,64.70.fm,67.85.-dThe possibility to manipulate Bose-Einstein conden-sates (BECs)and degenerate Fermi gases of cold atoms in optical traps opens up a wide variety of exciting re-search;prominent examples include spinor condensates [1],Feshbach resonance in cold collisions [2],and BECs of molecules [3,4].In many early experiments,condensates were first created in a magnetic trap and subsequently transferred to an optical dipole trap.These experiments could be greatly simplified after direct evaporation to BEC in optical traps was demonstrated [5].In this paper,we describe a further improvement on dipole-trap based evaporation,which allows for runaway cooling without significant increase in trap complexity.Evaporative cooling proceeds by lowering the depth of a confining potential,which allows atoms with high kinetic energy to escape and the remaining particles to acquire a lower temperature and higher phase space den-sity through rethermalization.Starting from a sample of precooled atoms in a dipole trap,one can in principle perform forced evaporative cooling on optically trapped atoms by constantly reducing the trap depth until quan-tum degeneracy is reached.This method has been suc-cessful in creating rubidium BEC in a dipole trap,and has become a critical component in recent experiments on quantum gases of Cs [6],Li [7],K [8]and Yb [9].In all these experiments,forced evaporative cooling in the dipole trap is realized by reducing the intensity of the trapping beam,and consequently also the restoring forces.In later discussion,we will refer to this approach as trap-weakening scheme.Evaporative cooling in optical traps remains one of the most time-consuming and technically challenging steps in condensate production.Fundamentally,this is due to the fact that cooling by weakening the trapping potential in-evitably reduces the collision rate.Here runaway (accel-erating)evaporation is essentially impossible even with perfect evaporation efficiency and purely elastic collisions [10].Within experimentally accessible times,the trap-weakening method puts a severe limit on the maximum gain in phase space density one can reach.Several auxil-iary schemes have been successfully implemented in order to increase the evaporation speed,including the dimple trap [6]and a zoom lens system [11].These methods of-online)Trap-tilt based evaporation and ex-apparatus.(a)Trap depth U decreases when an gradient is applied to the optically trapped Apparatus for evaporation of cesium atoms (black crossed-beam dipole trap.A strong,slowly-varying gradient B ′(t )over-levitates the atoms with against gravitational pull mg and evap-upward.ten increase the complexity of the apparatus or require delicate optical alignment or manipulation.In this paper,we report a new and simple evaporative cooling scheme which can be immediately implemented in many existing experiments.Instead of reducing the intensity of the trapping beam,we reduce the trap depth by applying an external force on the optically trapped atoms.This trap-tilting method entails only a weak re-duction in confinement strength over a large range of po-tential depth and can significantly speed up the cooling process.Using this method,we demonstrate runaway evaporative cooling in a large volume dipole trap and reach Bose-Einstein condensation of cesium significantly faster than previous results [12].Finally,we comment on the conditions for runaway evaporation in a tilted trap and investigate the dimensionality of atomic energy se-lection in the evaporation.For this study,cesium atoms are first slowed by a Zee-man slower,collected in a magneto-optical trap (MOT)for 2s,molasses precooled,and finally cooled and spin polarized by degenerate Raman-sideband cooling (dRSC)[13]to the lowest hyperfine ground state |F =3,m F =3 ,where F is the total angular momentum and m F is the magnetic quantum number;the apparatus for dRSC follows that in [13].A crossed dipole trap and magnetic field gradient are employed to levitate and collect the
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