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arXiv:1305.2313 · Journal of Physics B Atomic Molecular and Optical Physics

Accelerated thermalization of39K atoms in a magnetic trap with superimposed optical potential

Dipankar Nath, R Kollengode Easwaran, G Rajalakshmi, C S Unnikrishnan

Published Jul 30, 2013• 0 citations
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Abstract

We report the rapid accelerated thermalisation of Potassium 39 K atoms loaded in a magnetic trap, in the presence of a single dipole trap beam. More than an order of magnitude reduction in the thermalisation time, to less than a second, is observed with the focused off- resonant beam occupying only 0.01% of the volume of the magnetic trap. The cold atoms are loaded from a Magneto-Optical Trap(MOT) of 39 K that has gone through a compressed MOT and sub-Doppler cooling stage. The atoms are prepared in the magnetically stretched |F = 2, mF = 2> state prior to loading into the hybrid trap. We also report a direct loading of 39 K atoms, prepared in the state |F = 1>, into a single beam dipole trap.

DOI
10.1088/0953-4075/46/15/155303

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We report the rapid accelerated thermalisation of Potassium 39 K atoms loaded in a magnetic trap, in the presence of a single dipole trap beam. More than an order of magnitude reduction in the thermalisation time, to less than a second, is observed with the focused off- resonant beam occupying only 0.01% of the volume of the magnetic trap. The cold atoms are loaded from a Magneto-Optical Trap(MOT) of 39 K that has gone through a compressed MOT and sub-Doppler cooling stage. The atoms are prepared in the magnetically stretched |F = 2, mF = 2> state prior to loading into the hybrid trap. We also report a direct loading of 39 K atoms, prepared in the state |F = 1>, into a single beam dipole trap.
We demonstrate the direct loading of a D2_22​ MOT of potassium-39 atoms from a cryogenic buffer gas beam source. We load 10810^8108 atoms in a 10 ms pulse, with no degradation in performance up to a 10 Hz repetition rate. Observed densities reach ∼1011\sim10^{11}∼1011 atoms/cm3^33 in a single pulse, achieved with no sub-Doppler cooling or transverse compression. This system produces an ideal starting point for ultracold atom experiments where high experimental repetition rates are desirable and initial high densities are critical. Extension to other atomic species (e.g., refractory metals) that present technical challenges to high-yield production using oven-based sources is straightforward.
In this work, we have applied sub-Doppler laser cooling to a 39 K magneto-optical trap in order to load a 1071 nm crossed optical dipole trap. The number of atoms loaded into the dipole trap was characterized as a function of the frequency and intensity of the cooling and repump laser beams. For the optimum conditions, the dipole trap has about 2 × 10 6 atoms at an atomic density of 2 × 10 12 cm −3 , with a temperature of about 10 µK. This technique is a very simple procedure to load a 39 K optical dipole trap without a previous magnetic evaporative cooling step and may find application in other atomic physic systems.
The main goal of this research is to cool potassium-41 atoms to ultracold temperatures (<10 μK) in order to study the spin dynamics of atom to atom interactions. However, our current goal is to finish constructing the apparatus that will allow us to confine and cool these atoms in three dimensions. This apparatus will cool and confine the potassium atoms in 3 steps. First, the atoms will be cooled and trapped in our 3D magneto-optical trap. This step of the experiment will cool the atoms near the Doppler-cooling limit, which for potassium atoms is 145 μK. Second, atoms will be loaded into a FORT (far off resonance trap) with an expected depth of 25 μK. In order to cool the atoms below the Doppler-cooling limit, we will use sub-Doppler cooling techniques. What is particularly unique about our experiment is that we plan to use an octupole magnetic field to achieve sub-Doppler cooling. In typical laser cooling experiments sub-Doppler temperatures are achieved when the quadrupole magnetic field is off, which leaves the atoms untrapped. Third, once the atoms have been loaded into the FORT one last cooling mechanism will be implemented. This mechanism is referred to as evaporative cooling and will cool the atoms below 10 μK.
We discuss our experimental results on forced evaporative cooling of cold rubidium 87 Rb atoms to quantum degeneracy in an Optical Dipole Trap. The atoms are first trapped and cooled in a magneto-optical trap (MOT) loaded from a continuous beam of cold atoms [1]. More than 10 10 atoms are trapped in the MOT and then about 10 8 atoms are transferred to a Quasi-Electrostatic Trap (QUEST) formed by tightly focused CO2 laser (λ = 10.6μ m ) beams intersecting at their foci in an orthogonal configuration in the horizontal plane. Before loading the atoms into the dipole trap, the phase-space density of the atomic ensemble was increased making use of sub-doppler cooling at large detuning and the temporal dark MOT technique. In a MOT the phase-space density of the atomic ensemble is six orders of magnitude less than what is required to achieve quantum degeneracy. After transferring atoms into the dipole trap efficiently, phase-space density increases by a factor of 10 3 . Further increase in phase-space density to quantum degeneracy is achieved by forced evaporative cooling of atoms in the dipole trap. The evaporative cooling process involves a gradual reduction of the trap depth by ramping down the trapping laser intensity over a second. The temperature of the cold atomic cloud was measured by time-of-flight (TOF) technique. The spatial distribution of the atoms is measured using absorption imaging. We report results of evaporative cooling in a single beam and in a crossed double-beam dipole traps. Due to the large initial phase space density, and large initial number of atoms trapped, the quantum phase transition occurs after about 600 ms of evaporative cooling in our optimized crossed dipole trap.
We report a method for loading cold atoms into an optical trap with high initial phase-space density (PSD). When the trap beam is overlapped with atoms in optical molasses of optimized parameters including large cooling beam detuning compared with conventional detuning used for a magneto-optical trap (MOT), more than $3\ifmmode\times\else\texttimes\fi{}{10}^{6}$ rubidium atoms with an initial temperature less than 20 $\ensuremath{\mu}\mathrm{K}$ are loaded into a single beam trap. The obtained maximum initial PSD is estimated to be $1.1\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}$, which is one or two orders of magnitude greater than that achieved with the conventional loading into an optical trap from atoms in a MOT. The proposed method is promising for creating a quantum gas with a large number of atoms in a short evaporation time.
We report the sub-Doppler deep-cooled three-dimensional magneto-optical trap (3D MOT) of the fermionic 40K and bosonic 39K isotopes of potassium loaded by a very compact 2D+ MOT with a novel optical design feature. The set-up is meant for studies on the quantum dynamics of a few fermionic and bosonic atoms in an optical dipole trap near and well within quantum degeneracy. The loading rate and atom numbers achieved in the compact simple set-up are comparable to those in the previous set-ups with more elaborate vacuum design. We attained relatively low temperatures of 34 and 30 µK for 39K and 40K after the sub-Doppler cooling process.