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arXiv:physics/0010065 · Physical Review A

Optical trapping of cold fermionic potassium for collisional studies

G. Roati, W. Jastrzȩbski, Andrea Simoni, Giovanni Carlo Modugno, M. Inguscio

Published Apr 16, 2001• 18 citations
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Abstract

We report on trapping of fermionic ${}^{40}\mathrm{K}$ atoms in a red-detuned standing-wave optical trap, loaded from a magneto-optical trap. Typically, unpolarized samples of ${10}^{6}$ atoms are loaded at a density of ${10}^{12}{\mathrm{cm}}^{\ensuremath{-}3}$ and a temperature of $65\ensuremath{\mu}\mathrm{K},$ and trapped for more than 1 s. The optical trap appears to be the proper environment for performing collisional measurements on the cold atomic sample. In particular, we measure the spherically averaged elastic collision rate by detecting the rethermalization following an intentional parametric heating of the atoms. We also measure spherically averaged inelastic two-body collision rates for atoms in the ground hyperfine states, through detection of trap losses.

DOI
10.1103/physreva.63.052709

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We report on trapping of fermionic 40K{}^{40}\mathrm{K}40K atoms in a red-detuned standing-wave optical trap, loaded from a magneto-optical trap. Typically, unpolarized samples of 106{10}^{6}106 atoms are loaded at a density of 1012cm\ensuremath−3{10}^{12}{\mathrm{cm}}^{\ensuremath{-}3}1012cm\ensuremath−3 and a temperature of 65\ensuremathμK,65\ensuremath{\mu}\mathrm{K},65\ensuremathμK, and trapped for more than 1 s. The optical trap appears to be the proper environment for performing collisional measurements on the cold atomic sample. In particular, we measure the spherically averaged elastic collision rate by detecting the rethermalization following an intentional parametric heating of the atoms. We also measure spherically averaged inelastic two-body collision rates for atoms in the ground hyperfine states, through detection of trap losses.
We present measurements of loading and loss rates for a vapor-cell optical trap of the two naturally occurring potassium isotopes 39K and 41K. The unresolved excited-state hyperfine structure makes trapping of K fundamentally different from trapping of the other alkalis and leads to an enhanced loading rate. We measure the loading rate as a function of laser intensity, beam size, and detuning and find that the results are in reasonable agreement with a simple rate-equation model for the loading process. The dependence of the loss rate on trapped-atom density determines the contribution to the loss rates from excited-state collisions. We find a substantial difference between the collisional loss rates for the two isotopes.
Summary form only only given. We report on trapping of /sup 40/K atoms at low temperatures and high densities in a far-detuned optical trap. The trap is a vertical standing-wave created with about 550 mW of radiation tuned 10 nm on the red of the D/sub 2/ transition, and is loaded from a sample held in a magneto optical trap (MOT) at sub-Doppler temperatures, with the help of a temporal dark-spot phase and a magnetic compression phase. We are able to trap as many as 5/spl times/10/sup 6/ atoms with a lifetime of 200 ms in the F=9/2 ground state. From the temperature of the trapped atoms, which is around 30 /spl mu/K, and the measured trap frequencies (1 kHz radial, 500 kHz axial) we estimate a density of 5/spl times/10/sup 12/ cm/sup -3/.
Collisions between ultra-cold (T << 1K) atoms confined in a magneto- optical trap (MOT) may result in the ejection of atoms from the trap. These collisional loss processes, which involve ground state/excited state collisions, fundamentally limit the density attainable in MOT traps. Although accurate calculations of collision rates between ultra-cold atoms require precise knowledge of the molecular potentials over a long range, the loss rate for Li was recently predicted to be significantly less than for the other alkali metals. Motivated by this result, we are measuring the collisional loss rate of Li in a vapor-cell MOT using a red-detuned laser beam to catalyze the collisions, similar to recent experiments using Rb and Cs. We will report our progress to measure the trap loss rate for {sup 6}Li and {sup 7}Li, which will be compared to investigate an isotopic dependence, as was recently observed for Rb.
We present a detailed analysis of the cold collision measurements performed in a high-gradient magneto-optical trap with a few trapped Cs atoms first presented in Ueberholz et al ( J. Phys. B: At. Mol. Opt. Phys. 33 (2000) L135 ). The ability to observe individual loss events allows us to identify two-body collisions that lead to the escape of only one of the colliding atoms (up to 10% of all collisional losses). Possible origins of these events are discussed here. We also observed strong modifications of the total loss rate with variations in the repumping laser intensity. This is explained by a simple semiclassical model based on optical suppression of hyperfine-changing collisions between ground-state atoms.
Sodium atoms have been loaded from a slowed beam into a dark magneto-optical trap which confines atoms mostly in the non-absorbing F=1 ground state. This method has allowed us to achieve samples of more than 10{sup 10} atoms with optical densities in excess of 150. Temperatures around 1.2 mK and densities around 10{sup 12} cm{sup -3} have been measured using weak probe absorption. At this density an elastic collisions rate is already approximately 100 Hz. Our goal is to reach the regime where the rate is high enough to allow evaporative cooling of the trapped sample enroute to Bose-Einstein Condensation, for which yet higher densities and lower temperatures are required. We report the current status of experiments underway to further increase the phase space density of the trapped atoms by cooling with polarization-gradient molasses and reloading them into a magnetic trap with a spherical quandrupole field.
We have produced a magneto-optical trap that simultaneously confines two different atomic species: sodium and potassium. A measurement of the sodium trap loss rate constant due to collisions with cold potassium atoms was performed. We extract this rate constant by measuring the temporal behavior of the loading process of the sodium trap in the presence and absence of cold potassium. The constant that is obtained can be qualitatively explained using simple existing models. Future applications of simultaneous trapping are discussed.