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Journal of Optics B Quantum and Semiclassical Optics

Magnetic trapping and evaporative cooling of metastable triplet helium

Norbert Herschbach, Paul J. J. Tol, Andrey Tychkov, W. Hogervorst, W. Vassen

Published Apr 1, 2003• 11 citations
Publisher

Abstract

A cloverleaf magnetic trap is loaded from a magneto-optical trap containing 2 × 10 9 helium atoms in the metastable 2 3 S 1 state. With optical molasses and a spin polarization pulse up to 1.5 × 10 9 atoms are magnetically trapped. The vacuum limited trap lifetime is ∼12 s. Compression in 0.5 s yields a density increase to 8 × 10 9 cm −3 . With rf-forced evaporation using an exponentially shaped frequency sweep with a 5 s time constant the phase space density is increased from 1 × 10 −7 to 4 × 10 −3 . The central density increases to 3 × 10 11 cm −3 , while an increase in elastic collision rate from 40 to 220 s −1 indicates runaway evaporation.

DOI
10.1088/1464-4266/5/2/360

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arXiv:physics/0210060•Physical Review A•May 29, 2003•14 citations
A cloverleaf magnetic trap is loaded from a magneto-optical trap containing 2 × 10 9 helium atoms in the metastable 2 3 S 1 state. With optical molasses and a spin polarization pulse up to 1.5 × 10 9 atoms are magnetically trapped. The vacuum limited trap lifetime is ∼12 s. Compression in 0.5 s yields a density increase to 8 × 10 9 cm −3 . With rf-forced evaporation using an exponentially shaped frequency sweep with a 5 s time constant the phase space density is increased from 1 × 10 −7 to 4 × 10 −3 . The central density increases to 3 × 10 11 cm −3 , while an increase in elastic collision rate from 40 to 220 s −1 indicates runaway evaporation.
We describe a powerful scheme which combines laser cooling on two transitions of metastable helium to obtain a high phase-space density. By running a sequence of a large 1083nm1083\phantom{\rule{0.3em}{0ex}}\text{nm}1083nm magneto-optical trap (MOT) and a compressed 389nm389\phantom{\rule{0.3em}{0ex}}\text{nm}389nm MOT, a density increase of more than one order of magnitude is achieved within 5ms5\phantom{\rule{0.3em}{0ex}}\text{ms}5ms. After compression, $8\ifmmode\times\else\texttimes\fi{}{10}^{8}$ atoms at a central density of $5\ifmmode\times\else\texttimes\fi{}{10}^{10}\phantom{\rule{0.3em}{0ex}}{\text{cm}}^{\ensuremath{-}3}$ remain, while the temperature of the cloud has been reduced from 1mK1\phantom{\rule{0.3em}{0ex}}\text{mK}1mK to 0.4mK0.4\phantom{\rule{0.3em}{0ex}}\text{mK}0.4mK. The resulting phase-space density $(4.1\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}6})$ is more than one order of magnitude higher than what we achieved by 1083nm1083\phantom{\rule{0.3em}{0ex}}\text{nm}1083nm laser cooling only.
A quantitative discussion of several interesting and attractive properties of magneto-optical traps is presented. Calculations are given for the trap depth, sample size, and intrinsic cooling capability of this especially simple example of a radiative force trap. The discussion is directed toward the cooling and confinement of metastable 23S neutral helium, and a new loading procedure suitable for this atom is proposed. It is planned to operate the trap and to cool atoms in it with 1.08-μm light from solid-state, single-mode lasers pumped by efficient diode lasers. Because of the dissipative nature of the radiative force, a trapped sample of atoms may be magnetically compressed without increasing its millikelvin optical temperature, so that a subsequent adiabatic expansion can cool it to the nanokelvin temperature range.
Summary form only given. The importance of helium atoms in the metastable state 2/sup 3/S/sub 1/ (He*, lifetime 8000 s) for studies of ultracold atomic clouds in the quantum degeneracy regime has been pointed out. For /sup 4/He* Bose-Einstein condensation (BEC) is expected, whereas the /sup 3/He isotope allows the study of a fermionic system, if the technique of sympathetic cooling is feasible. We plan to reach the BEC transition using forced evaporative cooling in our magnetostatic trap (MST) of cloverleaf geometry.
A magneto-optical trap for He(2{sup 3}S) metastable atoms has been constructed, utilizing superconducting magnetic gradient coils and a Ti:Sapphire ring laser for pumping the helium 2{sup 3}S-2{sup 3}P transition. The He(2{sup 3}S) atoms are produced by a weak rf discharge in helium gas at a temperature of 1.4K. The discharge products flow through a small orifice into the trap cell, where a fraction of the He(2{sup 3}S) atoms are trapped and ground state helium atoms are rapidly cryopumped by zeolite pellets that cover most of the cell bottom. Preliminary experiments suggest that {approximately}10{sup 6} atoms are trapped in a small volume at {approximately}1 mK, with a trap lifetime of 10-100 msec limited by resonantly-enhanced He(2{sup 3}S)-He(2{sup 3}P) Penning reactions. Ultimately, it is estimated that a substantial number of atoms can be held at ultra-low temperature in near-perfect vacuum, in a dc magnetic trap. Measurements of decay times of the trapped atoms should yield rate coefficients in the quantum regime for He(2{sup 3}S)-He(2{sup 3}P) and He(2{sup 3}S)-He(2{sup 3}S) Penning reactions, and perhaps the He(2{sup 3}S) radiative lifetime.
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 constructed a magneto-optical trap (MOT) for metastable triplet helium atoms utilizing the $2{}^{3}{S}_{1}\ensuremath{\rightarrow}3{}^{3}{P}_{2}$ line at 389 nm as the trapping and cooling transition. The far-red-detuned MOT (detuning $\ensuremath{\Delta}=\ensuremath{-}41\mathrm{MHz})$ typically contains few times 107{10}^{7}107 atoms at a relatively high $(\ensuremath{\sim}{10}^{9}{\mathrm{cm}}^{\ensuremath{-}3})$ density, which is a consequence of the large momentum transfer per photon at 389 nm and a small two-body loss rate coefficient $(2\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}10}{\mathrm{cm}}^{3}/\mathrm{s}<\ensuremath{\beta}<1.0\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}9}{\mathrm{cm}}^{3}/\mathrm{s}).$ The two-body loss rate is more than five times smaller than in a MOT on the commonly used $2{}^{3}{S}_{1}\ensuremath{\rightarrow}2{}^{3}{P}_{2}$ line at 1083 nm. Furthermore, laser cooling at 389 nm results in temperatures somewhat lower than those achieved using 1083 nm. The 389-nm MOT exhibits small losses due to two-photon ionization, which have been investigated as well.