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a Permanent Magnet Trap for Cold Atoms

J. J. Tollett

Published Jan 1, 1995• 0 citations
OpenAlex

Abstract

Approximately$4\\times10\\sp8$ ground state lithium atoms have been confined in a non-zero magnetic field minimum produced by six permanent magnets in an Ioffe configuration. These atoms have a kinetic temperature of 1.1 mK and a peak density of $8\\times10\\sp9$ cm$\\sp{-3}$ The trapped atom lifetime is 240 seconds, limited by collisions with background gas. The tightly confining fields generated through the use of permanent magnets create a trap which is an excellent environment for the study of quantum statistical effects, atomic collisions, and other ultra-low temperature phenomena.

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Quantum Electronics•Aug 31, 2011•6 citations
Approximately4times10sp84\\times10\\sp84times10sp8 ground state lithium atoms have been confined in a non-zero magnetic field minimum produced by six permanent magnets in an Ioffe configuration. These atoms have a kinetic temperature of 1.1 mK and a peak density of 8times10sp98\\times10\\sp98times10sp9 cmsp−3\\sp{-3}sp−3 The trapped atom lifetime is 240 seconds, limited by collisions with background gas. The tightly confining fields generated through the use of permanent magnets create a trap which is an excellent environment for the study of quantum statistical effects, atomic collisions, and other ultra-low temperature phenomena.
We report the trapping of neutral atoms in a permanent magnet trap. Approximately 1\ifmmode\times\else\texttimes\fi{}108{10}^{8}108 ground-state lithium atoms have been confined in a nonzero magnetic-field minimum produced by six permanent magnets in an Ioffe configuration. These atoms have a kinetic temperature of 1.1 mK and a peak density of approximately 3\ifmmode\times\else\texttimes\fi{}109{10}^{9}109 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$. The trapped-atom lifetime is 240 s, limited by collisions with background gas. This trap provides an environment in which quantum-statistical effects, atomic collisions, and other ultralow-temperature phenomena can be studied.
We have constructed a permanent-magnet trap that provides a tightly confining potential useful for producing dense ultracold atomic vapors.1 This trap will be used to investigate quantum-statistical effects of 6Li and 7Li. Permanent magnets can produce field strengths and gradients comparable to those of superconducting-magnet traps but with reduced complexity and greater optical access. Our trap uses an arrangement of magnets in an Ioffe configuration,2 which features a nonzero magnetic-field minimum. Atoms are trapped about this minimum through the interaction of their total magnetic moment with the trap's magnetic field. The nonzero minimum prevents trap- loss-producing spin-flip (Majorana) transitions.
The aim of our experiment is to explore two methods of creating an ultracold dipolar gas which can subsequently be used to simulate quantum phenomena. The first method is to sympathetically cool polar molecules. In this case, the molecules are overlapped with ultracold lithium atoms, thus allowing the two clouds to thermalise through elastic collisions. The second method is to electrically polarise ultracold lithium atoms using an electric field of approximately 1MV/cm. This involves placing the atoms between two high voltage electrodes. This thesis describes and characterises the setup used to produce, trap and transport a cloud of lithium-7 atoms. The setup consists of a lithium oven, Zeeman slower, magneto-optical trap (MOT) and magnetic trap. Up to 2.3×10^8 atoms are loaded into the MOT with an initial temperature of 1.3 mK. By implementing a compressed MOT phase the temperature is reduced to 0.75 mK. Before transport, 23% of the MOT atoms are transferred into the magnetic trap, which has a lifetime of 1.53±0.01 s in the MOT chamber. Using a motorised translation stage to move the magnetic trapping coils, atomic transport over a distance of 44 cm from the MOT chamber to the science chamber has been demonstrated. The transport efficiency is 41%. In the science chamber the lifetime of the magnetic trap has been measured as 18.5±0.7 s. Experiments to optimise the absorption imaging system have also been carried out, highlighting the fact that a time and position dependent magnetic field is present after the trapping coils switch off. The feasibility of producing a 1MV/cm electric field has been investigated. By using indium tin oxide coated glass electrodes in an adjustable electrode mount, an electric field of approximately 0.2MV/cm has been generated. These electrodes were subsequently replaced with super-polished stainless steel electrodes which generated a field of 0.38MV/cm.
Lately there has been rapid progress in techniques for trapping samples of neutral atoms at elevated densities and extremely low temperatures. Now a group at the Joint Institute for Laboratory Astrophysics in Boulder, Colorado, has used a new type of magnetic trap to confine rubidium atoms at the lowest kinetic temperature yet recorded for an atomic gas. Compelling motivations for this effort include the use of such ultra-cold gases in high-precision spectroscopy, low-energy tests of the "standard model" of particle physics, and studies of atomic collisions at very low relative velocities. Particularly exciting is the prospect of studying quantum collective phenomena such as Bose-Einstein condensation (p37).
The purpose of our collaboration is to study ultracold collisions in a gas sample of atomic lithium. The associated experimental context is a scheme for producing such a gas sample in conditions where Bose-Einstein condensation (BEC) should be observable in the case of the {sup 7}Li isotope or Fermi degenerate behavior for the {sup 6}Li isotope. The plan is to load 10{sup 5}-10{sup 9} Li atoms from a laser cooled atomic beam into a magnetic trap with a diameter of about 1 mm. The fundamental limitation to achieving high density will be collisions of ground state atoms. We are interested in the loss of atomic density in a magnetic trap due to two-body dipolar collisions and spin-exchange collisions. Lithium atoms offer important advantages relative to other alkali atoms from the theoretical point of view. In the first place there is the greater accuracy of the available long-range potential parameters. Secondly, one avoids a complication of the heavier alkalis connected with the unknown spin-spin term following in second order from the electronic spin-orbit couplings In this contribution we present preliminary results for the above loss-mechanisms.
For the first time ultra-cold thulium atoms were trapped in a magnetic quadrupole trap with a small field gradient (20 Gs cm -1 ). The atoms were loaded from a cloud containing 4 × 10 5 atoms that were preliminarily cooled in a magneto-optical trap to the sub-Doppler temperature of 80 μK. As many as 4 × 10 4 atoms were trapped in the magnetic trap at the temperature of 40 μK. By the character of trap population decay the lifetime of atoms was determined (0.5 s) and an upper estimate was obtained for the rate constant of inelastic binary collisions for spin-polarised thulium atoms in the ground state (g in < 10 -11 cm 3 s -1 ).