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Bulletin of the American Physical Society

Experiments with atoms captured in a dark light trap

Joffe, Kendall B. Davis, Wolfgang Ketterle

Published May 1, 1993• 0 citations
OpenAlex

Abstract

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.

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Quantum Electronics and Laser Science Conference•Nov 13, 2002•0 citations
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 describe the setup to create a large Bose-Einstein condensate containing more than 120 x 10(6) atoms. In the experiment a thermal beam is slowed by a Zeeman slower and captured in a dark-spot magneto-optical trap (MOT). A typical dark-spot MOT in our experiments contains 2.0 x 10(10) atoms with a temperature of 320 microK and a density of about 1.0 x 10(11) atoms/cm(3). The sample is spin polarized in a high magnetic field before the atoms are loaded in the magnetic trap. Spin polarizing in a high magnetic field results in an increase in the transfer efficiency by a factor of 2 compared to experiments without spin polarizing. In the magnetic trap the cloud is cooled to degeneracy in 50 s by evaporative cooling. To suppress the three-body losses at the end of the evaporation, the magnetic trap is decompressed in the axial direction.
A detailed experimental and theoretical investigation of a magneto-optical trap for caesium atoms is presented. Particular emphasis has been placed on achieving high spatial number densities and low temperatures. Optimizing both of these together enables efficient evaporative cooling from a conservative trap, a procedure which has recently led to the first observations of Bose-Einstein condensation in a dilute atomic vapour. The behaviour of a magneto-optical trap is nominally determined by four independent parameters: the detuning and intensity of the light field, the magnetic field gradient and the number of trapped atoms. A model is presented which incorporates previous treatments into a single description of the trap that encompasses a wide range of its behaviour. This model was tested quantitatively by measuring the temperature of the cloud and its spatial distribution as a function of the four parameters. The maximum density was found to be limited both by the reabsorption of photons scattered within the cloud and by a reduction of the confining force at small light shifts. The nonlinear variation with position of the restoring force was found to be significant in limiting the number of atoms confined to a high density. A maximum density in phase space (defined as the number of atoms in a box with sides of dimension one thermal de Broglie wavelength) of (1.5 ± 0.5) x 10-5 was observed, with a spatial density of 1.5 x 1011 atoms per cm3. Cold collision losses from a caesium magneto-optical trap have been studied with the purpose of assessing their influence on spatial densities. In contrast to previous measurements of similar quantities, these measurements did not require the use of an ultra-low (< 10-10 Torr) background vapour pressure. The dependence of the cold collision loss coefficient β on the trapping intensity was measured to permit identification of the different cold collision processes. The largest loss rates observed were those due to hyperfine structure-changing collisions, with a coefficient β = (2±1) x 10-10cm3s-1. A study is presented of a modified magneto-optical trap in which a fraction of the population is shelved into a hyperfine level that does not interact with the trapping light. In this so-called "dark" magneto-optical trap, improved densities of nearly 1012cm-3 have been previously reported for sodium. The application of the technique to caesium is not straightforward due to the larger excited state hyperfine splittings. A simple theory for caesium is presented and its main predictions verified by measurements of density, number and temperature. A density of nearly 1012cm,-3 was indeed obtained but at a temperature substantially higher than in the conventional magneto-optical trap.
 A detailed experimental and theoretical investigation of a magneto-optical trap for caesium atoms is presented. Particular emphasis has been placed on achieving high spatial number densities and low temperatures. Optimizing both of these together enables efficient evaporative cooling from a conservative trap, a procedure which has recently led to the first observations of Bose-Einstein condensation in a dilute atomic vapour. The behaviour of a magneto-optical trap is nominally determined by four independent parameters: the detuning and intensity of the light field, the magnetic field gradient and the number of trapped atoms. A model is presented which incorporates previous treatments into a single description of the trap that encompasses a wide range of its behaviour. This model was tested quantitatively by measuring the temperature of the cloud and its spatial distribution as a function of the four parameters. The maximum density was found to be limited both by the reabsorption of photons scattered within the cloud and by a reduction of the confining force at small light shifts. The nonlinear variation with position of the restoring force was found to be significant in limiting the number of atoms confined to a high density. A maximum density in phase space (defined as the number of atoms in a box with sides of dimension one thermal de Broglie wavelength) of (1.5 ± 0.5) x 10-5 was observed, with a spatial density of 1.5 x 1011 atoms per cm3. Cold collision losses from a caesium magneto-optical trap have been studied with the purpose of assessing their influence on spatial densities. In contrast to previous measurements of similar quantities, these measurements did not require the use of an ultra-low ( A study is presented of a modified magneto-optical trap in which a fraction of the population is shelved into a hyperfine level that does not interact with the trapping light. In this so-called dark magneto-optical trap, improved densities of nearly 1012cm-3 have been previously reported for sodium. The application of the technique to caesium is not straightforward due to the larger excited state hyperfine splittings. A simple theory for caesium is presented and its main predictions verified by measurements of density, number and temperature. A density of nearly 1012cm,-3 was indeed obtained but at a temperature substantially higher than in the conventional magneto-optical trap.
Summary form only given. We report the observation of Bose-Einstein condensation of a very large number of /sup 87/Rb atoms in a time orbiting potential (TOP) trap. We found that for such large samples, efficient cooling in the TOP trap is complicated to some extent by undesired losses of atoms at the orbiting magnetic field zero point, but that these difficulties can be overcome. In our experiments, we first load /sup 87/Rb atoms from a slow beam into a magneto-optic atom trap (MOT). Second, we then compress the atoms with a forced dark MOT, further cool them with optical molasses, and optically pump them into the desired state.
Summary form only given.We have established an experimental and theoretical program focused on understanding photon atom interactions in condensates with the aim of probing the properties of condensates as well as testing our models of the interactions themselves. With a combination of laser and evaporative cooling, we are routinely creating million atom Bose-Einstein condensates of sodium atoms in the newly designed 4D magnet. Our experimental setup incorporates a candlestick atomic beam source, which provides for recirculation of uncollimated sodium atoms, The laser cooling step involves a zero-crossing Zeeman slower, magneto-optic trap, and polarization gradient cooling. The laser-cooled atom cloud is loaded into the 4D magnetic trap and adiabatically compressed to high density and an evaporative cooling process is initiated. Using a RF magnetic resonance technique, hot atoms are ejected through magnetic spin flips. After formation of the condensate in the tightly confining trap, we manipulate the condensate by adiabatically changing the trapping potential and through interaction with photons. We have developed a technique for probing the condensates directly inside the trap using near-resonant absorption imaging. This imaging technique gives high sensitivity to details of the condensate wavefunction and to its surface structure. Images of pure condensates are compared to calculations of the condensate wavefunction based on a Hartree mean field description of the two-body interaction between condensate atoms.
Summary form only given. Bose-Einstein condensates (BEC) are generally produced by evaporative cooling of laser cooled atomic samples in magnetic traps. Though magnetic traps are indispensable for the production of a BEC, they pose restrictions on the possible studies with a BEC. Previous experiments of our group have demonstrated that it is possible to trap a BEC in the circular focus of a far-detuned infrared laser beam. This first realization of an optical dipole trap (ODT) for BEC was used to prepare and investigate spinor condensates of Na atoms in the F=1 hyperfine manifold. However, the initial peak density in this ODT after loading from the magnetic trap reached 10/sup 15/ atoms/cm/sup 3/, leading to rapid losses due to three-body collisions. To realize an ODT capable of storing large condensates for a long time, we have used a laser beam with an elliptical beam profile. The optical trap is loaded by superimposing the light field onto the magnetic trap and then slowly turning off the magnetic fields. The transfer efficiency is close to 100%, and thus more than 10/sup 7/ Bose-condensed atoms can be loaded into the ODT.