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Journal of Experimental and Theoretical Physics

On the possibility of using dark magneto-optical lattices to achieve Bose condensation of atoms

А. В. Тайченачев, Anatolii M Tumaikin, Valeriy I. Yudin

Published Jun 1, 1998• 0 citations
Publisher

Abstract

We show that in dark magneto-optical lattices, effects associated with the Bose statistics of atoms can be observed even at laser cooling temperatures (10 −4 –10 −6 K), which exceed evaporative cooling temperatures in magnetic traps by several orders of magnitude. Quasicondensation occurs, i.e., the wave function is formed over the distances on which atoms are localized near the bottom of a separate potential well. In addition, switching off the magnetic field adiabatically reduces the temperature significantly, as a result of which Bose condensation in the entire volume of the gas can be observed. We propose a configuration of the light and magnetic fields in which the shape of the three-dimensional magneto-optical potential is independent of the phases of the emerging light waves.

DOI
10.1134/1.558582

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We show that in dark magneto-optical lattices, effects associated with the Bose statistics of atoms can be observed even at laser cooling temperatures (10 −4 –10 −6 K), which exceed evaporative cooling temperatures in magnetic traps by several orders of magnitude. Quasicondensation occurs, i.e., the wave function is formed over the distances on which atoms are localized near the bottom of a separate potential well. In addition, switching off the magnetic field adiabatically reduces the temperature significantly, as a result of which Bose condensation in the entire volume of the gas can be observed. We propose a configuration of the light and magnetic fields in which the shape of the three-dimensional magneto-optical potential is independent of the phases of the emerging light waves.
It is shown that in the dark magneto-optical lattices the effects connected with the Bose-statistics of particles can be observed under the laser cooling temperature (10-4 - 10-6 K), that is orders of magnitude higher than the evaporative cooling temperature in magnetic traps. There appears the quasicondensation, when the wave function is formed at the localization distance of atoms in a single well. Apart from this, we show that the adiabatic reducing of the magnetic field leads to the increasing of the temperature and the Bose-condensation in the whole lattice can take place. The field configuration, where the form of the three-dimensional magneto-optical potential does not depend on phases of light waves, is found.
The motion of atoms in a dark magneto-optical lattice is considered. This lattice is formed by a non-uniformly polarized laser field in the presence of a static magnetic field. Cold atoms are localized in the vicinity of points where dark states are not destroyed by a magnetic field. As a result, the optical interaction tends to zero (dark or grey lattices). Depending on the field configuration such a lattice can exhibit one-dimensional (1D) or two-dimensional (2D) periodicity. It is shown that in 1D and 2D dark magneto-optical lattices the effects connected with the Bose statistics of particles can be observed under temperatures 10 -5 -10 -6 K and densities 10 11 -10 12 cm -3 , i.e. those that are usual for current experiments on laser cooling.
The motion of atoms in a dark magneto-optical lattice is considered. This lattice is formed by a nonuniformly polarized laser field in the presence of a static magnetic field. Cold atoms are localized in the vicinity of points where dark states are not destroyed by a magnetic field. As a result, the optical interaction tends to zero (dark or gray lattices). Depending on the field configuration, such a lattice can exhibit one, two, or three-dimensional periodicity. It is shown that in 1D and 2D, mark magneto- optical lattices the effects connected with the Bose- statistics of particles can be observed under the temperature 10-5 - 10-6K and densities 1011 - 1012cm-3, those are usual for current experiments on laser cooling.
 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.
We present a new type of lattice for trapping cold atoms in nearly-dark states where they weakly interact with light. The periodic potential is obtained with fields in a lin ⊥ lin configuration tuned on the blue side of a J → J transition and in the presence of a uniform static magnetic field. A numerical analysis performed for J = 4 demonstrates the occurrence of very small absorption widths and large populations in the lowest vibrational states. These lattices should be interesting for increasing the atomic density and for observing delocalized atomic wave functions.
Presents work on the realization of Bose-Einstein condensates (BEC) in optical and electro-magnetic traps. We have done some theoretical and experimental work on the project. We have proposed an optical funnel or a hollow beam, which are produced from a short micron-sized hollow optical fiber or a set-up of lenses. The cold atoms extracted from an magneto-optical trap experience effective Sisyphus cooling in the dark hollow beam. We calculate the potential barriers for atomic guiding in the dark hollow beam, and analyze the atomic losses during the guiding process. Inside the hollow beam atoms experience an efficient Sisyphus cooling and repumping-beam induced geometric cooling. We have estimated the total loss rate of the cold atoms, and it shows that an ultracold and dense Rb atomic sample with a 3D temperature of /spl sim/1 K can be obtained and the appearance of an optically-trapped Bose-Einstein condensation may be possible in this optical trap. We have also made a proposal of a micro-wave electro-magnetic trap, which is working at the frequency which is very far from the optical transition of the cold atoms. The trapping force comes from the interaction between the electro-magnetic induced magnetic dipole moment with the electro-magnetic field. Theoretical calculation shows that the force can be used to trap the cold atoms in the trap.