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Yunnan Daxue xuebao. Shehui kexue ban

Basic notions of atom cooling and trapping——the Magneto-optical Trap (MOT)

Xiao Zhang

Published Jan 1, 2003• 0 citations
OpenAlex

Abstract

The theory of light force and the basic notions of atom cooling and trapping are discussed.The techrigue is by using a magnetic field gradient produced by a quadrupole field and three pairs of circularly polarized,counterpropagating optical beams.Then the atoms in the trap will be cooled by a dissipative force,and will be trapped in the center of the trap by a restoring force.The methods make it possible to realize BEC (BoseEinstein Condensate).

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•Jan 1, 2009•2 citations
The theory of light force and the basic notions of atom cooling and trapping are discussed.The techrigue is by using a magnetic field gradient produced by a quadrupole field and three pairs of circularly polarized,counterpropagating optical beams.Then the atoms in the trap will be cooled by a dissipative force,and will be trapped in the center of the trap by a restoring force.The methods make it possible to realize BEC (BoseEinstein Condensate).
We demonstrate a new magneto-optical trap (MOT) configuration using a simple pair of crossed wires rotated at 45 deg and an appropriate bias field to generate a MOT of >10^8 atoms. The same pair of wires, with slightly adjusted control parameters, is then used to magnetically trap the atoms and cool them via forced evaporative cooling into a Bose-Einstein condensate (BEC) with >10^4 atoms. We present the theoretical framework for generating a quadrupole field using a pair of crossed wires with arbitrary rotation angle, along with the atom chip design and fabrication. Finally, we describe the experimental protocols required for BEC production using only a single crossed-wire atom chip.
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.
This chapter discusses effective application of low power laser radiations for cooling and use of magnetic field gradient for trapping the atoms. It also discusses the basic principle of operation of Magneto-Optic Traps. Schawlow first showed in the seventies that the optical force of laser radiation can be used to decelerate an atom. This can be accomplished by the absorption of a photon by an atom through a process of stimulated absorption if the laser radiation has a frequency resonant with the energy level difference of the atom. Atoms can be trapped by static magnetic field or a combination of magnetic fields in multipolar, Helmholz or anti-Helmholz configuration or Joffe configuration. Since the atom beams are coherent in nature any beam of atoms from a Bose Einstein condensate can act like an atom laser having high energy that will have very strong penetrating power because of the coherent character of the beam.
We realized two species of magneto-optical traps (MOTs) by using linearly-polarized cooling beams. A mirror MOT was realized by using pairs of σ-π polarized cooling beams. Since the linearly-polarized field could act as opposite circular polarizations, atoms could be loaded on the mirror MOT with the elimination of one quarter-wave plate (QWP) among the four QWPs. However, for a typical MOT, counter-propagating cooling beams had to have opposite circular polarizations. The number of trapped atoms depended strongly on the polarization and optical alignment of the cooling beams when linear polarization with the elimination of one QWP was used. By using this anomalous alignment, we could increase the fluorescence of the mirror MOT. Furthermore, we were also able to realize a six-beam MOT with a configuration of pairs of orthogonal linearly-polarized cooling beams. The direction of the polarization depended strongly on the magnetic field for trapping.
We report on the observation of a new laser trap for neutral atoms. It uses three orthogonal pairs of counterpropagating laser beams having linear polarizations at 45° and a quadrupole magnetic field. 10 8 cesium atoms were thus confined in a 0.15 mm 3 volume at a temperature of 60 μK, a factor of 2 below the Doppler cooling limit. We interpret this trapping as being due to the new magneto-optical force recently observed by Grimm et al. and which is essentially a dipole force rectified by the magnetic field. This trap opens new possibilities for increasing the phase-space density of laser-cooled atoms.