Magneto-optical trap
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A magneto-optical trap (abbreviated MOT) is a device that cools down atoms to temperatures near absolute zero and traps them at a certain place using magnetic fields and circularly polarised laser light. Charged particles can be trapped in a Penning trap or a Paul trap using a combination of electric and magnetic fields, but these traps do not work for neutral atoms. In a magneto-optical trap, neutral atoms can be cooled down and stored by using the optical force of laser light.
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[edit] Laser cooling
Photons in a laser beam can transfer their momentum to atoms through excitation followed by spontaneous emission. Detuned laser beams can be used to decrease the momentum of the atoms. Lower momentum means lower kinetic energy and thus lower temperature: the atoms are cooled down. The laser light must be detuned for the following reason. For the atoms to be cooled down, the atoms moving towards the laser should be slowed down, but the atoms moving away from it should not be accelerated. Atoms moving towards the laser source or away from it see the laser light in a different wave length due to the Doppler effect (red shift and blue shift). Atoms have certain resonance frequencies for atomic transitions. Light with the resonance frequency can excite the atom. If the laser is detuned (its wavelength is increased), the atoms moving away from it will not see the resonance frequency, but the particles moving towards the light source will see the resonance frequency, can absorb quanta and will be cooled down. Six circularly polarised laser beams are used for the setup of a magneto-optical trap: three pairs of beams going in the opposite direction. The laser beams cool the atoms in the x-, y- and z-directions.
[edit] Implementing the trap
Using laser cooling, the atoms can be cooled down, but they cannot be kept in place. Diffusion will cause the cool atoms to move outside the laser beams. Therefore a mechanism is needed that pushes the atoms back towards the centre of the trap. In a MOT, magnetic fields are used to create a force field the strength of which depends on the position of the atoms.
The anti-helmholtz configuration of the coils is important because the magnetic field created by the wire will have a "zero-point" at which there is no field. Any gas particle moving out (axially or radially) from this point will experience a magnetic field gradient that, due to the magnetic dipole nature of the internal structure of the atom, forces it back to the center.
[edit] Application
Stepwise cooling:
- magnetic-optical trap
- magnetic trap with evaporative cooling
- Bose-Einstein condensate
[edit] References
- Phillips, William D. (1998): Laser cooling and trapping of neutral atoms, in: Reviews of Modern Physics 3 / 70, S. 721-741.
