Optical trap

Magneto-optical trap

A magneto-optical trap (abbreviated MOT) is a device that cools down non-charged 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 (non-charged) can be cooled down and stored by using the optical force of laser light. A MOT requires the atom to have a laser cooling transition in order to work. A typical MOT for Sodium atoms can trap and cool the atoms down to 300 μK or 0.0003 degrees above absolute zero.

Laser cooling

Photons in a laser beam can transfer their momentum to atoms through excitation followed by spontaneous emission. By detuning a laser beam, a laser is shifted away from an atomic energy resonance (usually shifted below the resonant energy). Detuned laser beams can be used to decrease the momentum of the atoms travelling directly towards the laser; the laser light is doppler shifted into resonance with the atom. 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 wavelength due to the Doppler effect (red shift and blue shift). Six circularly polarised laser beams are used for the setup of a magneto-optical trap: three pairs of perpendicular beams (the pair going in opposite directions). The laser beams cool the atoms in the x-, y- and z-directions. Often, a single laser properly tuned to cool one transition is split into 3 beams, and each beam after leaving the trap volume is passed through a half-wave plate before being passed into the trap a second time to change the circular polarisation.

Most often, since the MOT relies on spontaneous emission, the atom may decay to a different electronic state(for example, a different hyperfine ground state) which cannot be cooled by the cooling beams. A repump laser beam, or multiple repump beams, tuned to the transition(s) of the untrapped electronic state(s) is used to bring the atoms back in the cooling transition.

Most alkali atoms have two hyperfine ground states. While one of the ground states is used for the cooling transition, a repump is required to bring atoms which decayed to the other ground state back into the cooling cycle. In Sodium, one has the S1/2 F=1 and F=2 ground states which can couple to the P3/2 F'=0,1,2,3 excited states by the 589 nm D2 transition. The F=2 to F'=3 transition is used for cooling, while the F=1 to F'=2 transition is used to repump the atoms that decay into the F=1 hyperfine ground state.

Function of the trap

Using laser cooling with 3 perpendicular pairs of intersecting beams, termed an optical molasses, the atoms can be cooled down, but they cannot be kept in place deterministically. Slowly moving cooler atoms can drift out of the center as they will have too low a velocity to be in resonance with the red-detuned beams. In a MOT, magnetic fields are used to create a force field whose strength depends on the position of the atoms. Magnetic forces are not used in the MOT whatsoever. It is the spin-magnetic field coupling causing Zeeman-splitting that gives the force. As an atom travels farther and farther away from the magnetic field minimum, it's energy levels are split more and more. As the levels are split, an energy level of interest gets closer and closer to the laser's detuned frequency. This improves the laser cooling efficiency. As such, a "space dependent force" is indirectly created.

The anti-helmholtz configuration of the coils is a pair of two current loops with opposite current. This is important because the magnetic field created by the two coils of current will have a "zero-point" in the middle between them at which there is no field--more importantly, a minimum in the field, and thus a minimum energy state for the neutral atoms. Any particle moving out (axially or radially) from this point will experience a magnetic field gradient that causes the Zeeman splitting necessary for the MOT to function fully.

Application

A MOT of 133Cs was used to make some of the best measurements of CP violation.

MOTs tuned to different atoms can measure relative quantities of different isotopes.

Stepwise cooling:

  1. magnetic-optical trap
  2. magnetic trap with evaporative cooling
  3. Bose-Einstein condensate

References

Metcalf, Harold J. and Straten, Peter van der. Laser Cooling and Trapping. 1999 Springer-Verlag New York, Inc.

Foot, C.J. Atomic Physics. 2005 Oxford University Press.

C. Monroe, W. Swann, H. Robinson, and C. Wieman. Very Cold Trapped Atoms in a Vapor Cell. Physical Review Letters volume 65 number 13 page 1571. 24 September 1990.

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