Experiments to Control Atom Number and Phase-Space Density in ...

Optical molasses do not provide a spatial confinement, and are thus a cooling
and not a trapping method.
2.9 Magneto-Optical Trap
While optical molasses provides an effective method to cool atoms, it cannot
confine atoms for an extended period of time; optical molasses is not an optical trap for
atoms. The extension of the idea of optical molasses to create a trap is called magneto-
optical trap (MOT) [37].
Adding a quadrupole magnetic field, created by a pair of anti-Helmholtz coils, to
the three pairs of counter-propagating laser beams creates a magneto-optical trap. A
schematic of the setup is shown in figure 2.15. The restoring force towards the center
of the trap is again provided by photons scattered from the laser beams. Because
the magnetic field defines a quantization axis, the polarization of the laser beams is
important. The beams have to be circularly polarized, where opposing beams have
opposite handedness [22, 35].
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Figure 2.15: Schematic of a magneto-optical trap setup. A pair of anti-Helmholtz coils
produces a quadrupole magnetic field. Three pairs of counter-propagating laser beams
with opposite circular polarization cause a restoring force, pushing atoms towards the
center of the magnetic quadrupole field. Figure courtesy of Gabriel Price.
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