192 Laser cooling and trapping
Fig. 9.9 (a) The mechanism of a
magneto-optical trap illustrated for the
case of an atom with aJ =0to
J= 1 transition. In the magnetic field
gradient the Zeeman splitting of the
sub-levels depends on the atom’s posi-
tion. Two counter-propagating beams
of circularly-polarized light illuminate
the atom and the selection rules for
transitions between the Zeeman states
lead to an imbalance in the radiative
force from the laser beams that pushes
the atom back towards the centre of
the trap. (Not to scale; the Zeeman
energy is much smaller than the opti-
cal transition energy.) (b) A magneto-
optical trap is formed from three or-
thogonal pairs of laser beams, as in the
optical molasses technique, that have
the requisite circular polarization states
and intersect at the centre of a pair
of coils with opposite currents. The
small arrows indicate the direction of
the quadrupole magnetic field produced
by the coils (as shown in more detail in
Fig 9.8).
(a)(b)CoilsCoils