9.3 The optical molasses technique 1850 200 400Fig. 9.4The atomic velocity distri-
bution produced by chirp cooling cae-
sium atoms with radiation from semi-
conductor diode lasers. The trace
shows the experimentally observed flu-
orescence from the atoms as the laser
frequency was scanned over a frequency
range greater than the initial Doppler
shift of the atoms in the atomic beam.
From Steane (1991).the light. Nowadays, chirp cooling of heavy alkalis such as rubidium
and caesium can be carried out by directly scanning the frequency of
infra-red semiconductor diode lasers—Fig. 9.4 shows the results of such
an experiment. It can be seen that the laser cooling sweeps atoms to
lower velocities to produce a narrow peak in the velocity distribution. It
is the spread of the velocities within this peak that determines the final
temperature, not the mean velocity of these atoms. The atoms have a
much smaller spread of velocities than at room temperature so they are
cold.
9.3 The optical molasses technique
In an atomic beam the collimation selects atoms moving in one direction
that can be slowed with a single laser beam. Atoms in a gas move in all
directions and to reduce their temperature requires laser cooling in all
three directions by the configuration of three orthogonal standing waves
shown in Fig. 9.5—the light along the Cartesian axes comes from the
same laser and has the same frequency. At first, you might think that
this symmetrical arrangement has no effect on an atom since there are
equal and opposite forces on an atom. However, the radiation forces from
the laser beams balance each other only for a stationary atom, which is
what we want to achieve. For a moving atom the Doppler effect leads
to an imbalance in the forces. Figure 9.5(b) shows the situation for a
two-level atom in a pair of counter-propagating beams from a laser with
a frequency below the atomic resonance frequency (red frequency detun-
ing). Consider what happens in the reference frame of an atom moving
towards the right, as shown in Fig. 9.5(c). In this frame the Doppler
effect leads to an increase in the frequency of the laser beam propagat-
ing in the direction opposite to the atom’s velocity. This Doppler shift
brings the light closer to resonance with the atom and thereby increases
the rate of absorption from this beam. This leads to a resultant force
that slows the atom down.^14 Expressed mathematically, the difference^14 A similar situation arises for move-
between the force to the right and that to the left is ment in any direction.