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9.7 The Sisyphus cooling technique 203

9.7 The Sisyphus cooling technique

9.7.1 General remarks

The dipole force experienced by atoms in a light field can be stronger
than the maximum scattering force becauseFdipoledoes not saturate
with increasing intensities (whereasFscattdoes), but stimulated pro-
cesses alone cannot cool atoms. To dissipate energy there must be some
spontaneous emission to carry away energy from the atoms—this is true
for all cooling mechanisms, e.g. Doppler cooling by the scattering force,
and it is particularly apparent for the process described in this section.
The first experimental evidence that Doppler cooling does not give a
complete description of the laser cooling in a standing wave came from
measurements of the velocity distribution of atoms by the direct time-of-
flight method shown in Fig. 9.16. When William Phillips and co-workers
carried out such measurements they were pleasantly surprised to find
that the optical molasses technique can cool atoms below the Doppler

Atoms

(a) (b)

Atoms fall

under gravity

Atoms

launched

upwards

Probe

laser beam

Probe

laser beam

Laser-cooled

atoms

Detector of

fluorescent photons

Detector of

fluorescent photons

Microwave

cavity

, height

of fountain

Fig. 9.16(a) The temperature of a sample of atoms that has been cooled by the optical molasses technique is measured by
turning off the six laser beams (not shown) so that the cloud of cold atoms falls downwards to the bottom of the vacuum
chamber (because of gravity). The expansion of the falling cloud depends on the initial spread of the velocities. To observe
this expansion a horizontal probe laser beam is aligned several centimetres below the initial position of the cloud. This probe
beam has a frequency close to the atomic resonance frequencyω ω 0 so that atoms scatter light as they pass through and this
fluorescence signal is recorded (or absorption could be monitored). (b) Instead of just dropping the atoms, they can be launched
upwards to form an atomic fountain. This configuration is used for precision measurements, as described in Section 9.9.