Controlling the motion of an atom in an optical cavity

18 2.3. Experimental transit signals of single atoms
2.3 Experimental transit signals of single atoms
In the last section, it was shown theoretically that a single atom in a high-finesse cavity
can have a large influence on the transmission of the cavity. This is investigated experimentally
by sending slow atoms through the high-finesse cavity described in section 5.3
and measuring the transmission of the cavity. The density of the atomic cloud was chosen
so low that the mean number of atoms in the cavity is much less than one, so that only
once every while, a single atom is strongly coupled to the cavity mode. As the atoms
move independently from each other (MFPR99), the probability of finding two atoms at
the same time in the cavity is much smaller than the probability of finding a single atom.
If the probe laser is in resonance with both the atom and the empty cavity, i.e. ∆a =
∆c = 0, a drop in the transmission of the cavity is expected if an atom enters the cavity.
An experimental example is shown in Fig. 2.5. Most of the time, the transmission of
the cavity is high, but sometimes, there are sudden drops in the transmission, which are
caused by single atoms in the cavity. The noise in the transmission signal is mostly due
to shot noise. Remarkable is the high time resolution of 200 kHz, i.e. only 5 µs are needed
to detect a strongly coupled atom in the cavity.
In the next measurement, the probe laser was detuned from the empty cavity resonance.
In this case, the atom can shift the cavity into resonance, increasing the cavity
transmission. An example is shown in Fig. 2.6.
Thecomparisonwiththeexperimentshows that the classical theory can be used to
calculate the transmission of the cavity if the atoms are not saturated. However, the
motion of the atoms in the cavity is also important for the transmission signals: In
Fig. 2.5, a fast motion of the atoms across the antinodes over the standing wave had to be
assumed in order to explain the experimental signal, i.e. the transmission of the cavity
was averaged over the standing wave. This fast motion is due to heating of the atoms by
the light force in the cavity. The heating cannot be calculated correctly if the light field
in the cavity is assumed to be classical. Therefore, a quantum model for the light field is
considered in the next chapter. Quantum models are also needed to describe saturation
effects in a strongly coupled atoms-cavity system.
18