By performing QND counting of photons in a cavity, an ideal
quantum measurement on light is performed, which illustrates
all the postulates of the theory and tests accurately the
equations of quantum electrodynamics about field relaxation in
a cavity (rate equations). Observing single field trajectories
becomes possible and provides a new way to look at light.
By acquiring information on photon number, the conjugate
variable (field’s phase) is perturbed. This is related to the
preparation of Schrödinger cats (see lectures 2 and 4) and to
the quantum Zeno effect: measuring repeatedly the field’s
intensity randomizes the field’s phase and inhibits the
coherent process of field growth.
The knowledge of the photon number distribution provides
only partial information on the field state. To acquire info on
the coherences between Fock states, other kinds of
experiments are needed (wait for next lecture…)