  In the field of light-matter interactions, it is often assumed that a
classical light field that interacts with a quantum particle remains almost
unchanged and thus contains nearly no information about the manipulated
particles. To investigate the validity of this assumption, we develop and
theoretically analyze a simple Gedankenexperiment which involves the
interaction of a coherent state with a quantum particle in an optical cavity.
We quantify the resulting alteration of the light field by measuring the
fidelity of its initial and equilibrium states. Using Bayesian inference, we
demonstrate the information transfer through photon measurements. In addition,
we employ the concepts of quantum entropy and mutual information to quantify
the entropy transfer from the particle to the light field. In the weak coupling
limit, we validate the usually assumed negligible alteration of the light field
and entropy transfer. In the strong coupling limit, however, we observe that
the information of the initial particle state can be fully encoded in the light
field, even for large photon numbers. Nevertheless, we show that spontaneous
emission is a sufficient mechanism for removing the entropy initially stored in
the particle. Our analysis provides a deeper understanding of the entropy
exchange between quantum matter and classical light.
