  We develop a thermodynamical theory to describe the behavior of the
entanglement between a single two-level atom with a single mode of the
electromagnetic field. The resonant Jaynes-Cummings model is used to study both
the entanglement thermodynamics, in particular the entanglement temperature,
and its connection with the average number of photons in the optical cavity. We
find that this entanglement temperature has a strong dependence with the
initial conditions of the atom. We show that the entanglement temperature
between the photons and the atom defined in this work is the same temperature
obtained within the Jaynes-Cummings model at finite temperature developed in
the Thermo-Field Dynamics formalism.
