Nonresonant two-level transitions: Insights from quantum thermodynamics

Based on concepts from quantum thermodynamics, the two-level system coupled to a single electromagnetic
mode is analyzed. Focusing on the case of detuning, where the mode frequency does not match the transition
frequency, effective energies are derived for the levels and the photon energy. It is shown that these should be used
for energy exchange with fermionic and bosonic reservoirs in the steady state to achieve a thermodynamically
consistent description. While recovering known features such as frequency pulling or Bloch gain, this sheds light
on their thermodynamic background and allows for a coherent understanding.