Dynamical Casimir Effect in cavity with N-level detector or N-1   two-level atoms
  We study the photon generation from vacuum via the Dynamical Casimir Effect in a cavity containing a N-level detector in equally-spaced resonant ladder configuration or N-1 identical resonant two-level atoms. If the modulation frequency equals exactly twice the unperturbed cavity frequency, the photon growth goes on steadily for odd N, while for even N at most N-2 photons can be generated. This finding is corroborated by numerical calculations. In the limit $N\to\infty$ we obtain the harmonic oscillator model of the detector, which admits analytical results. 
A. V. Dodonov and V. V. Dodonov
quant-ph