Quantum Electrodynamics of Intense Photon Beams

A new formula for induced transition amplitudes in intense photon beams is derived which relates the photon-number description to the coherent-field description. According to this formula transition amplitudes describing processes in which $m$ photons are added to the intense one-mode beam or subtracted from that beam are equal to the Fourier coefficients of the corresponding amplitude in the external field, expanded with respect to the phase of the classical field. The same formula is used to study the expectation values of dynamical variables in $n$-photon states leading to the conclusion that for large $n$ such states are very much like classical statistical states with a given amplitude and evenly distributed phase.