Multiphoton excitation and harmonic generation from the quantum electrodynamic vacuum

The microscopic theory of the nonlinear interaction of coherent radiation with the quantum electrodynamic vacuum is developed. The evolution equation for a single-particle density matrix is solved in the multiphoton resonant approximation. The obtained solutions disclose Rabi oscillations and resonant multiphoton excitations of the Dirac sea. On the basis of the numerical simulations, we examine the rates of harmonics radiation at the electron–positron annihilation in the field of a strong pump wave. (letter)