The multiphoton electron-positron pair production through nonlinear channels by superintense opposed laser pulses of the same frequencies in vacuum is considered. On the basis of the Dirac model the resonance approximation for vacuum induced transitions is developed. The analytic formulas for energetic and angular distributions and total number of created particles in the limit of a short interaction time are obtained.

Nonlinear dynamics of establishment of electron-hole coherent superpositions states in graphene by multiphoton resonant excitation of interband transitions in laser fields is considered. The singleparticle time dependent density matrix for such a quantized system is calculated in the multiphoton resonant approximation. The dependence of Rabi oscillations of Fermi-Dirac sea in graphene on the time, momentum, and photon number at multiphoton laser-excitation is analyzed.

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A scheme of an intense coherent gamma-ray source based on the spontaneous radiation of positronium atoms in a Bose-Einstein condensate (BEC) due to two-photon collective annihilation decay is investigated analytically arising from the second quantized formalism. It is shown that because of the intrinsic instability of annihilation decay of BEC, the spontaneously emitted entangled photon pairs are amplified, leading to an exponential buildup of a macroscopic population into end-fire modes at a certain shape of the elongated condensate. The considered scheme may also be applied to a BEC of atoms or quasiparticles as a laser mechanism with double coherence to create entangled photonic beams with a macroscopic number of photons.

In this work a scheme of x-ray coherent radiation generation in the nonlinear quantum regime by means of mildly relativistic high density electron beams and a strong pump laser field is investigated. The consideration is based on a self-consistent set of Maxwell and relativistic quantum kinetic equations. The coupled equations are solved in the slowly varying envelope approximation.