Gauge-invariant quantum kinetic equations for electrons in classical electromagnetic fields

Gauge-invariant quantum kinetic equations for interacting electrons are deduced using the Keldysh diagrammatic technique. The Dyson equations are transformed using a special type of the Wigner representation that produces gauge-invariant Green functions. As a result, they depend on the variables having a meaning of position and kinetic momentum. The Wigner representation used makes it necessary to modify the diagrammatic technique in such a way that it will be able to account for the momentum-energy exchange between the system and the electromagnetic field. The formalism obtained makes it possible to carry out many-particle calculations for non-linear systems in arbitrary electromagnetic fields. Some particular simple cases are considered. A special discussion is given regarding the meaning of the detailed balance in such a formulation.