Quantization of systems with time-dependent constraints. Example of   relativistic particle in plane wave
  A modification of the canonical quantization procedure for systems with time-dependent second-class constraints is discussed and applied to the quantization of the relativistic particle in a plane wave. The time dependence of constraints appears in the problem in two ways. The Lagrangian depends on time explicitly by origin, and a special time-dependent gauge is used. Two possible approaches to the quantization are demonstrated in this case. One is to solve directly a system of operator equations, proposed by Tyutin and one of the authors (Gitman) as a generalization of Dirac canonical quantization in nonstationary case, and another to find first a canonical transformation, which makes it possible to discribe the dynamics in the physical sactor by means of some effective Hamiltonian. Quantum mechanics constructed in both cases proves to be equivalent to Klein-Gordon theory of the relativistic particle in a plane wave. The general conditions of unitarity of the dynamics in the physical sector are discussed. 
S.P. Gavrilov, D.M. Gitman