Hamiltonian models of multiphoton processes and four--photon squeezed   states via nonlinear canonical transformations
  We introduce nonlinear canonical transformations that yield effective Hamiltonians of multiphoton down conversion processes, and we define the associated non-Gaussian multiphoton squeezed states as the coherent states of the multiphoton Hamiltonians. We study in detail the four-photon processes and the associated non-Gaussian four-photon squeezed states. The realization of squeezing, the behavior of the field statistics, and the structure of the phase space distributions show that these states realize a natural four-photon generalization of the two-photon squeezed states. 
Silvio De Siena, Antonio Di Lisi, and Fabrizio Illuminati