Hi. I am going to start my MSc in a couple of months majoring in nonlinear quantum optics. I have a good basic in quantum mechanics, but have never looked at quantum optics before. My topic will be to investigate quantum properties of nonlinear optical coupler but i have problem with the construction of the Hamiltonian for interacting optical fields. For starting, i start with two mode nonlinear optical coupler H= hω(a1b1+a2b2)+hωg(a1^2 b1^2+a2^2 b2^2) + hk(a1b2+b1a2) h=planck constant (h bar) ω=angular frequency an= creation operator associated with the optical mode bn=annihilation operator associated with the optical mode g=nonlinear susceptibility k= coupling constant I will really appreciate it if anyone can explain the physical meaning of this Hamiltonian and how do we construct such equation to describe the system.