The coupled cluster method is improved with the random phase approximation (RPA) to calculate vacuum wave function and vacuum energy of 2 + 1-d SU (2) lattice gauge theory.
In particular, we have found that even at the mean field level the naive vacuum saturation approximation is not a good approximation when we consider nonlocal four quark condensate.
The squeezing of light in the system of two mode squeezed vacuum field interacting with a two level atom within and without the rotating wave approximation by means of quantum theory is studied.