I think I follow you. So, for instance, in the rotating wave approx. where E is the electric field operator in the Heisenberg picture,
E = \sumkl(akleklfkl(x)exp(-i\omegat) + a+kle*klf*kl(x)exp(i\omegat))
with <a|E2|a> as the the energy density expectation value, I could choose a state...