As usual in quantum optics, the density operators of light fields can be represented by normalized real-valued functions. There are three types of functions:
are the P-representation (Glauber-Sudarshan representation), the Q-representation and the Wigner-Weyl distribution, respectively. For example, the diagonal elements of the density operator
define the Q-representation:
is a real, well-behaved, non-negative definite and bounded function of the complex variable
. The set of states
forms the basis of coherent states, the eigenstates of the annihilation operator
. Note that the quasi-probability densities are not genuine probability densities, but they are suitable to calculate expectation values (mean values) of ordered operator products. In our case, expectation values of antinormal ordered operator products can be calculated with the help of the Q-representation.