  We investigate an ensemble of excitons in a coupled quantum well excited via
an applied laser field. Using an effective disordered quantum Ising model, we
perform a numerical simulation of the experimental procedure and calculate the
probability distribution function $P(M)$ to create $M$ excitons as well as
their correlation function. It shows clear evidence of the existence of two
phases corresponding to a liquid and a crystal phase. We demonstrate that not
only the correlation function but also the distribution $P(M)$ is very well
suited to monitor this transition.
