  Wigner functions generically attain negative values and hence are not
probability densities. We prove an asymptotic expansion of Wigner functions in
terms of Hermite spectrograms, which are probability densities. The expansion
provides exact formulas for the quantum expectations of polynomial observables.
In the high frequency regime it allows to approximate quantum expectation
values up to any order of accuracy in the high frequency parameter. We present
a Markov Chain Monte Carlo method to sample from the new densities and
illustrate our findings by numerical experiments.
