  The evolution of the discrete Wigner function is formally similar to a
probabilistic process, but the transition probabilities, like the discrete
Wigner function itself, can be negative. We investigate these transition
probabilities, as well as the transition rates for a continuous process, aiming
particularly to give simple criteria for deciding when a set of such quantities
corresponds to a legitimate quantum process. We also show how the transition
rates for any Hamiltonian evolution can be worked out by expanding the
Hamiltonian as a linear combination of displacement operators in the discrete
phase space.
