  Regular quasiprobabilities are introduced for the aim of characterizing
quantum correlations of multimode radiation fields. Negativities of these
quantum-correlation quasiprobabilities are necessary and sufficient for any
quantum correlation encoded in the multimode Glauber-Sudarshan P function. The
strength of the method is demonstrated for a two-mode phase randomized
squeezed-vacuum state. It has no entanglement, no quantum discord, a positive
Wigner function, and a classical reduced single-mode representation. Our method
clearly visualizes the quantum correlations of this state.
