  We study the quasiprobability representation of quantum light, as introduced
by Glauber and Sudarshan, for the unified characterization of quantum
phenomena. We begin with reviewing the past and current impact of this
technique. Regularization and convolution methods are specifically considered
since they are accessible in experiments. We further discuss more general
quantum systems for which the concept of negative probabilities can be
generalized, being highly relevant for quantum information science. For
analyzing quantum superpositions, we apply recently developed approaches to
visualize quantum coherence of states via negative quasiprobability
representations, including regularized quasiprobabilities for light and more
general quantum correlated systems.
