The
Glauber-Sudarshan P representation is a suggested way of writing down the
phase space distribution of a quantum system in the
phase space formulation of quantum mechanics. The P representation is the
quasiprobability distribution in which
observables are expressed in
normal order. In
quantum optics, this representation, formally equivalent to several other representations, is sometimes championed over alternative representations to describe
light in
optical phase space, because typical optical observables, such as the
particle number operator, are naturally expressed in normal order. It is named after
George Sudarshan and
Roy J. Glauber, who were working on the topic in 1963. It was the subject of a controversy when Glauber was awarded a share of the 2005
Nobel Prize in Physics for his work in this field and
George Sudarshan's contribution was not recognized. Despite many useful applications in laser theory and coherence theory, the
Glauber-Sudarshan P representation has the drawback that it is not always positive, and therefore is not a true probability function.