Wigner Distributions as Representations of the Density Matrix

Max L. Berkowitz

Yutaka Kano

The relation between the Wigner cistribution function and the P-representation of the density matrix for describing a boson system is investigated. The coherent-state representation of the density matrix is introduced for a Bose particle system. Especially the P-representation of the density matrix describing an ideal boson system in thermal equilibrium is obtained. It can be seen that the distribution function of the complex eigenvalues of the destruction operator is non-negative in this particular case. The coherent-state representation of the Wigner function is then obtained. The coherent-state representation of the components in the decomposition of the Wigner function is also found. The London-Placzek formula can be derived by using one of these components.

G. Goudaroulis, A. D. Jannussis, V. Papatheou

J. Ben-Benjamin, Moochan B. Kim, Wolfgang P. Schleich +2 more

Quantum phase‐space distributions offer a royal road into the fascinating quantum–classical interface; the Wigner function being the first and best example. However, the subject is frequent with subtleties and textbook‐level misinformation; e.g. “The Wigner distribution can give wrong answers for some operator expectation values” . Since the Wigner distribution is just another representation of the density matrix, it must yield correct answers. To that end, Marlan Scully has asked at several international conferences (the 2015 Prague conference being one of them) the following question: “Starting with the density matrix (not the Moyal characteristic function), could you give me a simple direct derivation of the Wigner distribution?” Section contains his answer. In Appendix D , we give a related treatment and make contact with other approaches. We hope that as a result of our studies, the Wigner distribution will become more deeply appreciated.