We show that the Wigner function corresponding to an electromagnetic field in a cavity or a vibronic state of a trapped ion can be directly measured by means of a simple experimental procedure. The proposed method is insensitive to the relatively low detection efficiency in cavity QED experiments and yields a direct physical meaning to the Wigner distribution.

• Received 18 October 1996

DOI:https://doi.org/10.1103/PhysRevLett.78.2547