Measurement of a negative value for the Wigner function of radiation - art. no. 054101

Following a proposal by two of us [L. G. Lutterbach and L. Davidovich, Phys . Rev. Lett. 78, 2547 (1997)], we have measured the Wigner function at the origin of phase space for a single photon field. Tts value is negative, exh ibiting the nonclassical nature of this state. The experiment is based on t he absorption-free detection of the microwave field stored in a superconduc ting cavity [G. Nogues et al., Nature (London) 400, 239 (1999)]. Extension to a measurement of the Wigner function over the complete phase space is di scussed.