We propose simple experiments in cavity quantum electrodynamics leadin
g, for the first time, to the measurement of negative values of the Wi
gner function of an electromagnetic field. We also show that the reali
zation of a controlled-not gate within the framework of cavity QED is
a special case of our proposal, and is equivalent to the measurement o
f the Wigner function of a one-photon field at the origin of phase spa
ce. (C) 1998 Optical Society of America.