Number 138 (Story #1), July 26, 1993 by Phillip F. Schewe and Ben Stein THE QUANTUM WAVE FUNCTION OF LIGHT has been measured experimentally by researchers at the University of Oregon and Oregon State (D.T. Smithey et al, Phys. Rev. Lett., 1 March 1993; contact Michael Raymer, 503-346-4785). Previous experiments on light and other quantum systems focused on measuring specific properties such as the mean strength of a light wave's electric field; none had directly investigated the underlying wave function, which in principle embodies all information about the quantum system and its properties. Armed with improvements in detector technology, the Oregon researchers applied a recent theoretical proposal for determining the wave function experimentally; they measured electric field amplitudes at different values of phase in a pulse of light. They made repeated measurements on identically prepared pulses, building up statistical distributions showing the range of amplitude values measured at each phase. From these measurements, they were able to reconstruct the Wigner function, a "quasiprobability" function which determines the probability distribution for one quantum property (in this case, the electric field amplitude in phase with a reference wave) or its complementary quantum property (the electric field amplitude 90 degrees out of phase). More significantly, the measured Wigner function for this system is essentially the Fourier transform of the wave function. The wave function, measured in this way, can in principle provide information about properties not even measured in the experiment, such as the photon number. These measurements promise to provide new insights into many quantum phenomena, such as "squeezed light." (Physics World, July 1993.)