Abstract

Usually, to determine the number of photons in an optical pulse, we direct it onto a photodiode or into a photomultiplier. In doing so, we destroy the pulse—the backaction of the measurement far exceeds thai-prescribed by Heisenberg's uncertainty principle. Quantum nondemolition (QND) measurements allow us to do much better—recent experiments have measured the photon number of optical signals in waveguides without losing photons or increasing photon-number noise.¹-² These measurements were carried out by scattering probe photons from signal photons by the Kerr nonlinearity of optical fibers.

© 1994 Optical Society of America