How large must the input signal to an amplifier be to dominate the quantum noise introduced during amplification? We attempt to answer this question for a Raman amplifier with an experiment that utilizes a modified Mach-Zender interferometer with an amplifier placed in each leg. The quantum noise added by the amplifiers manifests itself by degrading the visibility of the output fringes from the interferometer. Only an average of a few photons per mode were necessary to achieve good visibility.

• Received 26 December 1990

DOI:https://doi.org/10.1103/PhysRevLett.67.38