We propose and analyze a nonlinear optical amplifier that is phase insensitive but has no additive noise. When the input field is in a coherent state, the noise figure is -6 dB when the gain is high. However, unlike conventional linear amplifiers with -3 dB noise figures, the detectibility of the amplified signal is not degraded. The amplifier is constructed simply, in principle, by the same type of nonlinear interferometer used in photon number quantum nondemolition measurements.

• Received 17 May 1993

DOI:https://doi.org/10.1103/PhysRevLett.71.1709