QUANTUM-NOISE PROPERTIES OF A NONLINEAR AMPLIFIER

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