Quantum noise limits for nonlinear, phase-invariant amplifiers
Dmitri A. Kouznetsov, Roberto Ortega-Martı́nez, Daniel Rohrlich · Physical Review A · 1995
Any quantum device that amplifies coherent states of a field while preserving their phase generates noise. A nonlinear, phase-invariant amplifier may generate less noise, over a range of input field strengths, than any linear amplifier with the same amplification. We present explicit examples of such nonlinear amplifiers and derive lower bounds on the noise generated by a nonlinear, phase-invariant quantum amplifier.