Quantum Limits on Noise in Linear Amplifiers

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Professor Carlton Caves, University of New Mexico.
The approach of Josephson-effect linear amplifiers to the fundamental
quantum limit on noise temperature has sparked renewed interest in
low-noise linear amplifiers.  The standard, by now highly developed,
discussion of quantum limits on phase-preserving linear amplifiers
characterizes amplifier noise performance in terms of second moments
of the added noise, i.e., in terms of noise temperature or noise
power.  We have generalized this standard discussion to provide a
complete characterization of the quantum-mechanical restrictions on
the entire probability distribution of added noise.  Meanwhile, there
have been proposals for and experiments related to a different class
of linear amplifiers, which actually subtract noise from the output,
but which operate only part of time.  I discuss here what is known
about quantum limits on the operation of such probabilistic
amplifiers.

This work was carried out with Shashank Pandey, Zhang Jiang, and
Josh Combes, all of UNM, and parts were also done with Marco Piani
of the Perimeter Institute.