Abstract

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, so-called nondeterministic linear amplifiers, which operate only part of the time, but which, when they operate, take an input coherent state to an amplified output coherent state. We discuss here what is known about quantum limits on the operation of such 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.