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.