ABSTRACT
This chapter reviews and extends the work on the particular class of linear amplifiers known as inverted population amplifiers. It discusses first the fundamental requirements demanded by quantum mechanics from any theory of linear amplifiers, and then describes the basic model for the atomic amplifier in a closed cavity. The chapter considers the extension of this model to allow explicit incoherent coupling of the internal cavity field to external fields, and treats finally an open system with coherent field coupling. It investigates in particular the conditions under which the amplified field retains quantum properties such as squeezing. The chapter examines the ability of the phase-insensitive systems to enhance the signal-to-noise ratio of an injected signal, and finds that in general any signal is degraded by the amplification process. Phase-insensitive atomic amplifiers destroy all quantum statistical properties of the field if the gain exceeds the low value of two.
