Amplification and attenuation of quantum fields
As an application of the theory of open quantum systems, we consider the theory of optical amplifiers and absorbers. We would like to understand how an input field, particularly a nonclassical field, is amplified or attenuated. We specifically discuss changes in the quantum character of the field. This is especially important in connection with the amplification of fields with very low photon numbers since the amplifiers are known to add quantum noise [1–5]. Thus even if the input signal is shot noise limited, the output is not. We will also discuss how the entanglement is significantly affected by both amplifiers and attenuators.
10.1 Quantum theory of optical amplification