Feedback control of the squeezing of the fluorescence light

TL;DR

This paper develops a quantum feedback control method for a two-level atom using quantum trajectory theory, demonstrating how feedback influences fluorescence light properties like squeezing and spectral narrowing.

Contribution

It introduces a concrete implementation of fast feedback in quantum trajectory theory for a two-level atom, linking measurement-based control with quantum optical phenomena.

Findings

Demonstrates feedback control affects fluorescence light spectrum

Shows quantum squeezing and line-narrowing under feedback

Provides a framework for quantum measurement-based feedback

Abstract

Among the formulations of the theory of quantum measurements in continuous time, quantum trajectory theory is very suitable for the introduction of measurement based feedback and closed loop control of quantum systems. In this paper we present such a construction in the concrete case of a two-level atom stimulated by a coherent, monochromatic laser. In particular, we show how fast feedback \`a la Wiseman and Milburn can be introduced in the formulation of the theory. Then, the spectrum of the free fluorescence light is studied and typical quantum phenomena, squeezing and sub-natural line-narrowing, are presented.