Quantum measurements in continuous time, non Markovian evolutions and feedback

TL;DR

This paper develops a non-Markovian quantum trajectory theory incorporating continuous measurement and feedback, applied to a two-level atom to control properties like light squeezing and spectrum over time.

Contribution

It extends quantum trajectory theory to include non-Markovian effects with feedback, enabling realistic modeling of quantum systems with delays.

Findings

Successfully models non-Markovian quantum evolution with feedback.

Demonstrates control of emitted light properties such as squeezing and spectrum.

Provides a framework compatible with modern quantum mechanics axioms.

Abstract

In this article we reconsider a version of quantum trajectory theory based on the stochastic Schr\"odinger equation with stochastic coefficients, which was mathematically introduced in the '90s, and we develop it in order to describe the non Markovian evolution of a quantum system continuously measured and controlled thanks to a measurement based feedback. Indeed, realistic descriptions of a feedback loop have to include delay and thus need a non Markovian theory. The theory allows to put together non Markovian evolutions and measurements in continuous time in agreement with the modern axiomatic formulation of quantum mechanics. To illustrate the possibilities of such a theory, we apply it to a two-level atom stimulated by a laser. We introduce closed loop control too, via the stimulating laser, with the aim to enhance the "squeezing" of the emitted light, or other typical quantum properties. Note that here we change the point of view with respect to the usual applications of control theory. In our model the "system" is the two-level atom, but we do not want to control its state, to bring the atom to a final target state. Our aim is to control the "Mandel $Q$-parameter" and the spectrum of the emitted light; in particular the spectrum is not a property at a single time, but involves a long interval of times (a Fourier transform of the autocorrelation function of the observed output is needed).