Active versus Passive Coherent Equalization of Passive Linear Quantum Systems

TL;DR

This paper analyzes passive and active coherent quantum equalizers for passive linear quantum systems, highlighting how input field characteristics influence the choice and effectiveness of the equalization strategy, especially distinguishing between thermal and squeezed fields.

Contribution

It introduces a detailed analysis of active versus passive coherent quantum equalizers, emphasizing the importance of input field properties in filter selection and performance.

Findings

Processing both quadratures may not improve thermal field estimation.

Active equalizers can be beneficial for squeezed input fields.

Passive filters are often sufficient for thermal fields.

Abstract

The paper considers the problem of equalization of passive linear quantum systems. While our previous work was concerned with the analysis and synthesis of passive equalizers, in this paper we analyze coherent quantum equalizers whose annihilation (respectively, creation) operator dynamics in the Heisenberg picture are driven by both quadratures of the channel output field. We show that the characteristics of the input field must be taken into consideration when choosing the type of the equalizing filter. In particular, we show that for thermal fields allowing the filter to process both quadratures of the channel output may not improve mean square accuracy of the input field estimate, in comparison with passive filters. This situation changes when the input field is `squeezed'.