Harwood, Alfred; (2022) Coherent and Measurement-based Feedback in Quantum Mechanics. Doctoral thesis (Ph.D), UCL (University College London).

Preview

Text Thesis-Corrected-black.pdf - Accepted Version Download (4MB) | Preview

Abstract

This thesis is about the theory of quantum feedback control. In particular,it focuses on the theory of coherent feedback-a form of quantum feedback in which no measurements are performed- and how it can be compared with measurement-based feedback. After introducing the background concepts and formalisms, the first part of this thesis is concerned with coherent feedback in the regime of Gaussian quantum systems. We derive a general model for describing Gaussian coherent feedback, and use this to derive a compact description of passive, interferometric coherent feedback. The performance of this model is then evaluated for the task of squeezing a bosonic mode and it is shown that no setup of this kind is able to generate steady-state squeezing of a quadrature beyond the ‘3dB bound’. This performance is compared to the performance of homodyne monitoring which in certain circumstances can out perform the passive coherent feedback setups. After this, we apply our model of Gaussian coherent feedback to optomechanical systems. We investigate the tasks of cooling a mechanical oscillator, generating entanglement between optical and mechanical modes and generating optical and mechanical squeezing. Finally, we develop a unified model of coherent and measurement-based feedback, inspired by collision models and not restricted to Gaussian states. Within this model, we compare the two feedback methods for the tasks of generating low entropy steady-states and simulating unitary evolution on an unknown input state.

Download activity - last month

Export as XMLCSVJSON

Download activity - last 12 months

Export as JSONCSVXML

Downloads by country - last 12 months

Export as CSVXMLJSON

Archive Staff Only

View Item