Abstract : During the three-year performance period we have carried out a program of theoretical research on the foundations and potential applications of coherent-feedback quantum control. We have focused on potential applications in quantum-enhanced meterology and information processing, and on hardware platforms of an optical or opto-mechanical nature. The studies we have been performing begin to build a picture of how coherent feedback can provide a kind of circuit/network theory for quantum engineering, enabling rigorous analysis and numerical simulation of advanced devices and systems. We have begun to establish analytical approaches for comparing the performance of quantum-enhanced feedback control systems with classical counterparts, leveraging canonical results from classical control theory together with advanced methods for numerical simulations of proposed quantum systems. All of our results point to the interest of continued future study in this area, and of future investments in the experimental realization of the kinds of systems we propose.
Abstract : During the three-year performance period we have carried out a program of theoretical research on the foundations and potential applications of coherent-feedback quantum control. We have focused on potential applications in quantum-enhanced meterology and information processing, and on hardware platforms of an optical or opto-mechanical nature. The studies we have been performing begin to build a picture of how coherent feedback can provide a kind of circuit/network theory for quantum engineering, enabling rigorous analysis and numerical simulation of advanced devices and systems. We have begun to establish analytical approaches for comparing the performance of quantum-enhanced feedback control systems with classical counterparts, leveraging canonical results from classical control theory together with advanced methods for numerical simulations of proposed quantum systems. All of our results point to the interest of continued future study in this area, and of future investments in the experimental realization of the kinds of systems we propose.
In this paper, an overview of research in quantum control is presented. Examples of work in the first theme include numerous applications of semi-definite programming and relaxation methods in quantum information theory. Examples of the second include applications of feedback control in the development of new receiver designs for quantum-optical telecom. The third theme is just beginning to emerge as an active research area, linking fundamental issues in quantum field theory with new challenges for applied and computational mathematics in non-commutative stochastic analysis and optimization.
The subject of controlling quantum systems is not new, but concepts that have been introduced in the last decade and a half, especially that of coherent feedback, suggest new questions that broaden and deepen the field. Here we provide a concise overview and definition of quantum feedback control, both coherent and measurement-based, and discuss its relationship to "standard" time-dependent control; there is a sense in which the latter subsumes the rest. There are many open questions within quantum control and its subfields, and we highlight and discuss some of them here. These questions are of theoretical as well as practical interest: the answers will help to determine the relative power of the different methods of control, and the limits to our ability to control quantum systems imposed by available resources.
Quantum computing, leveraging the principles of quantum mechanics, offers the potential for unprecedented computational power. However, realizing this potential faces significant challenges related to qubit coherence, scalability, and control. This paper investigates a novel approach to quantum computing – Adaptive Self-Assembly of Quantum Mediums (ASAM) – that employs dynamic adjustment of material parameters to induce self-assembly of quantum bits. We propose a method that leverages a feedback loop to control the assembly process, resulting in enhanced quantum coherence and improved qubit performance. Traditional approaches often rely on fixed parameters, limiting design flexibility. Our research focuses on demonstrating the practical benefits of this adaptive approach through simulations and preliminary experimental validation. We present a framework for dynamically configuring material properties to guide the formation of highly coherent and controllable quantum states. The core mechanism involves a self-organizing process triggered by feedback, which enhances the quality of quantum information storage. This paper will detail the theoretical underpinnings of ASAM, present simulation results, and discuss the potential for future advancements.
In the last decade, there has been a growing interest in the integration of photonic components into VLSI circuits that in the long term may drive the need for systematic mixed electronic-photonic circuit design methods. On the other hand, theoretical and experimental investigations of coherent-feedback quantum control, the feedback control of a quantum system with another quantum system, have led to the development of a network synthesis theory for linear quantum stochastic models that are commonly employed in (linear) quantum optics, modelling such devices as optical cavities and optical parametric amplifiers. This paper makes theoretical contributions towards connecting these two independent developments, and anticipates potential future interest in systematic design methods for mixed linear electronic-photonic circuits, by establishing a network synthesis theory for linear stochastic systems with mixed quantum and classical degrees of freedom. It is shown how a physically realizable mixed quantum-classical linear stochastic system can be realized as a circuit composed of a feedback interconnection of a fully quantum linear subsystem, that can be implemented by quantum optical devices, and a classical linear sub-system, that can be implemented with standard electrical and electronic devices, together with appropriate interfaces that convert quantum signals to classical signals, and vice-versa. Two feedback architectures are proposed, and a decomposition lemma is derived that shows the structure of linear transformations of bosonic quantum signals into classical signals.
Quantum engineering has seen rapid growth in the past two decades. Physicists, mathematicians and engineers have been working in unison to control a number of diverse systems in the quantum regime. Quantum control involving feedback has become particularly topical, as using information gained from a system can lead to more stable operation of a control protocol. Quantum feedback can be split into two paradigms: measurement based feedback and coherent feedback. Although it is increasingly evident that retaining the coherence of the feedback signal provides an intrinsic advantage over measurement-based feedback, coherent feedback is still a new paradigm. In particular, there are a limited number of options for coherently estimating a state within a feedback loop. As a step towards better understanding and implementation of quantum feedback, this thesis reports on modelling, estimation and control of quantum systems. The first topic is physical realisability of a variety of quantum systems, especially finite level systems and their outgrowths. Bilinear quantum stochastic differential equations (QSDEs) have to be employed to characterise these systems, which is a significant complement to the previous work on linear QSDEs. In light of the fact that direct and indirect couplings play a vital role in quantum network and control, we also provide state-space models for different classes of coupled open quantum systems incorporating both bidirectional and directional interactions. The second topic is coherent observers and optimal filtering. It is well established that classically estimation using the Kalman filter can provide improved performance over direct feedback schemes, and similar demonstrations have been performed for measurement-based quantum feedback. In this thesis I present coherent observers, including least mean squares estimators, which are driven by the coherent output of a specified quantum plant and designed such that some subset of the observer and plant's expectation values converge in the asymptotic limit. Not only can estimators "observe" mean quantities of quantum plant, but also they can "estimate" quantum correlations such as entanglement. This is of much importance to coherent feedback design in the absence of measurement steps without loss of fidelity. Last but not least, several topics regarding stabilisation and control design of quantum systems are discussed in my thesis. To be specific, tools for quantum stability analysis (e.g. Lyapunov conditions) are provided. Both measurement-based feedback and coherent feedback control design are taken into account. A measurement-based optimal controller for opto-mechanical systems aimed at synchronising different mechanical modes is presented, which is application-oriented. Moreover, A general observer-based coherent feedback control framework is studied, and I demonstrate a pole-placement approach via coherent observers that can be applied to various scenarios.
The emerging discipline of coherent-feedback quantum control provides core concepts and methods for nanopho- tonic circuit theory, which can be assimilated within modern approaches to computer-aided design. Current research in this area includes the development of software tools to enable a schematic capture workflow for compilation and analysis of quantum stochastic models for nanophotonic circuits, exploration of elementary coherent-feedback circuit motifs, and laboratory demonstrations of quantum nonlinear photonic devices.