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IEEE Conference on Decision and Control (CDC)

Quantum feedback networks with uniform time delays

Hendra I. Nurdin, John Edward Gough

Published Dec 1, 2017• 2 citations
Publisher

Abstract

Quantum feedback networks with non-negligible propagation delays in the interconnecting edges have recently gained attention. Grimsmo has analysed the evolution of a Markov quantum system coupled to a single field via a doublepass interaction with a time delay and derived the evolution equation for the state (density operator) of the system (after tracing out the field) using a super-operator notion of partial tracing together with a tensor network representation. However, for the purposes of feedback control (both measurement-based and coherent feedback), the propagating output fields of the system can be used to facilitate feedback interconnections. Thus it is desirable to have an equivalent input-output description of quantum systems that incorporates time delays, without tracing out the field. Here, we first extend the analysis to the setting of a single system coupled to optical fields in multiple channels with a feedback structure in which the fields makes infinite repeated interactions with the system. Then we generalise this to the setting of quantum feedback networks with both instantaneous interconnections and uniform time delays in the interconnection.

DOI
10.1109/cdc.2017.8264586

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Quantum feedback networks with non-negligible propagation delays in the interconnecting edges have recently gained attention. Grimsmo has analysed the evolution of a Markov quantum system coupled to a single field via a doublepass interaction with a time delay and derived the evolution equation for the state (density operator) of the system (after tracing out the field) using a super-operator notion of partial tracing together with a tensor network representation. However, for the purposes of feedback control (both measurement-based and coherent feedback), the propagating output fields of the system can be used to facilitate feedback interconnections. Thus it is desirable to have an equivalent input-output description of quantum systems that incorporates time delays, without tracing out the field. Here, we first extend the analysis to the setting of a single system coupled to optical fields in multiple channels with a feedback structure in which the fields makes infinite repeated interactions with the system. Then we generalise this to the setting of quantum feedback networks with both instantaneous interconnections and uniform time delays in the interconnection.
A theory of time-delayed coherent quantum feedback is developed. More specifically, we consider a quantum system coupled to a bosonic reservoir creating a unidirectional feedback loop. It is shown that the dynamics can be mapped onto a fictitious series of cascaded quantum systems, where the system is driven by past versions of itself. The derivation of this model relies on a tensor network representation of the system-reservoir time propagator. For concreteness, this general theory is applied to a driven two-level atom scattering into a coherent feedback loop. We demonstrate how delay effects can qualitatively change the dynamics of the atom and how quantum control can be implemented in the presence of time delays.
Networks of open quantum systems with feedback have become an active area of research for applications such as quantum control, quantum communication and coherent information processing. A canonical formalism for the interconnection of open quantum systems using quantum stochastic differential equations (QSDEs) has been developed by Gough, James and co-workers and has been used to develop practical modeling approaches for complex quantum optical, microwave and optomechanical circuits/networks. In this paper we fill a significant gap in existing methodology by showing how trapped modes resulting from feedback via coupled channels with finite propagation delays can be identified systematically in a given passive linear network. Our method is based on the Blaschke-Potapov multiplicative factorization theorem for inner matrix-valued functions, which has been applied in the past to analog electronic networks. Our results provide a basis for extending the Quantum Hardware Description Language (QHDL) framework for automated quantum network model construction (Tezak et al. in Philos. Trans. R. Soc. A, Math. Phys. Eng. Sci. 370(1979):5270-5290, 2012 ) to efficiently treat scenarios in which each interconnection of components has an associated signal propagation time delay.
We present an elementary derivation and generalisation of a recently reported method of simulating feedback in open quantum systems. We use our generalised method to simulate systems with multiple delays, as well as cascaded systems with delayed backscatter. In addition, we derive a generalisation of the quantum regression formula that applies to systems with delayed feedback, and show how to use the formula to compute two-time correlation functions of the system as well as output field properties. Finally, we show that delayed coherent feedback can be simulated as a quantum teleportation protocol that requires only Markovian resources, pre-shared entanglement, and time travel. The requirement for time travel can be avoided by using a probabilistic protocol.
In this paper, we develop a novel method to solve problems involving quantum optical systems coupled to coherent quantum feedback loops featuring time delays. Our method is based on exact mappings of such non-Markovian problems to equivalent Markovian driven dissipative quantum many-body problems. In this work, we show that the resulting Markovian quantum many-body problems can be solved (numerically) exactly and efficiently using tensor network methods for a series of paradigmatic examples, consisting of driven quantum systems coupled to waveguides at several distant points. In particular, we show that our method allows solving problems in so far inaccessible regimes, including problems with arbitrary long time delays and arbitrary numbers of excitations in the delay lines. We obtain solutions for the full real-time dynamics as well as the steady state in all these regimes. Finally, motivated by our results, we develop a novel mean-field approach, which allows us to find the solution semianalytically, and we identify parameter regimes where this approximation is in excellent agreement with our tensor network results. Published by the American Physical Society 2024
Quantum nonlinear feedback control is developed for traveling optical fields. We first describe the discretization of the traveling optical fields. The discrete-time formulation is used to describe the stochastic master equation subject to homodyne measurement. Nonlinear feedback is formulated by directly feeding the measurement outcomes back to the traveling field through a multiplicative action. Since the measurement outcomes have a correlation with the system, the multiplicative feedback control can create nonlinear effects in the traveling field. In this formulation, a time delay is naturally introduced in the feedback loop. This is essentially different from instantaneous feedback in a continuous-time setting. As an example of the feedback scheme, a quantum nondemolition sum gate is considered. Numerical results show that quantum superposition state can be created by applying the feedback to a squeezed state.
We study the dynamics of photonic quantum circuits consisting of nodes coupled by quantum channels. We are interested in the regime where the time delay in communication between the nodes is significant. This includes the problem of quantum feedback, where a quantum signal is fed back on a system with a time delay. We develop a matrix product state approach to solve the quantum stochastic Schrödinger equation with time delays, which accounts in an efficient way for the entanglement of nodes with the stream of emitted photons in the waveguide, and thus the non-Markovian character of the dynamics. We illustrate this approach with two paradigmatic quantum optical examples: two coherently driven distant atoms coupled to a photonic waveguide with a time delay, and a driven atom coupled to its own output field with a time delay as an instance of a quantum feedback problem.