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arXiv:2408.03177 · IEEE Conference on Decision and Control (CDC)

On Poles and Zeros of Linear Quantum Systems

Zhiyuan Dong, Guo‐Feng Zhang, Joseph H. W. Lee

Published Dec 16, 2024• 1 citation
arXivHTMLPDF

Abstract

The non-commutative nature of quantum mechanics imposes fundamental constraints on system dynamics, which in the linear realm are manifested by the physical realizability conditions on system matrices. These restrictions endow system matrices with special structure. The purpose of this paper is to study such structure by investigating zeros and poses of linear quantum systems. In particular, we show that $-s_{0}^{*}$ is a transmission zero if and only if $s_{0}$ is a pole, and which is further generalized to the relationship between system eigenvalues and invariant zeros. Additionally, we study left-invertibility and fundamental tradeoff for linear quantum systems in terms of their zeros and poles.

DOI
10.1109/cdc56724.2024.10886374

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The non-commutative nature of quantum mechanics imposes fundamental constraints on system dynamics, which in the linear realm are manifested by the physical realizability conditions on system matrices. These restrictions endow system matrices with special structure. The purpose of this paper is to study such structure by investigating zeros and poses of linear quantum systems. In particular, we show that −s0∗-s_{0}^{*}−s0∗​ is a transmission zero if and only if s0s_{0}s0​ is a pole, and which is further generalized to the relationship between system eigenvalues and invariant zeros. Additionally, we study left-invertibility and fundamental tradeoff for linear quantum systems in terms of their zeros and poles.
The non-commutative nature of quantum mechanics imposes fundamental constraints on system dynamics, which, in the linear realm, are manifested through the physical realizability conditions on system matrices. These restrictions give system matrices a unique structure. This paper aims to study this structure by investigating the zeros and poles of linear quantum systems. Firstly, it is shown that−s0-s_{0}−s0​is a transmission zero if and only ifs0s_{0}s0​is a pole of the transfer function, and−s0-s_{0}−s0​is an invariant zero if and only ifs0s_{0}s0​is an eigenvalue of theAAA-matrix, of a linear quantum system. Moreover,s0s_{0}s0​is an output-decoupling zero if and only if−s0-s_{0}−s0​is an input-decoupling zero. Secondly, based on these pole-zero correspondences and inspired by a recent work on the stable inversion of classical linear systems [1], we show that a linear quantum system must be Hurwitz unstable if it is strongly asymptotically left invertible. Two types of stable input observers are constructed for unstable linear quantum systems. Finally, the sensitivity of a coherent feedback network is investigated; in particular, the fundamental tradeoff between ideal squeezing and system robustness is studied on the basis of system sensitivity analysis.
The non-commutative nature of quantum mechanics imposes fundamental constraints on system dynamics, which, in the linear realm, are manifested through the physical realizability conditions on system matrices. These restrictions give system matrices a unique structure. This paper aims to study this structure by investigating the zeros and poles of linear quantum systems. Firstly, it is shown that −s0-s_0−s0​ is a transmission zero if and only if s0s_0s0​ is a pole of the transfer function, and −s0-s_0−s0​ is an invariant zero if and only if s0s_0s0​ is an eigenvalue of the AAA-matrix, of a linear quantum system. Moreover, s0s_0s0​ is an output-decoupling zero if and only if −s0-s_0−s0​ is an input-decoupling zero. Secondly, based on these pole-zero correspondences and inspired by a recent work on the stable inversion of classical linear systems \cite{DD2023}, we show that a linear quantum system must be Hurwitz unstable if it is strongly asymptotically left invertible. Two types of stable input observers are constructed for unstable linear quantum systems. Finally, the sensitivity of a coherent feedback network is investigated; in particular, the fundamental tradeoff between ideal squeezing and system robustness is studied on the basis of system sensitivity analysis.
New fundamental limits on design and control of linear quantum systems are presented. A main difference between quantum and classical systems comes from the fact that quantum signals are represented by operator-valued vectors. The transfer function representation is useful to describe a system in the quantum setting for the same reason as it is useful classically. It is shown that the noncommutative property of quantum signals can be characterized by a specific relation between zeros and poles of a quantum transfer function. Then, two quantum mechanical tradeoffs follows from the transfer function constraint. One is a well known tradeoff, Heisenberg’s uncertainty relation, and the other is a tradeoff between the ability of noise reduction and sensitivity to modeling error. While these tradeoffs hold at each frequency, quantum systems have another constraint between different frequencies based on Bode integral as classical systems usually do. In the quantum case, however, this constraint is also dependent on quantum mechanical parameters.
The purpose of this paper is to characterize the structure of quantum linear systems. By means of a scattering representation, a relation between two systems GGG and Σ\SigmaΣ is established, by which we show that GGG is lossless bounded real if and only if Σ\SigmaΣ is lossless positive real. When GGG is a quantum linear system, we show that its fundamental symplectic structure and physical realizability conditions can be expressed in a simpler way in terms of Σ\SigmaΣ. Furthermore, when GGG is a passive quantum linear system, we show that its uncontrollable subspace and unobservable subspace are identical and they are spanned by the eigenvectors of the AAA matrix of GGG, whose corresponding eigenvalues are all on the imaginary axis. For the nonpassive case, several sufficient conditions are given. Finally, in terms of controllability and observability, the problem of quantum linear system decomposition is investigated. In particular, a quantum Kalman decomposition is proposed. This canonical decomposition naturally exposes decoherence-free modes and quantum non-demolished modes, which are useful resources for quantum information science.
This paper is concerned with a modified version of the frequency domain physical realizability (PR) condition for linear quantum systems. We consider open quantum systems whose dynamic variables satisfy the canonical commutation relations of an open quantum harmonic oscillator and are governed by linear quantum stochastic differential equations (QSDEs). In order to correspond to physical quantum systems, these QSDEs must satisfy PR conditions. We provide a relatively simple proof that the PR condition is equivalent to the frequency domain (J,J)-unitarity of the input-output transfer function and orthogonality of the feedthrough matrix of the system without the technical spectral assumptions required in previous work. We also show that the poles and transmission zeros associated with the transfer function of PR linear quantum systems are the mirror reflections of each other about the imaginary axis. An example is provided to illustrate the results.
The present series of papers is devoted to the localization problem in relativistic quantum mechanics. The consequences of imposing the condition that the descriptions of a localized state seen from two different frames of reference should be physically consistent were investigated in Paper II. Only instantaneous localization was studied. Considering elementary systems of nonzero mass and spin 0 and \textonehalf{}, the following results were found. (a) As regards the localization problem, one cannot avoid accepting at least one of the following strong departures from the usual ideas: (i) Position has no meaning; (ii) it violates the physical equivalence of inertial frames; (iii) it is the only quantum variable which cannot be represented by an operator; (iv) it is non-Hermitian; (v) some unusual interaction effects do not disappear when the interaction is switched off. (b) If a component of position is to be precisely measurable as a point, then for spin 0 there is only one possible position operator; for spin \textonehalf{} the operator is unique up to a parameter; in both cases the operators are non-Hermitian with respect to the relativistically invariant scalar product, but in spite of this the eigenvalues of all components are real. The components of position are compatible with each other for spin 0 and incompatible for spin \textonehalf{}. The communication relations of position with linear momentum are the standard ones. The velocity operator, which is Hermitian, has the expected form. Several authors have stated that the localization problem can have a solution for mass greater than zero, but for the zero-mass case a solution does not exist. In this paper the localization problem is studied for zero-mass elementary systems by only imposing, as in Paper II, the above requirement of physical consistency. We consider systems with spin 0, \textonehalf{}, and 1. We prove that the consequences are essentially the same as those obtained for systems of mass greater than zero. The parameter is no longer arbitrary.