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Proceedings of the IEEE

Steady-state quantum analysis of linear systems

H. A. Haus

Published Jan 1, 1970• 33 citations
Publisher

Abstract

A rigorous basis for the quantum analysis of the steady state of linear distributed systems is established. The analysis of a distributed system of finite length requires, for self-consistency, that excitations be stated at the boundaries of the system even in the absence of externally applied excitations. The commutators of the amplitudes at the boundaries are stated and a useful analogy with thermal noise of classical systems is established. The use of these boundary conditions enables one to formulate the theory of the steady state for distributed quantum systems. When the system under consideration is coupled to a dissipation mechanism, operator-noise sources have to be assigned to the dissipative elements. The commutation relations that must be obeyed by these noise sources are derived. This formalism enables one to analyze the steady-state operation of an attenuator and of a maser amplifier. Finally, properties of multiterminal-pair networks are discussed using the steady-state quantum approach.

DOI
10.1109/proc.1970.7984

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A rigorous basis for the quantum analysis of the steady state of linear distributed systems is established. The analysis of a distributed system of finite length requires, for self-consistency, that excitations be stated at the boundaries of the system even in the absence of externally applied excitations. The commutators of the amplitudes at the boundaries are stated and a useful analogy with thermal noise of classical systems is established. The use of these boundary conditions enables one to formulate the theory of the steady state for distributed quantum systems. When the system under consideration is coupled to a dissipation mechanism, operator-noise sources have to be assigned to the dissipative elements. The commutation relations that must be obeyed by these noise sources are derived. This formalism enables one to analyze the steady-state operation of an attenuator and of a maser amplifier. Finally, properties of multiterminal-pair networks are discussed using the steady-state quantum approach.
Non-relativistic quantum mechanics is treated using Feynman's space-time approach and Wigner's probablity density of position and momentum. The mechanics of a system is described by the probability of each path of the coordinates in space-time. The influence of one system on another is described by the probability of a force function. It is shown that linear systems follow only classical paths. The quantum theory merely puts uncertainty into their initial conditions, just like thermal noise. The formalism apparently does not simplify intrinsically non-linear problems. However, linear systems are sometimes coupled to non-linear quantum systems which operate only on a linear portion of their charactersitcs (like a vacuum tube in a class A amplifier). Then the reaction on the linear systems can be reduced to an equivalent impedence and noise power spectrum, so that the linear problem can be solved by classical methods. Solid state and beam type masers are treated as examples. Masers employ excited non-linear systems to acheive linear amplification of the electromagnetic signal. The modes of the waveguides and cavities are the linear systems. To them the amplifying systems appear as nagative resistance, reactance, and noise.
Non-relativistic quantum mechanics is treated using Feynman's space-time approach and Wigner's probablity density of position and momentum. The mechanics of a system is described by the probability of each path of the coordinates in space-time. The influence of one system on another is described by the probability of a force function. It is shown that linear systems follow only classical paths. The quantum theory merely puts uncertainty into their initial conditions, just like thermal noise. The formalism apparently does not simplify intrinsically non-linear problems. However, linear systems are sometimes coupled to non-linear quantum systems which operate only on a linear portion of their charactersitcs (like a vacuum tube in a class A amplifier). Then the reaction on the linear systems can be reduced to an equivalent impedence and noise power spectrum, so that the linear problem can be solved by classical methods. Solid state and beam type masers are treated as examples. Masers employ excited non-linear systems to acheive linear amplification of the electromagnetic signal. The modes of the waveguides and cavities are the linear systems. To them the amplifying systems appear as nagative resistance, reactance, and noise.
Fluctuation dissipation theorems (FDTs) connect the linear response of a physical system to a perturbation to the steady-state correlation functions. Until now, most of these theorems have been derived for finite-dimensional systems. However, many relevant physical processes are described by systems of infinite dimension in the Gaussian regime. In this work, we find a linear response theory for quantum Gaussian systems subject to time dependent Gaussian channels. In particular, we establish a FDT for the covariance matrix that connects its linear response at any time to the steady state two-time correlations. The theorem covers non-equilibrium scenarios as it does not require the steady state to be at thermal equilibrium. We further show how our results simplify the study of Gaussian systems subject to a time dependent Lindbladian master equation. Finally, we illustrate the usage of our new scheme through some examples. Due to broad generality of the Gaussian formalism, we expect our results to find an application in many physical platforms, such as opto-mechanical systems in the presence of external noise or driven quantum heat devices.
Abstract Fluctuation dissipation theorems (FDTs) connect the linear response of a physical system to a perturbation to the steady-state correlation functions. Until now, most of these theorems have been derived for finite-dimensional systems. However, many relevant physical processes are described by systems of infinite dimension in the Gaussian regime. In this work, we find a linear response theory for quantum Gaussian systems subject to time dependent Gaussian channels. In particular, we establish a FDT for the covariance matrix that connects its linear response at any time to the steady state two-time correlations. The theorem covers non-equilibrium scenarios as it does not require the steady state to be at thermal equilibrium. We further show how our results simplify the study of Gaussian systems subject to a time dependent Lindbladian master equation. Finally, we illustrate the usage of our new scheme through some examples. Due to broad generality of the Gaussian formalism, we expect our results to find an application in many physical platforms, such as opto-mechanical systems in the presence of external noise or driven quantum heat devices.
The exact response theory based on the Dissipation Function applies to general dynamical systems and has yielded excellent results in various applications. In this article, we propose a method to apply it to quantum mechanics. In many quantum systems, it has not yet been possible to overcome the perturbative approach, and the most developed theory is the linear one. Extensions of the exact response theory developed in the field of nonequilibrium molecular dynamics could prove useful in quantum mechanics, as perturbations of small systems or far-from-equilibrium states cannot always be taken as small perturbations. Here, we introduce a quantum analogue of the classical Dissipation Function. We then derive a quantum expression for the exact calculation of time-dependent expectation values of observables, in a form analogous to that of the classical theory. We restrict our analysis to finite-dimensional Hilbert spaces, for the sake of simplicity, and we apply our method to specific examples, like qubit systems, for which exact results can be obtained by standard techniques. This way, we prove the consistency of our approach with the existing methods, where they apply. Although not required for open systems, we propose a self-adjoint version of our Dissipation Operator, obtaining a second equivalent expression of response, where the contribution of an anti-self-adjoint operator appears. We conclude by using new formalism to solve the Lindblad equations, obtaining exact results for a specific case of qubit decoherence, and suggesting possible future developments of this work.