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Physical Review Letters

Ensemble-Dependent Bounds for Accessible Information in Quantum Mechanics

Christopher A. Fuchs, Carlton M. Caves

Published Dec 5, 1994• 90 citations
Publisher

Abstract

We simplify the derivation of the Holevo upper bound on the maximum information extractable from a quantum communication channel. This simplification leads to upper and lower bounds for binary channels, both of which depend explicitly on the message ensemble.
DOI
10.1103/physrevlett.73.3047

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We present a family of easily computable upper bounds for the Holevo (information) quantity of an ensemble of quantum states depending on a reference state (as a free parameter). These upper bounds are obtained by combining probabilistic and metric characteristics of the ensemble. We show that an appropriate choice of the reference state gives tight upper bounds for the Holevo quantity which in many cases improve the estimates existing in the literature. We also present an upper bound for the Holevo quantity of a generalized ensemble of quantum states with finite average energy depending on the metric divergence of an ensemble. In the case of a multi-mode quantum oscillator, this upper bound is tight for large energy. Upper bounds for the Holevo capacity of finite-dimensional quantum channels depending on metric characteristics of the channel output are obtained.
Quantum communication holds the potential to revolutionize information transmission by enabling secure data exchange that exceeds the limits of classical systems. One of the key performance metrics in quantum information theory, namely the Holevo bound, quantifies the amount of classical information that can be transmitted reliably over a quantum channel. However, computing and optimizing the Holevo bound remains a challenging task due to its dependence on both the quantum input ensemble and the quantum channel. In order to maximize the Holevo bound, we propose a unified projected gradient ascent algorithm to optimize the quantum channel given a fixed input ensemble. We provide a detailed complexity analysis for the proposed algorithm. Simulation results demonstrate that the proposed quantum channel optimization yields higher Holevo bounds than input ensemble optimization.
I derive a universal upper bound on the capacity of any communication channel between two distant systems. The Holevo quantity, and hence the mutual information, is at most of order EΔt/ℏ, where E is the average energy of the signal, and Δt is the amount of time for which detectors operate. The bound does not depend on the size or mass of the emitting and receiving systems, nor on the nature of the signal. No restrictions on preparing and processing the signal are imposed. As an example, I consider the encoding of information in the transverse or angular position of a signal emitted and received by systems of arbitrarily large cross section. In the limit of a large message space, quantum effects become important even if individual signals are classical, and the bound is upheld.
This paper is primarily concerned with the development and application of quantum bounds on mutual information, although some of the methods developed can be applied to any figure of merit indicating degree of correlation, such as coincidence rate. Three basic techniques for obtaining bounds are described: mappings between joint-measurement and communication correlation contexts; a duality relation for quantum ensembles and quantum measurements; and an information exclusion principle [M. J. W. Hall, Phys. Rev. Lett. 74, 3307 (1995)]. Results include a proof of Holevo's communication bound from a joint-measurement inequality; a measurement-dependent dual to Holevo's bound; lower bounds for mutual information under ensemble and measurement constraints; information exclusion relations for measurements described by probability-operator measures; a proof that Glauber coherent states are optimal signal states for quantum communication based on (noisy) optical heterodyne detection; and an information inequality for quantum eavesdropping. Relations between the three techniques are used to further obtain upper bounds for quantum information, and to extend the information exclusion principle to a joint-measurement context.