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Physical Review A

Derivation and physical interpretation of the optimum detection operators for coherent-state signals

Masao Osaki, Masashi Ban, Osamu Hirota

Published Aug 1, 1996• 76 citations
Publisher

Abstract

The optimum detection operators are derived for several linearly independent signal sets. The quantum minimax strategy is applied to the M-ary coherent state signals such as binary phase shift, ternary symmetric, ternary phase shift, and quadrature phase shift. It is shown that the detection process induces a kind of quantum interference between signal quantum states and measurement states. \textcopyright{} 1996 The American Physical Society.

DOI
10.1103/physreva.54.1691

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The optimum detection operators are derived for several linearly independent signal sets. The quantum minimax strategy is applied to the M-ary coherent state signals such as binary phase shift, ternary symmetric, ternary phase shift, and quadrature phase shift. It is shown that the detection process induces a kind of quantum interference between signal quantum states and measurement states. \textcopyright{} 1996 The American Physical Society.
Abstract A significant issue at present in the field of quantum information theory is the implementation of an optimum quantum receiver in which the error probability is minimized. Because a quantum receiver is represented by detection operators, in order to implement an optimum quantum receiver, it is important to derive optimum detection operators that minimize the error probability. Although it is generally difficult to analytically derive such a detection operator, optimum detection operators have been obtained on several specific signal systems in which signals are in the pure state. On the other hand, because mixed‐state signals are associated with more general signals, including classical noise, it is more important to address mixed‐state signals. However, for these signals, only basic optimum detection operators, such as binary signals in a two‐dimensional system and M‐ary symmetric signals, have been obtained. Regarding these optimum detection operators, it has been suggested that optimum detection operators for pure‐state signals can also be a reasonably good signal detection process for mixed‐state signals. Based on this suggestion, this paper examines the extent to which the optimum detection operators for pure‐state signals can be applied to mixed‐state signals. © 2003 Wiley Periodicals, Inc. Electron Comm Jpn Pt 3, 86(10): 8–18, 2003; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/ecjc.1153
The calculation method proposed here obtains an optimum detection process for an M-ary signal whose quantum states are linearly independent and nonorthogonal. Optimizations are carried out by minimizing the average probability of error or by maximizing the mutual information in the signal detection process. Quantum detection operators, in terms of which the signal detection process is mathematically described, are assumed to be projection operators defined on a Hilbert space H s spanned by the quantum states of the signal; that is, the best projection operators that describe the signal detection process are obtained. The optimization procedure is equivalent to finding an appropriate unitary transformation in the Hilbert space H s. For a binary quantum state signal, the optimum detection processes in which the average probability of error is minimized and in which the mutual information is maximized are obtained. It is found that the optimum detection process minimizing the average probability of error is the same as that maximizing the mutual information.
The mathematical structure of the quantum-optimum detection operators for optical signals is derived, and approximated in a three-dimensional number-state basis for the case of weak optical signals. It is shown that mathematical rotation of the received signal-states in Hilbert-space approaches the quantum-optimum detection performance for optical signals such as On-Off Keying (OOK), BPSK (binary phase-shift keying), and binary Pulse Position Modulation (2-PPM) signaling formats originating from interplanetary or even greater interstellar distances. The approximate three-dimensional number-state model is interpreted to help identify the physical operations required to implement the quantum-optimum receiver with well-known field-preparation and signal-processing techniques for these three modulation formats. Graphs demonstrating the practical performance of receivers approximated by three-dimensional detection operators are evaluated, and compared with the theoretical limit on ideal receiver performance dictated by the laws of quantum mechanics.
A new technique for evaluating the performance of quantum signals observed in the presence noise is described and evaluated. The quantum theory for detecting coherent-state signals has been developed previously, however the quantum "signal plus noise" problem has received little attention due to its complexity. Here we develop a discrete approximation to the coherent-state representation of signal-plus-noise density operators, and present solutions to optimum receiver performance in terms of quantum "measurement states" whose performance is optimized via generalized rotations in Hilbert space. An efficient algorithm for carrying out the required numerical optimization is described and applied to binary signals observed in the presence of noise, for which exact results are available for comparison. The algorithm is then applied to the detection of ternary signals observed in the presence of noise, a previously unsolved problem, and the performance of the optimum receiver characterized.
The problem of a correct quantum description of the phase difference is examined from the perspective of parameter estimation theory. It is shown that an optimum phase-shift measurement defines a phase difference operator which coincides with other approaches to the same problem. We also study the fundamental limit to the accuracy of a phase difference shift detection. We show that this limit can be reached by a measurement having countable outcomes despite the fact that a phase shift can take any value. We show that this is the case of the phase difference operator defined by an optimum phase-shift measurement. \textcopyright{} 1996 The American Physical Society.