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Europhysics Letters (EPL)

From asymmetry to correlations

Nan Li, Shunlong Luo, Yuan Sun

Published Jun 3, 2020• 11 citations
Publisher

Abstract

Symmetry, as well as the dual concept of asymmetry, are essential, powerful and ubiquitous in physics and nature. Motivated by the original Wigner-Yanase skew information and its extension by Dyson ( Wigner E. P. and Yanase M. M., Proc. Natl. Acad. Sci. U.S.A. , 49 (1963) 910), which quantifies the information content of a quantum state skew to a conserved observable and may be reinterpreted as asymmetry of a state with respect to an observable, we address the issue of quantifying asymmetry and symmetry of states with respect to Lie groups and Lie algebras. We elucidate that correlations arise naturally from the difference between asymmetries of global and local states. This provides a fundamental insight into correlations. The idea is further illustrated by several examples.

DOI
10.1209/0295-5075/130/30004

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Special Issue: “Symmetries in Quantum Mechanics and Statistical Physics”

Georg Junker

Symmetry is a fundamental concept in science and has played a significant role since the early days of quantum physics [...]

Symmetry•Oct 26, 2021•1 citation
Symmetry, as well as the dual concept of asymmetry, are essential, powerful and ubiquitous in physics and nature. Motivated by the original Wigner-Yanase skew information and its extension by Dyson ( Wigner E. P. and Yanase M. M., Proc. Natl. Acad. Sci. U.S.A. , 49 (1963) 910), which quantifies the information content of a quantum state skew to a conserved observable and may be reinterpreted as asymmetry of a state with respect to an observable, we address the issue of quantifying asymmetry and symmetry of states with respect to Lie groups and Lie algebras. We elucidate that correlations arise naturally from the difference between asymmetries of global and local states. This provides a fundamental insight into correlations. The idea is further illustrated by several examples.
Abstract Asymmetry is a useful physical resource which quantifies the extent to which a quantum state breaks a symmetry. Based on the generalization of Wigner–Yanase–Dyson skew information of quantum states for any operator (not necessarily Hermitian), we introduce an asymmetry measure of a quantum state with respect to a compact Lie group as the average skew information of this state for each operator in its unitary representation and prove that it meets the requirements for the resource theory of asymmetry. For several significant symmetry groups, such as the group U (1), the full unitary group, the local unitary group, the group UU , the group U U ̄ , and the orthogonal group OO , we calculate the asymmetry of quantum states. Furthermore, we compare the asymmetry measure of quantum states with respect to a compact Lie group with the asymmetry measure of quantum states with respect to the corresponding Lie algebras introduced in (2020 Europhys. Lett. 130 30004 ) and illustrate that they are closely related, although they capture different aspects of asymmetry of quantum states.
In order to quantify the information content of quantum states in the presence of conserved quantities, Wigner and Yanase introduced the notion of skew information [Proc. Natl. Acad. Sci. USA 49, 910, (1963)], which was later identified as a paradigmatic version of quantum Fisher information. The skew information is quite different from, yet deeply connected with, the ubiquitous quantum entropies and has important applications in quantum theory. In this paper, pursuing further the original idea of Wigner and Yanase, we propose a measure for correlations in terms of the skew information, investigate its fundamental properties, and elucidate its characteristics. An appealing feature of this measure for correlations is that its evaluation does not involve any optimization, in sharp contrast to the entanglement and discord measures, and can be straightforwardly calculated. In particular, the algorithm and explicit formulas for general bipartite states are prescribed, and simple analytical expressions for some special states, including arbitrary bipartite pure states, the Bell-diagonal states, and some highly symmetric states such as the Werner states and the isotropic states, are obtained.
While Bohr's complementarity principle constitutes a bedrock of quantum mechanics with profound implications, coherence, as a defining feature of the quantum realm originating from the superposition principle, pervades almost every quantum consideration. By exploiting the algebraic and geometric structure of state-channel interaction, we show that an information-theoretic measure of coherence and a quantitative symmetry-asymmetry complementarity emerge naturally from the formalism of quantum mechanics. This is achieved by decomposing the state-channel interaction into a symmetric part and an asymmetric part, which satisfy a conservation relation. The symmetric part is represented by the symmetric Jordan product, and the asymmetric part is synthesized by the skew-symmetric Lie product. The latter further leads to a significant extension of the celebrated Wigner--Yanase skew information, and has an operational interpretation as quantum coherence of a state with respect to a channel. This not only presents a basic and alternative framework for addressing complementarity, but also puts the study of coherence in a broad context involving channels. Fundamental properties of the symmetry-asymmetry complementarity are revealed, and applications and implications are illustrated via several prototypical channels as well as the Mach--Zehnder interferometry, in which the fringe visibility is linked to symmetry and the which-path is linked to asymmetry.
Skew information was introduced by Wigner and Yanase in the context of quantum measurements. It was later recognized that skew information in addition to its original significance as a measure of the information content of states admits several interpretations of a more physical and information theory nature. This quantity has now found many applications in quantum information. An intriguing and subtle feature of skew information is the involvement of the square root of quantum states (density operators). Comparing skew information with some of its natural modifications reveals mathematical and also physical reasons for using the square root. We further use skew information to quantify the asymmetry of quantum states with respect to group representations.