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Journal of Physics Conference Series

Quantum information and phase transitions: Fidelity and state distinguishability

Vitor Rocha Vieira

Published Mar 1, 2010• 14 citations
Publisher

Abstract

We review the definition and properties of fidelity as a measure of distinguishability of quantum states, presenting some of its geometric aspects. We apply then fidelity to the study of phase transitions in condensed matter systems, both at zero and finite temperatures. It does not rely on any already known order parameter, being useful not only in the usual Landau type phase transitions but also in phase transitions displaying topological order. The so called fidelity susceptibility and its relation to the usual thermodynamic susceptibilities and response functions are discussed. Finally, we consider the fidelity between different reduced density matrices for a subsystem of a larger system.

DOI
10.1088/1742-6596/213/1/012005

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In this dissertation, we address several problems in condensed matter physics, statistical mechanics, and quantum chaos from the perspective of the quantum information-theoretic notion of state distinguishability, as measured though evaluation of the appropriate fidelity measure. ❧ We find that the fidelity susceptibility, a measure for the response of the fidelity to infinitesimal changes of the Hamiltonian parameters, serves as a useful tool for studying the effects of disorder on quantum phase transitions. We study in detail the phase diagram of the well-known one-dimensional random quantum XY chain. Through an analysis of the fidelity susceptibility’s finite-size scaling and statistics for this model, we find that the fidelity approach reflects large variation between average and typical behavior, may detect the presence of Griffiths phases, and indicates the modifications to the phase diagram due to disorder. ❧ For this same model, though without disorder, we evaluate the dynamics following a sudden Hamiltonian quench using the Loschmidt echo, a form of the fidelity that compares the initial state with the time-evolved state. Through a central limit theorem-type argument we find that the long-time statistics of the Loschmidt echo takes one of two universal forms, even at finite temperature, depending on the system’s proximity to the quantum critical lines. ❧ Finally, for a canonical model exhibiting quantum chaos, the hydrogen atom in a uniform external magnetic field, we evaluate the operator fidelity susceptibility (OFS) between unitary operators generating the dynamics. We find that the part of the OFS which corresponds to the variation of the Hamiltonian eigenvectors serves to indicate the transition from the regular perturbative and n-mixing regimes to the quantum chaotic regime.

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We review the definition and properties of fidelity as a measure of distinguishability of quantum states, presenting some of its geometric aspects. We apply then fidelity to the study of phase transitions in condensed matter systems, both at zero and finite temperatures. It does not rely on any already known order parameter, being useful not only in the usual Landau type phase transitions but also in phase transitions displaying topological order. The so called fidelity susceptibility and its relation to the usual thermodynamic susceptibilities and response functions are discussed. Finally, we consider the fidelity between different reduced density matrices for a subsystem of a larger system.
In this dissertation, we address several problems in condensed matter physics, statistical mechanics, and quantum chaos from the perspective of the quantum information-theoretic notion of state distinguishability, as measured though evaluation of the appropriate fidelity measure. ❧ We find that the fidelity susceptibility, a measure for the response of the fidelity to infinitesimal changes of the Hamiltonian parameters, serves as a useful tool for studying the effects of disorder on quantum phase transitions. We study in detail the phase diagram of the well-known one-dimensional random quantum XY chain. Through an analysis of the fidelity susceptibility’s finite-size scaling and statistics for this model, we find that the fidelity approach reflects large variation between average and typical behavior, may detect the presence of Griffiths phases, and indicates the modifications to the phase diagram due to disorder. ❧ For this same model, though without disorder, we evaluate the dynamics following a sudden Hamiltonian quench using the Loschmidt echo, a form of the fidelity that compares the initial state with the time-evolved state. Through a central limit theorem-type argument we find that the long-time statistics of the Loschmidt echo takes one of two universal forms, even at finite temperature, depending on the system’s proximity to the quantum critical lines. ❧ Finally, for a canonical model exhibiting quantum chaos, the hydrogen atom in a uniform external magnetic field, we evaluate the operator fidelity susceptibility (OFS) between unitary operators generating the dynamics. We find that the part of the OFS which corresponds to the variation of the Hamiltonian eigenvectors serves to indicate the transition from the regular perturbative and n-mixing regimes to the quantum chaotic regime.
Quantum phase transition is one of the most important issues in condensed matter physics,and quantum fidelity is an important concept emerging from quantum information theory.In this paper,we describe as simply as possible the role of fidelity and its leading term,i.e.the so-called fidelity susceptibility,in quantum phase transitions.As a purely geometric quantity,the key advantage of the fidelity approach is that no a priori knowledge of the order parameter is assumed.Specifically,we discuss definitions of the fidelity,the fidelity susceptibility,and the corresponding quantum adiabatic dimension,as well as their role in quantum phase transitions.We also use the Lipkin-Meshkov-Glick model and the Kitaev honeycomb model as examples to clarify these issues.
Quantum phase transitions that take place between two distinct topological phases remain an unexplored area for the applicability of the fidelity approach. Here, we apply this method to spin systems in two and three dimensions and show that the fidelity susceptibility can be used to determine the boundary between different topological phases particular to these models, while at the same time offer information about the critical exponent of the correlation length. The success of this approach relies on its independence on local-order parameters or breaking symmetry mechanisms, with which nontopological phases are usually characterized. We also consider a topological insulator-superconducting phase transition in three dimensions and point out the relevant features of fidelity susceptibility at the boundary between these phases.
The Uhlmann connection is a mixed state generalization of the Berry connection.The latter has a very important role in the study of topological phases at zero temperature.Closely related, the fidelity is an information theoretical measure of distinguishability between quantum states.We show how one can use the fidelity and the Uhlmann connection to study phase transitions at finite temperature.We apply the analysis to free fermion Hamiltonians in 1D exhibiting symmetry protected topological order at zero temperature and also to the BCS theory of superconductivity.We show how one can study finite-temperature dynamical phase transitions by means of the fidelity and interferometric Loschmidt echoes.Moreover, we explain the physical and mathematical origin of the different behaviour of the two Loschmdit echoes by means of the associated susceptibilities.
We study the behavior of the fidelity and the Uhlmann connection in two-dimensional systems of free fermions that exhibit nontrivial topological behavior. In particular, we use the fidelity and a quantity closely related to the Uhlmann factor in order to detect phase transitions at zero and finite temperature for topological insulators and superconductors. We show that at zero temperature both quantities predict quantum phase transitions: a sudden drop of fidelity indicates an abrupt change of the spectrum of the state, while the behavior of the Uhlmann connection signals equally rapid change in its eigenbasis. At finite temperature, the topological features are gradually smeared out, indicating the absence of finite-temperature phase transitions, which we further confirm by performing a detailed analysis of the edge states. Moreover, we performed both analytical and numerical analysis of the fidelity susceptibility in the thermodynamic limit, providing an explicit quantitative criterion for the existence of phase transitions. The critical behavior at zero temperature is further analyzed through the numerical computation of critical exponents.
Quantum fidelity between two density matrices $F({\ensuremath{\rho}}_{1},{\ensuremath{\rho}}_{2})$ is usually defined as the trace of the operator $\mathcal{F}=\sqrt{\sqrt{{\ensuremath{\rho}}_{1}}{\ensuremath{\rho}}_{2}\sqrt{{\ensuremath{\rho}}_{1}}}$. We study the logarithmic spectrum of this operator, which we denote by the fidelity spectrum, in the cases of the XXXXXX spin chain in a magnetic field, a magnetic impurity inserted in a conventional superconductor, and a bulk superconductor at finite temperature. When the density matrices are equal, ${\ensuremath{\rho}}_{1}={\ensuremath{\rho}}_{2}$, the fidelity spectrum reduces to the entanglement spectrum. We find that the fidelity spectrum can be a useful tool in giving a detailed characterization of the different phases of many-body quantum systems.