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Physical review. B./Physical review. B

Quantum metric tensor of the Dicke model: Analytical and numerical study

Daniel Gutiérrez‐Ruiz, Jorge Chávez-Carlos, Diego González, Jorge G. Hirsch, J. David Vergara

Published Jun 15, 2022• 6 citations
Publisher

Abstract

We compute both analytically and numerically the quantum metric tensor and its scalar curvature for the Dicke model. In the analytical setting we consider the thermodynamic limit and carry out the computations by means of the truncated Holstein-Primakoff approximation. We also study the exactly solvable case ${\ensuremath{\omega}}_{0}=0$ and find that the corresponding non-Abelian QMT effectively reduces to just one metric tensor with zero determinant. In the numerical case we use an efficient basis to diagonalize the Hamiltonian for four different system's sizes. For the components of the quantum metric tensor and their derivatives, we find a remarkable agreement between the numerical and analytical results, with the metric's peaks signaling the precursors of the quantum phase transition. In the case of the scalar curvature, there are some differences between the numerical and analytical results that can be traced back to the behavior of the combination of the metric components' derivatives. Notably, the scalar curvature in the thermodynamic limit is continuous across the quantum phase transition and, in that zone, it approximately matches the numerical results.

DOI
10.1103/physrevb.105.214106

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We compute both analytically and numerically the quantum metric tensor and its scalar curvature for the Dicke model. In the analytical setting we consider the thermodynamic limit and carry out the computations by means of the truncated Holstein-Primakoff approximation. We also study the exactly solvable case \ensuremathω0=0{\ensuremath{\omega}}_{0}=0\ensuremathω0​=0 and find that the corresponding non-Abelian QMT effectively reduces to just one metric tensor with zero determinant. In the numerical case we use an efficient basis to diagonalize the Hamiltonian for four different system's sizes. For the components of the quantum metric tensor and their derivatives, we find a remarkable agreement between the numerical and analytical results, with the metric's peaks signaling the precursors of the quantum phase transition. In the case of the scalar curvature, there are some differences between the numerical and analytical results that can be traced back to the behavior of the combination of the metric components' derivatives. Notably, the scalar curvature in the thermodynamic limit is continuous across the quantum phase transition and, in that zone, it approximately matches the numerical results.
We study the classical analog of the quantum metric tensor and its scalar curvature for two well-known quantum physics models. First, we analyze the geometry of the parameter space for the Dicke model with the aid of the classical and quantum metrics and find that, in the thermodynamic limit, they have the same divergent behavior near the quantum phase transition, as opposed to their corresponding scalar curvatures which are not divergent there. On the contrary, under resonance conditions, both scalar curvatures exhibit a divergence at the critical point. Second, we present the classical and quantum metrics for the Lipkin-Meshkov-Glick model in the thermodynamic limit and find a perfect agreement between them. We also show that the scalar curvature is only defined on one of the system's phases and that it approaches a negative constant value. Finally, we carry out a numerical analysis for the system's finite sizes, which clearly shows the precursors of the quantum phase transition in the metric and its scalar curvature and allows their characterization as functions of the parameters and of the system's size.
We study the quantum metric tensor and its scalar curvature for a particular version of the Lipkin-Meshkov-Glick model. We build the classical Hamiltonian using Bloch coherent states and find its stationary points. They exhibit the presence of a ground-state quantum phase transition where a bifurcation occurs, showing a change in stability associated with an excited-state quantum phase transition. Symmetrically, for a sign change in one Hamiltonian parameter, the same phenomenon is observed in the highest-energy state. Employing the Holstein-Primakoff approximation, we derive analytic expressions for the quantum metric tensor and compute the scalar and Berry curvatures. We contrast the analytic results with their finite-size counterparts obtained through exact numerical diagonalization and find excellent agreement between them for large sizes of the system in a wide region of the parameter space except in points near the phase transition where the Holstein-Primakoff approximation ceases to be valid.
The geometric properties of quantum states are fully encoded by the quantum geometric tensor. The real and imaginary parts of the quantum geometric tensor are the quantum metric and Berry curvature, which characterize the distance and phase difference between two nearby quantum states in Hilbert space, respectively. For conventional Hermitian quantum systems, the quantum metric corresponds to the fidelity susceptibility and has already been used to specify quantum phase transitions from the geometric perspective. In this paper, we extend this wisdom to the non-Hermitian systems for revealing non-Hermitian critical points. To be concrete, by employing numerical exact diagonalization and analytical methods, we calculate the quantum metric and corresponding order parameters in various non-Hermitian models, which include two non-Hermitian generalized Aubry-Andr\'e models and non-Hermitian cluster and mixed-field Ising models. We demonstrate that the quantum metric of eigenstates in these non-Hermitian models exactly identifies the localization transitions, mobility edges, and many-body quantum phase transitions with gap closings, respectively. We further show that this strategy is robust against the finite-size effect and different boundary conditions.
We obtain the ground-state energy level and associated geometric phase in the Dicke model analytically by means of the Holstein-Primakoff transformation and the boson expansion approach in the thermodynamic limit. The nonadiabatic geometric phase induced by the photon field is derived with the time-dependent unitary transformation. It is shown that the quantum phase transition characterized by the nonanalyticity of the geometric phase is remarkably of the first order. We also investigate the scaling behavior of the geometric phase at the critical point, which can be measured in a practical experiment to detect the quantum phase transition.
The Dicke model generalized to the case where the dependence of the coupling constant on individual atoms is taken into account is investigated. It is shown that the generalized Dicke model exhibits a quantum phase transition from the normal phase to the superradiant phase in the thermodynamic limit, as well as the standard Dicke model. The mean photon number, the atomic inversion, the lowest excitation energy, and the entanglement entropy between the field and the atoms are evaluated analytically or numerically, and the critical behavior is examined. As a result, it turns out that the critical behavior in the generalized model is essentially the same as that in the standard model. This implies universality in the Dicke model.
The energy spectrum of Dicke Hamiltonians with and without the rotating wave approximation for an arbitrary atom number is obtained analytically by means of the variational method, in which the effective pseudo-spin Hamiltonian resulting from the expectation value in the boson-field coherent state is diagonalized by the spin-coherent-state transformation. In addition to the ground-state energy, an excited macroscopic quantum-state is found corresponding to the south- and north-pole gauges of the spin-coherent states, respectively. Our results of ground-state energies in exact agreement with various approaches show that these models exhibit a zero-temperature quantum phase transition of the second order for any number of atoms, which was commonly considered as a phenomenon of the thermodynamic limit with the atom number tending to infinity. The critical behavior of the geometric phase is analyzed.