Correlations in quantum systems with a high degree of symmetry

We study dynamical signatures of quantum chaos in one of the most relevant models in many-body quantum mechanics, the Bose-Hubbard model, whose high degree of symmetries yields a large number of invariant subspaces and degenerate energy levels. The standard procedure to reveal signatures of quantum chaos requires classifying the energy levels according to their symmetries, which may be experimentally and theoretically challenging. We show that this classification is not necessary to observe manifestations of spectral correlations in the temporal evolution of the survival probability, which makes this quantity a powerful tool in the identification of chaotic many-body quantum systems.We study dynamical signatures of quantum chaos in one of the most relevant models in many-body quantum mechanics, the Bose-Hubbard model, whose high degree of symmetries yields a large number of invariant subspaces and degenerate energy levels. The standard procedure to reveal signatures of quantum chaos requires classifying the energy levels according to their symmetries, which may be experimentally and theoretically challenging. We show that this classification is not necessary to observe manifestations of spectral correlations in the temporal evolution of the survival probability, which makes this quantity a powerful tool in the identification of chaotic many-body quantum systems.

arXiv:2005.06589Physical review. ESep 8, 202027 citations

Correlations in Quantum Mechanics

Richard Healey

Compendium of Quantum PhysicsJan 1, 20090 citations

Covariant catalysis requires correlations and good quantum reference frames degrade little

Lauritz van Luijk, Reinhard F. Werner, Henrik Wilming

Catalysts are quantum systems that open up dynamical pathways between quantum states which are otherwise inaccessible under a given set of operational restrictions while, at the same time, they do not change their quantum state. We here consider the restrictions imposed by symmetries and conservation laws, where any quantum channel has to be covariant with respect to the unitary representation of a symmetry group, and present two results. First, for an exact catalyst to be useful, it has to build up correlations to either the system of interest or the degrees of freedom dilating the given process to covariant unitary dynamics. This explains why catalysts in pure states are useless. Second, if a quantum system ("reference frame") is used to simulate to high precision unitary dynamics (which possibly violates the conservation law) on another system via a global, covariant quantum channel, then this channel can be chosen so that the reference frame is approximately catalytic. In other words, a reference frame that simulates unitary dynamics to high precision degrades only very little.Catalysts are quantum systems that open up dynamical pathways between quantum states which are otherwise inaccessible under a given set of operational restrictions while, at the same time, they do not change their quantum state. We here consider the restrictions imposed by symmetries and conservation laws, where any quantum channel has to be covariant with respect to the unitary representation of a symmetry group, and present two results. First, for an exact catalyst to be useful, it has to build up correlations to either the system of interest or the degrees of freedom dilating the given process to covariant unitary dynamics. This explains why catalysts in pure states are useless. Second, if a quantum system ("reference frame") is used to simulate to high precision unitary dynamics (which possibly violates the conservation law) on another system via a global, covariant quantum channel, then this channel can be chosen so that the reference frame is approximately catalytic. In other words, a reference frame that simulates unitary dynamics to high precision degrades only very little.

arXiv:2301.09877QuantumNov 6, 20239 citations

Quantum Correlations in a Multi-particle System

Jean-Louis Basdevant, Jean Dalibard

Springer eBooksJan 1, 20190 citations

Exploring Entropic Orders: High Temperature Continuous Symmetry Breaking, Chiral Topological States and Local Commuting Projector Models

Po-Shen Hsin, Ryohei Kobayashi

High temperature is usually expected to destroy order: as the Gibbs state approaches the infinite-temperature limit, it becomes an equal-weight ensemble over all states and the system is generically disordered. Recent works showed that entropic order can violate this expectation through coupling to bosons in classical lattice models and quantum field theories, where the ordered states have higher entropy. Here we present new analytic methods for constructing quantum lattice models that exhibit entropic orders. In particular, we construct quantum lattice models with continuous symmetry breaking at high temperature in 1+1 dimensions and clarify how entropic order can evade the Hohenberg-Mermin-Wagner theorems. We also construct high-temperature entropic $p+ip$ chiral topological superconducting states in 2+1 dimensions with temperature-independent anyon correlation functions. In addition, we obtain a broad family of high-temperature entropic non-chiral topological orders. We show that the entropic topological orders have strong higher form symmetries at high temperature unlike the conventional topological orders, and the symmetry is spontaneously broken. These results follow from two general constructions that couple a given lattice model with a low-temperature ordered phase either to ordered bosons or, for local commuting-projector Hamiltonians, to more general bosonic degrees of freedom.High temperature is usually expected to destroy order: as the Gibbs state approaches the infinite-temperature limit, it becomes an equal-weight ensemble over all states and the system is generically disordered. Recent works showed that entropic order can violate this expectation through coupling to bosons in classical lattice models and quantum field theories, where the ordered states have higher entropy. Here we present new analytic methods for constructing quantum lattice models that exhibit entropic orders. In particular, we construct quantum lattice models with continuous symmetry breaking at high temperature in 1+1 dimensions and clarify how entropic order can evade the Hohenberg-Mermin-Wagner theorems. We also construct high-temperature entropic $p+ip$ chiral topological superconducting states in 2+1 dimensions with temperature-independent anyon correlation functions. In addition, we obtain a broad family of high-temperature entropic non-chiral topological orders. We show that the entropic topological orders have strong higher form symmetries at high temperature unlike the conventional topological orders, and the symmetry is spontaneously broken. These results follow from two general constructions that couple a given lattice model with a low-temperature ordered phase either to ordered bosons or, for local commuting-projector Hamiltonians, to more general bosonic degrees of freedom.

cond-mat.str-elCross-listedarXiv:2604.18694Physical review. B./Physical review. BApr 22, 20260 citations

A dynamical mean-field theory approach to superconductivity and antiferromagnetism in a strongly correlated electron system

Hikaru Watanabe, Sebastian Doniach

We present the results of numerical studies of superconductivity and antiferromagnetism in a strongly correlated electron system. To do this we construct a Hubbard model on a lattice of self-consistently embedded multi-site clusters by means of a dynamical mean-field theory in which intra-cluster dynamics is treated essentially exactly. We show that a class of characteristic features which have been seen in the excitation spectra of high- $T_{c}$ cuprates (e.g., pseudogap and the spin-flip resonance), as well as their interplay with the onset of a pairing correlations, can be captured within a dynamical mean-field theory in which short-wavelength dynamics are rigorously treated. Thus we infer that the observation of the neutron scattering resonance in the superconducting state of the cuprate superconductors does not appear to be directly tied to their quasi-2D character. Although our approach is defined strictly in terms of fermion degrees of freedom, we show that we can readily identify the emergence of effective low energy bosonic degrees of freedom in the presence of a well-defined broken symmetry phase as long as their dynamics are dominated by short-range, short-wavelength fluctuations. Our results reveal that the dynamics of staggered spin degrees of freedom builds up coherence and a resonance-like sharp feature emerges as pairing correlations set in. Under conditions of superconducting broken symmetry our approach thus extends static BCS mean field theory to provide an exact treatment of quantum fluctuations of the BCS order parameter.We present the results of numerical studies of superconductivity and antiferromagnetism in a strongly correlated electron system. To do this we construct a Hubbard model on a lattice of self-consistently embedded multi-site clusters by means of a dynamical mean-field theory in which intra-cluster dynamics is treated essentially exactly. We show that a class of characteristic features which have been seen in the excitation spectra of high-$T_{c}$ cuprates (e.g., pseudogap and the spin-flip resonance), as well as their interplay with the onset of a pairing correlations, can be captured within a dynamical mean-field theory in which short-wavelength dynamics are rigorously treated. Thus we infer that the observation of the neutron scattering resonance in the superconducting state of the cuprate superconductors does not appear to be directly tied to their quasi-2D character. Although our approach is defined strictly in terms of fermion degrees of freedom, we show that we can readily identify the emergence of effective low energy bosonic degrees of freedom in the presence of a well-defined broken symmetry phase as long as their dynamics are dominated by short-range, short-wavelength fluctuations. Our results reveal that the dynamics of staggered spin degrees of freedom builds up coherence and a resonance-like sharp feature emerges as pairing correlations set in. Under conditions of superconducting broken symmetry our approach thus extends static BCS mean field theory to provide an exact treatment of quantum fluctuations of the BCS order parameter.

arXiv:cond-mat/0205086Physical review. D/Physical review. D.May 4, 20020 citations

p+ip

chiral topological superconducting states in 2+1 dimensions with temperature-independent anyon correlation functions. In addition, we obtain a broad family of high-temperature entropic non-chiral topological orders. We show that the entropic topological orders have strong higher form symmetries at high temperature unlike the conventional topological orders, and the symmetry is spontaneously broken. These results follow from two general constructions that couple a given lattice model with a low-temperature ordered phase either to ordered bosons or, for local commuting-projector Hamiltonians, to more general bosonic degrees of freedom.

T_{c}

cuprates (e.g., pseudogap and the spin-flip resonance), as well as their interplay with the onset of a pairing correlations, can be captured within a dynamical mean-field theory in which short-wavelength dynamics are rigorously treated. Thus we infer that the observation of the neutron scattering resonance in the superconducting state of the cuprate superconductors does not appear to be directly tied to their quasi-2D character. Although our approach is defined strictly in terms of fermion degrees of freedom, we show that we can readily identify the emergence of effective low energy bosonic degrees of freedom in the presence of a well-defined broken symmetry phase as long as their dynamics are dominated by short-range, short-wavelength fluctuations. Our results reveal that the dynamics of staggered spin degrees of freedom builds up coherence and a resonance-like sharp feature emerges as pairing correlations set in. Under conditions of superconducting broken symmetry our approach thus extends static BCS mean field theory to provide an exact treatment of quantum fluctuations of the BCS order parameter.