We explore the quantum criticality of a two-site model combining quantum Rabi models with hopping interaction. Through a combination of analytical and numerical approaches, we find that the model allows the appearance of a superradiant quantum phase transition (QPT) even in the presence of strong ${\mathbf{A}}^{2}$ terms, preventing single-site superradiance. In the two-site model the effect of ${\mathbf{A}}^{2}$ terms can be surmounted by the photon delocalization from hopping, and a reversed superradiant QPT occurs as a consequence of the competition between ${\mathbf{A}}^{2}$ terms and the hopping interaction. We characterize the phase diagram and scaling functions and extract the critical exponents in the vicinity of the critical point, thus establishing the universal behavior of the second-order phase transition. Remarkably the effective hopping strength will be enhanced if more cavities are cascaded. We also prove that the multiqubit counterpart of the quantum Rabi dimer, i.e., the Dicke dimer, has the same properties in beating the ${\mathbf{A}}^{2}$ effect. Our work provides a path to the study of phase transitions in the presence of the ${\mathbf{A}}^{2}$ terms and offers the prospect of investigating quantum-criticality physics and quantum devices in many-body systems.We explore the quantum criticality of a two-site model combining quantum Rabi models with hopping interaction. Through a combination of analytical and numerical approaches, we find that the model allows the appearance of a superradiant quantum phase transition (QPT) even in the presence of strong ${\mathbf{A}}^{2}$ terms, preventing single-site superradiance. In the two-site model the effect of ${\mathbf{A}}^{2}$ terms can be surmounted by the photon delocalization from hopping, and a reversed superradiant QPT occurs as a consequence of the competition between ${\mathbf{A}}^{2}$ terms and the hopping interaction. We characterize the phase diagram and scaling functions and extract the critical exponents in the vicinity of the critical point, thus establishing the universal behavior of the second-order phase transition. Remarkably the effective hopping strength will be enhanced if more cavities are cascaded. We also prove that the multiqubit counterpart of the quantum Rabi dimer, i.e., the Dicke dimer, has the same properties in beating the ${\mathbf{A}}^{2}$ effect. Our work provides a path to the study of phase transitions in the presence of the ${\mathbf{A}}^{2}$ terms and offers the prospect of investigating quantum-criticality physics and quantum devices in many-body systems.

arXiv:2003.01325Physical Review AJun 29, 202018 citations

Quantum Criticality in Coupled Spin-Dimer Systems

Stefan Weßel, S. C. Wenzel

Bulletin of the American Physical SocietyMar 17, 20100 citations

Quantum criticality in the Hubbard model

Mark Jarrell

Bulletin of the American Physical SocietyMar 25, 20111 citation

Phase-driven unconventional superradiance phase transition in non-Hermitian cascaded quantum Rabi cavities

Shujie Cheng, Shuai-Peng Wang, G. D. de Moraes Neto +1 more

This study investigates phase-driven symmetry breaking leading to superradiance phase transitions in cascaded non-Hermitian quantum Rabi cavities. Non-Hermiticity is introduced via the phase coupling $\ensuremath{\varphi}$ between the atom and the optical field. In the thermodynamic limit of the quantum harmonic oscillator, we analytically derive the superradiance phase boundary, validated by observables. An unconventional quantum phase transition without a Hermitian analog arises when $|\ensuremath{\varphi}|=\frac{\ensuremath{\pi}}{4}$ or $\frac{3\ensuremath{\pi}}{4}$, where the phase boundary is uniquely determined by the cavity coupling, at $\mathcal{J}=\frac{1}{2}$ ( $\mathcal{J}$ is the ratio of the cavity coupling strength $J$ to the optical field frequency ${\ensuremath{\omega}}_{p}$, and $\mathcal{J}=\frac{J}{{\ensuremath{\omega}}_{p}}$), independent of the atom-photon coupling strength $g$ . For other $\ensuremath{\varphi}$, the phase boundary relies on both $\mathcal{J}$ and $g$ , similar to the scenario observed in Hermitian systems. Furthermore, we identify phase-driven first- and second-order superradiance phase transitions, focusing on the quantum criticality of the second-order transition by determining the critical exponents and the universality class. The feasibility of experimental realization is also discussed, aiming to inspire further studies on non-Hermitian superradiance quantum phase transitions.This study investigates phase-driven symmetry breaking leading to superradiance phase transitions in cascaded non-Hermitian quantum Rabi cavities. Non-Hermiticity is introduced via the phase coupling $\ensuremath{\varphi}$ between the atom and the optical field. In the thermodynamic limit of the quantum harmonic oscillator, we analytically derive the superradiance phase boundary, validated by observables. An unconventional quantum phase transition without a Hermitian analog arises when $|\ensuremath{\varphi}|=\frac{\ensuremath{\pi}}{4}$ or $\frac{3\ensuremath{\pi}}{4}$, where the phase boundary is uniquely determined by the cavity coupling, at $\mathcal{J}=\frac{1}{2}$ ($\mathcal{J}$ is the ratio of the cavity coupling strength $J$ to the optical field frequency ${\ensuremath{\omega}}_{p}$, and $\mathcal{J}=\frac{J}{{\ensuremath{\omega}}_{p}}$), independent of the atom-photon coupling strength $g$. For other $\ensuremath{\varphi}$, the phase boundary relies on both $\mathcal{J}$ and $g$, similar to the scenario observed in Hermitian systems. Furthermore, we identify phase-driven first- and second-order superradiance phase transitions, focusing on the quantum criticality of the second-order transition by determining the critical exponents and the universality class. The feasibility of experimental realization is also discussed, aiming to inspire further studies on non-Hermitian superradiance quantum phase transitions.

arXiv:2406.16576Physical Review AMay 1, 20253 citations

Dynamic sensitivity of quantum Rabi model with quantum criticality

Ying Hu, Huang Jian, Jin‐Feng Huang +2 more

We study the dynamic sensitivity of the quantum Rabi model, which exhibits quantum criticality in the finite-component-system case. This dynamic sensitivity can be detected by introducing an auxiliary two-level atom far-off-resonantly coupled to the cavity field of the quantum Rabi model. We find that when the quantum Rabi model goes through the critical point, the auxiliary atom experiences a sudden decoherence, which can be characterised by a sharp decay of the Loschmidt echo. Our scheme will provide a reliable way to observe quantum phase transition in ultrastrongly coupled quantum systems.We study the dynamic sensitivity of the quantum Rabi model, which exhibits quantum criticality in the finite-component-system case. This dynamic sensitivity can be detected by introducing an auxiliary two-level atom far-off-resonantly coupled to the cavity field of the quantum Rabi model. We find that when the quantum Rabi model goes through the critical point, the auxiliary atom experiences a sudden decoherence, which can be characterised by a sharp decay of the Loschmidt echo. Our scheme will provide a reliable way to observe quantum phase transition in ultrastrongly coupled quantum systems.

arXiv:2101.01504Jan 5, 20212 citations

{\mathbf{A}}^{2}

terms, preventing single-site superradiance. In the two-site model the effect of

{\mathbf{A}}^{2}

terms can be surmounted by the photon delocalization from hopping, and a reversed superradiant QPT occurs as a consequence of the competition between

{\mathbf{A}}^{2}

terms and the hopping interaction. We characterize the phase diagram and scaling functions and extract the critical exponents in the vicinity of the critical point, thus establishing the universal behavior of the second-order phase transition. Remarkably the effective hopping strength will be enhanced if more cavities are cascaded. We also prove that the multiqubit counterpart of the quantum Rabi dimer, i.e., the Dicke dimer, has the same properties in beating the

{\mathbf{A}}^{2}

effect. Our work provides a path to the study of phase transitions in the presence of the

{\mathbf{A}}^{2}

terms and offers the prospect of investigating quantum-criticality physics and quantum devices in many-body systems.

\mathcal{J}=\frac{1}{2}

(

\mathcal{J}

is the ratio of the cavity coupling strength

J

to the optical field frequency ${\ensuremath{\omega}}_{p}$, and $\mathcal{J}=\frac{J}{{\ensuremath{\omega}}_{p}}$), independent of the atom-photon coupling strength

g

. For other $\ensuremath{\varphi}$, the phase boundary relies on both

\mathcal{J}

and

g

, similar to the scenario observed in Hermitian systems. Furthermore, we identify phase-driven first- and second-order superradiance phase transitions, focusing on the quantum criticality of the second-order transition by determining the critical exponents and the universality class. The feasibility of experimental realization is also discussed, aiming to inspire further studies on non-Hermitian superradiance quantum phase transitions.