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
A2 terms, preventing single-site superradiance. In the two-site model the effect of
A2 terms can be surmounted by the photon delocalization from hopping, and a reversed superradiant QPT occurs as a consequence of the competition between
A2 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
A2 effect. Our work provides a path to the study of phase transitions in the presence of the
A2 terms and offers the prospect of investigating quantum-criticality physics and quantum devices in many-body systems.
Abstract We investigate the quantum phase transitions (QPTs) of the two-qubit quantum Rabi model with staggered qubit biases. In the limit of an infinite qubit-to-cavity frequency ratio, we analytically derive the mean-field Hamiltonian and the order-parameter-dependent energy density functional, which yields the ground-state energy and order parameter. The rich superradiant phase transitions (SRPTs), including both second- and first-order QPTs and a tricritical point (TCP), are analytically derived. Specifically, we derive the analytical expressions for all phase transition points, including the nonperturbative point of the first-order SRPT. The analytical findings are further corroborated by numerical finite-size scaling analysis. It is found that both the critical correlation-length and order-parameter exponents at the TCP differ from those of the original second-order SRPTs, implying that the TCP belongs to a new universality class. This work provides a reliable theoretical framework for designing new, simple experimental platforms to explore the rich QPTs.