Thermodynamic Limit in the Two-qubit Quantum Rabi Model with Spin-Spin Coupling

TL;DR

This paper reveals a second-order superradiant quantum phase transition in a two-qubit system with spin-spin interaction, introducing a thermodynamic limit based on coupling ratios.

Contribution

It introduces a novel thermodynamic limit for the two-qubit quantum Rabi model with spin-spin interaction, enabling analysis of phase transitions.

Findings

Identification of a second-order superradiant phase transition.

Definition of a thermodynamic limit based on coupling ratios.

Insights into quantum phase transitions in multi-qubit systems.

Abstract

The occurrence of a second-order superradiant quantum phase transition is brought to light in a quantum system consisting of two interacting qubits coupled to the same quantized field mode. We introduce an appropriate thermodynamic-like limit for the integrable two-qubit quantum Rabi model with spin-spin interaction. Namely, it is determined by the infinite ratios of the spin-spin and the spin-mode couplings to the mode frequency, regardless of the spin-to-mode frequency ratios.