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.
@article{arxiv.2310.19595,
title = {Thermodynamic Limit in the Two-qubit Quantum Rabi Model with Spin-Spin Coupling},
author = {R. Grimaudo and G. Falci and A. Messina and E. Paladino and A. Sergi and E. Solano and D. Valenti},
journal= {arXiv preprint arXiv:2310.19595},
year = {2025}
}