2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/24288We show that the quantum phase transition arising in a standard radiation-matter model (Dicke model) belongs to the same universality class as the infinitely-coordinated, transverse field XY model. The effective qubit-qubit exchange interaction is shown to be proportional to the square of the qubit-radiation coupling. A universal finite-size scaling is derived for the corresponding two-qubit entanglement (concurrence) and a size-consistent effective Hamiltonian is proposed for the qubit subsystem.4 pages, 3 figures. Minor changes. Published versionMesoscale and Nanoscale PhysicsStatistical MechanicsQuantum Physicstext