Ultrastrong-coupling quantum-phase-transition phenomena in a few-qubit circuit QED system

Abstract

We study ultrastrong-coupling quantum-phase-transition phenomena in a few-qubit system. In the one-qubit case, three second-order transitions occur and the Goldstone mode emerges under the condition of ultrastrong-coupling strength. Moreover, a first-order phase transition occurs between two different superradiant phases. In the two-qubit case, a two-qubit Hamiltonian with qubit-qubit interactions is analyzed fully quantum mechanically. We show that the quantum phase transition is inhibited even in the ultrastrong-coupling regime in this model. In addition, in the three-qubit model, the superradiant quantum phase transition is retrieved in the ultrastrong-coupling regime. Furthermore, the N-qubit model with U(1) symmetry is studied and we find that the superradiant phase transition is inhibited or restored with the qubit-number parity.

7 pages, 5 figures

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• Universal dynamics of superradiant phase transition in the anisotropic quantum Rabi model
• Thermodynamic Limit in the Two-qubit Quantum Rabi Model with Spin-Spin Coupling
• Critical phenomena in light-matter systems with collective matter interactions
• Effects of counter-rotating-wave terms on the noisy frequency estimation