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

TL;DR

This paper investigates quantum-phase-transition phenomena in few-qubit circuit QED systems under ultrastrong coupling, revealing different phase transition behaviors depending on qubit number and interactions.

Contribution

It provides a comprehensive quantum mechanical analysis of phase transitions in multi-qubit circuit QED systems, highlighting conditions for their occurrence or inhibition.

Findings

One-qubit system exhibits three second-order and one first-order phase transitions.

Two-qubit system shows inhibited phase transition due to qubit-qubit interactions.

Three-qubit system restores superradiant phase transition in ultrastrong-coupling regime.

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.