Quantum phases in circuit QED with a superconducting qubit array

TL;DR

This paper demonstrates a circuit QED system with superconducting qubits that exhibits four distinct quantum phases, including two novel phases, due to the competition between spin-photon and spin-spin interactions.

Contribution

The work introduces a realization of a Dicke-Ising model in circuit QED and predicts four quantum phases, notably including two new phases in many-body quantum optics.

Findings

Identification of four quantum phases in the system.

Prediction of two new antiferromagnetic phases.

Unconventional photon signature in the antiferromagnetic superradiant phase.

Abstract

Circuit QED on a chip has become a powerful platform for simulating complex many-body physics. In this report, we realize a Dicke-Ising model with an antiferromagnetic nearest-neighbor spin-spin interaction in circuit QED with a superconducting qubit array. We show that this system exhibits a competition between the collective spin-photon interaction and the antiferromagnetic nearest-neighbor spin-spin interaction, and then predict four quantum phases, including: a paramagnetic normal phase, an antiferromagnetic normal phase, a paramagnetic superradiant phase, and an antiferromagnetic superradiant phase. The antiferromagnetic normal phase and the antiferromagnetic superradiant phase are new phases in many-body quantum optics. In the antiferromagnetic superradiant phase, both the antiferromagnetic and superradiant orders can coexist, and thus the system possesses $Z_{2}^{z}\otimes Z_{2}$\ symmetry. Moreover, we find an unconventional photon signature in this phase. In future experiments, these predicted quantum phases could be distinguished by detecting both the mean-photon number and the magnetization.