Quantum phase transitions play an important role in many-body systems and have been a research focus in conventional condensed matter physics over the past few decades. Artificial atoms, such as superconducting qubits that can be individually manipulated, provide a new paradigm of realising and exploring quantum phase transitions by engineering an on-chip quantum simulator. Here we demonstrate experimentally the quantum critical behaviour in a highly-controllable superconducting circuit, consisting of four qubits coupled to a common resonator mode. By off-resonantly driving the system to renormalise the critical spin-field coupling strength, we have observed a four-qubit non-equilibrium quantum phase transition in a dynamical manner, i.e., we sweep the critical coupling strength over time and monitor the four-qubit scaled moments for a signature of a structural change of the system's eigenstates. Our observation of the non-equilibrium quantum phase transition, which is in good agreement with the driven Tavis-Cummings theory under decoherence, offers new experimental approaches towards exploring quantum phase transition related science, such as scaling behaviours, parity breaking and long-range quantum correlations.
@article{arxiv.1406.1436,
title = {Exploring the quantum critical behaviour in a driven Tavis-Cummings circuit},
author = {M. Feng and Y. P. Zhong and T. Liu and L. L. Yan and W. L. Yang and J. Twamley and H. Wang},
journal= {arXiv preprint arXiv:1406.1436},
year = {2015}
}