title: Exploring the quantum critical behaviour in a driven Tavis-Cummings circuit.
abstract: 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 renormalize the critical spin-field coupling strength, we have observed a four-qubit nonequilibrium quantum phase transition in a dynamical manner; that is, 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 nonequilibrium 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.
authors.affiliation: State Key Laboratory of Magnetic Resonance and Atomic and Molecular Physics, Wuhan Institute of Physics and Mathematics, Chinese Academy of Sciences, Wuhan 430071, China.
authors.forename: M
authors.initials: M
authors.lastname: Feng
authors.affiliation: Department of Physics, Zhejiang University, Hangzhou 310027, China.
authors.forename: Y P
authors.initials: YP
authors.lastname: Zhong
authors.affiliation: 1] State Key Laboratory of Magnetic Resonance and Atomic and Molecular Physics, Wuhan Institute of Physics and Mathematics, Chinese Academy of Sciences, Wuhan 430071, China [2] The School of Science, Southwest University of Science and Technology, Mianyang 621010, China.
authors.forename: T
authors.initials: T
authors.lastname: Liu
authors.affiliation: State Key Laboratory of Magnetic Resonance and Atomic and Molecular Physics, Wuhan Institute of Physics and Mathematics, Chinese Academy of Sciences, Wuhan 430071, China.
authors.forename: L L
authors.initials: LL
authors.lastname: Yan
authors.affiliation: State Key Laboratory of Magnetic Resonance and Atomic and Molecular Physics, Wuhan Institute of Physics and Mathematics, Chinese Academy of Sciences, Wuhan 430071, China.
authors.forename: W L
authors.initials: WL
authors.lastname: Yang
authors.affiliation: Department of Physics and Astronomy, ARC Centre for Engineered Quantum Systems, Macquarie University, Sydney, New South Wales 2109, Australia.
authors.forename: J
authors.initials: J
authors.lastname: Twamley
authors.affiliation: 1] Department of Physics, Zhejiang University, Hangzhou 310027, China [2] Synergetic Innovation Center of Quantum Information and Quantum Physics, University of Science and Technology of China, Hefei, Anhui 230026, China.
authors.forename: H
authors.initials: H
authors.lastname: Wang