RQC Seminar

214th RQC Seminar

• Speaker

  Mr. Zheng Ri-Hua
  ( Fuzhou University, China )

• Date

  16:00-17:00 (4:00 p.m. - 5:00 p.m.), August 12, 2025 (Tuesday)

• Venue

  Hybrid(ZOOM・ Wako Main Research Bldg. 3F 345-347 Seminar Room / 研究本館3階 セミナー室 (345-347) (C01))

• Title

  Observation of quantum phase transitions with emergent entanglement in circuit QED systems

• Inquiries

  rqc_info[at]ml.riken.jp

Abstract
This report focuses on quantum phase transitions (QPTs) accompanied by emergent entanglement in circuit QED systems. Two sets of experiments are presented to demonstrate the emergence of nonclassical entangled states along with the QPTs. In the first set of experiments [1,3], a controllable quantum Rabi model is constructed. As the system is tuned across the critical point, the cavity field undergoes a transition from a normal phase to a superradiant phase characterized by Schrödinger cat state superpositions with pronounced quantum interference. Wigner tomography is employed to comprehensively characterize the light–matter hybrid state, revealing strong nonclassical features of the field and its entanglement with the qubit. In the second experiment [2], a multiqubit Lipkin-Meshkov-Glick (LMG) model is realized, where spontaneous symmetry breaking (SSB), purely due to quantum effects, is observed alongside the generation of multiqubit GHZ-state entanglement. The Hamiltonian of the system captures the competition between continuous driving and intra-qubit interactions. In this experiment, six Xmon qubits are nearly homogeneously coupled via a common bus resonator. By slowly decreasing the drive strength, a quasi-adiabatic evolution is achieved, in which the initial symmetric product state evolves into a superposition of two degenerate symmetry-broken eigenstates. The gradual enhancement of Z₂ symmetry breaking is observed in real time by measuring average two-qubit longitudinal correlations. The quantum nature of the SSB process is further confirmed via measurements of multiqubit transverse correlations and Wigner function. The results highlight the fundamental difference between the SSB in QPT of many-body systems and that in classical phase transitions, offering new insights into QPTs in finite-size quantum systems: (1) In finite-size systems, SSB corresponds to GHZ state generation (SSB = GHZ); (2) In the thermodynamic

limit (N → ∞), decoherence scales linearly with N, leading to the loss of quantum coherence;

(3) GHZ-type entanglement provides a criterion for distinguishing QPTs in finite quantum systems from those in the thermodynamic limit.