Conical intersections and Berry phase in deep-strongly coupled superconducting qubit-resonator system*

Conical intersections, or Dirac cones, have been observed in various modern physical systems. Measuring Berry phases and curvatures are interesting themes [1-2]. Flux-qubit resonator circuits can be well described by a simple model, the quantum Rabi-model, even in the deep strong coupling regime [3]. Numerical analysis reveals that conical intersections occur not only in the symmetric quantum Rabi-model which has clear parity symmetry, but also in the asymmetric quantum Rabi-model which has no obvious symmetry [4]. The occurrence of conical intersections in the asymmetric Rabi-model provides indirect evidence that there should be a hidden symmetry. Here, we propose an experimental scheme to observe conical intersections in the energy landscape and the Berry phase in ultra- or deep-strongly coupled qubit-resonator systems. This will also provide a clue to understand the hidden symmetry in the system.
[1] P. Roushan et al., Nature 515, 241 (2014). [2] X. Tan et al., Phys. Rev. Lett. 122, 210401 (2019). [3] F. Yoshihara, T. Fuse, S. Ashhab et al., Nature Physics 13, 44 (2017). [4] M. T. Batchelor, et al., J. Phys. A 49, (2016) 01LT01.