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Effects of an environment on the ground state of circuit QED systems in the deep-strong coupling regime

  • Tomohiro Shitara
  • ,
  • Motoaki Bamba
  • ,
  • Fumiki Yoshihara
  • ,
  • Tomoko Fuse
  • ,
  • Sahel Ashhab
  • ,
  • Kouichi Semba
  • ,
  • Kazuki Koshino


We investigate theoretically how the ground state of a qubit-resonator system
in the deep-strong coupling (DSC) regime is affected by the coupling to an
environment. We employ a superposition of coherent states displaced in the
qubit-state-dependent directions as a variational ansatz for the ground state
of the qubit-resonator-environment system. We show that the reduced density
matrix of the qubit-resonator system strongly depends on types of the
resonator-waveguide and resonator-qubit coupling, i.e., capacitive or
inductive, because of the broken rotational symmetry of the eigenstates of the
DSC system in the resonator phase space. When the resonator couples to the
qubit and the environment in different ways (for instance, one is inductive and
the other is capacitive), the system is almost unaffected by the
resonator-waveguide coupling. In contrast, when the types of two couplings are
the same (for instance, both are inductive), by increasing the
resonator-waveguide coupling strength, the average number of virtual photons
increases and the quantum superposition realized in the qubit-resonator
entangled ground state is partially degraded. Since the superposition becomes
more fragile when the qubit-resonator coupling strength gets large, there
exists an optimal strength of the qubit-resonator coupling to maximize the
nonclassicality of the qubit-resonator system.

http://arxiv.org/abs/2006.16769 本文へのリンクあり