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Published in New Journal of Physics in collaboration with Tomohiro Shitara, et al.

Nonclassicality of open circuit QED systems in the deep-strong coupling regime

We investigate theoretically how the ground state of a qubit–resonator (Q–R) system in the deep-strong coupling (DSC) regime is affected by the coupling to an environment. We employ as a variational ansatz for the ground state of the Q–R–environment system a superposition of coherent states displaced in qubit-state-dependent directions. We show that the reduced density matrix of the Q–R system strongly depends on how the system is coupled to the environment, 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 (R–W) coupling. In contrast, when the two couplings are of the same type (for instance, both are inductive), by increasing the R–W coupling strength, the average number of virtual photons increases and the quantum superposition realized in the Q–R entangled ground state is partially degraded. Since the superposition becomes more fragile with increasing the Q–R coupling, there exists an optimal coupling strength to maximize the nonclassicality of the Q–R system. 

Full Text

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

Nonclassicality of open circuit QED systems in the deep-strong coupling regime 

New Journal of Physics 23 103009 (2021)