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

TL;DR

This paper explores how the ground state of a qubit-resonator system in the deep-strong coupling regime is influenced by environmental interactions, revealing the importance of coupling types and strengths on nonclassicality.

Contribution

It introduces a variational approach to analyze the ground state of open circuit QED systems in the DSC regime, highlighting the impact of coupling symmetry on nonclassical features.

Findings

The system's state depends on whether the environment couples capacitively or inductively.

Matching coupling types enhances the effect of the environment on virtual photon number.

An optimal coupling strength maximizes the nonclassicality of the ground state.

Abstract

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 as a variational ansatz for the ground state of the qubit-resonator-environment system a superposition of coherent states displaced in qubit-state-dependent directions. We show that the reduced density matrix of the qubit-resonator 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 coupling. In contrast, when the two couplings are of the same type (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 with increasing the qubit-resonator coupling, there exists an optimal coupling strength to maximize the nonclassicality of the qubit-resonator system.