Hamiltonian suitable for various quasi-degenerate \textit{non-Hermitian} systems. We apply our results to an exemplary –symmetric circuit QED composed of two non-Hermitian qubits embedded in a lossless resonator. We consider a resonant quantum circuit as |ωr−Ω|≪ωr, where Ω and ωr are qubits and resonator frequencies, respectively, providing well-defined groups of quasi-degenerate resonant states. For such a system, using direct numerical diagonalization we obtain the dependence of the low-lying eigenspectrum on the interaction strength between a single qubit and the resonator, g, and the gain (loss) parameter γ, and compare that with the eigenvalues obtained analytically using the effective Hamiltonian of resonant states. We identify –symmetry broken and unbroken phases, trace the formation of Exceptional Points of the second and the third order, and provide a complete phase diagram g−γ of low-lying resonant states. We relate the formation of Exceptional Points to the additional -pseudo-Hermitian symmetry of the system and show that non-hermiticity mixes the „dark“ and the „bright“ states, which has a direct experimental consequence.
Schrieffer-Wolff transformation for non-Hermitian systems: application for -symmetric circuit QED
Combining non-hermiticity and interactions yields novel effects in open quantum many-body systems. Here, we develop the generalized Schrieffer-Wolff transformation and derive the effective