Ultranarrow spectral line of the radiation in double qubit-cavity ultrastrong coupling system

Abstract

We investigate the ultranarrow spectrum and non-Gaussian feature of a circuit QED system with two qubits ultrastrongly coupled to a single-mode cavity. By adjusting the frequency of the driving field to resonate with the cavity frequency at the point of avoided-level crossing, we can get an equivalent four-level dressed state model. The vacuum-induced quantum interference between two transition pathways couples the slow decaying incoherent components of the density matrix into the equations of the sidebands, thereby an ultranarrow peak appeared at the line center of the cavity emission spectrum. This result provides a possibility for the study of quantum interference effect in the USC system. Moreover, the two metastable states with electronic shelving in the nondegenerate four-level system enhance the effect of spectral line narrowing. Finally, we also explore the intensity-amplitude correlation of the system, and the non-Gaussian feature of the ultrastrong coupling system can be identified, indicating that the system has broad application prospects in the field of nonlinear optics and preparation of non-classical states.