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

Authors：Zhao, Teng; Wu, Shao-ping; Yang, Guo-qing; Huang, Guang-ming; Li, Gao-xiang^*

Source：Optics Express, 2021, 29(15): 23939-23952.

DOI：10.1364/OE.432358

• ERROR-CORRECTION
• QUANTUM-STATE
• FLUORESCENCE
• LINEWIDTH
• LASER

The ultrastrongly coupling (USC) system has very important research significance in quantum simulation and quantum computing. In this paper, the ultranarrow spectrum of a circuit QED system with two qubits ultrastrongly coupled to a single-mode cavity is studied. In the regime of USC, the JC model breaks down and the counter-rotating terms in the quantum Rabi Hamiltonian leads to the level anti-crossing in the energy spectrum. Choosing a single-photon driving field at the point of avoided-level crossing, we can get an equivalent four-level dressed state model, in which the dissipation of the two intermediate states is only related to the qubits decay. Due to the electron shelving of these two metastable states, a narrow peak appears in the cavity emission spectrum. Furthermore, we find that the physical origin for the spectral narrowing is the vacuum-induced quantum interference between two transition pathways. And this interference effect couples the slowly decaying incoherent components of the density matrix into the equations of the sidebands. This result provides a possibility for the study of quantum interference effect in the USC system.