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

TL;DR

This paper investigates the ultranarrow spectral line in a double qubit-cavity ultrastrong coupling system, revealing quantum interference effects that lead to spectral narrowing, with implications for quantum simulation and computing.

Contribution

It introduces a four-level dressed state model in USC regime showing how quantum interference causes spectral narrowing in cavity emission.

Findings

Spectral narrowing due to vacuum-induced quantum interference.

Identification of a four-level dressed state model in USC.

Narrow peak in cavity emission spectrum linked to metastable states.

Abstract

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.