Few-Qubit lasing in circuit QED

TL;DR

This paper analyzes the mechanisms and spectral properties of lasing in circuit QED systems with few qubits, comparing different theoretical models and exploring effects of strong coupling and noise.

Contribution

It extends quantum optics models to account for strong qubit-resonator coupling and noise effects in few-qubit lasing systems, providing detailed spectral analysis.

Findings

Population inversion mechanisms identified

Spectral properties characterized in lasing state

Phase diffusion increases with coupling strength

Abstract

Motivated by recent experiments, which demonstrated lasing and cooling of the electromagnetic modes in a resonator coupled to a superconducting qubit, we describe the specific mechanisms creating the population inversion, and we study the spectral properties of these systems in the lasing state. Different levels of the theoretical description, i.e., the semi-classical and the semi-quantum approximation, as well as an analysis based on the full Liouville equation are compared. We extend the usual quantum optics description to account for strong qubit-resonator coupling and include the effects of low-frequency noise. Beyond the lasing transition we find for a single- or few-qubit system the phase diffusion strength to grow with the coupling strength, which in turn deteriorates the lasing state.