Single-qubit lasing in the strong-coupling regime

TL;DR

This paper investigates the quantum fluctuations and spectral properties of single-qubit lasers in the strong coupling regime, revealing effects like linewidth enhancement and non-exponential decay near the lasing transition.

Contribution

It introduces two complementary theoretical approaches to analyze single-qubit lasing across different coupling strengths and photon number regimes, highlighting quantum fluctuation effects.

Findings

Quantum fluctuations dominate linewidth in strong coupling.

Amplitude and phase fluctuations are coupled when detuned.

Pronounced linewidth enhancement near lasing transition.

Abstract

Motivated by recent ``circuit QED'' experiments we study the lasing transition and spectral properties of single-qubit lasers. In the strong coupling, low-temperature regime quantum fluctuations dominate over thermal noise and strongly influence the linewidth of the laser. When the qubit and the resonator are detuned, amplitude and phase fluctuations of the radiation field are coupled, and the phase diffusion model, commonly used to describe conventional lasers, fails. We predict pronounced effects near the lasing transition, with an enhanced linewidth and non-exponential decay of the correlation functions. We cover a wide range of parameters by using two complementary approaches, one based on the Liouville equation in a Fock state basis, covering arbitrarily strong coupling but limited to low photon numbers, the other based on the coherent-state representation, covering large photon numbers but restricted to weak or intermediate coupling.