Circuit QED Spectra in the Ultrastrong Coupling Regime: How They Differ from Cavity QED

TL;DR

This paper analyzes the spectral differences between circuit QED and cavity QED in the ultrastrong coupling regime, highlighting how coupling to the output port influences observable features despite similar underlying models.

Contribution

It provides a detailed analysis of spectra in circuit QED systems in the ultrastrong coupling regime, revealing distinctive features based on system coupling configurations.

Findings

Spectral features depend on how the system is coupled to the output port.

Distinctive spectral features emerge in the ultrastrong coupling regime.

Circuit QED spectra show differences from cavity QED despite similar models.

Abstract

Cavity quantum electrodynamics (QED) studies the interaction between resonator-confined radiation and natural atoms or other formally equivalent quantum excitations, under conditions where the quantum nature of photons is relevant. Phenomena studied in cavity QED can also be explored using superconducting artificial atoms and microwave photons in superconducting resonators. These circuit QED systems offer the possibility to reach the ultrastrong coupling regime with individual artificial atoms, unlike their natural counterparts. In this regime, the light-matter coupling strength reaches a considerable fraction of the bare resonance frequencies in the system. Here, we provide a careful analysis of both incoherent and coherent spectra in circuit QED systems consisting of a flux qubit interacting with an LC resonator. Despite these systems can be effectively described by the quantum Rabi model, as the corresponding cavity QED ones, we find distinctive features, depending on how the system is coupled to the output port, which become evident in the ultrastrong coupling regime.