Frustration-induced anomalous transport and strong photon decay in waveguide QED

TL;DR

This paper investigates how ultrastrong frustrated coupling in waveguide QED causes anomalous photon transport, strong photon decay, and breakdown of traditional quasiparticle descriptions, with implications for quantum optics experiments.

Contribution

It reveals the effects of ultrastrong frustrated coupling on photon transport, showing significant inelastic scattering and photon decay phenomena not captured by existing models.

Findings

Weak elastic response at the spin frequency

Strong broadband inelastic response at higher energies

Photon decay into multiple lower-energy photons

Abstract

We study the propagation of photons in a one-dimensional environment consisting of two non-interacting species of photons frustratingly coupled to a single spin-1/2. The ultrastrong frustrated coupling leads to an extreme mixing of the light and matter degrees of freedom, resulting in the disintegration of the spin and a breakdown of the "dressed-spin", or polaron, description. Using a combination of numerical and analytical methods, we show that the elastic response becomes increasingly weak at the effective spin frequency, showing instead an increasingly strong and broadband response at higher energies. We also show that the photons can decay into multiple photons of smaller energies. The total probability of these inelastic processes can be as large as the total elastic scattering rate, or half of the total scattering rate, which is as large as it can be. The frustrated spin induces strong anisotropic photon-photon interactions that are dominated by inter-species interactions. Our results are relevant to state-of-the-art circuit and cavity quantum electrodynamics experiments.