Dynamics of many-body photon bound states in chiral waveguide QED

TL;DR

This paper investigates the formation, propagation, and scattering of multi-photon bound states in chiral waveguide QED, revealing their role in quantum many-body phenomena and soliton dynamics.

Contribution

It introduces a comprehensive theoretical analysis of photon bound states in chiral waveguides, connecting few-photon quantum effects to many-body and classical soliton behaviors.

Findings

Photon bound states propagate with group delay scaling as 1/n^2.

Incident coherent pulses break into bound state components during propagation.

Bound states can undergo elastic scattering with additional photons.

Abstract

We theoretically study the few- and many-body dynamics of photons in chiral waveguides. In particular, we examine pulse propagation through a system of $N$ two-level systems chirally coupled to a waveguide. We show that the system supports correlated multi-photon bound states, which have a well-defined photon number $n$ and propagate through the system with a group delay scaling as $1/n^2$. This has the interesting consequence that, during propagation, an incident coherent state pulse breaks up into different bound state components that can become spatially separated at the output in a sufficiently long system. For sufficiently many photons and sufficiently short systems, we show that linear combinations of $n$-body bound states recover the well-known phenomenon of mean-field solitons in self-induced transparency. For longer systems, however, the solitons break apart through quantum correlated dynamics. Our work thus covers the entire spectrum from few-photon quantum propagation, to genuine quantum many-body (atom and photon) phenomena, and ultimately the quantum-to-classical transition. Finally, we demonstrate that the bound states can undergo elastic scattering with additional photons. Together, our results demonstrate that photon bound states are truly distinct physical objects emerging from the most elementary light-matter interaction between photons and two-level emitters. Our work opens the door to studying quantum many-body physics and soliton physics with photons in chiral waveguide QED.