Generating spatially separated correlated multiphoton states in nonlinear waveguide quantum electrodynamics

TL;DR

This paper proposes a scalable method to generate correlated multi-photon states in nonlinear waveguides using cascaded inelastic scattering, enabling deterministic creation of complex quantum states for advanced quantum technologies.

Contribution

It introduces a novel cascade scattering scheme with pseudo-giant atoms to produce controllable, spatially separated multi-photon entangled states in a scalable manner.

Findings

Achieved unidirectional photon conversion with multiple coupling points.

Generated programmable superpositions of multi-photon states.

Enabled on-demand multi-photon resource generation for quantum applications.

Abstract

Strongly correlated multi-photon states are indispensable resources for advanced quantum technologies, yet their deterministic generation remains challenging due to the inherent weak nonlinearity in most optical systems. Here, we propose a scalable architecture for producing correlated few-photon entangled states via cascaded inelastic scattering in a nonlinear waveguide. When a single photon scatters off a far detuned excited two-level emitter, it coherently converts into a propagating doublon, a bound photon pair with anomalous dispersion. This doublon can subsequently scatter off a downstream excited emitter to further convert into a triplon, and so on, thereby establishing a photon-number amplification cascade $|\cdot \rangle \!\! \rightarrow \!\! |\!\!: \rangle \!\! \rightarrow \! \! |\!\!\therefore \rangle \!\! \to \!\! ...$ Central to this process is the concept of a pseudo-giant atom, which we introduce here to capture the non-local scattering potential emergent from the wave functions of bound states. By implementing this scheme using a real giant atom with multiple engineered coupling points, we achieve unidirectional and full controllable photon conversion without backscattering. The resulting output state forms a programmable superposition of spatially and temporally isolated photon-number components, automatically sorted by their distinct group velocities. This work opens a new paradigm in quantum state engineering, enabling on-demand generation of complex multi-photon resources for quantum simulation, metrology, and scalable quantum networks.