Robust continuous-variable multipartite entanglement in circular arrays of nonlinear waveguides

TL;DR

This paper proposes a robust method for generating scalable multipartite entanglement in circular arrays of nonlinear waveguides using spontaneous parametric down-conversion, with a comprehensive theoretical framework ensuring resilience against experimental variations.

Contribution

It introduces a new protocol for multipartite entanglement generation in circular waveguide arrays, including analytical solutions and configurations for scalable, robust quantum information processing.

Findings

Identifies regimes for full inseparability across arbitrary waveguide numbers

Provides analytical propagation equations for the system

Demonstrates robustness against sample variations

Abstract

Encoding continuous-variable quantum information in the optical domain has recently enabled the generation of large entangled states, yet robust implementation remains a challenge. Here, we present a straightforward protocol for generating multipartite entanglement based on spontaneous parametric down-conversion in a circular array of quadratic nonlinear waveguides. We provide a rigorous theoretical framework, including comprehensive derivations of the propagation equations and the identification of regimes where analytical solutions are possible. Crucially, our approach identifies the pump and detection configurations required to sustain and measure multipartite full inseparability across arbitrary propagation distances and for any number of waveguides $N=4 n$. This regime, elusive to standard numerical methods, represents a key requirement for scalable quantum protocols. Our scheme is inherently robust as it relies on phase-matched propagation eigenmodes, making it resilient against variations in sample length, coupling, and nonlinearity.