Propagation-invariant high-dimensional orbital angular momentum states

TL;DR

This paper introduces a method to generate propagation-invariant high-dimensional orbital angular momentum states using Laguerre-Gaussian modes of the same order, addressing diffraction issues in free-space quantum communication.

Contribution

The authors propose and experimentally demonstrate a new encoding technique using same-order LG modes to create propagation-invariant OAM states, overcoming diffraction challenges.

Findings

Successfully demonstrated propagation-invariant OAM qudits

Eliminated asynchronous diffraction effects without imaging systems

Enhanced robustness for free-space quantum communication

Abstract

Photonic states encoded in spatial modes of paraxial light fields provide a promising platform for high-dimensional quantum information protocols and related studies, where several pioneering theoretical and experimental demonstrations have paved the path for future technologies. Crucially, critical issues encountered in free-space propagation still represent a major challenge. This is the case of asynchronous diffraction between spatial modes with different modal orders, which experience variations in their transverse structure upon free-space propagation. Here we address this issue by proposing an encoding method based on the use of Laguerre-Gaussian (LG) modes of the same modal order N to define a N + 1 dimensional space. Noteworthy, such modes endowed with orbital angular momentum (OAM) experience the same propagation aberrations featuring an identical Gouy phase and wavefront curvature. We demonstrate our proposal experimentally by using time-correlated-single-photon imaging combined with a digital propagation technique. Importantly, our technique allows to eliminate, without the use of imaging systems, all issues related to asynchronous diffraction, providing an accessible way to generate propagation-invariant OAM qudits for quantum optical protocols.