High-Dimensional Orbital Angular Momentum Entanglement in Optical Fibers

Quantum states encoded in high-dimensional Hilbert spaces provide advantages over conventional two-level systems, including increased information capacity per photon, enhanced robustness against noise, and stronger violations of local realism in entanglement tests. The photon’s orbital angular momentum (OAM), with its unbounded integer values, is a natural platform for such encoding. Traditionally, OAM states are generated through spontaneous parametric downconversion (SPDC) in bulk crystals, but the resulting free-space modes are not directly compatible with optical fiber networks and typically require lossy post-selection to equalize mode-dependent emission amplitudes. This work investigates the generation of high-dimensional OAM states in multimode ring-core fibers (RCFs) using intermodal spontaneous four-wave mixing (SFWM). The stability and scalability of OAM propagation in RCFs are analyzed, followed by the principles of SFWM between guided modes. A versatile inverse-design algorithm enables complete control over the amplitude and phase of the pump field, supporting arbitrary OAM superpositions and processing of the generated states. The spectral correlations of the emitted photon pairs, described by the joint spectral amplitude (JSA), are shown to depend on mode selection and can be tuned nondestructively from correlated to uncorrelated to anti-correlated distributions. High single-photon performance is demonstrated with heralded second-order correlations below 0.005 and coincidence-to-accidental ratios above 4000, which to the best of current knowledge represent the highest values reported for fiber-based systems using avalanche photodetectors. Furthermore, through enforcement of JSA overlap, OAM correlations spanning 15 transverse dimensions in fiber are identified, each maintaining high single-photon performance. This result establishes a foundation for fiber-based high-dimensional transverse-mode entanglement and outlines a scalable pathway toward quantum networks.