High-dimensional entanglement for quantum communication in the frequency domain

High-dimensional photonic entanglement is a promising candidate for error-protected quantum information processing with improved capacity. Encoding high-dimensional qudits in the carrier frequency of photons combines ease of generation, universal single-photon gates, and compatibility with fiber transmission for high-capacity quantum communication. Recent landmark experiments have impressively demonstrated quantum interference of a few frequency modes, yet the certification of massive-dimensional frequency entanglement has remained an open challenge. This study shows how to harness the large frequency-entanglement inherent in standard continuous-wave spontaneous parametric down-conversion processes. It further reports a record certification of discretized frequency entanglement, combined with a novel approach for certification that is both highly efficient and nonlocally implementable. This technique requires very few measurements and does not require assumptions on the state. The work opens the possibility for utilizing this encoding in quantum communications and in quantum information science in general.