biostudies-literatureTsujimoto YMEXT | Japan Society for the Promotion of Science (JSPS)8720https://www.ebi.ac.uk/biostudies/studies/S-EPMC12504699biostudies-literatureUnknown16(1)In photonic quantum information processing, quantum operations using nonlinear photon-photon interactions are vital for implementing two-qubit gates and enabling faithful entanglement swapping. However, due to the weak interaction between single photons, the all-photonic realization of such quantum operations has remained out of reach so far. Herein, we demonstrate an entanglement swapping using sum-frequency generation between single photons in a χ⁽²⁾-nonlinear optical waveguide. We show that a high signal-to-noise ratio (SNR), stable sum-frequency-generation-based entanglement heralder with an ultralow-dark-count superconducting single-photon detector can satisfy the unprecedented SNR requirement indispensable for the swapping protocol. Furthermore, the system clock is enhanceNature communicationsExperimental entanglement swapping through single-photon χ<sup>(2)</sup> nonlinearity.PMC12504699JP22K03490JP18K13487JP20K14393Kato GMiki STsujimoto YFujiwara MTerai HYabuno MWakui KKishimoto TfalseExperimental entanglement swapping through single-photon χ<sup>(2)</sup> nonlinearity.In photonic quantum information processing, quantum operations using nonlinear photon-photon interactions are vital for implementing two-qubit gates and enabling faithful entanglement swapping. However, due to the weak interaction between single photons, the all-photonic realization of such quantum operations has remained out of reach so far. Herein, we demonstrate an entanglement swapping using sum-frequency generation between single photons in a χ⁽²⁾-nonlinear optical waveguide. We show that a high signal-to-noise ratio (SNR), stable sum-frequency-generation-based entanglement heralder with an ultralow-dark-count superconducting single-photon detector can satisfy the unprecedented SNR requirement indispensable for the swapping protocol. Furthermore, the system clock is enhance2025-01-01T00:00:00Z2025 Oct2026-06-04T06:24:30.862Z2026-05-06T03:13:27.248ZS-EPMC125046994105731210.1038/s41467-025-63785-5