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 enhanced by utilizing ultrafast telecom entangled photon-pair sources that operate in the GHz range. Our results confirm a lower bound 0.770(76) for the swapped state's fidelity, surpassing the classical limit of 0.5 successfully. Our findings highlight the strong potential of broadband all-single-photonic nonlinear interactions for further sophistication in long-distance quantum communication and photonic quantum computation.
@article{arxiv.2411.17267,
title = {Experimental entanglement swapping through single-photon $\chi^{(2)}$ nonlinearity},
author = {Yoshiaki Tsujimoto and Kentaro Wakui and Tadashi Kishimoto and Shigehito Miki and Masahiro Yabuno and Hirotaka Terai and Mikio Fujiwara and Go Kato},
journal= {arXiv preprint arXiv:2411.17267},
year = {2025}
}