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arXiv · 2411.17267

Experimental entanglement swapping through single-photon $\chi^{(2)}$ nonlinearity

Yoshiaki Tsujimoto·Kentaro Wakui·Tadashi Kishimoto·Shigehito Miki·Masahiro Yabuno·Hirotaka Terai·Mikio Fujiwara·Go Kato
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Abstract

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 $\chi^{(2)}$-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.

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Yoshiaki Tsujimoto, Kentaro Wakui, Tadashi Kishimoto, Shigehito Miki, Masahiro Yabuno, Hirotaka Terai, Mikio Fujiwara, Go Kato. 2024-11-26. Experimental entanglement swapping through single-photon $\chi^{(2)}$ nonlinearity. https://doi.org/10.1038/s41467-025-63785-5

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