NICT achieves first entanglement swapping by sum-frequency generation between single photons

by Riko Seibo
Tokyo, Japan (SPX) Nov 12, 2025

The National Institute of Information and Communications Technology (NICT) has demonstrated, for the first time, entanglement swapping using sum-frequency generation (SFG) between single photons - a breakthrough for quantum communication protocols. NICT researchers combined advanced entangled photon-pair sources, low-noise superconducting nanowire detectors, and a highly efficient nonlinear optical crystal, achieving SFG between single photons with an unprecedented signal-to-noise ratio.

This experiment enables high-fidelity entanglement swapping without destroying entangled photon pairs, addressing the major limitation of conventional two-photon interference in quantum computing. Direct detection of SFG photons allows the identification of successful swapping events while preserving the entangled states, opening prospects for loophole-free Bell tests and extended-distance quantum key distribution.

The team's setup delivered SFG photon detection with nearly an order of magnitude improvement in signal-to-noise ratio compared with previous studies. The measured fidelity to the maximally entangled state reached a lower bound of 0.770 +/- 0.076, confirming robust entanglement in the final state.

The result, published in Nature Communications on October 7, 2025, marks the first experimental realization of this quantum protocol and sets a benchmark for future miniaturized and efficient photonic quantum information circuits.

Ongoing work aims to further enhance nonlinear optical efficiency and signal-to-noise ratio, key steps for deploying more advanced quantum communication technologies.

Research Report:Experimental entanglement swapping through single-photon X(2) nonlinearity

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