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
