Signal Scientists in Japan Instantly Detect Quantum "W States" — Breakthrough for Quantum Internet

Summary

A Kyoto University and Hiroshima University research team, led by corresponding author Shigeki Takeuchi with Geobae Park, Holger F. Hofmann and Ryo Okamoto, achieved the first instantaneous detection of quantum W states using three single photons. The team exploited the "cyclic shift symmetry" of W states, building a photonic quantum circuit that performs quantum Fourier transformation applicable to W states with any number of photons, converting the hidden W-state structure into measurable signals. The demonstration used highly stable optical quantum circuits that operated without active control over extended periods. The work was published in Science Advances (volume 11, issue 37, 2025, DOI 10.1126/sciadv.adx4180) and announced by Kyoto University on 13 May 2026. Takeuchi said, "More than 25 years after the initial proposal concerning the entangled measurement for GHZ states, we have finally obtained the entangled measurement for the W state as well." The result is described as the first experimental demonstration of entangled measurements identifying W states, a fundamental form of quantum entanglement relevant to quantum internet development.