25 Years in the Making: Quantum Physicists Finally Measure Elusive “W” Entanglement in One Go

25 Years in the Making: Quantum Physicists Finally Measure Elusive “W” Entanglement in One Go

Scientists achieve first single‑shot measurement of a three‑photon W state, ending a 25‑year quest and opening new avenues for quantum teleportation and scalable quantum networks.
LS
Linsey Smith
May 15, 2026
1 min read

Researchers from Kyoto University and Hiroshima University have performed the first-ever single‑shot measurement of a three‑photon W state, one of the two fundamental forms of multi‑partite entanglement. The work, published in Science Advances on 12 September 2025, solves a problem that had remained open for more than 25 years.

Conventional quantum tomography requires an exponentially growing number of measurements as more photons are added, a serious bottleneck for quantum technologies. By exploiting the cyclic shift symmetry of W states, the team built a photonic quantum circuit that implements a quantum Fourier transformation for any number of photons. In their proof‑of‑principle experiment with three photons, the device identified different W states with a fidelity of 0.871 ± 0.039.

“More than 25 years after the initial proposal … we have finally obtained the entangled measurement for the W state as well,” says corresponding author Shigeki Takeuchi. The team notes that the method works with 100% success probability and is scalable to larger W states. This breakthrough opens the door for quantum teleportation, new quantum communication protocols, and measurement‑based quantum computing. The team now plans to extend the method to larger systems and to integrate the circuits onto photonic chips.

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