Achieving the entanglement measurement of the W state. Credit: KyotoU / Takeuchi lab via Science News
A team from Kyoto University and Hiroshima University has achieved a major breakthrough in quantum physics: they successfully measured a W-state, a special type of entanglement involving three particles.
In quantum mechanics , entanglement links particles in ways that make their states connected, even across distances. When three or more particles are involved, there are different types of entangled states. The GHZ state is fragile, if one particle is lost, the entanglement collapses. The W-state, however, is robust: even if one particle is lost, the others remain entangled . This makes it very useful for quantum computing, teleportation, and communication networks.
Until now, measuring a W-state in real experiments was extremely difficult because traditional methods require huge numbers of measurements as more particles are added. The Kyoto-Hiroshima team solved this by using a new measurement technique that efficiently reconstructs the full quantum state. They created three entangled photons using spontaneous parametric down-conversion and confirmed the presence of a genuine W-state.
This is the first time a W-state has been directly observed and verified in a lab. The result is a big step toward building more stable quantum networks and understanding how complex quantum systems behave. It also opens the door to experimenting with even larger entangled systems in the future.
Reference: Kyoto University & Hiroshima University. (2025, September 12). First direct measurement of a three-photon W-state in quantum entanglement. Science Daily. https://www.sciencedaily.com/releases/2025/09/250912195122.htm