Scientists Solve 25-Year Quantum Entanglement Mystery with New W-State Measurement Technique

Physicists published a study in September 2025 detailing a new technique to measure multi-particle entangled W states using three photons, sciencealert.com reported. The milestone arrives more than 25 years after scientists first proposed entangled measurements for the Greenberger-Horne-Zeilinger, or GHZ, states, addressing a long-standing challenge in quantum information technology.

Physicists Solve 25-Year Measurement Puzzle

Quantum entanglement links fundamental particles like electrons or photons so deeply that they share a quantum identity and cannot be described as individual entities, sciencealert.com reported. While scientists previously mapped GHZ states, measuring the more complex W state had remained out of reach for a quarter-century. Shigeki Takeuchi, a quantum information researcher cited by sciencealert.com, noted that the team finally obtained the entangled measurement for the W state with a genuine experimental demonstration for 3-photon W states.

Unlike GHZ states, W states possess a distinct property: if one of the entangled particles is lost, the remaining particles retain their useful entangled state, sciencealert.com reported.

One-Shot W State Measurements

The research team employed a one-shot measurement that can identify an entangled state, sciencealert.com reported.

This setup allowed the team to explore the cyclic shift symmetry of the W state, representing a structural fingerprint where the system’s structural description does not change when its individual photons are shifted in a cyclical fashion, sciencealert.com reported. Through this method, the team achieved an averaged measurement discrimination fidelity of 0.871 ± 0.039, correctly identifying the W state 87 percent of the time. That performance exceeded the mathematical threshold of 66.7 percent required to demonstrate that three-particle entanglement measurement has been achieved, with the team attributing the shortfall to imperfections in photon preparation and the measurement setup.

Quantum Teleportation and Communication Protocols Await Broader Multi-Qubit Scaling

The advance carries implications for several areas of quantum technology, including quantum teleportation, which uses entanglement to transfer the quantum state that contains the information from one location to another without physically transporting matter. The new measurement approach could also support new quantum communication protocols, methods for transferring multi-photon quantum entangled states, and new forms of measurement-based quantum computing. While these results advance quantum communications and computing, translating fundamental findings into commercial technologies requires further experimentation and development.

Following the three-photon demonstration, the researchers plan to extend their approach to larger-scale and more general multi-photon quantum entangled states. Takeuchi emphasized that deepening an understanding of basic concepts is crucial to accelerating the research and development of quantum technologies.