After 25 years of effort, physicists from Kyoto University and Hiroshima University have successfully measured the W state of quantum entanglement using a one-shot method, achieving 87% accuracy in identifying 3-photon W states, as reported by multiple outlets including ScienceAlert.
Physicists from Kyoto University and Hiroshima University have cracked a 25-year-old challenge in quantum physics by demonstrating a breakthrough method to measure the W state of entangled photons, a feat described as a real demonstration
by researcher Shigeki Takeuchi. The team’s work, published in Science Advances in September 2025, uses a discrete Fourier transform (DFT) optical circuit to identify W states in a single step, bypassing the inefficiencies of traditional quantum tomography. This development marks a critical advance for quantum computing, communication, and sensing technologies.
The W State Challenge and Its Significance
Quantum entanglement, where particles share a linked quantum state regardless of distance, underpins emerging technologies like quantum computing and secure communication. While Greenberger-Horne-Zeilinger (GHZ) states—often called the vanilla ice cream
of multi-particle entanglement—were measurable decades ago, W states posed a persistent challenge. Unlike GHZ states, W states retain their entangled properties even if one particle is lost, making them more robust but harder to measure. Takeuchi noted, 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, with genuine experimental demonstration for 3-photon W states,
according to Yahoo.
The team’s approach leveraged the cyclic shift symmetry
of W states, a mathematical property where shifting photons in a cycle preserves their quantum structure. By designing a photonic circuit that reorganizes quantum information, the researchers could identify W states with an averaged measurement discrimination fidelity (MDF) of 0.871 ± 0.039, exceeding the 66.7% threshold required to confirm three-particle entanglement. In order to accelerate the research and development of quantum technologies, it is crucial to deepen our understanding of basic concepts to come up with innovative ideas,
Takeuchi emphasized reported by ScienceDaily.
How the Method Works and Its Limitations
The researchers injected three photons with known polarization into a DFT-based optical circuit, which split and recombined their wave functions to detect interference patterns. This allowed them to analyze the W state’s cyclic symmetry without requiring multiple identical entangled systems, a bottleneck in traditional quantum tomography. However, the method achieved 87% accuracy, falling short of 100% due to imperfections in photon preparation and measurement setup. “Why not 100%?” the team asked in their paper cited by ScienceAlert. The team attributes the shortfall to imperfections in photon preparation and the measurement setup itself.
The breakthrough could streamline quantum teleportation, a process that transfers quantum information across distances using entanglement. By simplifying the identification of complex W states, the method may accelerate the development of quantum communication protocols and measurement-based quantum computing. In order to accelerate the research and development of quantum technologies, it is crucial to deepen our understanding of basic concepts to come up with innovative ideas,
Takeuchi emphasized in a Yahoo Tech report.
Advancements in Quantum Computing and Real-World Applications
The team plans to scale their technique to larger multi-photon systems and develop on-chip photonic circuits for practical applications. Such advancements could enhance quantum computing’s role in drug discovery, personalized healthcare, and secure data transmission. While the research is still in its early stages, the success in measuring W states opens new avenues for harnessing quantum mechanics in real-world technologies.
The research underscores the importance of fundamental quantum science in driving technological progress. As Takeuchi noted, The W state differs from GHZ states in that after the loss of one entangled particle, the others can retain a useful entangled state.
This resilience could prove vital for building reliable quantum networks and error-resistant systems, marking a pivotal step toward a quantum future.