Physicists at Kyoto University and Hiroshima University have experimentally measured the W-state entanglement of three photons, solving a multi-photon measurement challenge that had remained open for more than 25 years using a custom discrete Fourier transform circuit.
Quantum entanglement links fundamental particles so deeply that their properties cannot be described as independent entities, a phenomenon with its disconnect from classical physics. While particles like electrons or photons share a unified quantum identity across physical distances, turning that bizarre connection into functional next-generation technology requires both creating multi-photon states and measuring them reliably. For over two decades, scientists relied heavily on Greenberger-Horne-Zeilinger states, often described as the foundational vanilla ice cream of multi-particle entanglement systems.
Kyoto University and Hiroshima University Researchers Break a 25-Year Bottleneck
Measuring multi-particle entanglement has traditionally relied on a technique called quantum tomography. Much like assembling a massive jigsaw puzzle, quantum tomography requires gathering a massive collection of measurements to reconstruct an entangled state. That traditional approach creates practical hurdles because observing a quantum system snaps it out of its entangled state, forcing researchers to create many identical systems. Furthermore, the amount of required data rises exponentially as the number of photons increases.
A research team spanning Kyoto University and Hiroshima University sidestepped those bottlenecks by demonstrating a one-step measurement that determines an entire entangled system at once. Described in a paper published in Science Advances in September 2025, the team achieved a direct experimental measurement of three-photon W states.
Shigeki Takeuchi, quantum information researcher
“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,”
Cyclic Shift Symmetry and the Discrete Fourier Transform Optical Circuit
Unlike GHZ states, W-state entanglements exhibit greater complexity alongside a unique advantage: if an entangled particle is lost, the others can retain a useful entangled state. To capture this configuration without falling into the exponential data trap of tomography, the researchers capitalized on a mathematical property known as cyclic shift symmetry.
The team constructed a custom optical device acting as an advanced interferometer based on a discrete Fourier transform optical circuit. By injecting three photons of known polarization into the device, the system split them along different paths before crashing them back together to observe how the peaks and valleys of their wave functions combined or canceled out. This approach allowed the researchers to utilize cyclic shift symmetry, where shifting the position of each bulb preserves the overall light pattern—photons in a W state share a similar mathematical balance.
The experimental setup revealed an averaged measurement discrimination fidelity of 0.871 ± 0.039. The team correctly identified the W state condition 87 percent of the time, clearing the mathematical threshold of 66.7 percent required to prove that three-particle entanglement measurement was successfully achieved according to the reporting. The researchers attributed the remaining margin away from absolute perfection to imperfections during photon preparation and within the physical measurement setup.
Applications in Quantum Communication and Scaling to Larger Systems
While this milestone breakthrough proves the concept using three photons, translating foundational physics into commercial hardware requires further development. The research team is already working toward scaling the technique to larger-scale multi-photon systems, with plans to shrink these optical circuits onto microchips.
By clearing a decades-long hurdle in multi-photon analysis, the advance holds practical implications for quantum teleportation—transferring data across distances using entanglement as a bridge rather than physically moving matter—as well as secure data transfer protocols and advanced quantum computing.
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