A research team from Kyoto University and Hiroshima University in Japan has successfully realized an entanglement measurement method capable of identifying the three-photon W state. This marks the first time such a measurement has been achieved for the W state, 25 years after the proposal of entanglement measurement for the GHZ state. The W state is a major type of multiphoton entanglement that, until now, could not be identified through a single entanglement measurement.

Quantum entanglement is one of the most peculiar features of the quantum world, describing deep correlations between particles such as photons, such that the system must be treated as a whole. This view conflicts with the classical physics notion that each particle exists independently. Entanglement has become a core element of future technologies such as quantum computing, quantum communication, and quantum teleportation. To build these technologies, scientists need reliable methods to accurately determine the type of entangled state produced. The standard method, quantum tomography, can estimate quantum states, but as the number of photons increases, the required number of measurements grows dramatically, creating a severe bottleneck. Entanglement measurement, on the other hand, can identify a specific entangled state in one shot. Scientists had previously achieved such measurements for the GHZ state, but the W state remained elusive.

The research team focused on the cyclic shift symmetry of the W state. Exploiting this property, they proposed a photonic quantum circuit that can perform a quantum Fourier transform on W states of any number of photons, converting hidden structures into measurable signals. They constructed a device for three photons using a highly stable optical quantum circuit, a system capable of long-term operation without active control. The researchers injected three single photons with carefully chosen polarization states into the device, which successfully distinguished different types of 3-photon W states, each representing specific non-classical correlations among the three incident photons. The team also evaluated the fidelity of the entanglement measurement, i.e., the probability that the device gives the correct result when the input is a pure W state.

"More than 25 years after the initial proposal of entanglement measurement for GHZ states, we have finally obtained one for W states as well, and conducted a real experimental demonstration for the three-photon W state," said corresponding author Shigeki Takeuchi. This achievement could advance quantum teleportation, novel quantum communication protocols, and measurement-based quantum computing. Takeuchi said, "To accelerate the research and development of quantum technologies, it is crucial to deepen the understanding of fundamental concepts to propose innovative ideas." This work moves quantum communication and photonic quantum systems from laboratory demonstrations toward more scalable platforms. Following the W state research in 2025, progress in the field has continued: in late 2025, researchers achieved all-photonic quantum teleportation using photons from different quantum dots; in 2026, another team reported an integrated photonic chip capable of generating, manipulating, and measuring multipartite cluster state entanglement; the same year, researchers tested a three-node quantum network over fiber optic cables in New York, using entanglement swapping to connect quantum links. These advances highlight the long-term need for precise entanglement measurement. The Kyoto University and Hiroshima University team plans to extend their method to larger, more general multiphoton entangled states and to develop on-chip photonic quantum circuits for entanglement measurement.

Publication Details: Title: "Quantum breakthrough could revolutionize teleportation and computing"; Published in: Science Advances (2025); Journal Information: Science Advances