Scientists demonstrated the first entangled measurement for W states, a breakthrough for quantum information transfer and computing.
Quantum entanglement highlights the profound divide between classical and quantum physics. In this phenomenon, the state of each photon cannot be described independently, challenging the classical view that every particle has its own distinct reality—an idea that deeply troubled Einstein. Grasping the significance of entanglement is vital for advancing next-generation quantum technologies.
To build such technologies, researchers must be able to reliably generate multi-photon entangled states and accurately determine which type of state has been produced. Conventional quantum tomography, the standard method for analyzing these states, faces a major obstacle: the number of required measurements increases exponentially as the number of photons grows, creating a severe challenge for data collection.
Pursuing entangled measurement for W states
When available, an entangled measurement makes it possible to determine the type of entangled state in a single step. Such a measurement had already been achieved for the Greenberger-Horne-Zeilinger (GHZ) entangled quantum state, but for the W state—another fundamental form of multi-photon entanglement—it had neither been theoretically proposed nor experimentally demonstrated until now.
This challenge was taken up by a team of researchers from Kyoto University and Hiroshima University, who successfully developed a new entangled measurement method capable of identifying the W state.
“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,” says corresponding author Shigeki Takeuchi.
The researchers based their approach on the W state’s cyclic shift symmetry and introduced a theoretical method to build an entangled measurement using a photonic quantum circuit that performs quantum Fourier transformation for W states of any photon number.
To validate the method, they constructed a device designed for three photons, employing highly stable optical quantum circuits that could operate for long periods without active control. By sending in three single photons prepared with specific polarization states, the device successfully distinguished different types of three-photon W states, each tied to a unique non-classical correlation among the input photons. The team also assessed the fidelity of the entangled measurement, defined as the likelihood of obtaining the correct outcome when provided with a pure W-state input.
Future applications in quantum technologies
This achievement opens the door for quantum teleportation, or the transfer of quantum information. It could also lead to new quantum communication protocols, the transfer of multi-photon quantum entangled states, and new methods for 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,” says Takeuchi.
In the future, the team aims to apply their method to a larger-scale, more general multi-photon quantum entangled state, and plans to develop on-chip photonic quantum circuits for entangled measurements.
Reference: “Entangled measurement for W states” by Geobae Park, Holger F. Hofmann, Ryo Okamoto and Shigeki Takeuchi, 12 September 2025, Science Advances.
DOI: 10.1126/sciadv.adx4180
Funding: Japan Science and Technology Agency, Japan Society for the Promotion of Science
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