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The “other” kind is W-state entanglement, a multipartite form usually discussed alongside GHZ-state entanglement. In a paper published in Science Advances on September 12, 2025, researchers demonstrated an entangled measurement for W states using three photonic qubits. They reported an average measurement discrimination fidelity of 0.871 ± 0.039—not a teleportation success rate or a general measure of how entangled the photons were.
What are W states, and how do they differ from GHZ states?
W and GHZ states are distinct classes of multipartite entanglement: quantum correlations shared across multiple particles. The 2025 paper treats W states as a counterpart to GHZ states, which have been the focus of earlier work on scalable multipartite entangled measurements. It describes W states through cyclic shift symmetry and connects them with Dicke states, which represent collective excitations of two-level systems.
The distinction matters because a measurement method designed for one state class does not automatically identify the other. The researchers set out to turn a symmetry characteristic of W states into a detectable pattern in measurement outcomes.
How did the researchers measure a W state?
Park, Hofmann, Okamoto and Takeuchi used a three-mode optical circuit implementing a discrete Fourier transform (DFT). In the proposed method, the DFT transforms the optical modes so that measurement outcomes reveal the W state’s cyclic shift symmetry and project multiqubit states onto W-state components. The experiment demonstrated this approach with three photonic qubits; it did not image photons as objects or test every possible form of entanglement.
What the experiment demonstrated
The team reported discrimination among three-qubit W states, with an average measurement discrimination fidelity (MDF) of 0.871 ± 0.039. The paper compares that result with a maximum MDF of two-thirds for a biseparable measurement; the reported experimental value exceeded that benchmark, supporting the authors’ conclusion that their measurement was entangled. MDF here describes the reported W-state discrimination, not the fidelity of a teleportation protocol.
What remains theoretical
The authors say their ideal proposed setup could detect a W state with 100% efficiency in principle. That is a theoretical claim, separate from the experiment’s measured MDF. The work demonstrates a three-photon measurement; it does not experimentally demonstrate the method at higher photon counts.
Why is this called a 25-year challenge?
The researchers frame the result as a counterpart to entangled-measurement work on GHZ states. Corresponding author Shigeki Takeuchi said: “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.” The quotation was reported in Kyoto University-originated coverage.
Does the result enable quantum teleportation?
No teleportation experiment is reported in this paper. Entangled measurements are relevant to quantum-information protocols: Bell-state measurements, for example, are used in teleportation and entanglement swapping. The authors discuss possible relevance of W-state measurements to photonic quantum computation, quantum communication and sensing, but these are prospective directions rather than deployed applications demonstrated by this work.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.What is established—and what is not
- Established: an experimental entangled measurement for three-photon W states using a DFT optical circuit, with the reported average MDF of 0.871 ± 0.039.
- Not demonstrated in this paper: a many-photon implementation, a quantum teleportation system, or a practical network, computer or sensor based on the result.
- Not a matched comparison: the paper places the result in context with GHZ-state measurement work, but it does not provide a head-to-head benchmark of W- and GHZ-state measurements under identical laboratory conditions.
The result fills in a measurement capability for a different class of multipartite entanglement. Its practical reach will depend on work beyond this three-photon demonstration.
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