An entangled measurement offers a potentially far more efficient alternative. Instead of collecting a huge set of measurements and reconstructing the state afterward, this approach can identify an entangled state in a single shot.

Scientists had already achieved such a measurement for the Greenberger-Horne-Zeilinger (GHZ) state, one of the best-known forms of multi-photon entanglement. But no comparable method had been proposed, let alone experimentally demonstrated, for the W state — another important class of entangled multi-photon states. That is the gap the Japanese team set out to close.

How does the new technique work?

The researchers built their approach around a mathematical property of the W state known as cyclic shift symmetry. In plain terms, the arrangement of the photons can be shifted in a repeating cycle while an important underlying pattern stays intact.

Exploiting that symmetry, the team theoretically designed an entangled measurement based on a photonic quantum circuit. The circuit performs a quantum Fourier transformation — a mathematical operation that reorganizes quantum information in a way that reveals patterns that are otherwise hard to detect. In principle, the method can apply to W states containing any number of photons.

To test the idea, the researchers constructed a device and sent three individual photons into it, each prepared with carefully selected polarization states. The results showed the system could distinguish among different types of three-photon W states. Each of those states represents a particular non-classical correlation shared by the three incoming photons.

A key practical detail: the device used high-stability optical quantum circuits that kept operating for long periods without active control — a meaningful engineering requirement for laboratory quantum optics.

The team also measured the fidelity of the entangled measurement. Fidelity indicates how reliably a quantum system performs its intended task; here, it corresponds to the probability that the device produces the correct result when given a pure W-state input.

What could this mean for quantum technologies?

The advance could touch several areas of quantum technology. One is quantum teleportation — a process that transfers quantum information from one location to another. Despite the name, quantum teleportation does not physically transport matter. It uses entanglement to transfer the quantum state carrying the information.

The researchers see potential applications in:

• New quantum communication protocols
• Methods for transferring multi-photon quantum entangled states
• New forms of measurement-based quantum computing

Takeuchi framed the result as part of a broader effort to strengthen the foundations of the field. "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," he said.

What comes next?

The three-photon demonstration is a proof of principle, and the researchers now plan to extend the approach to larger and more general multi-photon entangled states. They also intend to develop on-chip photonic quantum circuits capable of performing entangled measurements — a step that could make the technology more compact and easier to integrate into future quantum systems.

The usual caveats apply. The experimental demonstration covered only three photons, and the general, any-number-of-photons version of the method remains, so far, a theoretical result. Still, after a quarter-century wait, the toolkit for identifying multi-photon entanglement now covers both of its best-known families — GHZ states and, at last, W states.