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Quantum computing

A 25-year-old quantum puzzle, solved with one shot

Physicists in Japan built an optical circuit that identifies a three-photon W state, a kind of shared entanglement, in a single measurement instead of many, and it works in principle for any number of photons.

Peer-reviewed

Checking what kind of entanglement a group of photons shares normally takes a long series of measurements, and the work explodes as photons are added. A team at Kyoto and Hiroshima Universities has now shown how to identify one important kind in a single shot, a problem that had stayed open since a similar method for another kind was proposed more than 25 years ago. The paper was published in Science Advances in September 2025 and highlighted by ScienceDaily this week.

One unit of light, shared by three

Entangled photons are linked, so none of them can be described on its own. In a W state, one unit of light (one excitation) is shared by three photons: it is in the first, the second or the third, all at once and in equal parts. Written out, that is

The W state written as one over root three times the sum of 100, 010 and 001, shown as three rows of three photons with one lit in each.

  • W⟩ = ( 100⟩ + 010⟩ + 001⟩)/√3: each term puts the excitation on a different photon.*

The old way: measure, measure, measure

The standard way to work out which state a set of photons is in is quantum state tomography: measure many copies in many different ways and reconstruct the state from the statistics. The number of measurements needed grows exponentially with the number of photons. In a typical scheme, each extra photon triples the number of measurement settings.

A graph of measurement settings against the number of photons, rising steeply as three to the power n. The cost of tomography grows exponentially (a typical scheme shown).

The trick: rotate it, and it looks the same

An entangled measurement skips the reconstruction and asks the question directly, identifying the state in one go. Such a measurement for GHZ states, another family of multi-photon entanglement, was proposed more than 25 years ago, but none existed for W states. The team, led by Shigeki Takeuchi, used a property of W states called cyclic shift symmetry: if you shift the three photons round in a circle, the state looks the same. A photonic circuit that performs a quantum Fourier transform reorganises the photons so that this symmetry reveals which W-type state came in, in a single shot. They demonstrated it with three photons and measured how reliably it gave the right answer.

Three photons on a ring with the shared light spread over all three; turning the ring leaves the state the same, and an optical circuit reads it in one shot. The symmetry the circuit uses.

Why it matters

Quantum teleportation and future quantum networks need quick ways to identify and pass on entangled states. A one-shot measurement for W states, which in principle extends to any number of photons, adds a tool for teleporting multi-photon states, new communication protocols and measurement-based quantum computing.

The demonstration used three photons; larger versions and on-chip circuits are still to come. The paper itself is from 2025 and reached wide attention only this week.

One measurement can now recognise this kind of shared entanglement, a small but real step towards quantum networks.

Sources

  1. Entangled measurement for W states, Park, Hofmann, Okamoto & Takeuchi, Science Advances 11(37) (published 12 Sep 2025) (details checked via Crossref)
  2. Quantum puzzle solved with a single measurement, ScienceDaily (29 Sep 2026) (Kyoto University news release)