For 25 years, physicists have been able to measure one famous type of quantum entanglement in a single shot. The other major type had no equivalent method — until now.

  • 25 years since the matching measurement method was first proposed for the other entangled state
  • 3 photons successfully entangled and identified in the new experiment
  • 2 universities involved: Kyoto University and Hiroshima University
  • 1 new photonic circuit design its creators say can scale to any number of photons

Two Kinds of Entanglement, One Missing Tool

Quantum entanglement links particles so tightly that they can no longer be described independently — a property Einstein famously distrusted, but one that underpins emerging quantum computing and communication technology. Building real quantum systems requires not just creating entangled photons, but quickly figuring out exactly which entangled state you've made.

The standard method, quantum tomography, works but scales badly: the number of measurements needed explodes as more photons get added. A faster alternative, called an entangled measurement, can identify a state in a single shot instead. Scientists had already built one for the GHZ state, a well-known form of multi-photon entanglement. No one had built an equivalent for the W state, the other major type — despite the GHZ method being proposed a quarter-century ago.

Researchers at Kyoto University and Hiroshima University set out to close that gap.

How the Kyoto Team Cracked It

The team built its method around a mathematical property of the W state called cyclic shift symmetry — the idea that shifting the photons' arrangement in a repeating cycle preserves an underlying pattern. Using that symmetry, they designed a photonic quantum circuit that performs a quantum Fourier transformation, a mathematical operation that reveals patterns in quantum information that would otherwise stay hidden.

They then built a working device and tested it with 3 photons, using high-stability optical circuits that could run for extended periods without active recalibration. By sending in individually polarized photons, the system correctly distinguished between different types of three-photon W states — each representing a distinct, non-classical correlation between the particles. Full details of the method appear in the journal Science Advances.

"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," said corresponding author Shigeki Takeuchi.

What It Means for Quantum Teleportation

The new measurement technique isn't just a theoretical curiosity. It has direct implications for quantum teleportation — the process of transferring quantum information between locations using entanglement, without physically moving matter. It could also feed into new quantum communication protocols and measurement-based quantum computing, an alternative approach to building quantum computers.

The team says their method can, in principle, scale to W states containing any number of photons, not just three. Their next goal is proving that at larger scale, and building compact, on-chip photonic circuits that could perform these entangled measurements outside a specialized optics lab.

"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," Takeuchi said. A 25-year-old gap in the toolkit is now closed — and quantum engineers have one more way to check their work.