Quantum physicists have finally done something that has been on their to-do list for more than two decades: they have measured a specific kind of multi-photon entanglement in the lab. The work comes from scientists at Kyoto University and Hiroshima University, and it could make quantum technologies far easier to build.
The achievement is a genuine milestone. For over 25 years, researchers have been able to measure one kind of entangled state but not another. Now it is closed.
The Problem With Quantum Measurement
Quantum entanglement is weird. In an entangled system, particles lose their separate identities and behave as one unit. Measure one particle, and you know something about the other, even if it is light-years away. That violates classical physics, which is why Albert Einstein famously found it troubling.
Measuring entanglement is hard. One common method is quantum tomography, which involves making many measurements and then piecing together the results. The problem is that the amount of data needed grows exponentially with the number of particles. Add a few photons, and the workload explodes.
There is a better way. Instead of collecting a mountain of measurements, you can make one smart measurement that tells you what the system is doing. Scientists have been able to do this for one famous kind of entangled state, called the GHZ state. But no one had managed it for another important state, the W state.
Until now.
Shigeki Takeuchi’s Three-Photon Breakthrough
The Kyoto team focused on the W state. The W state is an entangled state where the photons share a particular kind of correlation, and it is central to many quantum protocols. The team’s key insight was to use a mathematical property of the W state called cyclic shift symmetry.
In simple terms, the photons in a W state can be rotated in a repeating cycle without changing the underlying pattern. The team built a photonic quantum circuit that performs a quantum Fourier transformation, a mathematical operation that reorganizes quantum information. That transformation revealed the W state’s hidden structure.
The researchers tested their idea with three photons. They sent individual photons into the device with carefully chosen polarization states, and the system distinguished among different types of three-photon W states. Each state represents a particular non-classical correlation shared by the three incoming photons.
They also measured the fidelity of the entangled measurement. Fidelity tells you how reliably a quantum system performs its intended task. In this case, it is the probability that the device produces the correct result when given a pure W-state input.
What This Means for Quantum Technology
The measurement could speed up several fields. Quantum teleportation, for example, relies on entanglement to move quantum information from one place to another. The new measurement approach could make that process faster and more reliable.
The technique could also help with quantum communication protocols, which transfer multi-photon entangled states. And it could aid measurement-based quantum computing, where computations are performed by manipulating entangled states.
Shigeki Takeuchi, the corresponding author, put the stakes plainly. “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,” he said.
He also stressed the importance of basic research. “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.
From Three Photons to Many
The current demonstration works with three photons. The team plans to scale up. Their longer-term goal is to apply the method to larger-scale and more general multi-photon quantum entangled states.
They also want to build on-chip photonic quantum circuits capable of performing entangled measurements. That would make the technology more compact and easier to integrate into future quantum systems.
Here is what the team is working on next:
- Scaling the technique to larger quantum systems
- Developing on-chip photonic quantum circuits for entangled measurements
- Applying the method to more general multi-photon states
| Comparison | GHZ state | W state |
|---|---|---|
| Measurement status | Already demonstrated | Just demonstrated |
| Key property | Not specified in the source | Cyclic shift symmetry |
| Use cases | Not specified in the source | Quantum teleportation, computation |
The difference between the two states matters. The Kyoto team has shown that the W state is measurable.
The Road Ahead
The Kyoto team’s work closes a significant gap in quantum physics. It shows that the measurement techniques that work for one kind of entanglement can be adapted to others.
The next step is scaling. Three photons is a proof of concept.
The research was published in Science Advances in 2025. The paper is titled “Entangled measurement for W states.” The authors are Geobae Park, Holger F. Hofmann, Ryo Okamoto, and Shigeki Takeuchi.
The measurement approach is a tool, not a finished product. But tools like this one make the finished products possible. The Kyoto team has shown that the W state is measurable, and that changes what engineers can build.
The road from three photons to many is long, but it starts somewhere. The Kyoto team started it.
Source material: “Quantum teleportation breakthrough: Scientists crack a 25-year entanglement challenge,” ScienceDaily.
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