Physicists Finally Measure the "Other" Kind of Quantum Entanglement After 25 Years
Quantum entanglement is the foundation of quantum computing, quantum cryptography, and other future technologies, but entangled states are notoriously difficult to control. Physicists usually measure them with quantum tomography: they prepare many identical copies of a particle, measure each copy at a different angle, and combine the results to reconstruct the full quantum state.
To picture it, imagine being asked to take a family photo — but instead of one group picture, you must photograph each family member individually and stitch the portraits together. Worse, adding just a few extra photons makes the required data explode exponentially.
Scientists had already invented a shortcut — the entangled measurement, which identifies an entangled state in a single shot. But that trick had only been worked out decades ago for one of the two representative multi-photon entangled states: the GHZ state. The other major type, the W state, had never been measured directly — for more than 25 years, no comparable method had even been proposed.
A team at Kyoto University and Hiroshima University has finally closed the gap. "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," says corresponding author Shigeki Takeuchi.
The team built its approach around a mathematical property of the W state known as cyclic shift symmetry: the arrangement of the photons can be shifted in a repeating cycle while preserving the underlying correlation pattern. Using this symmetry, the researchers theoretically designed a photonic quantum circuit that performs a quantum Fourier transformation — reorganizing quantum information so that otherwise hard-to-detect patterns reveal themselves. In principle, the method applies to W states with any number of photons.
To test the idea, the team constructed a device using high-stability optical quantum circuits that could operate for long periods without active control. By sending three individual photons into the device with carefully selected polarization states, they showed that it could distinguish among different types of three-photon W states — each representing a particular non-classical correlation shared by the three incoming photons.
The researchers also evaluated the fidelity of the entangled measurement: the probability that the device produces the correct result when given a pure W-state input. According to Physics World, the achievement was reported in the journal Science Advances. "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 says.
An entangled measurement captures the whole family in one shot, far more efficiently than tomography. The breakthrough opens the door for quantum teleportation — the transfer of quantum information without moving physical matter — and could lead to new quantum communication protocols, methods for transferring multi-photon entangled states, and new approaches to measurement-based quantum computing. The team is now working to scale the method beyond three photons and to miniaturize the circuits onto photonic chips.