Four users exchange entangled photons over six pairwise channels at a distance of 20 km, and yet no one trusts the measurement devices — the result still confirms genuine quantum entanglement.
A Chinese team led by He Lu has experimentally realized measurement-device-independent verification and quantification of entanglement in a fully connected time-bin quantum network. The scientists used a broadband source on periodically poled lithium niobate on insulator and dense wavelength-division multiplexing to distribute all six pairwise entangled channels among four users over 20 km of optical fiber. The trusted input states were encoded in the polarization degree of freedom of the same photons that carried the time-bin entanglement — this made it possible to avoid additional photons and active stabilization of long lines.
Conventional entanglement witnesses can be mistaken if the measurement instruments are unreliable. Here, however, the measurements were performed independently of the devices: polarization encoding served as a kind of "internal reference" that does not depend on exactly how the detectors register photons. Imagine a scale whose readings can be faked — yet you still learn the exact weight, because you built a known reference standard directly into the object being measured.
All six links showed negative values of the MDI witness, exceeding the separability threshold by more than one hundred standard deviations. The entanglement was preserved without active phase correction in the fiber, which simplifies scaling.
The results, published in Physical Review Letters on 30 September 2026, demonstrate a practical path toward reliable characterization of entanglement in large networks where measurement devices may be compromised.
Now quantum networks can grow without requiring absolute trust in every detector on the line.
