A research team has demonstrated a distributed spectroscopy technique that uses entangled photon pairs to measure signals along 400 km of optical fiber. Photon pairing enhances the system's sensitivity compared with conventional distributed spectroscopy methods, which lose usefulness over long distances due to signal attenuation in the fiber.

Distributed spectroscopy makes it possible to characterise physical and chemical properties along a fibre cable without the need for point sensors, which is useful for monitoring infrastructure, detecting leaks or observing environmental changes. Until now, the maximum achievable distance was limited by photon loss in the guiding medium, which degrades the signal-to-noise ratio and makes measurement beyond tens of kilometres unfeasible.

The new approach is based on generating correlated photon pairs and using one of them as a probe while the other acts as a reference. This correlation allows background noise to be filtered out and spectral information to be recovered even when the returning signal is extremely weak. The combination of photon pairing with the extended fibre architecture is what enables the record 400 km reach.

The result expands the horizon of distributed optical metrology and opens the door to monitoring systems at telecommunications-network scale. Open questions include the scalability of the scheme to real field conditions and its integration with already deployed fibre infrastructures. Future work will need to confirm the method's performance in uncontrolled environments and assess its compatibility with the standard transmission windows of optical networks.