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A flashlight in thick fog is useless: the beam scatters, and the reflected light drowns in noise. But if you tie two photons with an invisible thread, one can pass through the fog and come back, while the other stays safe. This connection is called quantum entanglement — when two particles behave as a single whole, even when far apart.

In the new experiment, pairs of entangled photons were born in a special optical setup. One photon stayed in the lab, while the other was sent to an object nearly a kilometer away. The reflected light returned through a turbid medium, where most photons were lost. Surprisingly, even when only a few dozen particles reached the detector, the entanglement did not vanish. The scientists measured its strength and got a value that, in quantum mechanics, means "the connection is definitely there."

This result is an important step toward a quantum radar. A conventional radar sends a signal and waits for the reflection, but in noise it's easy to mistake an echo for interference. Entangled photons allow you to distinguish "your" reflected light from random noise: coincidences between partners give away the target. Even if only a handful of photons return, their link with the partners left in the lab confirms that the signal is real.

This approach could be useful for seeing through fog, smoke, or underwater, where conventional radars go blind. Quantum entanglement, once considered a fragile laboratory curiosity, turns out to be robust enough for real-world distances.

Original abstract

as written by the authors arXiv:2602.08947

Using polarization entangled photon pairs, we demonstrate a robust scheme for quantum illumination and ranging in a lossy environment. Entangled photon pairs are generated in a Sagnac interferometer configuration, yielding high-visibility two-photon polarization entanglement with a measured CHSH parameter of $$S =2.802\pm0.002$$. One of the photons from the entangled pair is retained as idler and the other one is directed into either of the two paths, namely reference and probe, of which probe is sent toward a distant object through a lossy free-space channel, and the reflected photons are collected after round-trip free-space propagation over distances approaching $$1$$ km. Remarkably, strong correlations are observed with CHSH values $$S >2.6$$ even when only a few tens of probe photons are returned, confirming the robustness of polarization entanglement under long-distance free-space propagation. This work reports the robustness of encoding photons in different basis before it is sent towards the object and recovery of polarization entanglement even after a kilometer-scale scattering from the objects, establishing a practical foundation for scalable quantum-assisted object detection and ranging.