Quantum entanglement refers to a very surprising phenomenon in which two systems form a linked state, with the state of each individual system remaining undefined. Regardless of the distance that separates them, these systems possess correlated physical properties between them.

Continuing to marvel at this, physicists have learned to use entanglement and to imagine technological applications that have no classical equivalent. One of them consists in using entanglement as a way to develop secure communication networks, essentially a kind of quantum internet with unprecedented security guarantees.

Whereas the most classical scheme is based on a pair of photons whose polarization states are correlated, the researchers have chosen to work with only one photon. To produce the single-photon entanglement, it is "sufficient" for them to have a single-photon source, a semi-reflecting strip and two optical fibers. Where previously two photons share a polarization state, two "optical paths" share a single photon.

While the production of single-photon entanglement is much simpler than that of two photons, the same cannot be said for the detection. How can we highlight the correlation between the properties of the two optical fibers – i.e., in terms of presence, absence or both presence and absence of photons? The physicists have shown that by adding a little bit of light into the two optical fibers, it is possible to detect these three configurations that indicate the desired correlation.

This method applies not only locally, near the separator strip, but also at the extremities of the fibers, at a distance of two kilometers! The experiment reproduces a "complete" elementary quantum network link, including the device for "announcing" the entanglement. At this stage, it does not appear impossible to extend this link to several hundred kilometers, by using quantum repeaters.

Compared to its two-photon counterpart, single-photon entanglement has a much higher resistance to loss. For a 100-km link, the probability that the entanglement is preserved is 10% for one photon and only 1% for two photons. Moreover, the entanglement availability announcement is easier to generate in the one-photon case. There really are a lot of advantages to this brand new modality!