Entanglement is one of the main properties of quantum which leads to strong correlations between two quantum objects even if they are far apart.
Entanglement is also at the heart of many quantum technologies which aim at using quantum resources to provide new capabilities in computing, simulation, communication and sensing that are not possible with classical resources. In particular, the entanglement between light and matter is an enabling resource for the realization of a quantum internet and of long-distance quantum communication using quantum repeaters. The matter system allows the storage of quantum bits, providing a quantum memory, while the photon can be used as communication qubit and be sent over long distances. . Various demonstrations of light-matter entanglement have been made in the laboratory, but to reach the next steps of development, it is crucial to demonstrate that the technology also works outside the laboratory, which represents a significant challenge.
Here, we report on the first demonstration of the distribution of entanglement between a photon and a solid.-state quantum memory using the installed optical fiber network. Entangled photon pairs were created by spontaneous parametric down conversion and one photon was stored in the quantum memory while the other photon, at telecommunication wavelength was sent into an optical fiber. The photon travelled a distance of up to 50 km in a fiber loop in the Barcelona metropolitan area. We showed that the entanglement was preserved after the transmission in the fiber, showing that the photon did not suffer significant decoherence during the transmission. This experiment represents an important step towards the deployment of quantum repeater technologies outside the laboratory.
