Quantum mechanics permits the existence of unique correlations, or entanglement, between individual particles. For
a pair of entangled photons, this means that performing a measurement on one photon appears to affect the state of
the other. The ability of entangled particles to act in concert is preserved even when they are separated by large
distances and serves as a resource for numerous applications. For example, distributing entangled photon pairs over
fiber-optic cables enables secure communication between two remote parties or could offer the possibility of
interconnecting quantum computers. The vast transparency band of the installed global fiber-optic network,
consisting of over a Gigameter of optical fiber cables, presents a particularly attractive opportunity for this task. The
bond between entangled photons is, however, very fragile and could be lost.