VIEWPOINT A Solid Footing for a Quantum

Q uantum secure communication relies on sharing of entangled states between parties. Over short distances (less than 100 km), these states can be distributed by sending photons over optical fibers, but losses in those fibers limit long-distance sharing. One solution is to use a sequence of “quantum repeaters” along the optical fiber connection [1]. These devices can store the quantum information in an excited state of matter. The states of two relatively close repeaters can be entangled through photon emission. This process can conceivably be repeated over and over to entangle more distant repeaters together, until the entanglement extends from one continent to another. Previous work on repeaters has focused on trapped gases [2, 3], but solid-state solutions would be easier to incorporate into existing optical networks. Three new experiments demonstrate the feasibility of using crystals doped with rare-earth ions [4–6]. They each show quantum correlations between photons and collective excitations of ionic dopants inside a crystal. This is an important step in developing solid-state quantum repeaters for a global network of quantum communication.