Quantum communication networks and other quantum information processing will require coherent and efficient transfer of information between light and matter, and the realm of light-matter interfaces is an active area of research. Much of the activity has focused on the mapping of quantum information onto atomic systems (see, for instance, Physics Today, March 2001, page 17). Nicolas Gisin and colleagues at the University of Geneva in Switzerland have now demonstrated the coherent storage and retrieval of information using a solid-state system. The team’s quantum memory was an ensemble of roughly 107 neodymium ions trapped in a crystal of yttrium vanadium oxide (YVO4). In such an environment, the resonant frequencies of the rare-earth atoms are inhomogeneously shifted, which broadens the absorption spectrum. That’s normally undesirable, but the researchers turned it to their advantage. By optically pumping some of the Nd atoms out of the ground state,...

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