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QUANTUM INFORMATION
Spin systems have now been found that have lifetimes of up to six hours. They could be used to build quantum-communication networks and, if optical transmission fails, could even be shipped as a 'quantum memory stick'. See Letter p.177
It is just a little past the time of year when many children write letters to Father Christmas. While being typed, these electronic letters exist in their computers' dynamic memories, before being transmitted down optical fibres to the North Pole, where they are stored on hard disks and backed up using magnetic-tape storage (or so we are reliably informed). Quantum information offers the prospect of fundamentally secure communication, but constructing practical quantum networks relies on developing a similar set of interconnected, long-lived memories storing quantum bits (qubits; the smallest logical units of quantum information). On page 177 of this issue, Zhong et al.1 show how qubits encoded in the magnetic moment (spin) of atomic nuclei in inorganic crystals can have lifetimes of up to six hours, and also offer a route to strong coupling with light. Six hours may seem short compared to storage times of conventional memories, but it should be plenty to develop quantum memories for the nodes that plug together quantum-communication networks. It is also striking that, using this technology, we could, rather straightforwardly, send a qubit from London to New York by air freight - something currently impossible using optical links.
Quantum cryptography is a way to communicate using information encoded in the states of quantum systems. It exploits the fact that measurements of quantum systems unavoidably affect their state, allowing communicating agents to validate that there is no eavesdropper on the line. Quantum states are notoriously fragile, and hence communication of qubits typically relies on using the states of nature's fastest and least-interacting particles: photons. However, even photons are eventually scattered or absorbed, which has so far limited quantum communication based on transmission of individual photons to distances of 200 kilometres in optical fibres2 and 140 km through free space3.
Extending such demonstrations to create a secure...