By encoding messages using the quantum states of photons, quantum cryptography offers the prospect of completely secure data transmission. However, physicists have found it difficult to make the single-photon sources needed in most variants of quantum cryptography. Now, Frédéric Grosshans of the Institute of Optics in Orsay, France, and colleagues have shown experimentally how to encode data using pulses containing several hundred photons. Their technique remains secure even when the pulses are degraded during transmission (F Grosshans et al. 2003 Nature 421 238).
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