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Practical quantum cryptography systems have developed rapidly over the past decade and are now available commercially. There is a continuous drive to make these systems work faster and over longer distances. One of the main concepts underlying quantum cryptography is quantum key distribution (QKD) — the generation of a perfectly random key, known only by the sender and receiver, to encode and subsequently decode a message, making it completely secure. QKD has been performed at distances in excess of 100 km and driven by clock pulses at frequencies as high a 1 GHz. However, further improvements have been limited by both erroneous signal pulses, known as dark counts, and random variations in the timing of pulses, referred to as jitter, that are inherent to the single-photon detectors used. Now, a report by Hiroki Takesue and co-workers1 could herald a large step forward in the design of these systems.

Two main improvements to their method have allowed Takesue et al. to demonstrate the differential phase shift QKD protocol at a clock frequency of 10 GHz1. The first improvement was the laser used to generate single-photon pulses. By using a mode-locked laser to create very short pulses, the effect of the random dark counts on the real signal was reduced. Secondly, the detectors’ jitter was reduced to 30 ps, an order of magnitude smaller than in previous QKD experiments. The system was used to transfer the key over a 105-km optical-fibre link at a rate of 3,700 bits per second. With such improvements, quantum cryptography could soon become a viable real-world technique for secure communication.