Cryptography gets quantum boost

The distances for securely transmitting information via quantum mechanical effects, or quantum cryptography, is limited to100km. Any longer and the signal is lost to noise. A team of researchers have formulated a protocol to sidestep this hurdle, publishing their work last week in Nature Photonics¹.

In their setup, the researchers used quantum superposition – which allows particles to hold multiple states simultaneously – and entanglement – which ties together separate quantum entities, to overcome the distance limitation. They managed to noiselessly amplify the distance travelled by coherent states – which are the information carriers in current optical communication infrastructures.

The receiving end first prepares a light wave using quantum superposition so that it encodes a binary code of both 0 and 1 simultaneously. This state is broken into two separate, entangled pulses via a beam splitter, and one of these pulses is sent to the sender. The sender then combines this pulse with a weak input light signal and performs a measurement, forcing the superposition to collapse into either a 0 or 1. Due to the entangled state of both pulses, the pulse kept at the receiver instantly also collapses, receiving the message regardless of distance.

Currently, the researchers' setup of the transmitter and the receiver is still a few meters square in size and remains impractical. Their next step is to use integrated optics to make it more compact.

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