Quantum communication starts to repeat itself

A new technique that extends the range of quantum-secured communication has been unveiled by a team of British researchers in the journal Nature. Known as twin-field quantum key distribution (TF-QKD), the method makes it more practical to send the “unhackable” quantum signals that are regarded as the precursors of a quantum internet.

Standard quantum key distribution (QKD) allows unbreakable encryption for ultra-secure communication. It works by transmitting photons of laser light that share a quantum connection, known as “entanglement”, which makes it impossible for any eavesdroppers to go undetected.

These entangled photons are then employed as keys that can be used to encrypt a subsequent message. Only someone who knows the keys can decrypt the message, and the quantum entanglement makes sure that only the intended recipient can have them.{%recommended 591%}

Unlike many existing encryption techniques, QKD is safe against hacking by quantum computers that may exist in the future.

Its weakness, however, is distance: the longer the connection, the lower the bandwidth. Over 50 kilometres, you can send 1.26 megabits per second, but even using the highest quality optic fibre the bandwidth drops off to an almost useless one bit per hour when the line is stretched to 400 kilometres. And for greater distances, QKD is practically impossible.

Most research into extending the range revolves around building a gadget called a “quantum repeater”: a device that can receive a signal, amplify it and re-transmit it, all while preserving entanglement. 

Quantum repeaters are still only theoretical, however. The advance of twin-field QKD, according to Marco Lucamarini and colleagues at Toshiba Research in Cambridge, UK, is to use off-the-shelf equipment to transmit keys between parties up to 550 km apart, while maintaining a relatively high bandwidth.

The new method requires two parties who wish to communicate (usually referred to as Alice and Bob) to each send a signal to a third party halfway between them. (The third party is called Charlie.) Charlie will conduct a measurement that combines the two signals and announce the result. Alice and Bob can then use this announcement to construct a key that only the two of them will know.

“This scheme is a promising step towards overcoming the rate-distance limit of QKD,” the researchers write, “and greatly extending the range of secure quantum communications.”