China reports key advance in encrypted long-range communications

A quantum key for encrypting and decrypting messages has been shared between two ground stations about 700 miles apart, a team of researchers in China reported this week.

Why it matters: It's the latest milestone in an effort to create a long-range and theoretically ultra-secure quantum communications network.

• Researchers in the U.S., Singapore, Canada and elsewhere are trying to build similar systems but the team in China, led by Jian-Wei Pan of the University of Science and Technology in Hefei, is at least several years ahead.

How it works: Photons can be entangled in pairs so that the state of one photon is tied to that of another no matter the distance between them.

• Quantum cryptography leverages entanglement to distribute quantum keys for communications.
• If there is an attempt to intercept a message sent with a quantum key, the photons would be altered and the key would no longer work.
• Fiber optic cables can carry photons but only short distances so researchers want to use satellites to relay quantum keys between ground stations.

What they did: Building on past experiments, Pan and his colleagues report using China's Micius satellite — built specifically for basic research on quantum satellite communications — to send a quantum key that was shared between two newly built observatories.

• Key advance: The satellite is a blind transmitter and has no information about the key.
• Previously, the longest distance for distributing a quantum key on the ground was about 62 miles.
• In the new paper, the researchers report increasing the efficiency of the transmission by four fold compared to previous experiments and decreasing the error rate (to about 4.5%).

What's new: The team reports using a model to extract an encryption key from a relatively small number of received quantum signals. "This new method seems to be a major improvement over the previous models," says Alexander Ling at the National University of Singapore's Centre for Quantum Technologies, who was not involved in the work. "If this new model can be confirmed by other scientists, it will be a major advance."

• The researchers say the error rate is low enough that attempts to eavesdrop on the message would be detected from other noise, "enabling the realization of satellite-based entanglement quantum key distribution."

What's next: A global network would require a constellation of satellites to transmit entanglement signals around the globe, says Ling, who is working on creating quantum-enabled nanosatellites.

Go deeper: China takes the lead in building quantum data security networks