You can’t hack it (Image: Chris Stowers/Panos)

QUANTUM networks are quietly spreading across the world. With secure quantum connections linking up cities, people can communicate in the knowledge that the laws of physics will prevent eavesdropping. Eventually, there may even be a global quantum internet.

When former US National Security Agency contractor Edward Snowden uncovered the extent of government spying, that underlined the need for more secure communications, says Don Hayford of Battelle, a research organisation in Columbus, Ohio. “Even before Snowden we decided there were things coming up in the future that meant people should start looking at something better.”

That something is called quantum key distribution (QKD). The technique transmits photons in particular quantum states to generate a secure cryptographic key, with which you can encrypt data sent over an ordinary, non-quantum connection. QKD is far more secure than standard cryptography, which relies on hard mathematical problems that can theoretically be cracked, given enough computing power. Any attempt to intercept a quantum key, however, will disturb the photon’s quantum states, alerting users not to use the key (see “Unbeatable security”).


Any attempt to intercept the key will disturb the quantum states, alerting users not to use the key

Since December, Battelle has operated a quantum link between its Columbus headquarters and manufacturing offices 62 kilometres away in Dublin, Ohio – the first commercial link of its kind in the US. They are working with ID Quantique in Geneva, Switzerland, which sells QKD technology and helped to keep the results of a 2007 Swiss election secure.

Now, Battelle has announced plans to use an existing fibre-optic network running through Dublin to test a larger quantum network. The long-term aim is to link up with their offices in Washington DC, more than 650 kilometres away.

Researchers in China are also linking up cities. A group led by Zheng-Fu Han of the University of Science and Technology of China in Hefei has just released details of the first experimental wide-area QKD network, which ran from December 2011 to July 2012 (arxiv.org/abs/1409.1568). The network connected five computers or nodes in Hefei to three more in Wuhu, 150 kilometres away, via another in a third city, Chaohu. “From the coverage area point of view, it is the largest quantum network built to date,” says group member Shuang Wang.

A 2000-kilometre link between Beijing and Shanghai is due for completion by 2016. The Chinese government is already using QKD to protect its secrets, including discussions during the 18th National Congress in 2012 as new leaders took over the ruling party.

Building such an extensive QKD network was not without problems, however. Local roadworks severed a fibre in Wuhu three times while Wang’s network was active, and a power outage at a Hefei node split the network in half just 10 days into the project. Another Hefei node had to be placed in a makeshift kitchen – the only space with access to the necessary optical fibre link. The large temperature variations in the kitchen weren’t necessarily a bad thing, says Wang. “This harsh environment also provided us with a chance to test the robustness of the QKD devices.”

Long-range networks come at a cost. The performance of Wang’s network declined as the distance between the nodes increased. While two nodes in Hefei were able to conduct secure, real-time voice transmission, Hefei-to-Wuhu links could only send new keys three times a second.

One possible solution involves a device called a quantum repeater. At the moment, extending a quantum link beyond 100 kilometres or so requires a trusted node to sit between the two parties and establish a link with each of them. Each user has a secure quantum connection with this node, but they aren’t able to communicate directly because the laws of quantum mechanics prevent the trusted node from copying a state to relay it. A quantum repeater would solve this problem by linking states over long distance using a property called entanglement, but entanglement is fragile and no one has yet built a successful repeater.

Quantum repeater

Eventually though, Hayford says repeaters could bring QKD to ordinary internet users. You won’t have a direct optical fibre link to your home, but will instead visit a kind of quantum post office that is plugged in to the network. There, you’ll pick up a memory stick filled with quantum codes to use with Google, Amazon and other internet companies on your home PC or smartphone. Such quantum keys could even travel via the undersea cables that the internet uses today – though installing repeaters at the bottom of the Atlantic will be a challenge, says Saikat Guha of BBN Technologies in Cambridge, Massachusetts.

Anyone who has been forced to change their password after an online leak might be sceptical that QKD can deliver totally secure communications. Researchers have already shows that it is possible to hack QKD systems, but this was due to flaws in hardware. Hayford says Battelle is working with hacking researchers to fix such problems. “It’s something that we continually address.”

Even with a perfect system, QKD can’t hide the fact that you are talking to someone. Snowden told the world that the NSA gathers “metadata” like the time, location and parties involved in a conversation, from spying on optic cables. This enables the agency to glean information about people without ever reading their messages.

Following the Snowden leak, Guha and colleagues decided to look into the metadata problem. It turns out it is possible to send a small amount of data without anyone else realising, providing you disguise the message in a quantum link’s inherent noise (arxiv.org/abs/1404.7347). The idea has only been tested on a small scale in the lab, but as quantum networks roll out across the globe, perhaps it will provide the ultimate way to send covert messages. “This is one step higher security than secure communication,” says Guha.

This article appeared in print under the headline “Quantum internet to keep spies at bay”

When this article was first published, the leader of the group at the University of Science and Technology of China in Hefei was misnamed.