Quantum leap

When buying an item online, we voluntarily hand over our credit card information. But how do we know that it’s safe? Most sensitive information sent over the internet is secured through encryption, a process that converts information into a code that can only be unlocked by those with the encryption key. Currently, encryption keys are essentially impossible to break with conventional computing equipment – it would simply take too long and too much computing power to do the mathematical calculations that could reveal the key.

But in the coming decades, all that could change. Google, IBM and many other companies are all working to build a quantum computer that would outperform contemporary computers by taking advantage of the ability of subatomic particles to exist in more than one state at a time. A quantum computer could enable us to make calculations and solve problems that are well out of reach of even the most powerful supercomputers, but in the wrong hands, they could also crack encryption keys with relative ease.

So how can individuals, corporations and governments keep information safe in the face of this potential threat?

A group of researchers in Cambridge’s Department of Engineering are working to defend against the security threats posed by quantum computers by developing ‘unhackable’ encryption keys hidden inside particles of light, or photons, and sent over optical fibres.

Quantum keys are generated randomly through quantum mechanics, taking advantage of a property of photons that prevents them being cloned. The real strength of quantum links, however, is that if an attacker attempts to intercept the key, the quantum state of the photons changes and they cannot be used as part of the key, rendering the information carried by the stolen photons worthless.

“This means that we can send single photons over our networks and end up with keys at each end which are fundamentally secure,” says Professor Ian White, Head of the Photonics group in Cambridge’s Department of Engineering.

In June 2018, White and his colleagues Professor Richard Penty and Dr Adrian Wonfor started putting these ideas into practice with the launch of the UK’s first quantum network. The ‘metro’ network provides secure quantum communications between the University’s Electrical Engineering Division in West Cambridge, the city centre and Toshiba Research Europe Ltd (TREL) on the Cambridge Science Park. It was built with corporate partners including ADVA and Toshiba.

The network has since been extended and connected to other sites around the country, including BT’s research and development centre in Ipswich, and is currently being extended to the National Physical Laboratory in London and the University of Bristol, creating the first UK quantum network.

The quantum network is a project of the Quantum Communications Hub, a consortium of eight UK universities led by the University of York, as well as private sector companies and public sector stakeholders. It’s funded by the Engineering and Physical Sciences Research Council (EPSRC) through the UK’s National Quantum Technologies Programme.

“This network provides us with a UK facility where we can test ideas that until now have been research-based, and to get users used to the concepts behind quantum communications so they can translate this technology into practice,” says Penty. “There’s a world of difference between transmitting quantum keys over a coil of fibre in the lab and actually putting it in the ground.”

The network has the highest quantum key rate in the world. This secures a data network in Cambridge that runs at roughly five times the capacity of the entire University network, and the link to BT in Ipswich operates at five times that again. The link to BT is comparable with the highest data capacity links in the UK, and has the possibility for scale-up in future.

“For us, it’s really important to get this right as it’s our first chance to start doing very detailed studies and see how these systems really work in the field,” says White. “This is only the start, however.”

In addition to the continued growth and development of the quantum network, the researchers are also investigating other ways that quantum technology could be used to secure information. For example, instead of counting individual photons, it could be possible to measure the amplitude and phase properties of pulses. “This way, you could use a type of hardware that’s not so different from conventional networks, so it would dramatically reduce the cost,” says Wonfor. “In theory, this would represent a huge step towards commercialising quantum technology, because it would effectively rely on technology that people are already used to.”

The researchers are also looking at turning the entire concept on its head, and instead of relying on quantum mechanics for encryption key distribution, it could be used as a type of quantum alarm. In this scenario, the quantum signal would be in the background, buried inside a classical data signal, and would detect when an intruder attempts to break into the fibre.

“At the moment, it’s not easy to detect whether someone is tapping into the actual fibre, but with this kind of system working at the level of single photons, it would be much easier to do,” says Penty.

Another possibility is that of an entirely optical quantum-secured network. The Cambridge researchers have been developing optical switches that work with quantum signals so that everything stays in the optical domain. “Effectively, this would mean that quantum IP routers should be possible, a concept that is now testable thanks to the quantum network,” says Wonfor.

So where else might quantum encryption be used? According to White, it could go into space. At the moment, quantum keys can be distributed up to a maximum distance of approximately 100 km of fibre, which is why the quantum network is built on a series of nodes, with a new quantum key being generated at each node. This setup works well in urban areas with a high number of users but is not ideal for rural areas with few users. It also makes it impractical to send a quantum link across the Atlantic.

“An interesting movement within the field of quantum communications is to start involving satellites so that you could produce a quantum communications link for two remote sites,” says White. These satellites would work in parallel with fibre networks, sending quantum links to one of the trusted nodes within the network, where they could be managed, stored and distributed as needed.

The Cambridge group, along with several other academic and industrial collaborators, have recently secured several parallel funding bids from Innovate UK to develop both lower cost terrestrial and space-based quantum communications.

“The main thrust of all of this work has been to develop technologies that can be commercialised and put into regular use,” says White. “Cybersecurity is such an important issue, and we think that the laws of physics can be used to make our data transmission as secure as possible.”