Important Step Towards Quantum Cryptography between Many Users

This central entangled photon source powers the quantum network. (Source: ÖAW, K. Pichler / U. Bristol)

In the quest to create a secure internet and find an answer to the growing threat of cyber-attacks, scientists now have created a unique prototype which could transform how we communicate online. The invention, led by the University of Bristol, has the potential to serve millions of users, is understood to be the largest-ever quantum network of its kind, and could be used to secure people’s online communication.

By deploying a new technique, it can make messages completely safe from interception while also overcoming major challenges which have previously limited advances in this technology. Team lead Siddarth Joshi at the university’s Quantum Engineering Technology Labs, said: “This represents a massive breakthrough and makes the quantum internet a much more realistic proposition. Until now, building a quantum network has entailed huge cost, time, and resource, as well as often compromising on its security which defeats the whole purpose. Our solution is scalable, relatively cheap and, most important of all, impregnable. That means it’s an exciting game changer and paves the way for much more rapid development and widespread rollout of this technology.”

The current internet relies on complex codes to protect information, but hackers are increasingly adept at outsmarting such systems leading to cyber-attacks across the world which cause major privacy breaches and fraud running into trillions of pounds annually. With such costs projected to rise dramatically, the case for finding an alternative is even more compelling and quantum has for decades been hailed as the revolutionary replacement to standard encryption techniques.

So far physicists have developed a form of secure encryption, known as quantum key distribution, exchanged by photons. The process allows two parties to share, without risk of interception, a secret key used to encrypt and decrypt information. But to date this technique has only been effective between two users.

“Until now efforts to expand the network have involved vast infrastructure and a system which requires the creation of another transmitter and receiver for every additional user. Sharing messages in this way, known as trusted nodes, is just not good enough because it uses so much extra hardware which could leak and would no longer be totally secure,” Dr Joshi said.

The team’s quantum technique applies entanglement. This process presents far greater opportunities for quantum computers, sensors, and information processing. “Instead of having to replicate the whole communication system, this latest methodology, called multiplexing, splits the light particles, emitted by a single system, so they can be received by multiple users efficiently,” Dr Joshi said.

The team created a network for eight users using just eight receiver boxes, whereas the former method would need the number of users multiplied many times – in this case, amounting to 56 boxes. As the user numbers grow, the logistics become increasingly unviable – for instance 100 users would take 9,900 receiver boxes.

To demonstrate its functionality across distance, the receiver boxes were connected to optical fibers via different locations across Bristol and the ability to transmit messages via quantum communication was tested using the city’s existing optical fiber network.

“Besides being completely secure, the beauty of this new technique is its streamline agility, which requires minimal hardware because it integrates with existing technology,” Dr Joshi said. The team’s unique system also features traffic management, delivering better network control which allows, for instance, certain users to be prioritized with a faster connection.

Whereas previous quantum systems have taken years to build, at a cost of millions or even billions of pounds, this network was created within months for less than 300,000 pounds. The financial advantages grow as the network expands, so while a hundred users on previous quantum systems might cost in the region of 5 billion, Dr Joshi believes multiplexing technology could slash that to around 4.5 million pounds, less than one percent.

In recent years quantum cryptography has been successfully used to protect transactions between banking centers in China and secure votes at a Swiss election. Yet its wider application has been held back by the sheer scale of resources and costs involved.

“With these economies of scale, the prospect of a quantum internet for universal usage is much less far-fetched. We have proved the concept and by further refining our multiplexing methods to optimize and share resources in the network, we could be looking at serving not just hundreds or thousands, but potentially millions of users in the not too distant future,” Dr Joshi said.

“The ramifications of the Covid-19 pandemic have not only shown importance and potential of the internet, and our growing dependence on it, but also how its absolute security is paramount. Multiplexing entanglement could hold the vital key to making this security a much-needed reality.”

The research received funding from the Quantum Communications Hubs of the Engineering and Physical Science Research Council (EPSRC), Ministry of Science and Education (MSE) of Croatia, and the Austrian Research Promotion Agency (FFG). Collaborating institutions with the University of Bristol are the University of Leeds, Croatia’s Ruder Boskovic Institute (RBI) in Zagreb, Austria’s Institute for Quantum Optics and Quantum Information (Iqoqi), in Vienna, and China’s National University of Defence Technology (NUDT) in Changsha. (Source: U. Bristol)

Reference: S. K. Joshi et al.: A trusted node–free eight-user metropolitan quantum communication network, Sci. Advances 6(36), 2 Sep 2020; DOI: 10.1126/sciadv.aba0959

Link: Quantum Communication and Networks, Quantum Engineering Technology Labs (QET Labs) at the University of Bristol, Bristol, UK

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