Could an entire city run on a quantum network? A new study says yes

Quantum key distribution (QKD) brings the power of quantum computing to communication, using the unique properties of quantum physics to make an unhackable system. Now, a new network developed by an international team sets a path to make a dream come true: an entire city operating through quantum communication.

The quantum world is vastly different from the natural world we see around us every day, which is controlled by what quantum scientists refer to as “classical physics.” Under classical physics, an object, like the key to your house, can only be in one place at a time. Yet at the smallest quantum level, photons of light can take on what is known as a superposition, meaning that they exist in multiple places at the same time.

But superpositions come with a fascinating qualification: they fall apart if directly observed. If a scientist tried to directly observe quantum superposition, it would fall apart into two separate positions again.

So, the thinking goes, what’s true for the quantum scientist is equally true for hackers trying to push their way into an email. In QKD, those photons are sent through a quantum channel with a secret key that can be used to decrypt the message. Until now, it had only been possible between two users.

The team’s work, published Wednesday in Science Advance, has expanded that to eight users, with simulations showing as many as 32 users. The system can operate over a distance of a little over 10.5 miles or 17 kilometers.

Two to eight might seem like a long way to go until QKD is ready to handle a city’s communications, and it is. But Dr. Siddarth Joshi, who headed the project at the University of Bristol’s Quantum Engineering Technology (QET) Labs, says in a press statement that the jump “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.”

Talking to Inverse via email, Dr. Joshi estimates that his team’s QKD process could be rolled out in a city as big as London, with nearly 9 million people, within 20 years. Half of that timeframe is not because of the technology itself, but because of the lack of fiber optic infrastructure in homes. If fiber optic technology was widely available for all citizens, “then it would take about 10 years,” he says.

And if the technology was instead applied to a widespread venture, like connecting ATM machines to their home banks, than Dr. Joshi again knocks down the timeline to “as little as 5-6 years.” Considering that there are over 80,000 ATM machines vulnerable to hacking in the United States alone, the change would mark a huge security upgrade. Overall, Dr. Joshi believes that this project has hastened a full quantum Internet’s arrival “by at least 5 years.”

That quickening stems from a change in tactics. Previously, Dr. Joshi says in a press statement, quantum networks “involved vast infrastructure.” But through multiplexing, which splits light particles emitted by a single system, photons can be “received by multiple users efficiently,” he says.

“The thing that I really like about our network is how simple the fundamental principles behind it are….The dramatic reduction in the complexity and effort needed to build a large network is what allowed us to perform the experiment...Our team was able to assemble and operate the network within a month,” Joshi tells Inverse. “This was one of the rare experiments that worked the first time around.”

Abstract: Quantum communication is rapidly gaining popularity due to its high security and technological maturity. However, most implementations are limited to just two communicating parties (users). Quantum communication networks aim to connect a multitude of users. Here, we present a fully connected quantum communication network on a city-wide scale without active switching or trusted nodes. We demonstrate simultaneous and secure connections between all 28 pairings of eight users. Our novel network topology is easily scalable to many users, allows traffic management features, and minimizes the infrastructure as well as the user hardware needed.