Quantum communication could enable uncrackable transfer of information, but most approaches rely on trusted devices. Researchers have now demonstrated that a new method that does away with this challenging requirement can operate over distances as large as 62 miles.

One of the central promises of a future quantum internet is provably secure communication. That’s thanks to one of the quirks of quantum physics: Observing a quantum state inevitably changes it. So if anyone attempts to intercept and read a message encoded in the quantum states of particles, they will alter it in the process, alerting the receiver to the breach.

Quantum communication speeds are too slow to transmit large amounts of information, so most schemes instead rely on an approach known as quantum key distribution. This involves using the quantum communication channel to share an encryption key between two parties, which they use to encode and decode messages sent over classical communication networks.

There have been impressive demonstrations of the technology’s potential, including an effort that beamed keys more than 8,000 miles via satellite and another that transmitted them more than 620 miles over optical fiber. But these feats used communication schemes relying on assurances the devices used had no technical flaws and hadn’t been tampered with. This is hard to guarantee.

New research from China’s quantum communications supremo, Jian-Wei Pan, who was also behind the previous record-breaking research, has shown the ability to securely transmit keys over a distance of more than 62 miles even if the equipment used is compromised.

“The demonstration of device-independent [quantum key distribution] at the metropolitan scale helps close the gap between proof-of-principle quantum network experiments and real-world applications,” the researchers write in a paper reporting the results in Science.

Most quantum key distribution schemes send photons encoding quantum information over a series of trusted relays. In contrast, the device-independent scheme uses a pair of entangled photons, one of which stays with the sender while the other is sent to the receiver.

By carrying out a series of measurements on the entangled photons and subjecting them to a statistical test, the sender and receiver can verify if the particles are truly entangled and then use the data to extract a secret key only they can access. Crucially, the approach doesn’t rely on assumptions about the hardware used to generate the results.

But the scheme has struggled to scale because it places strict demands on the efficiency with which quantum particles are detected and the strength of their entanglement. Any loss or noise can undermine security, so earlier experiments only operated over distances of a few hundred feet.

To achieve their latest results, Pan’s team used two network nodes, each consisting of an individual rubidium atom trapped by lasers. These atoms are encoded into a specific quantum state and then excited to produce an entangled photon. Photons from each node are then transmitted over optical fiber to a third node where they interfere with each other and entangle the two atoms.

In a series of innovations, the team improved the creation and measurement of the entangled atoms. The changes resulted in reliable entanglement above 90 percent even at distances of up to 62 miles.

This enabled them to produce a positive key rate—essentially a guarantee that the protocol produces the secret bits that make up the key faster than they must be discarded due to error, noise, or interception by an adversary—up to the maximum distance they tested.

Calculating a positive key rate typically relies on the assumption that the system can send an unlimited amount of data and therefore doesn’t always guarantee the scheme will be practical. But the researchers also tested how their protocol worked when restricted to a finite amount of data and found it could transmit a secure key over almost seven miles.

Steve Rolston, a quantum physicist at the University of Maryland, College Park, told The South China Morning Post that the work is a significant advance over previous efforts. However, he also noted that the data rates remain “abysmally small”—producing less than one bit of secure key every 10 seconds. The tests were also done on a coil of fiber in a laboratory rather than real-world telecom networks subject to environmental noise and temperature swings that can disrupt quantum states.

Even so, the results mark an important milestone. By demonstrating device-independent quantum key distribution at city-scale distances, the study shows that the strongest known form of quantum-secure communication is no longer limited to tabletop experiments.