A Chinese research team has developed a new method for quantum communication by designing an open configuration of twin-field quantum key distribution (QKD), achieving secure communication at a distance of more than 615 kilometers.

Transmission using the new configuration only requires half the amount of optical fiber normally required by conventional closed channels, an innovation that shows promise for wide area quantum network construction in the country, according to the study findings recently published in the journal Nature Communications.

One of the main methods in quantum communication, QKD can exchange cryptographic keys securely only known between shared parties.

Of all the QKD protocols, twin-field is the most viable solution for long-distance secure fiber communication. The communication users of both parties need to transmit their own optical fields independently to meet at the intermediate station for interference in a twin-field QKD configuration.

Then, the interference outcome is used by the two users for their information reconciliation, so it is important to keep the mutual phase stable. However, the phase varies violently in practice, caused by the frequency difference between the lasers from both parties and the rapid phase fluctuation of the long fiber.

To remedy this problem, the previous configurations of twin-field QKD had adopted a gigantic and resource-inefficient interferometer structure, which means that besides the essential quantum fiber between the QKD's two parties, an additional fiber called the service fiber is needed for optical frequency dissemination, said Yuan Zhiliang, chief scientist at the Beijing Academy of Quantum Information Sciences (BAQIS).

The research team led by Yuan introduced a new technique that stabilizes an open channel without using a closed interferometer or the service fiber, and successfully achieved quantum communication at a distance of 615.6 km.

Zhou Lai, a team member from the BAQIS, explained their findings, noting that in the traditional methods, two lines are necessary for quantum communication between two sites about 615 km apart, while with the new method, one line is enough.

In the twin-field QKD with their open configuration, the team adopted the optical frequency comb technology to replace the conventional service fiber to calibrate the frequency of lasers from both parties in the quantum communication, realizing accurate information transmission.

Zhou illustrated the frequency comb as turning a beam of light with a single frequency into multiple beams of light with different frequencies, which are "separated like the row of teeth on a comb."

The frequency comb technology serves the additional function of solving the problem of fiber drifts, which inevitably occur during a long-distance quantum communication, Zhou said, explaining that the comb works by greatly reducing the impact of noise on quantum signals, thus ensuring the accuracy of information transmission over long distances.

During an interview with Xinhua, the team demonstrated their research results through an optical fiber with a core diameter of about 10 microns and a length of 615 km, successfully completing the long-distance quantum communication.

The twin-field QKD system with the open configuration is still relatively large, and the team is currently in the process of developing a photonic chip measuring 1 square centimeter to integrate various device modules used in the QKD system, Yuan said.

"Once the chip has been successfully developed, devices used for quantum communication could be carried around like portable laptops," he added.