A Chinese research team has made a significant breakthrough in the field of quantum communication, developing a new method that enables secure communication over a distance of more than 615 kilometers. The researchers achieved this feat by utilizing a newly designed open configuration of twin-field quantum key distribution that requires only half the amount of optical fiber usually needed by conventional closed channels.
Quantum key distribution is a process that allows for the exchange of cryptographic keys securely between two parties, making it an essential component of quantum communication. Among all the QKD protocols, twin-field is the most viable solution for long-distance secure fiber communication. The users of both parties transmit their own optical fields independently, which meet at an intermediate station for interference in a twin-field QKD configuration. The interference outcome is then used by the two parties for information reconciliation.
However, maintaining a stable mutual phase is essential for successful communication, and this is where the research team’s innovative method comes into play. Previous configurations of twin-field QKD had used a gigantic and resource-inefficient interferometer structure that required an additional fiber called the service fiber for optical frequency dissemination. The new technique stabilizes an open channel without using a closed interferometer or the service fiber, making it a significant breakthrough in the field.
The research team led by Yuan Zhiliang, chief scientist at the Beijing Academy of Quantum Information Sciences, used optical frequency comb technology to replace the conventional service fiber in their open configuration. The technology works by 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 also helps solve the problem of fiber drifts that occur during long-distance quantum communication by reducing the impact of noise on quantum signals.
The team successfully demonstrated their research results through an optical fiber with a length of 615 km, completing the long-distance quantum communication. The next step for the team is to develop a photonic chip measuring 1 square centimeter to integrate various device modules used in the QKD system. Yuan Zhiliang believes that once the chip has been successfully developed, devices used for quantum communication could be carried around like portable laptops. This significant innovation in quantum communication holds promise for the building of a wide area quantum network in the country.