Quantum Photonics Group of BAQIS has recently published a research article entitled "Twin-field quantum key distribution without optical frequency dissemination", reporting their invention that has substantially simplified long-distance secure quantum communications.

Fiber Loss limits the communication distance of conventional quantum key distribution (QKD) to around 400 km. Twin-field quantum key distribution (TF-QKD) can break this limit and allows secure quantum communication with favourable rate-loss scaling, but requires cumbersome interferometric implementations which are often impractical for network deployment.  To resolve this problem, Yuan & colleagues develop a simple and versatile TF-QKD setup that can operate over an open fiber link and achieve secure communication over a distance of 615 km.

In TF-QKD, the communication users (Alice and Bob) need to transmit independently their own optical fields to meet at the intermediate measurement station for interference.  The interference outcome is used by Alice and Bob for information reconciliation, and therefore it is of utter importance to keep the mutual phase stable.  However, the phase varies violently, caused by the frequency difference between the users’ lasers and the rapid phase fluctuation of the long fibers.Consequently, all existing long-haul TF-QKD setups have to adopt essentially a gigantic Mach–Zehnder interferometer (MZI) configuration with half of its fiber used for optical frequency dissemination, see figure a.  Such closed setups have two drawbacks.  First, the setups are fiber-resource inefficient and require additional frequency

locking hardware and often optical amplifiers along the service fiber. Second, the closed fiber configuration is inherently incompatible with optical switching, which could restrict TF-QKD’s scalability into a larger network.

a） Traditional Closed configuration；b）Open TF-QKD proposed by the BAQIS

Using locally generated frequency combs to establish mutual coherence, Yuan & colleagues develop a simple and versatile TF-QKD setup that does not need service fiber, see figure b. They confirm the setup’s repeater-like behavior and overcome the fundamentally secure key rate bounds at distances between 403 and 615 km. The setup can also operate over links of 100 km asymmetry.

Dr. Lai Zhou is the first author, and the coauthors include Mr. Jinping Lin and Dr. Yumang Jing.

Paper link：https://www.nature.com/articles/s41467-023-36573-2