Designed by scientists from the Beijing Academy of Quantum Information Sciences and Tsinghua University, a new system has increased the transmission distance of quantum secure direct communication (QSDC) to more than 100 km in fiber, setting a new world record.

　　Different from previous systems, which used only phase states for both eavesdropping detection and information transmission, the new system adopted both photonic time-bin and phase states, the former for eavesdropping detection and the latter for communicating the message. This enables an ultra-low quantum bit error rate (QBER) and long-term stability against environmental noise.

　　This new system also increased the secrecy capacity greatly by achieving an ultra-low QBER of less than 0.1 percent, one order of magnitude smaller than that of existing systems, according to the research results.

　　Long Guilu, a professor in the department of physics at Tsinghua University, said that the new system lifted the maximum tolerable loss from 5.1 dB to 18.4 dB under the pulse-repetition rate of 50？MHz and the transmission distance from 18 km to 102.2 km. In addition, the system can communicate at 22.4 kbps through about 30？km of commercial fiber.

　　Long and his then doctorate candidate Liu Xiaoshu, proposed the first QSDC protocol in 2000. In 2019, Long and his team developed the world's first QSDC system, realizing a transmission rate of 50bps through 1.5 km of fiber. One year later, they released the first practical QSDC prototype at the Zhongguancun Forum, increasing the speed to 4 kbps in 10 km of fiber. Later in 2020, the transmission distance reached 18 km.

　　With this latest research result, point-to-point QSDC between cities is feasible through the application of current advanced technologies. However, there is still much space for the improvement of the new system in terms of pulse-repetition frequency, which could further enhance the corresponding transmission distance and speed, and satisfy the need for certain scenarios, according to Long.