A team of scientists from Long Guilu's group and Lu Jianhua's group of Tsinghua University and Beijing Academy of Quantum Information Sciences, China, designed a new Quantum Secure Direct Communication (QSDC) with mixed encoding of phase quantum states and time-bin quantum states with a communication distance of up to 100 kilometers, which is the longest quantum direct communication distance in the world.

Such indicators can realize point-to-point quantum direct communication between some cities without relays, and can support some applications of wide-area quantum networks based on secure classical relays.

Schematic diagram of the new system of quantum direct communication [Photo provided to chinadaily.com.cn]

QSDC transmits secure information directly using quantum states, which was proposed by Long Guilu and his then Ph.D. student Liu Xiaoshu in 2000. 

The new system uses photonic time-bin states for monitoring, and phase states for communication respectively. 

The transmission rate of the new QSDC system is 0.54 bps at 100 km. The transmission rate strongly depends on the transmission distance. At shorter distances, the transmission rate is much higher. It is 22.4 kbps at 30 km of fiber, which will satisfy the rate requirement of many practical applications.