Recently, the Team of Tsinghua University has for the first time realized a new system of quantum direct communication with a communication distance of 100 kilometers, which is currently the longest quantum direct communication distance in the world, which is conducive to the realization of intercity quantum direct communication without relay.
Professor Long Guilu of the Department of Physics of Tsinghua University proposed the first protocol for quantum direct communication in 2000. Quantum direct communication uses quantum states as a carrier to encode and transmit information, which is a new paradigm of quantum secure communication. Quantum direct communication evolves reliable communication under noise channels into reliable and secure communication under noise and eavesdropping channels.
In 2016, a joint team of Tsinghua University and Shanxi University completed the first experimental demonstration of quantum direct communication. Three years later, Professor Long Guilu's team and Professor Lu Jianhua's team of the Department of Electronics of Tsinghua University cooperated to successfully develop the world's first quantum direct communication system, achieving a secure communication rate of 50bps (bits per second) at a distance of 1.5 km of optical fiber. They also released the world's first practical quantum direct communication prototype at the Zhongguancun Forum (China's national platform for global scientific and technological innovation exchanges and cooperation), achieving a transmission rate of 4kbps (kilobits per second) in a 10-kilometer optical fiber and further increasing the communication distance to 18 kilometers.
This time, the team of Long Guilu and Professor Lu Jianhua designed and implemented a new quantum direct communication system with mixed coding of phase quantum state and timestamp quantum state, with a quantum direct communication distance of 100 kilometers for the first time, which can realize point-to-point quantum direct communication between cities without relay, and can support some applications of wide-area quantum networks based on secure classical relay. The results were published in the journal Light-Science & Applications.
Experimental setup, picture from the paper
In previous systems, sampling detection and information transmission used phase qubits. This time, the new system developed by the Tsinghua team uses a mixture of phase quantum states and timestamp quantum states. Timestamp quantum states can be used for sampling detection, greatly reducing the impact of noise, and communication still uses phase qubits with self-compensating performance. As a result, the new system has a high degree of stability and a very low eigen-bit error rate (i.e., bit error rate without eavesdropping), combined with very low bitrate LDBCH encoding with stronger error correction capabilities, effectively improving the secure communication capacity, distance and rate.
In addition, the new system increases the maximum tolerable loss from 5.1dB (dB) to 18.4dB at the 50MHz (hertz) laser pulse frequency, and the longest communication distance in commercial low-loss single-mode fiber reaches 100 kilometers, breaking the previous longest distance of 18 kilometers. Its communication rate has also been improved, reaching 22.4kbps over a distance of 30 km of fiber. At the same time, the system still has room to improve the laser pulse frequency, and the corresponding communication distance and rate are expected to be further improved to meet the application needs of some scenarios.
Zhang Haoran (supervisor Academician Wang Chongyu) and Sun Zhen (supervisor Academician Lu Jianhua) of Tsinghua University are the co-first authors of the paper. Professor Long Guilu of the Department of Physics and Yin Liuguo, researcher of the National Research Center for Information Sciences, are co-corresponding authors.