For the first time, the quantum direct communication distance reached 100 kilometers

Recently, the team of Long Guilu, vice president of Beijing Institute of Quantum Information Science and professor of Tsinghua University, and Lu Jianhua, professor of Tsinghua University, have cooperated to design and implement a new quantum direct communication system with mixed coding of phase quantum states and timestamp quantum states, with a communication distance of 100 kilometers, which is the longest quantum direct communication distance in the world. Such an indicator can achieve point-to-point quantum direct communication between cities without relay, and can support some applications of wide-area quantum networks based on secure classical relays. The results were published online on April 6 in Light:Science & Applications.

As early as 2000, Long Guilu’s team proposed the first protocol for quantum direct communication. 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. In 2017, the relevant teams of the University of Science and Technology of China, Nanjing University of Posts and Telecommunications and Tsinghua University completed the demonstration experiment of entanglement-based quantum direct communication.

In 2019, Long Guilu’s team and Lu Jianhua’s team cooperated to successfully develop the world’s first quantum direct communication system, achieving a secure communication rate of 50bps at a distance of 1.5km fiber. In 2020, the Long Guilu team and the Lu Jianhua team released the world’s first practical quantum direct communication prototype at the Zhongguancun Forum, achieving a transmission rate of 4kbps in a 10-kilometer optical fiber; in the same year, they further increased the communication distance to 18 km. In 2021, the joint team of Shanghai Jiao Tong University and Jiangxi Normal University realized a point-to-point quantum direct communication network with 15 users.

In previous systems, sampling detection and information transmission were all phase qubits. The new system uses a mixed encoding of phase qubits and timestamp quantum states, and timestamp quantum states are used for sampling detection, which greatly reduces the noise impact. Communication still uses phase qubits with self-compensating properties. Therefore, the new system has a high degree of stability and a very low intrinsic bit error rate, that is, the bit error rate without eavesdropping, combined with the very low bit rate LDBCH encoding with stronger error correction capabilities, effectively improving the secure communication capacity, distance and rate.

The new system increases the maximum tolerable loss from 5.1dB to 18.4dB at a 50MHz laser pulse frequency, and the longest communication distance in commercial low-loss single-mode fiber reaches 100 kilometers, breaking through the previous longest distance of 18 kilometers. The communication rate of the new system has also been improved, reaching 22.4 kbps over a distance of 30 km of fiber.

Schematic of quantum direct communication. Courtesy of respondents

However, Long Guilu also said that the new system still has a large room for improvement in 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.

The research results show that using existing mature technical means, point-to-point quantum direct communication between cities is feasible. Long Guilu said that using the recent safe classical relay quantum network technology proposed by Chinese and British scholars such as the Beijing Institute of Quantum Information Science and Tsinghua University, scientists can build a secure relay quantum network to support a variety of applications. (Source: China Science Daily Zheng Jinwu)

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