近日,Light报导了清华龙桂鲁教授团队在量子安全直接通信方面的实验工作(。工作发表仅半个月,便受到了国际科技媒体的热切关注,包括、Serendeputy、Semantics Scholar、Doradus Labs、Voat、Oxford academic、INFOSCUM、Innovation Toronto、Quantum Mechanics、UK Anygator、Live Science、FlipBoard、TodayChan、Parallel State等在内的十多家国际媒体对该项工作进行了重点推介。

Implementation of a Practical Quantum Secure Direct Communication

      Present communication is secured through encryption techniques, relying on pre-shared key and cryptographic protocols built on the computational difficulty of certain mathematical problems. However, emerging supercomputers and quantum computers pose serious threats to these encryption methods, there is increasing need for new cryptographic techniques, such as quantum communication.

   Quantum secure direct communication (QSDC) is one important branch of quantum communication, which was proposed in 2000 by Gui-Lu Long and co-workers of Tsinghua University. QSDC transfers directly information without a pre-shared key. Recently, Gui-Lu Long and co-workers at Tsinghua University and Beihang University, Beijing, China, have demonstrated the DL04 QSDC protocol with practical environmental conditions. The scientists developed a coding scheme of concatenation of low-density parity-check (LDPC) codes and the universal hashing families (UHF), and maintained a secure communication rate of 50 bps to send text messages, reasonably sized images and sounds over 1.5 kilometers through fiber . Their research results were published in Light: Science & Applications.

     A strongly attenuated 1550 nm laser is used as an approximate single-photon source with a systematic pulse-repetition frequency of 1 MH. The communication scheme is based on the DL04 protocol, which is proposed by Deng and Long in 2004. The LDPC coding scheme is specifically designed for operating in the high loss and high error-rate regime, unique for quantum communication. Though only a fraction of photons can reach the legitimate receiver’s site, the information can be decoded successfully by the legitimate receiver, even when the situation was made realistic by causing high photon loss or introducing errors due to noise.

      Long et al. analyzed the security of their system, based on the Wyner wiretap channel theory. Secrecy capacity is defined as the supremum of all the achievable transmission rates with security and reliability. Usually, estimation of the secrecy capacity in a practical communication system is hard. When quantum systems such as single photons are used to transmit digital information, quantum physics principles give rise to novel capability unachievable with classical transmission media. It is impossible in principle for Eve to eavesdrop without disturbing the transmission so as to avoid detection. The first quantum communication protocol, proposed by Bennett and Brassard, showed how to exploit quantum resources for secure key agreement. In 2000, quantum secure direct communication was proposed to transfer information directly by Gui-Lu Long and Xiao-shu Liu using Einstein-Podolsky-Rosen pairs and encoding the informationin quantum states. Shortly after, Fu-Guo Deng, Gui-Lu Long and Xiao-shu Liu generalized the protocol by encoding the information in quantum operations. In 2004, Fu-Guo Deng and Gui-Lu Long, designed a QSDC protocol using single photons, known nowadays as DL04 protocol.

     After nearly two decades of development, QSDC has made great progress. Three proof-of-principle experiments were completed in recent years. In 2016, a collaboration of Shanxi Universityand Tsinghua University demonstrated a single-photon based DL04 protocol. In 2017, a collaboration of USTC and Nanjing University of Posts and Telecommunication demonstrated the entangled-based QSDC protocols, one with state encoding and one with operation encoding, using real quantum memory. In the same year, a collaboration Tsinghua University and Nanjing University of Posts and Telecommunication demonstrated the two QSDC protocols using fiber entanglement source and devices.

    The current implementation has considered the conditions of practical environments and analyzed thesecurity of QSDC. The implementation has shown that QSDC platform could effectively function in a realistic environment. It is a milestone in quantum communication.