Recently, Pan Jianwei team of China University of Science and Technology and Wang Xiangbin team of Tsinghua University once again joined forces to break through the long-range independent laser phase interference technology and realize the two-field quantum key distribution (TF-QKD) of 500 km of real-world optical fiber, thus once again breaking the world record for the transmission distance of quantum key distribution.
February 20, 2020 local time, the above team named Send-or-Not-Send with Laser: Secure Twin-Field Quantum Key Distribution ove The research on r 509 km (“Send-Do Not Send” stand-alone laser: 509 km secure two-field quantum key distribution) was published in The Physical Review Letters, a world-renowned academic journal of physics, and was selected as an “editor recommendation” article.
“Send -Not Send” Two-Field Quantum Key Distribution Scheme
In cryptography, key refers to the secret information used to complete cryptographic applications such as encryption, decryption, integrity verification, etc. In fact, since the use of language, the technology of encrypting information by key has evolved.
In 1984, physicist Bennett and cryptographer Brassard proposed the world’s first quantum key distribution protocol (BB84 protocol) based on the principle of quantum mechanics measurement, designed to increase secure communication distances and security. The quantum key distribution (QKD, or quantum key distribution) proposed under this protocol can provide unconditional secure sharing keys for users who separate the two places, which fundamentally guarantees the security of the key.
However, due to limitations such as communication fiber loss, detector noise, etc., quantum key distribution systems typically achieve a high code-generation rate within 100 km – in the absence of quantum trunking (relay refers to a transmission path between two switching centers), the security code-forming rate is bound by linear boundaries.
Anti-hacking quantum key distribution schematic
Based on this, a new quantum key distribution scheme, the “send-do not send” two-field quantum key distribution scheme, was proposed by Wang Xiangbin, professor of physics at Tsinghua University, deputy director of the Center for Cryptographic Theory of Tsinghua University, director of Jinan Institute of Quantum Technology, and one of the authors of the paper.
It is reported that this scheme cleverly utilizes the characteristics of single photon interference, greatly improves the tolerance of phase noise and security, so that the code rate of quantum key distribution in long-distance situations also maintain a high level.
Simply put, a two-field quantum key is like a twin working hand in hand, even easily breaking the code-based linear limit without relaying.
In 2019, Pan Jianwei, the “father of quantum” in China and a member of the Chinese Academy of Sciences, and his team completed a two-field quantum key distribution experiment in a 300-kilometer fiber optic in a real-world environment.
However, Lei Feng learned that the technology of dual-field quantum key distribution is quite demanding:
The single-photon level interference of two remote independent lasers is required.
The precise estimation of the relative phase of the long-distance fiber link is accurately estimated by the single photon detection result.
Single photon detectors that require high count, high efficiency and ultra-low dark count situated at the same time.
Quantum Key Distribution Transmission Distance From World Record
This time, the research team applied the above solution, experimenting on the 509 km long ultra-low-loss fiber to prove a secure key distribution, breaking the absolute key rate limit of unrelayed quantum keys, and successfully setting a new world record for the longest distance distribution of ground-based quantum keys.
Specifically, Pan Jianwei’s team based on Wang Xiangbin’s “send-do not send” two-field quantum key distribution scheme, the development of time-frequency transmission technology and laser injection locking technology, the two independent remote laser wavelength stab to the same, and the use of additional phase reference light to estimate the relative phase of the fiber rapid drift. At the same time, the team combined the high-count low-noise single-photon detector developed by the Shanghai Microsystem of the Chinese Academy of Sciences to push the safe code distance of quantum key distribution to 500 km.
The following illustration shows the experimental device, Alice and Bob are remote lysing-frequency stable continuous wave lasers, modulated by the phase modulator (PM) and three intensity modulators (IM1, IM2, IM3) for phase randomization, coding, and bait intensity modulation.
In addition, the ATT is the attenuator, the PC is the polarization controller, the PBS is the polarization divider, the DWDM is the dense wave divider, the CIR is the circulator, the BS is the beam divider, the FM is the Faraday mirror, the PD is the photodiode.
According to the paper, the 509 km fiber code-generation rate is more than 7 times higher than the relative limit of traditional unrelayed quantum key distribution, i.e. exceeds the unrelayed quantum key distribution code threshold under the ideal detection device (the detector efficiency is 100%).
In addition, if the system repeat frequency is upgraded to 1GHz in long-distance quantum communication networks such as the Beijing-Shanghai mainline, the code-forming rate of 300 km can reach 5kbps, thereby greatly reducing the number of trusted trunks in the backbone fiber quantum communication network and greatly improving the security of the optical fiber quantum confidential communication network.
In fact, on March 2, 2020, local time, the international academic journal Nature Photonics published another study by Academician Pan Jianwei and others, called Implementation of Quantum key distrib the operation of the velocity the linear rate-transmittance bound (implementation of quantum key distribution beyond the linear rate-transmission limit).
Lei Feng noted that the study is based on the phase-matching quantum key distribution (PM-QKD) protocol proposed by Tsinghua University professor Ma Xiongfeng, which further advances the development of remote quantum communication and lays the foundation for the burgeoning quantum Internet.
On September 10, 2019, at the first bilateral seminar organized jointly by the Chinese Academy of Sciences and the German National Academy of Sciences (Leopoldina), academician Pan Jianwei said:
There are two major challenges in the journey of long-range secure quantum communication, namely, security problems under realistic conditions and long-distance transmission.
Visible, this research for quantum communication is of great significance, in this Lei Feng network also look forward to more breakthroughs brought by Chinese researchers!