Ma Xiongfeng's group at Tsinghua University proposes a pattern-pairing quantum key distribution protocol
Recently, Professor Xiongfeng Ma and his research group members Pei Zeng, Hongyi Zhou, and Weijie Wu at Tsinghua University's Institute of Cross-Information have designed a novel measurement device-independent protocol that breaks through the code formation rate of point-to-point quantum key distribution. The protocol does not require complex laser phase locking and channel phase monitoring, which significantly improves the performance of measurement device-independent key distribution protocols and provides a theoretical basis for the practicalization of high-performance and high-security quantum key distribution.
Quantum key distribution is currently one of the most successful applications in the field of quantum information and the first step in building quantum communication networks. Based on the existing commercial optical equipment, quantum key distribution can already be practically applied in some commercial environments, such as China's Beijing-Shanghai trunk line project, Hefei's urban key distribution network and so on.
To promote the practicalization of quantum key distribution, it is necessary to further improve the security distance and code-forming rate of its actual devices. On the one hand, the security distance and code-forming rate of quantum key distribution are severely limited by the transmission loss of optical channels. For a general point-to-point quantum key distribution protocol, the code formation rate R is a linear function O(η) of the channel transmission efficiency η, and the code formation rate R decays exponentially with increasing distance due to the exponential relationship between distance and transmission efficiency. Quantum repeaters can solve the problem of quantum signal transmission loss over long distances, however, they are still in the research stage and are still far from practicalization.
On the other hand, in the process of implementing quantum key distribution with actual devices, the devices often have vulnerabilities that do not conform to the theoretical assumptions. These vulnerabilities can be used by eavesdroppers, thus reducing the practical security of quantum key distribution protocols. A general quantum key distribution system consists of three parts: quantum source, channel and measurement device. Among them, the quantum channel is usually not assumed in the theoretical analysis; the assumption of the quantum source is relatively simple and easy to calibrate; while the quantum measurement equipment is often relatively complex and difficult to calibrate. To solve the security problem of measurement devices, Professor Hoi-KwongLo et al. proposed a measurement-device-independent quantum key distribution protocol (measurement-device-independent quantum key distribution) at the University of Toronto in 2012. As shown in Figure 1a, both Alice and Bob, the communication sides of the protocol, generate signals to send to the untrusted relay Charlie, thus eliminating the assumptions about the measurement device and improving the practical security of the quantum key distribution protocol.
There are two main design schemes for current measurement device-independent protocols: 1) In the scheme of Figure 1b, Alice (and Bob) encodes relative information in two optical modes. Such a protocol is collectively referred to as a dual-mode protocol. This scheme is experimentally easier to implement because it uses relative information as the encoding and thus does not require laser phase locking and channel phase monitoring on both sides of the communication. However, since only two simultaneous successful interferences by Charlie, the detector, can produce a successful detection response signal, the code rate of the dual-mode protocol is still severely limited by the total transmission efficiency of the Alice to Bob channel η. 2) In the scheme of Fig. 1c, Alice (and Bob) encodes the information in a single optical mode. Such a protocol is collectively referred to as a single-mode protocol. In this kind of scheme, the detector Charlie only needs to perform one successful interference to associate Alice and Bob's information, thus greatly improving the long-range code-formation rate. However, the interferometric stability of a single optical mode is more dependent on the phase stability of the light source and channel, thus requiring long-range laser phase-locking techniques for Alice and Bob to ensure the interferometric stability, including light source locking and channel phase monitoring techniques. However, this also poses a challenge for broader experimental replication.
Figure 1. Comparison of different measurement device-independent protocols (dual-mode protocol, single-mode protocol, and mode-pairing protocol)
Xiongfeng Ma's group proposed a novel mode-pairing protocol that combines the high performance of a single-mode protocol with the practicality of a dual-mode protocol. As shown in Figure 1d, in this protocol, Alice (and Bob) first encodes the signal in a single optical mode. Then, based on the results of Charlie's detection response, Alice and Bob pair the transmitted signals to extract the relative encoded information. Since Charlie only needs to perform a single interference to correlate Alice's and Bob's signals, the protocol obtains a high code-forming rate similar to that of the single-mode protocol. On the other hand, since the code-forming information is encoded in the relative information of the paired signals, the encoding of the pattern pairing protocol can tolerate higher source and link phase variations, thus eliminating the need for complex long-range laser phase-locking techniques and significantly reducing the requirements of single-mode protocols for source and link phase control.
Figure 2 illustrates the code formation rate of the mode-pairing protocol. According to the stability of the laser and the channel itself in the experiment, we can adjust the maximum pairing interval l. At l>1000, the code rate of the protocol can break the linear code rate limit of the point-to-point protocol; at l taken above 10^4, the code rate of the protocol is basically the same as that of the current single-mode protocol. Experimental verification shows that the protocol still has a low error rate when the pair length l=10^4 in the environment without laser phase locking.
Figure 2. Numerical comparison of code-forming rates for pattern pairing protocols
This work has important implications for the realization of high-rate, practical and secure quantum key distribution protocols and quantum communication networks.
The result was published in Nature Communication on July 7, entitled "Mode-pairing Quantum Key Distribution".
The corresponding author is Prof. Xiongfeng Ma from the Institute of Cross-Information, Tsinghua University. Co-authors are Hongyi Zhou, a 2019 PhD graduate of the Institute of Cross-Information and currently an assistant professor at the Institute of Computing, Chinese Academy of Sciences, and Weijie Wu, a 2020 undergraduate of the Department of Physics.
This paper was supported by the National Natural Science Foundation of China.
Link to the paper：
https://www.nature.com/articles/s41467-022-31534-7
