First-of-its-kind achievement! Chinese scientists achieve pattern-matching quantum key distribution

Recently, Pan Jianwei and Chen Teng-Yun of the University of Science and Technology of China (USTC), in collaboration with Ma Xiong-Feng of Tsinghua University, have experimentally realized mode-pairing quantum key distribution (Mode-pairing QKD) for the first time, and the related research results were published in Physical Review Letters on January 17 [1].

Mode-pairing quantum key distribution

Quantum key distribution (QKD) is based on the basic principles of quantum mechanics and can achieve theoretically unconditionally secure and confidential communication, so it has been a hot research topic in academia in recent decades.

Pattern matching quantum key distribution protocol (MP-QKD) is a new measurement device-independent quantum key distribution protocol proposed by Ma Xiongfeng's research group at Tsinghua University in 2022, which requires both communicating parties to first encode information in a single optical pattern, and based on the detection response results, both communicating parties pair according to certain rules, and then perform post-processing operations such as basis vector comparison and parameter estimation according to the pairing situation to generate the final security key.

Compared with the original measurement device-independent protocol (MDI-QKD), MP-QKD can use more detection events for code-forming, which can largely improve the code-forming rate. Compared with the two-field quantum key distribution protocol (TF-QKD) and the phase-matching protocol (PM-QKD), MP-QKD does not require complex laser frequency-locking and phase-locking technology, which saves cost and reduces practical application difficulties. It also has better anti-interference capability against environmental noise.

Comparison of different MDI-QKD schemes. In the conventional MDI-QKD scheme using dual-mode coding, the key information is encoded into two predefined pulses. Only when Charlie detects these two pulses, Alice and Bob can know the encoded information, indicated as gray arrows. In a two-field QKD scheme using single-mode coding, the key information is encoded as one pulse, which is susceptible to interference in the channel. When Charlie announces a click on a pulse, denoted as a green arrow, Alice and Bob can derive a key bit. In pattern pairing MDI-QKD, Alice and Bob encode the information into a pulse to get rid of the overlap detection requirement and pair the clicked pulses based on the detection result. They can extract a key bit from any two paired successful detections, indicated by the red arrow, which is robust to channel interference.

Secure code-forming over long distances without laser frequency-locked phase-locking

Over the past two decades, telecommunication fiber-based quantum key distribution networks have been implemented at the metropolitan and intercity scale. One of the bottlenecks is that the key rate decays exponentially with the increase of transmission distance.

This time, Pan's team's solution focuses on achieving longer distances by creating a long-arm single-photon interferometer on both communication sides. While they have advantageous performance in long-range communication, the phase-locking requirement between two independent lasers is technically challenging. By using the recently proposed idea of mode pairing, the team achieved high-performance quantum key distribution without global phase locking.

The experimental setup. the setups of Alice and Bob are identical, but their coded modulations are independent. The continuous wave laser is cut into discrete pulses by an intensity modulator (IM). These pulses were then randomly modulated into one of four intensities with the help of two Sagnac rings (SR1, SR2): a strong pulse, a signal pulse, a decoy pulse, and a vacuum pulse. Three phase modulators (PM1, PM2, PM3) are used for phase encoding and active phase randomization. The encoded pulses are attenuated to single photon level by an electrically variable optical attenuator (EVOA) and transmitted to Charlie. the polarization of the pulses is adjusted by an electrical polarization controller (EPC) and a polarization beam splitter (PBS) before performing interference measurements. Finally, the signal is detected by a superconducting nanowire single photon detector (SNSPD). SNSPD1 and SNSPD2 are used for interference detection, and SNSPD3 and SNSPD4 are used for polarization feedback and arrival time feedback.

Using two independent off-the-shelf lasers, the team also showed a four-fold improvement in key rate over the traditional measurement device-independent scheme in the regime of city and inter-city distances. For longer distances, the experimental team also improved key rate performance by three orders of magnitude with 304 km of commercial fiber and 407 km of ultra-low loss fiber.

The X-error rate and contrast ratio for different l (a) X-base error rate of the intense optical pulse, which can be used to quantify the extent of Alice and Bob's estimate of the phase reference. It shows that the X-base error rate also varies for different fiber lengths and pairing intervals. Larger pair length intervals lead to larger phase fluctuations. Therefore, the X-base error rate increases with the increase of communication distance. The minimum pair length is chosen as 63 to avoid errors caused by the post-pulse and detector dead time. (b) When the maximum pairing length is chosen as 3000, the pairing ratio falling into certain pairing length intervals varies with the fiber distance. The interval [63, 500) means that only two click pulses with index differences between 63 and 500 are counted. Other intervals are defined similarly. For short communication distances, the ratio of low intervals like [63, 500) is high because the average pair length is small. While for long distances, the ratio of high intervals like [1500, 2000) becomes larger.

Comparative plot of code-forming rates for pattern matching protocols

MP-QKD can achieve safe code formation at long distances without laser frequency locking and has a high code formation rate at metropolitan distances, which greatly reduces the difficulty of protocol implementation. This ready-to-implement high-performance scheme is expected to be widely used in future intercity quantum communication networks [2].

Reference links：

[1]https://arxiv.org/abs/2208.05649

[2]http://www.quantumcas.ac.cn/2023/0204/c20525a591070/page.htm