In 20 12, device-independent quantum key distribution (MDIQKD) introduces an untrusted intermediate node to measure the Bell state, which can close all probe holes in the quantum network. Because of its excellent practical security, easy deployment in star network and mature general technology, MDIQKD is considered as an important and efficient architecture module in the future quantum network. However, MDIQKD requires two photons to arrive at the intermediate node at the same time to interfere, and its key rate is limited by the rate-distance of the quantum channel without relay, so it is difficult to be applied to the actual intercity quantum network.
20 18 Two-field Quantum Key Distribution (TFQKD) breaks the rate-distance limit of quantum channel without relay by using long-distance single-photon interference, and greatly improves the secure key rate of inter-city quantum communication. Using the most advanced technology, our country has realized the TFQKD experiment of optical fiber transmission over 830 kilometers. However, in order to compensate the rapid phase drift of long-distance quantum channel and realize the phase locking of long-distance independent lasers, complex and expensive phase locking and tracking technologies must be used in the experiment, which greatly increases the experimental complexity and commercial cost, and lax implementation may lead to security risks.
Therefore, it is an arduous and important task to propose a new protocol that can integrate the excellent inter-city code rate performance of TFQKD and the mature general technology of MDIQKD at the same time. In addition, the universal restriction theory of quantum key distribution network points out that the MDIQKD protocol of two-photon interference cannot break the rate-distance restriction of unrepresented quantum channels.
This time, inspired by the double entanglement, Yin Hualei and Chen Zengbing of the State Key Laboratory of Solid Microstructure Physics of the Collaborative Innovation Center for Artificial Microstructure Science and Technology skillfully converted synchronous time coding into asynchronous time coding through the post-matching method, and designed the asynchronous MDIQKD protocol.
Using identical particles's double entanglement, the team decoupled two consecutive time boxes in quantum key distribution of measuring equipment independent of synchronous phase coding, and proposed an asynchronous protocol through post-matching. Moreover, using the general MDIQKD technology, the asynchronous protocol is time-multiplexed by classical post-processing, so as to construct a two-photon bell state, which increases the key rate of intercity transmission by several orders of magnitude, greatly increases the transmission distance, and establishes a bridge between MDIQKD and TFQKD. The related results were recently published in american physical society magazine PRX Quantum.
In the specific experiment, the aforementioned team randomly matched the time boxes associated with two detection phases to establish asynchronous two-photon Bell states, which made the two-photon interference double-track MDIQKD protocol break the rate-distance limit of the unrepeatered quantum channel and made the theoretical impossibility possible.
In addition, because the phase noise difference between each time is approximately equal in a short time interval, two phase-related time boxes can be matched after the phase tracking and locking technology is not used, which greatly reduces the experimental difficulty. No matter what asymmetric source parameters are selected, the component density matrix of each user's single photon pair is always the same in time and phase basis vectors, so the new protocol is suitable for quantum networks that users can dynamically access, without considering the existing user source parameters.
The experimental simulation results show that the transmission distance of the above protocol can reach 450 kilometers for 1 GHz (gigahertz) system without phase tracking. After removing the phase tracking and locking technology at the same time, the protocol can still break the rate-distance limit of the unrepeatered quantum channel at a distance of 270 kilometers under the finite key effect. In the intercity distance, the key rate of this protocol is tens of thousands of times higher than that of the original MDIQKD protocol. For example, when transmitting 300 kilometers, the key rate can reach 0. 15 Mbit/s (megabits per second), which is enough to complete various tasks including audio and video encryption at one time.
At the same time, because the new protocol does not need phase tracking, all the detection and counting capabilities of its single photon detector can be used for quantum signal measurement, which makes it possible to realize the key rate one order of magnitude higher than TFQKD protocol by using the same frequency strong reference light phase tracking technology in intercity distance.
The above research results break through the rate-distance limit of independent quantum key distribution of "dual-track" measuring equipment, which is helpful to large-scale quantum communication networks. Commentators commented, "This is an ingenious way to connect the measurement device with the two-field quantum key distribution" and "this work provides important suggestions for the improvement of TFQKD system".
The first author of the paper * * * is Xie and Lu, graduate students of the School of Physics of Nanjing University, and the corresponding authors are Yin Hualei, associate professor of Nanjing University, and Chen Zengbing, a professor. The research work has been supported by the National Natural Science Foundation of China, the Natural Science Foundation of Jiangsu Province, the basic scientific research business expenses of central universities and the key projects of R&D plan of Nanjing Jiangbei New District.
Proofreading: Zhang Yan