Break the bitrate-distance limit! Nanjing University proposes new quantum key distribution protocol

Recently, the latest research results of the research groups of Yin Hualei and Chen Zengbing from the School of Physics of Nanjing University, the State Key Laboratory of Solid State Microstructure Physics, and the Collaborative Innovation Center for Artificial Microstructure Science and Technology have broken through the bitrate-distance limit of independent quantum key distribution of "dual-rail" measuring equipment. Using the dual entanglement property of identical particles, they decoupled two consecutive time bins in the measurement-device-independent quantum key distribution of synchronous time-phase encoding, and proposed an asynchronous protocol through a post-matching method. Using a general measurement device-independent quantum key distribution technology, the asynchronous protocol realizes time multiplexing through classical post-processing, thereby constructing a two-photon Bell state, increasing the intercity transmission key rate by multiple orders of magnitude, and greatly improving the transmission distance. A bridge between measurement device-independent quantum key distribution and dual-field quantum key distribution is established. The related research results were published in the  American Physical Society Journal [PRX Quantum 3, 020315 (2022)] on April 21, 2022 under the title "Breaking the Rate-Loss Bound of Quantum Key Distribution with Asynchronous Two-Photon Interference".

The co-first authors of the paper are Xie Yuanmei and Lu Yushuo, postgraduate students of the School of Physics of Nanjing University, and the corresponding authors are Associate Professor Yin Hualei and Professor Chen Zengbing of Nanjing University. The research work was 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 research and development plan of Nanjing Jiangbei New Area.

In 2012, Measurement Device Independent Quantum Key Distribution (MDIQKD) could close all probe-side loopholes in quantum networks by introducing an untrusted intermediate node for Bell state measurements. Due to its excellent practical security, ease of deployment in star networks, and mature general-purpose technology, MDIQKD is regarded as an important and efficient architectural module in future quantum networks. However, MDIQKD requires two photons to reach the intermediate node at the same time, and its key rate is limited by the bitrate-distance limit of the unrelayed quantum channel [Nat. Commun. 5, 5235 (2014); Nat. Commun. 8, 15043 (2017) )], which is difficult to apply to practical intercity quantum networks. In 2018, two-field quantum key distribution (TFQKD) used long-distance single-photon interference [Nature 557, 400 (2018)] to break the code rate-distance limit of non-relay quantum channels, greatly improving the security of intercity quantum communication key rate. Using state-of-the-art technology, China has achieved a TFQKD experiment of over 830 kilometers of fiber-optic transmission [Nat. Photonics 16, 154 (2022)].

However, in order to compensate for the fast drift of the long-distance quantum channel phase and achieve phase-locking of long-distance independent lasers, complex and expensive phase-locking and phase-tracking techniques must be used in the experiment, which greatly increases the experimental complexity and commercial cost of TFQKD. And lax implementation can lead to security risks. Therefore, it is a arduous but important task to propose a new protocol that integrates the excellent intercity bit rate performance of TFQKD and the mature general technology of MDIQKD at the same time. In addition, in 2021, the universal limit theory of quantum key distribution network [PRX 11, 041016 (2021)] pointed out that the dual-rail (dual-rail) MDIQKD protocol of two-photon interference cannot break the rate-distance limit of non-relay quantum channel.

Inspired by dual entanglement, an asynchronous MDIQKD protocol is designed by cleverly transforming synchronous time coding into asynchronous time coding by using post-matching method. By randomly matching the time bins associated with the two detected phases to establish an asynchronous two-photon Bell state, the two-photon interferometric two-orbit MDIQKD protocol breaks the bitrate-distance limit of the unrepeatered quantum channel, making the theoretically impossible became possible. Furthermore, since the phase noise difference between each time instant is approximately equal over a short time interval, the two phase-dependent time bins can be post-matched without the use of phase tracking and phase locking techniques, which greatly increases the Reduced the difficulty of the experiment. Since the single-photon pair composition density matrix of each user is always the same under the time and phase basis no matter what asymmetric source parameters are chosen, this protocol is suitable for users that can dynamically access the source parameters regardless of the existing user source parameters. Quantum Network.

   Figure 1(a): Phase tracking technique removed; 1(b): Phase tracking and phase locking techniques removed simultaneously.

The simulation results show that for a 1 GHz system, the transmission distance of the protocol can reach 450 km without phase tracking; after removing the phase tracking and phase locking techniques at the same time, the protocol can reach a distance of 270 km with limited key effect. Breaking the bitrate-distance limit of unrepeatered quantum channels. At intercity distances, the key rate of this protocol is improved by tens of thousands of times compared to the original MDIQKD protocol. For example, a key rate of 0.15 Mbit/s can be achieved over 300 kilometers, which is sufficient for a variety of tasks including audio and video one-time pads. At the same time, since the protocol does not require phase tracking, all the detection counting capacity of its single-photon detector can be used for quantum signal measurement, making it more feasible than TFQKD using the same-frequency strong reference optical phase tracking technology at intercity distances. The protocol has an order of magnitude higher key rate.

The theoretical work was highly valued by the reviewers, who praised: "This is a clever way to bridge MDI and TF-QKD with measurement device independence and two-field quantum key distribution", "This work is This work provides an important proposal for an improvement for TF-QKD systems which should be relevant for anyone working in the field".

Link:

https://journals.aps.org/prxquantum/abstract/10.1103/PRXQuantum.3.020315