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Applied Optics

Robust aperiodic synchronous scheme for satellite-to-ground quantum key distribution

Chao-Ze Wang, Yang Li, Wenqi Cai, Meng Yang, Wei-Yue Liu, Sheng‐Kai Liao, Cheng-Zhi Peng

Published May 11, 2021• 15 citations
Publisher

Abstract

Time synchronization is essential for quantum key distribution (QKD) applications, not only in fiber links and terrestrial free-space links but also in satellite-to-ground links. To compensate for the time drift caused by the Doppler effect and adapt to the unstable optical link in satellite-to-ground QKD, previous demonstrations adopted a two-stage solution, combining a global navigation satellite system (GNSS) and light synchronization. In this paper, we propose a novel aperiodic synchronization scheme that can achieve high-precision time synchronization by encoding time information into pseudo-random laser pulse positions. This solution can simplify the use of GNSS hardware, thus reducing the complexity and cost of the system. Successful experiments have been conducted to demonstrate the feasibility and robustness of the presented scheme, resulting in a synchronization precision of 208-222 ps even when 90% of the light signals are lost. Further analysis of the Doppler effect between the satellite and the ground station is also given. The presented robust aperiodic synchronization can be widely applied to future satellite-based quantum information applications.

DOI
10.1364/ao.425085

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Time synchronization is essential for quantum key distribution (QKD) applications, not only in fiber links and terrestrial free-space links but also in satellite-to-ground links. To compensate for the time drift caused by the Doppler effect and adapt to the unstable optical link in satellite-to-ground QKD, previous demonstrations adopted a two-stage solution, combining a global navigation satellite system (GNSS) and light synchronization. In this paper, we propose a novel aperiodic synchronization scheme that can achieve high-precision time synchronization by encoding time information into pseudo-random laser pulse positions. This solution can simplify the use of GNSS hardware, thus reducing the complexity and cost of the system. Successful experiments have been conducted to demonstrate the feasibility and robustness of the presented scheme, resulting in a synchronization precision of 208-222 ps even when 90% of the light signals are lost. Further analysis of the Doppler effect between the satellite and the ground station is also given. The presented robust aperiodic synchronization can be widely applied to future satellite-based quantum information applications.
Time synchronization is one of the most crucial issues that must be addressed in developing quantum key distribution (QKD) systems. It not only lets the transmitter and the receiver to assign a sequence number to each event and then do correct basis reconciliation, but also allows to increase signal-to-noise ratio. Time synchronization in satellite communications is especially complicated due to such factors as high loss, signal fading, and Doppler effect. In this work, a simple, efficient, and robust algorithm for time synchronization is proposed. It was tested during experiments on QKD between Micius, the world’s first quantum communications satellite, and an optical ground station located in Russia. The obtained synchronization precision lies in the range from 467 to 497 ps. The authors compare their algorithm for time synchronization with the previously used methods. The proposed approach can also be applied to terrestrial QKD systems.
Time synchronization is crucial for quantum key distribution (QKD) systems. In order to compensate for the time drift caused by the Doppler effect and adapt to the unstable optical link in satellite-to-ground QKD, previous demonstrations generally adopted synchronization methods requiring additional hardware. In this paper, we present a novel synchronization method based on the detected quantum photons, thus simplifying additional hardware and reducing the complexity and cost. This method adopts target frequency scanning to realize fast frequency recovery, utilizes polynomial fitting to compensate for the Doppler effect, and takes use of the vacuum state in the decoy-state BB84 protocol to recover the time offset. This method can avoid the influence of synchronization light jitter, thus improving the synchronization precision and the secure keys as well. Successful satellite-to-ground QKD based on this new synchronization scheme has been conducted to demonstrate its feasibility and performance. The presented scheme provides an effective synchronization solution for quantum communication applications.
Abstract Time synchronization is one of the most crucial issues that must be addressed in developing quantum key distribution (QKD) systems. It not only lets the transmitter and the receiver to assign a sequence number to each event and then do correct basis reconciliation, but also allows to increase signal-to-noise ratio. Time synchronization in satellite communications is especially complicated due to such factors as high loss, signal fading and Doppler effect. In this work, a simple, efficient and robust algorithm for time synchronization is proposed. It was tested during experiments on QKD between Micius, the world’s first quantum communications satellite, and an optical ground station located in Russia. The obtained synchronization precision lies in the range from 467 to 497 ps. The authors compare their algorithm for time synchronization with the previously used methods. The proposed approach can also be applied to terrestrial QKD systems.
Abstract Global Navigation Satellite Systems (GNSSs), such as GPS and Galileo, provide precise time and space coordinates globally and constitute part of the critical infrastructure of modern society. To reliably operate GNSS, a highly accurate and stable system time is required, such as the one provided by several independent clocks hosted in Precise Timing Facilities (PTFs) around the world. The relative clock offset between PTFs is periodically measured to have a fallback system to synchronize the GNSS satellite clocks. The security and integrity of the communication between PTFs is of paramount importance: if compromised, it could lead to disruptions to the GNSS service. Therefore, securing the communication between PTFs is a compelling use-case for protection via Quantum Key Distribution (QKD), since this technology provides information-theoretic security. We have performed a field trial demonstration of such a use-case by sharing encrypted time synchronization information between two PTFs, one located in Oberpfaffenhofen (Germany) and one in Matera (Italy)—more than 900 km apart. To bridge this large distance, a satellite-QKD system is required, plus a “last-mile” terrestrial link to connect the optical ground station (OGS) to the actual location of the PTF. In our demonstration, we have deployed two full QKD systems to protect the last-mile connection at both locations and have shown via simulation that upcoming QKD satellites will be able to distribute keys between Oberpfaffenhofen and Matera, exploiting already existing OGSs.
High-resolution time synchronization is the crucial technology of quantum key distribution experiment from ground to satellite.A new scheme of time synchronization was presented.In this scheme,a synchronous pulse which has the same wavelength with the quantum signal was adopted and single photo detector was used to detect the synchronous pulse.High-resolution time measurement module was used to measure the instance of synchronous pulse and signal pulse.This scheme overcomes the problem that the the synchronization laser requires much more resources of satellite and the isolation between the synchronization pulse and quantum single pulse.This system was successfully applied to the 16 km quantum key distribution in free space channel.Synchronous resolution of the system is 1.023 ns and bit rate of the raw key for the quantum key distribution is 8.27 kHz.
We propose a novel synchronization scheme using a periodic light pulse for free-space quantum key distribution (QKD) system. For our 10MHz system, the repetition rate of timing pulse in the novel synchronization scheme can be reduced to 100 KHz or even lower. It allows employing a KHz repetition rate ultrashort pulse laser as timing synchronization source to implement high precision time synchronization. In this paper, short term time drift induced by atmospheric turbulence is analyzed, and how the time deviation impacts on the final secure key generation rate in our QKD system is calculated. In addition, we carried out the QKD experiments over the distance of 1.38 km. The results of the experiments indicate that our scheme is feasible.