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Quantum Information and Measurement (QIM)

Point-ahead demonstration of a transmitting antenna for satellite quantum communication

Xuan Han, Hai-Lin Yong, Ping Xu, Weiyang, Wang Kui-Xing Yang, Hua-Jian Xue, Wenqi Cai, Ji-Gang Ren, Cheng-Zhi Peng, Jian-Wei Pan

Published Jan 1, 2019• 1 citation
Publisher

Abstract

We design a novel transmitting antenna with a point-ahead function, and provide an easy-to-perform calibration method with an accuracy better than 0.2 μ rad. Subsequently, our antenna establishes an uplink to the quantum satellite, Micius, with a link loss of 41-52 dB over a distance of 500–1,400 km.
DOI
10.1364/qim.2019.t5a.58

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A low-divergence beam is an essential prerequisite for a high-efficiency long-distance optical link, particularly for satellite-based quantum communication. A point-ahead angle, caused by satellite motion, is always several times larger than the divergence angle of the signal beam. We design a novel transmitting antenna with a point-ahead function, and provide an easy-to-perform calibration method with an accuracy better than 0.2 μrad. Subsequently, our antenna establishes an uplink to the quantum satellite, Micius, with a link loss of 41-52 dB over a distance of 500-1,400 km. The results clearly confirm the validity of our model, and provide the ability to conduct quantum communications. Our approach can be adopted in various free space optical communication systems between moving platforms.
To meet the requirement of high-accuracy pointing of quantum signals in satellite-to-ground quantum communication, this paper proposes a flexible satellite-based pointing method that changes the fine tracking point to solve the problem from point-ahead angle and ground beacon laser offset. This method does not require the use of a point-ahead mechanism and can detect the pointing angle in real time. Detailed algorithms and analysis are given. The method has been verified in orbit on the quantum science satellite Micius. The satellite-to-ground test results show that the quantum signal pointing accuracy is between 0.5∼1.0 µrad, which meets the efficiency requirements of satellite-to-ground quantum communication.
Losses limit the maximum distance of ground based quantum communication to « 400 km. Satellites overcome this limitation. Till date there have been numerous feasibility studies and two successful, large-scale quantum communications satellite missions. Achieving similar performance with a tiny, low-cost CubeSat in the uplink scenario is a major challenge. Here we present a feasibility study for a fully functional 3U-CubeSat-based quantum receiver. The advantage of this scheme is twofold: first, the majority of complex and critical equipment is ground based; second, one satellite design is compatible with a variety of quantum optical experiments. We provide a complete link loss analysis, count rate estimations and preliminary design. Further we discuss solutions to key problems such as satellite pointing errors and measurement/detection issues. Using current technology, we show that the CubeSat is feasible and can be used to violate a Bell-like inequality over a free-space distance of 500 km.
High-fidelity transmission of polarization encoded qubits plays a key role in long distance quantum communication. By establishing the channel between ground and satellite, the communication distance can even exceed thousands of kilometers. Aimed to achieve the efficient uplink quantum communication, here we describe a high-fidelity polarization design of a transmitting antenna with an average polarization extinction ratio of 887:1 by a local test. We also implement a feasible polarization-compensation scheme for satellite motions with a fidelity exceeding 0.995 ± 0.001. Based on these works, we demonstrate the ground-to-satellite entanglment distribution with a violation of Bell inequality by 2.312±0.096, which is well above the classic limit 2.
Quantum communication rates in terrestrial quantum networks are fundamentally limited by fiber loss, even in the presence of quantum repeaters. Satellites offer a solution for long-distance communication, with the most commonly explored scenario involving prepare-and-measure protocols connecting from orbit to a trusted-node ground station via free-space down-links. In contrast, up-link scenarios allow for entanglement to be distributed between a satellite and remote end-users in terrestrial networks, eliminating any trust requirement on the ground station. Here, we demonstrate an ultra-bright source of far-non-degenerate entangled photons and perform quantum key distribution in emulated high-loss satellite scenarios. With a loss profile corresponding to that of one of the pioneering Micius up-link experiments, and a terrestrial end-user separated by 10 km of telecom fiber, we achieve secure key bit accumulation of 5.2 kbit in a single emulated overpass in the asymptotic limit. Our results confirm the viability of upcoming low-Earth orbit receiver satellite missions.
Quantum communication is proven to be the only unconditionally secure method for information exchange, which could well be the first commercial application of quantum information science. Compared with fiber-based demonstrations, free-space links could provide a more promising approach because photon loss and decoherence are almost negligible in the atmosphere. Furthermore, by using satellites, ultra-long-distance quantum communication and tests of quantum foundations could be achieved on a global scale. Here we report a serial of recent experimental progresses on free-space quantum communication over long distance in the Qinghai Lake area. In the meantime, besides the experimental efforts on the ground, Chinese Quantum Science Satellite has been launched on August 16th2016. And we will introduce the latest achievements in satellite-based quantum communication and large-scale tests of quantum foundations obtained through Micius.