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International Conference on Communication Systems & Networks (COMSNETS)

Design and Modeling of a PAT System for Freespace Optical Links in Quantum Key Distribution

Mallikarjuna Korrapati, Pranab Dutta, Kushal Patel, Korak Biswas, S. Sagar Maurya, Deepika Aggarwal, Sruthi Chennuri, Rajesh Kumar Krishnan

Published Jan 3, 2024• 2 citations
Publisher

Abstract

This study introduces a novel free-space optical (FSO) communication system, designed for quantum key distribution (QKD) effective up to 2 kilometers and extendable to 10 kilometers. Suitable for terrestrial and unmanned aerial vehicle (UAV) applications, the system centers around an advanced Pointing, Acquisition, and Tracking (PAT) model, specifically tailored for the demands of free-space quantum key distribution (FSQKD) communication. The PAT system, with its dual coarse and fine-tracking capabilities, is crucial in establishing and sustaining robust, quantum-secured optical channels. The fine tracking module, in particular, achieves an impressive 0.11 milliradian accuracy and a swift 3.5 millisecond response time. Key to the system's performance is the optimization of beam wavelengths to minimize beam divergence and diffraction losses, essential for the stable operation of the QKD system. A detailed link budget analysis identifies specific losses: diffraction loss at 5dB for 2 kilometers, escalating to 18.4 dB at 10 kilometers, and receiver insertion losses, including optical, coupling, and wavefront losses, at 10.5 dB. The system's modular design enhances its adaptability for various QKD scenarios. This study gives a detailed approach towards practical, scalable quantum communication networks.

DOI
10.1109/comsnets59351.2024.10427419

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This study introduces a novel free-space optical (FSO) communication system, designed for quantum key distribution (QKD) effective up to 2 kilometers and extendable to 10 kilometers. Suitable for terrestrial and unmanned aerial vehicle (UAV) applications, the system centers around an advanced Pointing, Acquisition, and Tracking (PAT) model, specifically tailored for the demands of free-space quantum key distribution (FSQKD) communication. The PAT system, with its dual coarse and fine-tracking capabilities, is crucial in establishing and sustaining robust, quantum-secured optical channels. The fine tracking module, in particular, achieves an impressive 0.11 milliradian accuracy and a swift 3.5 millisecond response time. Key to the system's performance is the optimization of beam wavelengths to minimize beam divergence and diffraction losses, essential for the stable operation of the QKD system. A detailed link budget analysis identifies specific losses: diffraction loss at 5dB for 2 kilometers, escalating to 18.4 dB at 10 kilometers, and receiver insertion losses, including optical, coupling, and wavefront losses, at 10.5 dB. The system's modular design enhances its adaptability for various QKD scenarios. This study gives a detailed approach towards practical, scalable quantum communication networks.
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Here, we demonstrate a field-deployable free-space optical link for quantum key distribution (QKD) between a ground station and radio-controlled electric vehicle (RCEV). Compact and high-performance acquisition, pointing and tracking (APT) system is developed for small tracking error down to 3.8 μrad and average link loss of 16 dB. Such link is within the requirement of normal QKD system. With automatic acquisition and coarse tracking, the APT system can automatically establish the free-space optical link in 10 minutes. It is the first step towards mobile quantum secure communication network. Based on RCEV, the quantum optical link can be reconfigured at different link distances. This result greatly improves the flexibility of the link construction for the future QKD applications.
This study investigates the design and performance of a transmitter optical system optimized for Low-Earth-Orbit (LEO) satellite-based Free-Space Quantum Key Distribution(FS-QKD). The architecture integrates a 120 mm aperture telescope, a beam expander, and an optical-axis alignment module, complemented by a pointing control unit to maintain link stability. Performance was evaluated across 660 nm, 810 nm, and 1,550 nm wavelengths. Link budget analysis for a 10 km channel with a 200 mm receiver aperture demonstrated that a 660 nm beacon laser achieves a diffraction loss of 4.71 dB and a received power of 15.5 dB, with the beam expanding to approximately 440 mm. Notably, the integration of a Fast Steering Mirror(FSM) enhanced the average Signal-to-Noise Ratio(SNR) by nearly ninefold. These results validate the system's efficacy in mitigating pointing errors and optimizing optical coupling for robust inter-satellite quantum communication.
Unmanned aerial vehicles (UAVs)-based free-space optics (FSO)/quantum key distribution (QKD) systems require high-precision pointing mechanisms. This increases system complexity and limits rapid deployment for lightweight and energy-constrained UAVs. This paper proposes a modulating retroreflector (MRR)-equipped UAV architecture for BB84-QKD systems that enables simplified yet accurate tracking while relaxing pointing requirements. A realistic quantum channel model is developed, for which we newly derive the channel probability distribution of transmittance (PDT). Capitalizing on the derived channel PDT, several QKD performance metrics are analytically obtained. Numerical results verify the feasibility of the proposed MRR-aided UAV for practical QKD deployment, highlight its effectiveness over conventional UAV-ground systems, and validate the accuracy of the developed analytical framework.
Quantum cryptography is a promising technology that achieves unconditional security, which is essential to a wide range of sensitive applications. In contrast to optical fiber, the free-space optical (FSO) link is efficiently used as a quantum channel without affecting the polarization of transmitted photons. However, the FSO link has several impairments, such as atmospheric turbulence and pointing errors, which affect the performance of the quantum channel. This paper proposes a quantum key distribution (QKD) scheme that uses a time-bin entanglement protocol over the FSO channel that suffers from various channel impairments. Due to the interest in unmanned aerial vehicles (UAVs) and their usefulness for many social, internet-of-things (IoT), civil, and military applications, the proposed QKD-FSO system is integrated with the ground-to-UAV platform. Furthermore, variances in the position and orientation of the UAV are investigated using a tracking system. These variances are considered when evaluating the overall performance of the proposed integrated system. For this purpose, closed-form expressions are obtained for the system average symbol error rate (ASER) and outage probability. The Monte Carlo simulation is used to verify the validity of the proposed expressions. The system security is investigated assuming photon number splitting (PNS) attack. Moreover, the transmit power, time-bin number, and modulation order are optimized to maximize the raw and secret key rates. The results show that for 500 m link, ASER22 mrad and signal-to-noise ratio > 7.5 dB which leads SNRth < 2.5 dB and raw key rate maximized by adjusting the number of time bins according to the received SNR.