Optical transmission over space links between satellites in intra-orbital constellations or inter-orbital constellations and optical ground stations are currently attracting a lot of attention, both for their ultra-high capacities reaching multi-tera-bits/sec and for their use in quantum key distribution for coding to transmitting over public channels.This paper outlines the followings: (i) Multi-dimensional considerations to reach Tera-bits/sec that include multiplexing of polarized modes, high-order modulation formats, high speed integrated photonic complex modulations, digital signal processing techniques etc.; (ii) Continuous variable quantum key distribution (CV-QKD) achieved through constant amplitude zero auto-correlation (CAZAC) sequence for quantum level signaling with phase-coded BB84protocol, decorrelation , high order sampling rate and digital signal processing for quantum key recovery.(iii) Discussion of advantages and disadvantages of both coherent and self-coherent reception techniques for space transmission channels for multi-photons to subphoton energy reception and transmission; (iv) Presentation of initial transmission results and Terabits/s optical transmission systems; (vi) Analytical considerations of CV-QKD space self-homodyne and homodyne reception system employing CAZAC and crosscorrelation decoding as a photonic-pre-processor.
Optical transmission over space links between satellites in intra-orbital constellations or inter-orbital constellations and optical ground stations are currently attracting a lot of attention, both for their ultra-high capacities reaching multi-tera-bits/sec and for their use in quantum key distribution for coding to transmitting over public channels.This paper outlines the followings: (i) Multi-dimensional considerations to reach Tera-bits/sec that include multiplexing of polarized modes, high-order modulation formats, high speed integrated photonic complex modulations, digital signal processing techniques etc.; (ii) Continuous variable quantum key distribution (CV-QKD) achieved through constant amplitude zero auto-correlation (CAZAC) sequence for quantum level signaling with phase-coded BB84protocol, decorrelation , high order sampling rate and digital signal processing for quantum key recovery.(iii) Discussion of advantages and disadvantages of both coherent and self-coherent reception techniques for space transmission channels for multi-photons to subphoton energy reception and transmission; (iv) Presentation of initial transmission results and Terabits/s optical transmission systems; (vi) Analytical considerations of CV-QKD space self-homodyne and homodyne reception system employing CAZAC and crosscorrelation decoding as a photonic-pre-processor.
We present the transmission of continuous variable quantum key distribution (CV-QKD) qubits, ultra-high capacity of multi-tera bits over atmospheric and inter-satellite channels via the C-band. With the aid of advanced modulation formats and modern photonic technologies, we argue that multi-satellite jumps are both feasible and realizable. Photonic transceivers utilizing photonic integrated circuits in Silicon and InP technology of electro-absorption modulation lasers are presented. It is shown that optical amplification techniques, booster power amplifiers, and highly sensitive optical preamplifiers can compensate for exceedingly high attenuation of links between optical ground stations and satellites with all optical routing and link distances of 12,000 to 45,000 km. The digital signal processing algorithms in coherent receivers for ultra-high capacity and ultra-low power quantum bit systems are demonstrated to play a critical role in system performance. We specifically provide a Nyquist-equivalent sampling theorem that studies the quantum bit error rate based on both the Nyquist sampling theorem and the Heisenberg uncertainty principle. Furthermore, the Nyquist pulse shaping method and constellation probability shaping are used to maximize tera-bit transmission over spatial channels.
Continuous variable quantum key distribution (CV-QKD) has emerged as a promising candidate for quantum-secure communication due to its experimentally demonstrated high key rates in fiber-optic channels. However, the feasibility of discrete modulation CV-QKD in satellite-to-ground downlinks remains an open question due to practical challenges such as high transmission loss, limited communication windows, and atmospheric turbulence. In this work, a comprehensive framework is proposed to evaluate the feasibility of discrete modulation CV-QKD by integrating orbital dynamics and atmospheric channel models, and to comprehensively analyze the influence of the parameter space on free-space discrete modulation CV-QKD. To achieve this, a free-space CV-QKD simulation platform is employed, which calculates the elevation angle and transmission distance based on precise orbital models, thereby providing a more practical assessment of the key rate for discrete modulation CV-QKD. Simulation results verify the feasibility and practicality of discrete modulation CV-QKD in satellite-based quantum communication systems. Furthermore, the critical factors influencing the key rate performance are identified, and parameter optimization strategies are proposed, providing theoretical support for realizing the future satellite-to-ground discrete modulation CV-QKD.
Losses and attenuation are the main limitations to implementing secure quantum networks based on fiber or free-space optical links over long distances. A hybrid satellite–ground approach offers an effective solution to overcome these constraints and achieve higher key transmission rates. This article presents a systematic review of publications on satellite quantum communication, providing an overview of quantum key distribution with satellites, the use of satellite networks with emphasis on commonly adopted orbits, and the role of satellites in such architectures. Special attention is given to the implementation of continuous-variable QKD (CV-QKD) in satellite-based channels, highlighting its challenges and potential for integration into future satellite-based quantum networks.
Optical communications have been widely deployed all over the world. In state-of-the-art optical communication systems, security and high capacity are the most important factors to be considered. As the rapid development of quantum computing, traditional computational complexity based encryption faces unprecedented challenges. To guarantee the unconditional security, quantum key distribution (QKD) has been proposed and now is attracting increasing attentions. Classical optical communications have been exponentially growing for decades. The telecommunications industry is always in favor of high-speed, long-haul, and cost-efficient optical transports. Lots of advanced techniques have been applied to meet the requirements, e.g., multi-dimensional multiplexing, advanced modulation formats, coherent detection, forward error correction (FEC) coding, constellation shaping, and adaptive coding. In this dissertation, I will investigate continuous-variable QKD (CV-QKD) protocols to enable unconditional security and study the reliable classical advanced optical transmission systems. In a dynamic fiber-optic transmission system, the system performance can be improved by the proposed self-adaptive coding approach. We also experimentally and numerically compare the mutual information (MI) performances of regular/probabilistically shaped/geometrically shaped (PS/GS)-8/16/32 quadrature amplitude modulation (QAM) formats. Therefore, the industry can select the best shaping scheme accordingly in a specific situation. Furthermore, we propose a novel hybrid PS/GS scheme, which can minimize non-Gray mapping penalty as well as be tolerant to the implementation penalty. In addition, free-space optical (FSO) communication systems are also studied here. We emulate the outdoor atmospheric turbulence by indoor turbulence emulator, which is based on split-step beam propagation method (SSBPM). Turbulence mitigation methods, including adaptive optics (AO), channel coding, spatial offset, and Huffman coding, are proposed and experimentally demonstrated to enable high-speed FSO communications. At the end, we propose novel four-state and eight-state CV-QKD systems, where the phase noise can be effectively mitigated, and power fluctuation caused by turbulence can be accurately monitored. Thereafter, beyond 1 Gb/s CV- QKD systems can be realized by controlling phase noise, leveraging reconciliation efficiency, monitoring power fluctuation, and multiplexing quantum channels.
This article presents an analysis of the practical challenges and implementation perspectives of point-to-point continuous-variable quantum key distribution (CV-QKD) systems over optical fiber. The study addresses the physical layer, including the design of transmitters, quantum channels, and receivers, with emphasis on impairments such as attenuation, chromatic dispersion, polarization fluctuations, and coexistence with classical channels. We further examine the role of digital signal processing (DSP) as the bridge between quantum state transmission and classical post-processing, highlighting its impact on excess noise mitigation, covariance matrix estimation, and reconciliation efficiency. The post-processing pipeline is detailed with a focus on parameter estimation in the finite-size regime, information reconciliation using LDPC-based codes optimized for low-SNR conditions, and privacy amplification employing large-block universal hashing. From a hardware perspective, we discuss modular digital architectures that integrate dedicated accelerators with programmable processors, supported by a reference software framework (CV-QKD-ModSim) for algorithm validation and hardware co-design. Finally, we outline perspectives for the deployment of CV-QKD in Brazil, starting from metropolitan testbeds and extending toward hybrid fiber/FSO and space-based infrastructures. The work establishes the foundations for the first point-to-point CV-QKD system in Brazil, while providing a roadmap for scalable and interoperable quantum communication networks.
Photonic entanglement is a fundamental resource in quantum information processing and its distribution between distant parties is a key challenge in quantum communications. Optical satellite links allow transmitting entangled photons over longer distances than currently possible on ground and could provide a path towards global quantum communication networks. The majority of current photonic free-space quantum communication systems, including two recent quantum satellite experiments, use two-dimensional polarization encoding, where each photon can carry at most a single bit of quantum information. Encoding additional qubits in the high-dimensional spatio-temporal degrees of freedom could not only increase channel capacities but also improve robustness with respect to noise and eavesdropping in quantum cryptography. Here, we give an overview of our efforts towards exploiting high-dimensional entanglement in long-distance free-space quantum communications. We report on the results of a first feasibility study, in which we used polarization / energy-time hyperentanglement to increase the dimensionality of the state space and transmit genuine 4-dimensional entanglement via an intra-city free-space link. We discuss how we intend to extend this approach to applications in noise-tolerant large-alphabet quantum key distribution via free-space links, and, ultimately, experiments with satellites.