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.
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.
Free-space optical (FSO) communication provides a promising alternative solution to the traditional radio frequency (RF) communications. Compared to the RF technology, there are many advantages of FSO communications, such as ultra-high bandwidth, license-free spectrum, low inter-channel interference, and energy-efficient transmission. The potential applications for FSO communication include the last-one-mile access link from the fiber-based backbone network, low earth orbit (LEO) satellite communications, high-altitude base station platforms or vehicles, deep-space communications, and in-building automation connections. As the FSO technology becomes mature and increasingly involved, the security and reliability requirements of such applications also become crucial. Regarding the security issues, although the FSO communication is inherently more secure than the RF counterpart due to high directivity of its line-of-sight (LOS) link, it can still suffer from optical wire-tapping attack. There are many different application schemes that an eavesdropper can still probe the FSO link. Traditionally, to protect the communication, many works have been done on encryption layer and upper network layer, such as computationally intensive cryptographic algorithms and message exchanging protocols. In recent years, physical-layer security (PLS) has been getting more attentions as it provides an extra layer of security against eavesdropping. The PLS schemes, in general can be categorized into two main categories: classical PLS and quantum key distribution (QKD) protocols. The classical PLS is based on the information theoretic security analysis, while the QKD security is guaranteed by the quantum mechanics laws. Both techniques have been studied for FSO channels. In the first half of this dissertation, we study the PLS in a LOS FSO channel using orbital angular momentum (OAM) multiplexing and show that higher secrecy capacity can be achieved using OAM multiplexing technology in the presence of atmospheric turbulence effects. Then, we proposed an adaptation scheme for QKD system over FSO channel with different QKD protocols, i.e. BB84 and decoy state protocols. By adapting the source brightness based on the channel condition, we optimized the secret key rate (SKR) of the QKD system. Furthermore, we proposed a multiple spatial modes-based QKD system with backpropagation method to increase the total SKR through parallel channels. Regarding the reliability of an optical communication system as well as information reconciliation for both PLS and QKD schemes, the forward error correction (FEC) becomes an essential technique to enable high-speed transmission. The FEC with low-density parity-check (LDPC) codes has been studied for decades from LDPC block codes to the most recent spatially-coupled (SC) LDPC codes. For FSO communications, due to the time-varying nature of FSO channels, the capability of rate adaptation is important to achieve a consistent reliable transmission. In the second half of this dissertation, we proposed unified FPGA-based architecture to implement both quasi-cyclic (QC) LDPC codes and SC-LDPC codes with rate adaptation capability. The performance is verified through FPGA emulation to show good error floor properties. To further validate the rate adaptation capability, we have built a spatial light modulator (SLM)-based FSO channel emulator to test the adaptive coding scheme. Furthermore, we have conducted a fiber-based experiment with our rate adaptative coding scheme to show that the proposed adaptive coding scheme can be also used in dynamic fiber-based networks.
We propose and experimentally investigate a high-speed Kramers-Kronig (KK) scheme based continuous-variable quantum key distribution (CV-QKD) system over a free-space optical (FSO) channel. The atmospheric turbulence channel is emulated by three spatial light modulators on which three randomly generated azimuthal phase patterns yielding Andrews' spectrum are recorded. The proposed KK scheme enables a low-complexity implementation, insensitivity to laser phase noise, low detection noise, and accurate channel transmittance monitoring. In addition, eight state CV-QKD protocols and 10-channels wavelength-division multiplexing (WDM) scheme are experimentally verified to obtain a high secure key rate. In our experiment, the minimum transmittance of 0.6 is required to guarantee the secure transmission, and a total SKR of 2.1 Gb/s can be obtained at the mean transmittance in weak turbulence regime.
A high-speed four-state continuous-variable quantum key distribution (CV-QKD) system, enabled by wavelength-division multiplexing, polarization multiplexing, and orbital angular momentum (OAM) multiplexing, is studied in the presence of atmospheric turbulence. The atmospheric turbulence channel is emulated by two spatial light modulators (SLMs) on which two randomly generated azimuthal phase patterns yielding Andrews' spectrum are recorded. The phase noise is mitigated by the phase noise cancellation (PNC) stage, and channel transmittance can be monitored directly by the D.C. level in our PNC stage. After the system calibration, a total SKR of >1.68 Gbit/s can be reached in the ideal system, featured with lossless channel and free of excess noise. In our experiment, based on commercial photodetectors, the minimum transmittances of 0.21 and 0.29 are required for OAM states of 2 (or -2) and 6 (or -6), respectively, to guarantee the secure transmission, while a total SKR of 120 Mbit/s can be obtained in case of mean transmittances.
Quantum information processing uses the laws of quantum physics to perform tasks that are infeasible or impossible classically. Within this broader programme, quantum communication has emerged as one of the most practically mature sub-fields of quantum information science, with several proposed protocols having already been demonstrated over deployed optical fibre and free-space links. Of the various platforms available for encoding quantum information for communication, the continuous-variable (CV) framework encodes information in the quadratures of the electromagnetic field and is therefore directly compatible with the existing machinery of classical coherent optical communications. The most prominent CV protocol is continuous-variable quantum key distribution (CV-QKD), which establishes an information-theoretically secure cryptographic key between two spatially separated parties. Because CV-QKD reuses much of the hardware of classical coherent telecommunications, including off-the-shelf lasers, modulators, and balanced detectors, it is a leading candidate for the deployment of quantum-secured communication over standard optical fibre networks. The development of CV-QKD has typically proceeded with the underlying physics dictating the engineering. Security proofs, often relying on the Gaussian optimality of collective attacks, set the structure of the protocol, and the practical system is then built to satisfy the assumptions on which those proofs depend. Contributions of this thesis adopt the opposite view, taking the practical engineering of high-rate CV-QKD systems as the starting point and developing the theoretical machinery required to support it. The first contribution is the formalisation of one-round sliced reconciliation with guard-band post-selection as a key distillation scheme designed for real-time, high-throughput operation. Originally introduced experimentally as a means to stabilise bit error rates and enable the use of fixed-rate error correction codes at high symbol rates, guard-band post-selection is shown here to identify the guard-band span as a free parameter that can be tuned to hold the quantum bit error rate approximately constant across a range of channel conditions. However, conditioning on accepted data produces non-Gaussian post-selected statistics that lie outside the reach of standard Gaussian-optimality security arguments, which motivates the remaining two contributions. The second contribution is an asymptotic security analysis of guard-band post-selected CV-QKD against arbitrary collective attacks, no longer restricted to Gaussian adversaries. By using a quantum channel estimate as side information and by lifting reduced-state distances to purification distances via entropic continuity bounds, security is certified against the class of channels lying close to a Gaussian thermal-loss reference, with a key-rate penalty that scales with the energy-constrained diamond distance to that reference. The third contribution is the experimental basis required to apply this framework in practice. A lightweight three-stage protocol for quantum channel estimation is proposed and demonstrated, which repurposes the Gaussian probe constellation already present in the CV-QKD signal and requires no additional hardware or modifications to the underlying protocol. Over single-mode optical fibre at distances from 10 km to 50 km, the protocol produces physically valid process tensor estimates whose energy-constrained diamond distances to a Gaussian reference channel remain on the order of 10^(-3) throughout. Together, these three contributions provide a treatment of post-selected Gaussian-modulated CV-QKD in which the engineering, the security theory, and the channel characterisation are designed to operate together within a deployable system.
Quantum key distribution (QKD) leverages quantum information theory to enable two remote parties to establish a random key with unconditional secrecy. Performing QKD with coherent light states integrates with modern optical communication techniques but presents new challenges. This survey reviews advancements in applying digital signal processing (DSP) techniques, transformative in coherent optical communications, to continuous-variable QKD (CV-QKD). We discuss how DSP methods enhance CV-QKD performance, security, and feasibility. The areas examined include phase estimation, channel equalization, error correction, and noise reduction, highlighting associated challenges and future research directions.
When light is propagated through a channel that is not a waveguide, such as a fiber, it is known as a free-space optical channel. When used in communication systems, free-space optical channels provide the advantages of high communication rates, lower power, and minimal infrastructure installation. When the free-space optical channel has a random medium in it, such as the atmosphere, the distortions will be applied to the phase. The phase distortions can lead to intensity fluctuations and additional beam spreading. To compensate for the distortions adaptive optics can be used, allowing for the stability of the system to be improved. Additionally, post processing techniques on transmitted data can also be used to improve the stability of the channel. By significantly decreasing the number of photons transmitted, a classical communication channel can be considered as a quantum channel. Encoding information into the quantum states of the transmitted photons, quantum communication can be performed. When transmitting on the order of 1 photon per pulse, classical post processing techniques cannot be used to improve the stability of the channel. To stabilize the free-space optical channel, adaptive optics can be used at the transmitter or receiver. As no measurement of the quantum state is performed by the adaptive optics system, no information in the photon is lost. In this work, quantum key distribution will be implemented over the quantum channel. The focus of this work is on the design and study of how the channel degradations can be overcome, as well as the establishment of a free-space optical channel with adaptive optics in a quantum networking testbed. Quantum key distribution systems were simulated over a 30 km maritime channel as well as over a underwater channel. For the 30 km channel, the use of multiple parallel spatial modes results in the highest increases of the secure key rate at the cost of transmitting a lower average number of photons. When adaptive optics is applied, the secure key rates have a small increase, however as the strength of the turbulence increases from weak to moderate, the relative improvement is higher. For the underwater channel, operation at lower depths is optimal as fewer background photons reach the receiver and the channel attenuation is lower. In the experimental setup, it was found that the single photon detectors that are being used are the limiting the channel loss to be at most 6 dB. The free space optical link established between the Electrical and Computer Engineering building and College of Optical Sciences uses a corner cube retro-reflector to yield a distance of 1.45 km. The channel has been characterized at various times of day to determine the strength of the atmospheric turbulence, which ranges from a scintillation index of 0.07 to 2.21. A classical communication experiment was performed, resulting in error-free transmission while the input power to the EDFA before the photodetector was consistently above -25 dBm.