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Exploring the Performance of Structured Light Modes and Novel Protocols for Quantum Key Distribution

Abolhassani, Rojan

Published Dec 12, 2025• 0 citations
Publisher

Abstract

This thesis includes two main projects that explore the use of structured light, especially photonic orbital angular momentum (OAM), in quantum key distribution protocols. Particular attention has been paid to how different modes behave in turbulent channels and how adaptive optics systems can mitigate the distortions induced in the beam. Based on the results of these experiments, we propose a new protocol designed to combine the qualities that make a light mode ideal for turbulent channels and adaptive optics systems. The security analyses and performance evaluations of this new protocol are discussed. In the first chapter, a brief overview of the field is provided. Following that, the second chapter aims to explore the necessary background for understanding the two projects in more detail. We discuss the different degrees of freedom of light and how they can be utilised to help achieve our goals. In the third chapter, the mathematical background relevant to the first project is covered. This includes the modes investigated in the turbulent channel and AO system. In the fourth chapter, an overview of the theory of security in quantum key distribution is presented, followed by an article on a new protocol for QKD. Finally, the conclusions and outlooks are outlined in the last chapter.

DOI
10.20381/ruor-31612

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This thesis includes two main projects that explore the use of structured light, especially photonic orbital angular momentum (OAM), in quantum key distribution protocols. Particular attention has been paid to how different modes behave in turbulent channels and how adaptive optics systems can mitigate the distortions induced in the beam. Based on the results of these experiments, we propose a new protocol designed to combine the qualities that make a light mode ideal for turbulent channels and adaptive optics systems. The security analyses and performance evaluations of this new protocol are discussed. In the first chapter, a brief overview of the field is provided. Following that, the second chapter aims to explore the necessary background for understanding the two projects in more detail. We discuss the different degrees of freedom of light and how they can be utilised to help achieve our goals. In the third chapter, the mathematical background relevant to the first project is covered. This includes the modes investigated in the turbulent channel and AO system. In the fourth chapter, an overview of the theory of security in quantum key distribution is presented, followed by an article on a new protocol for QKD. Finally, the conclusions and outlooks are outlined in the last chapter.
Quantum key distribution is a promising candidate to ensure secure communication in the future. Higher-dimensional state spaces as provided by the orbital angular momentum of photons (OAM) could further improve the security and robustness to noise. When transmitted through free-space, these photons experience atmospheric turbulence which introduces distortions to the encoded OAM. A technologoy called adaptive optics has been developed in astronomy to counter-act turbulence-induced distortion. While the application of adaptive optics has been considered in the context of classical OAM communication, this thesis presents the first results for OAM-based quantum key distribution. The main result of this thesis is that the losses of the quantum mechanical properties of OAM photons in atmospheric can be mitigated by adaptive optics. Both the entanglement and the trace of the final state's density matrix can be significantly improved. Furthermore, the security of the quantum key distribution protocol can be ensured for two to three times stronger turbulence as measured by the Fried parameter. The presented results demonstrate the great potential of adaptive optics for OAM quantum cryptography.
Quantum key distribution (QKD) allows secure key exchange based on the principles of quantum mechanics, with higher-dimensional photonic states offering enhanced channel capacity and resilience to noise. Free-space QKD is crucial for global networks where fibres are impractical, but atmospheric turbulence introduces severe states distortions, particularly for spatial modes. Adaptive optics (AO) provides a pathway to correct these errors, though its effectiveness depends on the encoding basis. Here, we experimentally evaluate a high-speed AO system for orbital angular momentum (OAM) modes, mutually unbiased bases (MUB), and symmetric, informationally complete, positive operator-valued measures (SIC-POVM) up to dimension d=8d=8d=8 in a turbulent free-space channel. While OAM states are strongly distorted, their cylindrical symmetry makes them optimally corrected by AO, yielding error rates below QKD security thresholds. MUB and SIC-POVM exhibit greater intrinsic robustness to turbulence but are less precisely corrected, though their performance remains within protocol tolerances. These results establish AO as a key enabler of secure, high-dimensional QKD and highlight the role of basis choice in optimizing resilience and correction.
Quantum key distribution (QKD) allows secure key exchange based on the principles of quantum mechanics, with higher-dimensional photonic states offering enhanced channel capacity and resilience to noise. Free-space QKD is crucial for global networks where fibres are impractical, but atmospheric turbulence introduces severe states’ distortions, particularly for spatial modes. Adaptive optics (AO) provides a pathway to correct these errors, though its effectiveness depends on the encoding basis. Here, we experimentally evaluate a high-speed AO system for orbital angular momentum (OAM) modes, mutually unbiased bases (MUB), and symmetric, informationally complete, positive operator-valued measures (SIC-POVM) up to dimension d = 8 in a turbulent free-space channel. While OAM states are strongly distorted, their cylindrical symmetry makes them optimally corrected by AO, yielding error rates below QKD security thresholds. MUB and SIC-POVM exhibit greater intrinsic robustness to turbulence but are less precisely corrected; however their performance remains within protocol tolerances. These results establish AO as a key enabler of secure, high-dimensional QKD and highlight the role of basis choice in optimizing resilience and correction.
This Chapter presents a review of the quantum mechanical properties of spatially structured light fields, specifically those fields carrying orbital angular momentum (OAM). This review is concerned both with the conceptual understanding of the quantum features of these light fields and with the use of these features for applications. We describe how to produce spatially entangled light fields by means of the nonlinear optical process of parametric downconversion. We ask and provide a tentative answer to the question of how much information can be encoded into a single photon. As an example, we review a recent experiment that demonstrated the ability to discriminate among four target objects using only one photon for illumination. We also present a description of the concept of the OAM of light, and we describe means to generate and detect OAM. We then present a brief survey of some recent studies of the fundamental quantum properties of structured light beams. Much of this work is aimed at studying the nature of entanglement for the complementary variables of angular position and OAM. Finally, as a realworld application, we describe a secure communication system based on quantum key distribution (QKD). This key distribution system makes use of encoding information in the OAM modes of light and hence is able to transmit more than one bit of information per photon. These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.
Quantum key distribution (QKD) technology has demonstrated remarkable practicality and sustained developmental trends in the current field of quantum communication. This paper focuses on a novel and potent quantum communication protocol, specifically quantum key distribution utilizing the encoding of information through orbital angular momentum (OAM) of photons. OAM states, owing to their distinctive properties, exhibit significant advantages in terms of information capacity and channel robustness. However, the sensitivity of OAM states to atmospheric conditions and challenges arising from wavefront distortions necessitate in-depth investigation. Additionally, the unique structure of OAM beams has raised concerns about geometric losses. By analyzing the impact of atmospheric turbulence and geometric losses on the transmission performance when applying the OAM-QKD protocol in free space, this paper explores variations in system losses under different OAM orders. Simulation results indicated that as the channel distance increases, system losses gradually escalate. Furthermore, with an increase in OAM order, although the system's sensitivity to atmospheric turbulence diminishes, geometric losses exhibit an exponential increase. Addressing this issue, our research suggested that when considering free-space quantum key distribution with OAM encoding, selecting an OAM order of 1 for the beam achieves minimal transmission losses. Therefore, in practical applications, employing OAM beams with <?TeX l=1l = 1l=1 ?> emerges as an effective strategy to optimize the performance of the OAM-QKD protocol in free space.
Abstract In the last two decades, structured light patterns with orbital angular momentum (OAM) have been extensively utilized in quantum cryptography or quantum key distribution, which is generally considered an unconditionally secure method of secret key transmission between two distant communicating entities. High-dimensional encoding of quantum bits in quantum key distribution improves the security and information-carrying capability of a quantum signal. Significant degradation in the efficiency of such communication is observed due to the aberration of OAM states under atmospheric turbulence. In this paper, we focus on analyzing the impact of atmospheric turbulence on the error-rates of quantum key distribution, which is considered the benchmark in analyzing the security of such communication over the quantum channel in the presence of an intruder (man-in-the-middle). Our results show that turbulence under normal conditions does not significantly affect the successful transmission of secret quantum bits between sender and receiver. Furthermore, we identify an interesting variation in the signature of error-rates to clearly detect the presence of an intruder as a result of medium-level turbulence.