In this work we study the underwater optical channel, making a feasibility analysis on the development of a high-speed communication system with security guaranteed by Quantum Key Distribution (QKD) protocol. In particular, we introduce a new model for the attenuation of the underwater medium which includes the effect of turbulence within the marine environment.We report the performance of the system focusing on the most important figures of merit, such as the secret key rate of the quantum channel and the bit rate of the classical channel at the physical layer as a function of the total link range. We carried out these analyses depending on various environmental conditions, such as the geographical area, depth, path-loss and turbulence models. The communication protocols considered are the BB84 with time-bin encoding for QKD and baseband On-Off Keying (OOK) modulation for classical data communication for a maximum link range of 100 meters.
In this work we study the underwater optical channel, making a feasibility analysis on the development of a high-speed communication system with security guaranteed by Quantum Key Distribution (QKD) protocol. In particular, we introduce a new model for the attenuation of the underwater medium which includes the effect of turbulence within the marine environment.We report the performance of the system focusing on the most important figures of merit, such as the secret key rate of the quantum channel and the bit rate of the classical channel at the physical layer as a function of the total link range. We carried out these analyses depending on various environmental conditions, such as the geographical area, depth, path-loss and turbulence models. The communication protocols considered are the BB84 with time-bin encoding for QKD and baseband On-Off Keying (OOK) modulation for classical data communication for a maximum link range of 100 meters.
The current literature on quantum key distribution is limited mainly to transmissions over fiber optic, atmospheric, or satellite links and is not directly applicable to underwater environments with different channel characteristics. In this paper, we analyze the quantum bit error rate (QBER) and secret key rate (SKR) performance of the well-known BB84 protocol in underwater channels. As a path loss model, we consider a modified version of the Beer–Lambert formula, which takes into account the effect of scattering. We derive a closed-form expression for the wave structure function to determine the average power transfer over a turbulent underwater path and use this to obtain an upper bound on QBER as well as a lower bound on SKR. Based on the derived bounds, we present the performance of the BB84 protocol in different water types including clear, coastal, and turbid and under different atmospheric conditions such as clear, hazy, and overcast. We further investigate the effect of system parameters such as aperture size and detector field of view on QBER and SKR performance metrics.
Underwater wireless communication is rapidly advancing, finding applications in diverse fields such as oceanography, defense, and commercial ventures. However, ensuring security in such transmissions is crucial due to the sensitive nature of the data involved and the challenges posed by the underwater environment. While classical encryption techniques provide some level of security, the emergence of quantum computing presents opportunities and challenges. Quantum key distribution (QKD) offers theoretically unbreakable encryption, making it an attractive solution. Extending QKD capabilities to underwater environments is a significant endeavor in this context. This paper explores the feasibility of applying an entanglement-based non-coherent QKD protocol inspired by the BBM92 protocol to underwater visible light communication (VLC)/QKD systems. We investigate the system’s design criteria and analyze its secret key performance, addressing challenges such as water absorption and turbulence-induced fading, focusing on addressing unauthorized receiver attacks. Through analysis and the considered case study, the feasibility and efficacy of this approach are explored, contributing to the advancement of secure underwater communications.
The current literature on quantum key distribution (QKD) is mainly limited to the transmissions over fiber optic, atmospheric or satellite links and are not directly applicable to underwater environments with different channel characteristics. Absorption, scattering, and turbulence experienced in underwater channels severely limit the range of quantum communication links. In the first part of this thesis, we analyze the quantum bit error rate (QBER) and secret key rate (SKR) performance of the well-known BB84 protocol in underwater channels. As path loss model, we consider a modified version of Beer-Lambert formula which takes into account the effect of scattering. We derive a closed-form expression for the wave structure function to determine the average power transfer over turbulent underwater path and use this to obtain an upper bound on QBER as well as a lower bound on SKR. In the second part of this thesis, as a potential solution to overcome range limitations, we investigate a multi-hop underwater QKD where intermediate nodes between the source and destination nodes help the key distribution. We consider the deployment of passive relays which simply redirect the qubits to the next relay node or the receiver without any measurement. Based on the near-field analysis, we present the performance of relay-assisted QKD scheme in terms of QBER and SKR in different water types and turbulence conditions. In the last part of this thesis, we investigate the fundamental performance limits of decoy BB84 protocol over turbulent underwater channels and provide a comprehensive performance characterization. Based on near field analysis, we utilize the wave structure function to determine the average power transfer over turbulent underwater path and use this to obtain a lower bound on key generation rate. Based on this bound, we present the performance of decoy BB84 protocol in different water type.
Quantum key distribution (QKD) has been identified as a viable solution for secure underwater communication. This study aims to characterize the impact of natural turbulent waters on systems. The performance of QKD systems is analyzed in terms of the quantum bit error rate and the secret key rate. We expand upon previous studies which only considered large separations by including large and small separations by comparing the turbulence length scale with the distance between two observation points. To that end, we obtain the analytical expression of the wave structure function (WSF) for small separation for propagating Gaussian beams. Since the WSF describes how the amplitude and phase of an optical wave change as it travels through the varying refractive index of the turbulent medium over a given distance, the accurate characterization remains essential in terms of revealing the beam wavefront distortion. Leveraging the practical parameters of underwater environment and communication systems, we demonstrate that the average temperature, average salinity concentration, temperature-salinity gradient, temperature and energy dissipation rates of turbulent water impact the performance and security of the QKD systems to a nonnegligible extent. It is also observed that the waters having high chlorophyll concentrations, e.g., coastal waters, drastically decrease the performance of the QKD systems due to strong absorption and scattering effects. The findings and insights gained from this study may help advance secure underwater communication, which in turn will aid in establishing future QKD networks. Potential applications of this research include secure underwater communication for defense, data gathering for marine environmental monitoring, remote data transmission, and deep-sea exploration.
Decoy state quantum key distribution protocols have been studied for atmospheric, fiber, and satellite links; however, those results are not directly applicable to underwater environments with different channel characteristics. In this paper, we investigate the fundamental performance limits of decoy state BB84 protocol over turbulent underwater channels and provide a comprehensive performance characterization. We adopt a near field analysis to determine the average power transfer over a turbulent underwater path and use this to obtain a lower bound on the secret key rate. We quantify the performance of decoy BB84 protocol in different water types assuming various turbulence conditions. We further investigate the effect of system parameters such as transmit aperture size and detector field of view on the performance.
The channel fading induced in the ocean quantum links is a major obstacle restricting the performance of ocean quantum key distribution. To determine the ultimate limits at which secret bits (and entanglement) can be distributed via a seawater medium, we propose an analytical model for ocean quantum links to derive the probability distribution of the transmittance, which characterizes the evolution of the quantum state over the fading channel. The proposed model includes the effects of absorption, scattering, turbulence, and ocean thermal noise due to background light. Our comprehensive numerical analysis of several practical scenarios encompasses both pure-loss bounds and thermal-loss bounds, with a submarine at various depths and in various oceans on earth. Besides identifying ultimate limits, we present a security analysis of coherent-state and squeezed-state protocols, showing the secret key rate of both the asymptotic case and finite-size regimes.