Considering quantum signal transmission in the underwater links with scattering, we investigate the quantum channel model, the related capacity and secrecy performance. Monte Carlo simulation results demonstrate that the polarization angle change within [−20, 20] degrees accounts for 98%. We propose a quantum channel model for underwater link with scattering and theoretically investigate the capacity, where a larger proportion of scattered photons and a larger variation range of the polarization angle leads to lower quantum classical capacity. We analyze the secret key rate of BB84 protocol under unauthorized receiver attack (URA). The results show the maximum secure transmission distance under the given parameters of seawater decreases from 113 m to 58 m as the probability that the eavesdropper receives scattered photons increases from 0.1 % to 10%.
Considering quantum signal transmission in the underwater links with scattering, we investigate the quantum channel model, the related capacity and secrecy performance. Monte Carlo simulation results demonstrate that the polarization angle change within [−20, 20] degrees accounts for 98%. We propose a quantum channel model for underwater link with scattering and theoretically investigate the capacity, where a larger proportion of scattered photons and a larger variation range of the polarization angle leads to lower quantum classical capacity. We analyze the secret key rate of BB84 protocol under unauthorized receiver attack (URA). The results show the maximum secure transmission distance under the given parameters of seawater decreases from 113 m to 58 m as the probability that the eavesdropper receives scattered photons increases from 0.1 % to 10%.
Underwater quantum key distribution (QKD) has potential applications in absolutely secure underwater communication. However, the performance of underwater QKD is limited by the optical elements, background light, and dark counts of the detector. In this paper, we propose a modified formula for the quantum bit error rate (QBER), which takes into account the effect of detector efficiency on the QBER caused by the background light. Then we calculate the QBER of the polarization encoding BB84 protocol in Jerlov-type seawater by analyzing the effect of the background light and optical components in a more realistic situation. Finally, we further analyze the final key rate and the maximum secure communication distance in three propagation modes, i.e., upward, downward, and horizontal modes. We find that secure QKD can be performed in the clearest Jerlov-type seawater at a distance of hundreds of meters, even in the worst downward propagation mode. Specifically, by optimizing the system parameters, it is possible to securely transmit information with a rate of 67 kbits/s at a distance of 100 m in the seawater channel with an attenuation coefficient of 0.03/m at night. For practical underwater QKD, the performance can also be improved by using decoy states. Our results are useful for long-distance underwater quantum communication.
Considering the ocean water’s optical attenuation and the roughness of the sea surface, we analyze the security of continuous-variable (CV) quantum key distribution (QKD) based air-to-water channel. The effects of the absorption and scattering on the transmittance of underwater quantum channel and the maximum secure transmission distance are studied. Considering the roughness of the sea surface, we simulate the performance bounds of CV QKD with different wind speeds using the Monte Carlo method. The results show that even if the secret key rate gradually reduces as the wind speed increases, the maximum transmission distance will not be affected obviously. Compared to the works regarding short-distance underwater optical communication, our research represents a significant step towards establishing secure communication between air platform and submarine vehicle .
There is a growing interest in the security of underwater communication with the increasing demand for undersea exploration. In view of the complex composition and special optical properties of seawater, this paper deals with a performance analysis for continuous-variable quantum key distribution (CVQKD) over an underwater link. In particular, we focus on analyzing the channel transmittance and detection efficiency based on Monte Carlo simulation for different water types, link distances and transceiver parameters. A comparison between the transmittance obtained by simple Beer’s law and Monte Carlo simulation reveals that the transmittance of underwater link may be severely underestimated in the previous underwater CVQKD research. The effect of the receiver aperture and field of view (FOV) on detection efficiency under different water types is further evaluated based on Monte Carlo. Simulation results show that the transmission distance of the underwater CVQKD system obtained by Monte Carlo simulation in pure sea water, clear ocean water and coastal ocean water is larger than that obtained by Beer’s law, while the key rate of the system in all types of water is smaller than that obtained by Beer’s law because the size and FOV of the receiver aperture are taken into account. By considering the practical system parameters, this paper establishes a comprehensive model for evaluating the security of underwater CVQKD systems with different system configurations.
Underwater wireless optical communication (UWOC) possesses significant advantages, such as high bandwidth, low latency, and low power consumption, making it a key technology for building information networks in marine environments. However, due to the scattering effect of seawater, some photons carrying information inevitably scatter out of their predetermined paths, leading to the possibility for information leakage. Therefore, we propose a physical-layer security analysis model for UWOC systems based on the wiretap channel model. The model evaluates the security of the communication system by calculating the capacity difference between the legitimate channel and the eavesdropping channel in the UWOC system. Specifically, the model first constructs the three-dimensional intensity distribution of scattered photons in the underwater channel via Monte Carlo simulations and experimental measurements. Then, it calculates the capacities of both the legitimate and eavesdropping channels based on the decoding results. Finally, the three-dimensional distribution of secrecy capacity is derived to assess the security of the communication system. In this work this model is used to analyze the security of the UWOC system in clear seawater environments. The results show that the secrecy capacity of the system is zero within a certain range near the transmission path, demonstrating that scattered photons can cause information leakage. We recommend that, in practical applications, monitoring the non-signal transmission area near the transmitter is essential to ensure communication security. This research provides a solution for analyzing the quantitative security of UWOC, which can strongly support the design of UWOC systems and encoding/decoding schemes.
In the context of secure underwater optical communications, this study examines the performance of the entanglement-based BBM92 protocol when the source generates non-maximally entangled photon pairs. The impact of seawater optical properties, as well as source correlation characteristics, on transmission efficiency is analyzed for three types of nonturbulent channels: clear, coastal, and turbid water. The main performance metrics, including quantum bit error rate (QBER), secret key rate (SKR), and correlation probability, are analytically derived and evaluated as functions of transmission distance between two communicating parties. Monte Carlo simulation results validate our analytical models and quantify performance under different channel conditions.
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.