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Optical and Quantum Electronics

Prediction of transmittance for a free space quantum channel and improving quantum Keyrate in adverse atmospheric condition

Debarpita Paul Choudhury, Dalia Nandi

Published May 3, 2024• 2 citations
Publisher

Abstract

Abstract Free Space Optical Quantum Communication (FSOQC) is a new wireless approach that allows users to communicate in a secure and faster manner. The working mode of quantum communication is based on quantum physics and is somewhat probabilistic in nature. In order to carry quantum signal in a scattered environment, it is needed to derive the probability distribution of transmittance for a specified region at a desired communication link distance. To understand the performance of FSOQC under adverse weather condition, the effects of various atmospheric parameters such as fog, rain, and turbulence need to be analyzed. In the present study, an attempt is made to predict transmittance for a tropical location, Kolkata, India in real-time conditions, and Keyrate is estimated based on the predicted transmittance. The diurnal, monthly and yearly variation of transmittance is analyzed which is required for effective implementation of a quantum communication system. Two alternative situations, with and without sunlight are considered for Keyrate computation. A combination of two techniques, Difference Frequency Generation and Spontaneous Parametric Down Conversion has been applied to increase Keyrate under adverse weather condition. The operating wavelength is 800 nm, and combined technique is developed around that wavelength.

DOI
10.1007/s11082-024-06959-7

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Abstract Free Space Optical Quantum Communication (FSOQC) is a new wireless approach that allows users to communicate in a secure and faster manner. The working mode of quantum communication is based on quantum physics and is somewhat probabilistic in nature. In order to carry quantum signal in a scattered environment, it is needed to derive the probability distribution of transmittance for a specified region at a desired communication link distance. To understand the performance of FSOQC under adverse weather condition, the effects of various atmospheric parameters such as fog, rain, and turbulence need to be analyzed. In the present study, an attempt is made to predict transmittance for a tropical location, Kolkata, India in real-time conditions, and Keyrate is estimated based on the predicted transmittance. The diurnal, monthly and yearly variation of transmittance is analyzed which is required for effective implementation of a quantum communication system. Two alternative situations, with and without sunlight are considered for Keyrate computation. A combination of two techniques, Difference Frequency Generation and Spontaneous Parametric Down Conversion has been applied to increase Keyrate under adverse weather condition. The operating wavelength is 800 nm, and combined technique is developed around that wavelength.
One of the most rapidly developing communication technologies is quantum communication. Quantum Free-Space Optical Communication (FSO) provides unlimited bandwidth for data transmission. A key barrier to FSO links is associated with the losses caused by atmospheric absorption and scattering of molecules and aerosols. In the present study, we analyze the performance of a free-space classical link for 650 nm, 785 nm, 850 nm and 1550 nm wavelengths for a link range of 0.5 Km and 50 Km. The losses due to atmospheric components such as rain, fog, snow and turbulence are predicted for different wavelengths. Also, we presented similar analysis for free space quantum link using difference frequency generation(DFG) method for both short range (0.5 Km) and long-range link (50 Km) under diverse weather conditions. It is done by combining weak wavelengths of 650 nm, 785 nm, 850 nm and 1550 nm, along with a tunable pump LASER (Krypton) to generate IR B waves (1400 nm - 3000 nm). The total atmospheric losses with DFG is compared with the losses generated without using DFG.
Quantum key distribution (QKD) provides secure keys with information-theoretic security ensured by the principles of quantum mechanics. The flexibility and efficient deployment of free-space optical (FSO) quantum key distribution systems can make them highly advantageous. Despite the benefits of free-space transmission, there is still a chance of running into unfavorable weather conditions like fog. According to this study, free-space continuous variable (CV) QKD systems may be applicable when fog is present. Using attenuation based on the Kim and Kruse empirical visibility models, the performance of the models that explain the relationship between fog attenuation and air visibility is examined in order to evaluate its impact on the secure key rate (SKR) at wavelengths of 850 and 1550 nm. A comparative analysis between the two wavelengths is carried out to highlight their suitability under different weather conditions. Based on the results, 1550 nm performs better in foggy situations, reaching an SKR of 1.389379 bits/symbol, because of lower scattering losses, whereas 850 nm achieves a longer range of 12 km in clear air because of smaller diffraction divergence. The results demonstrate that wavelength selection plays a critical role in enhancing the robustness of CV-QKD systems that operate over FSO links. In addition, optimizing reconciliation efficiency (β) under thick-fog conditions greatly improved the SKR, indicating that 1550 nm operation has the ability to achieve reliable and secure quantum communication. Results show that varying β from 0.90 to 1.00 for both 850 nm and 1550 nm produces SKR enhancement of (0.63-0.80 bits/symbol) and (0.97-1.13 bits/symbol), respectively.
Free-Space Optical (FSO) communication technology can cater to various types of connectivity requirements within optical networks, including core, edge, and access networks. However, the performance of FSO communication links can be adversely affected by severe atmospheric conditions, such as turbulence and weather-related factors. In this study, we have investigated the benefits of photon entangled 16-QAM- FSO link in improving the performance of FSO communication under diverse weather and atmospheric turbulence conditions. The efficient implementation of a practical secure quantum communication network may revolutionize the terrestrial and non-terrestrial communication both form speed and security point of view.
Many fundamental and applied experiments in quantum optics require transferring nonclassical states of light through large distances. In this context the free-space channels are a very promising alternative to optical fibers as they are mobile and enable to establish communications with moving objects, using satellites for global quantum links. For such channels the atmospheric turbulence is the main disturbing factor. The statistical properties of the fluctuating transmittance through the turbulent atmosphere are given by the probability distribution of transmittance (PDT). We derive the consistent PDTs for the atmospheric quantum channels by step-by-step inclusion of various atmospheric effects such as beam wandering, beam broadening and deformation of the beam into elliptic form, beam deformations into arbitrary forms. We discuss the applicability of PDT models for different propagation distances and optical turbulence strengths in the case when the receiver module has an annular aperture. We analyze the optimal detection and correction strategies which can improve the channel transmission characteristics. The obtained results are important for the design of optical experiments including postselection and adaptive strategies and for the security analysis of quantum communication protocols in freespace.
Quantum Communication (QC) represents a promising futuristic technology, revolutionizing secure communication. Photon-based Quantum Key Distribution (QKD) is the most widely explored area in QC research, utilizing the polarisation degree of freedom of photons for both fibre and free-space communication. In this work, we investigate and mitigate the challenges posed by fibre birefringence and atmospheric effects on QKD, using a 505050-meter free-space optical link and entanglement-based BBM92 QKD protocol. We implement a passive polarisation correction scheme to address the critical issue of polarisation scrambling induced by fibre birefringence and the difference in the frame of reference between Alice and Bob. This scheme effectively mitigates these adverse effects, ensuring reliable polarisation control over the quantum channel. Furthermore, we conduct QKD experiments in both day and night conditions, encountering challenges such as high background noise levels and dynamic environmental changes. To overcome these issues, we employ various filtering techniques to enhance signal quality and security. Our results demonstrate the successful implementation of QKD over a free-space optical link by producing information-theoretic secure QBER of <11%<11\%<11% on an average and high keyrate, even under varying lighting and weather conditions. Over one 24 hour cycle of data acquisition, we measured an average daylight keyrate and QBER of (3.9118±0.7339KHz3.9118\pm0.7339 KHz3.9118±0.7339KHz and 10.5518±1.3428%10.5518\pm1.3428\%10.5518±1.3428%) respectively and night time keyrate and QBER of (4.6118±0.8088KHz4.6118\pm0.8088 KHz4.6118±0.8088KHz and 10.3545±1.2501%10.3545\pm1.2501\%10.3545±1.2501%) respectively.
Subject of study. This study examined the applicability of quantum key distribution (QKD) technology under free-space atmospheric conditions. Aim of study. The objective was the development of a hardware and software system for investigating the applicability of QKD technology in wireless communication systems based on commercially available modules. The system includes optical blocks for atmospheric optical communication and a training setup for studying QKD in fiber-optic communication lines. Method. The characteristics of a quantum communication channel in free-space conditions were studied through experiments. The channel was formed by the QKD units of the EMQOS 1.0 scientific and educational complex, integrated with optical blocks for atmospheric optical communication. The systems were also upgraded to minimize losses in the quantum channel. Main results. A hardware and software system was developed, within which the transmission of a quantum key through free-space atmospheric conditions over a distance of 180 m was successfully demonstrated. The quantum bit error rate did not exceed 6%. This system enabled the study of the effects of weather conditions on optical losses in the quantum communication channel and the quantum key generation rate. Practical significance. The developed hardware and software system allows the monitoring of the parameters of both classical and quantum communication channels in free space under various weather conditions. Its modular structure enables the integration of other quantum equipment for comparative testing of the performance of quantum communication systems under real atmospheric transmission conditions.