Quantum Key Distribution Achieves Secure Data Transfer over Turbulent Air

Free-space optical communication offers a promising solution for high-bandwidth data transmission, but its vulnerability to eavesdropping and atmospheric disturbances presents a significant challenge. Sushil Kumar, Soumya P. Dash, and George C. Alexandropoulos investigate methods to secure these systems using quantum key distribution, specifically continuous-variable protocols, and enhance performance in realistic conditions. Their work centres on multiple-input multiple-output (MIMO) free-space optical systems, where they propose innovative one-way and two-way protocols to improve the secure exchange of encryption keys, even when faced with atmospheric turbulence and the threat of an eavesdropper. By mathematically modelling the communication channel and deriving new expressions for key exchange rates, the researchers demonstrate how MIMO technology and their two-way protocol significantly boost security and performance in free-space optical quantum communication systems.

MIMO CV-QKD for Free-Space Security

This research explores continuous-variable quantum key distribution (CV-QKD) in free-space optical (FSO) communication systems, focusing on multiple-input multiple-output (MIMO) configurations and realistic noise models. The work aims to improve the security and performance of quantum communication over open-air links, which offer high bandwidth but are vulnerable to eavesdropping and atmospheric disturbances. By employing MIMO techniques, the system increases channel capacity and enhances the potential secret key rate. Detailed noise models accurately represent the characteristics of FSO channels, crucial for evaluating the practical feasibility of these systems.

The investigation also explores two-way quantum communication protocols, which can potentially improve both the secret key rate and the overall security of the communication link. The research demonstrates the benefits of using a hybrid quantum noise model, combining different types of noise to more accurately represent the complexities of the FSO channel. Detailed analysis reveals how MIMO configurations impact performance, considering channel characteristics, noise levels, and system parameters. Evaluating two-way quantum communication protocols within MIMO FSO systems provides valuable insights into optimizing key exchange rates.

Furthermore, the study investigates the use of intelligent reflecting surfaces (RIS) to enhance the FSO link, improving signal quality and extending communication range. The primary metric used to assess performance is the secret key rate, which quantifies the amount of secure key generated between communicating parties. This work builds upon CV-QKD, which uses continuous variables, such as the amplitude and phase of light, to encode quantum information. Gaussian modulation is employed, and the research investigates two-way quantum communication protocols to further enhance security. Intelligent reflecting surfaces are integrated to improve the FSO link, and MIMO techniques are used to increase channel capacity and improve system performance. This combination of technologies offers a promising approach to secure communication in challenging environments.

FSO Channel Modelling for CV-QKD Security

This research focuses on a multiple-input multiple-output (MIMO) free-space optical (FSO) communication system designed for secure key distribution between two parties, Alice and Bob, using continuous-variable quantum key distribution (CV-QKD). The study meticulously models the wireless channel, accounting for atmospheric turbulence, which causes beam spreading, pointing errors, and fading, alongside hybrid quantum noise that degrades key exchange. To assess system security, the research incorporates an eavesdropper, Eve, who attempts to intercept the key exchange using a collective Gaussian attack. The team developed a mathematical formulation of transmissivity, considering atmospheric absorption, turbulence-induced fading, and detector efficiency to accurately simulate the FSO channel.

This transmissivity is modified by factors accounting for atmospheric attenuation and turbulence, modeled using a lognormal distribution to precisely represent turbulence strength. Beyond atmospheric effects, the research accounts for noise at the receiver, modeling it as a hybrid quantum noise resulting from a Poisson and Gaussian mixture. This noise combines Poisson noise and Gaussian noise, and the combined effect is calculated through a convolution of these distributions, providing a comprehensive model of the noise impacting key exchange. To enhance key rates, the team proposed both one-way and two-way protocols.

In the one-way protocol, Alice prepares and transmits coherent states, modulated with position and momentum quadratures sampled from a Gaussian distribution. These signals are precoded before transmission, and Eve attempts to intercept them using an entangled EPR state and beam splitters. The received signal at Bob’s end is modeled as a transformation of the transmitted signal, incorporating Eve’s intervention and the additive hybrid noise. The two-way protocol builds upon this framework, further optimizing key exchange.

Secure Key Rates in Turbulent Free Space

This work investigates a multiple-input multiple-output (MIMO) free-space optical (FSO) communication system designed for secure key transmission between two users, Alice and Bob, using continuous-variable quantum key distribution (CV-QKD). The research focuses on enhancing security in the presence of atmospheric turbulence, which causes beam spreading, pointing errors, and fading, alongside hybrid quantum noise that degrades signal quality. Scientists proposed both one-way and two-way protocols to improve the security of transmitted keys, and mathematically formulated the transmissivity of the FSO channels to establish bounds on mutual information between transmitted and received coherent states. The team derived novel expressions for secret key rates (SKRs) for both the one-way and two-way protocols, and presented asymptotic expressions for SKRs alongside numerical results validating the analytical framework.

Experiments demonstrate that employing MIMO technology and the two-way protocol significantly improves SKR performance, particularly in mitigating the effects of atmospheric disturbances and quantum noise on key exchange. The analysis considers the limitations of single-input single-output (SISO) configurations, which are susceptible to beam deflection and spreading, and leverages MIMO to enhance signal reliability.