Software Enhances Quantum-Classical Communication with Gaussian Post-selection, Optimising Key Rates in Fluctuating Environments

Simultaneous quantum-classical communication offers a promising route to secure communication by encoding both quantum and classical information within the same optical signal, but its performance typically suffers when faced with real-world channel fluctuations. Ozlem Erkilic, Biveen Shajilal from A*STAR Quantum Innovation Centre, Nicholas Zaunders and Timothy C. Ralph from the University of Queensland now demonstrate a significant improvement to this technology through the innovative application of Gaussian post-selection. Their research introduces a software-based method to optimise signal modulation after assessing channel conditions, enhancing key rates and extending communication distances without requiring any changes to existing hardware. This passive approach proves effective in both theoretical and practical scenarios, even when accounting for imperfections in receiver technology, and substantially boosts the reliability and range of simultaneous quantum-classical communication over fibre optic cables and, crucially, in challenging free-space environments like satellite links. The team’s findings highlight the potential for practical, robust quantum communication systems capable of operating effectively across terrestrial networks and in demanding satellite-to-ground applications.

Scientists explore techniques to enhance QKD systems, addressing limitations caused by noise, signal loss, and imperfect devices. A significant focus lies on integrating QKD with classical communication to create more efficient and secure data transmission systems. The work concentrates on CV-QKD, which encodes quantum information using continuous variables like the amplitude and phase of light. Researchers investigate Gaussian states, frequently used in CV-QKD due to their mathematical simplicity and ease of generation.

They explore techniques like Gaussian postselection, which filters measurement outcomes to improve the signal-to-noise ratio and enhance key rates. Virtual noiseless amplification, a method to amplify the quantum signal without adding significant noise, is also examined, crucial for long-distance QKD. Scientists also investigate non-Gaussian states and operations like photon addition or subtraction, which potentially improve security and performance. Entangled states, adaptive signal processing, and quantum catalysis are also explored as methods to improve key rates and security. The research emphasises the importance of rigorous security proofs for QKD protocols and analyses potential attacks.

Key security concepts like the Holevo bound and collective attacks are considered. A significant portion of the work explores simultaneous quantum and classical communication, aiming to improve communication efficiency by transmitting classical data alongside the quantum key while maintaining security. The study addresses practical challenges such as signal loss, noise, and imperfections in detectors, and explores the use of satellites to extend the range of QKD systems. Ultimately, this research aims to develop more secure communication systems resistant to attacks, enable QKD over longer distances, increase communication efficiency, and contribute to the development of future quantum networks for secure communication and distributed quantum computing.

Gaussian Post-Selection Enhances QKD Performance

Scientists have pioneered a new approach to continuous-variable key distribution (CV-QKD) by integrating Gaussian post-selection into the Simultaneous-classical (SQCC) protocol, significantly improving its performance in fluctuating channels. The team engineered a passive optimisation technique, enabling software-based adjustment of modulation variance after channel estimation, without requiring any hardware modifications. This innovative method involves applying a Gaussian filter to Alice’s measurement outcomes, effectively tuning the modulation variance to match optimal conditions. The experimental setup builds upon the SQCC scheme, where Alice encodes coherent states and sends them through a thermal-loss channel, simulating realistic transmission conditions.

Crucially, the team implemented a post-selection process where Alice retains only a subset of her modulation data based on the Gaussian filter, communicating the retained indices to Bob. This filtering step, performed after channel estimation, allows the system to adapt to changing channel characteristics, maintaining higher key rates and extending transmission distances. Researchers meticulously modelled the channel with parameters representing transmittivity and thermal noise, ensuring the accuracy of the simulation. To ensure the security of the protocol, the study incorporates a finite-size composable security analysis tailored to the Gaussian post-selection within the SQCC framework. This involved a detailed examination of additional error sources and renormalisation steps, providing a robust assessment of the system’s resilience against potential attacks. This passive approach avoids the need for hardware modifications and remains effective even when accounting for imperfections in receiver components, demonstrating a substantial improvement in transmission distance and robustness across both fibre and free-space channels. The team’s method involves a Gaussian filter that effectively modifies the modulation variance of the SQCC protocol, aligning it with the optimal variance used in established GG02 protocols. This is particularly beneficial in fluctuating channels, where pre-optimising modulation variance is unreliable due to changing conditions.

By implementing this post-selection technique, scientists circumvent the need to emulate any underlying physical operation for filtering, offering a flexible and adaptable solution for dynamic environments. Experiments confirm that the Gaussian post-selection method successfully adjusts the modulation variance, matching the performance of the optimal GG02 protocol even as channel conditions change. This breakthrough delivers a practical solution for integrating quantum and classical signals onto the same optical link, eliminating the need for separate channels and enabling cost-effective deployment of quantum-enhanced security. The research demonstrates the potential for seamless upgrades to existing infrastructure and the immediate extension of quantum communication capabilities to metropolitan and transnational networks. Measurements confirm that the technique significantly improves the performance of SQCC in challenging free-space environments, paving the way for robust and reliable quantum communication over long distances and in dynamic conditions.