Practical continuous variable quantum key distribution with squeezed light

Quantum key distribution (QKD), which provides information-theoretic security grounded in quantum physics, is one of the most advanced applications of quantum technology. Continuous-variable quantum key distribution (CV-QKD), encoding information in the quadratures of the optical field, is particularly appealing due to its ease of implementation, compatibility with existing telecommunications infrastructure, and potential to achieve high secret key rates (SKRs). While most state-of-the-art CV-QKD systems rely on coherent states, squeezed states of light have been theoretically shown to offer significant advantages, including enhanced SKRs, improved resilience to excess noise, and increased performance under imperfect information reconciliation.
However, practical implementations of CV-QKD with squeezed states entail significant technical challenges, with prior experiments primarily relying on free-space channels with emulated loss. This thesis focuses on the practical realization of squeezed-state CV-QKD, demonstrating its advantages experimentally over optical fiber channels.
First, we introduce a novel concept of digitally reconstructing squeezed light through digital signal processing (DSP). Using this approach, we achieve the asynchronous distribution of squeezed light over 10 km of fiber without active control of phase, polarization, or clock synchronization. Additionally, we establish secure communication using squeezed vacuum states between two laboratories connected via deployed fiber. Building on these advances, we develop a practical CV-QKD system based on squeezed states for fiber channels. Experimental results showed that squeezed states nearly double the secret key rate over a 50 km fiber channel. Further investigation revealed the reduction in the penalty from imperfect information reconciliation, enabling secret key generation at a reconciliation efficiency of 60.7% with just 20 decoder iterations—something unattainable with coherent states. We also showed that the squeezed states protocol can tolerate more excess noise, over 1.6 times in this work, compared to the coherent states protocol. This clearly demonstrates the advantages of using squeezed states in CV-KD. To lay the groundwork for future developments, we demonstrate an initial experiment exploring multi-user squeezed-state CV-QKD. Furthermore, we develop a broadband squeezed light source and perform preliminary experiments to enable high-speed CV-QKD with squeezed states.
This work represents a significant step toward the practical deployment of CV-QKD systems leveraging the unique advantages of squeezed states in real-world quantum communication networks.