Efficient system implementation and security analysis for continuous-variable quantum key distribution

Abstract

Quantum key distribution (QKD) is a cryptography technology that utilizes the principles of quantum mechanics. It has gained significant attention as a potential replacement for modern cryptography systems that are vulnerable to emergence of quantum computer. Among QKD, continuous-variable quantum key distribution (CVQKD) is actively being researched due to its potential for high key transmission rates over short distances, compatibility with existing communication infrastructure, and suitability for wavelength-division multiplexing environments. This dissertation focuses on the practical implementation of CVQKD, building upon its advantages, and conducts research in two main parts. Firstly, we introduce implementation research findings on performance improvement of CVQKD systems. So far, most CVQKD systems have been implemented based on homodyne detection. Heterodyne detection, which simultaneously measures two orthogonal quadrature components, can generate secret keys faster than homodyne detection, particularly in short-distance range. However, due to the high implementation complexity of the heterodyne detection system, we were not able to find experimental studies on CVQKD systems based on heterodyne detection where the transmitter sends the local oscillator (LO) along with the quantum signal. Therefore, we propose a new detection method named time-division dual-quadrature detection including its corresponding key technologies for the transmitter and receiver. This approach achieves equivalent performance to heterodyne detection systems while simplifying the experimental implementation of the system. Especially, we apply a Faraday-Michelson interferometer to the receiver, which is challenging to implement in conventional CVQKD systems. We show the time-division dual-quadrature detection system, capable of measuring both divided quantum signals to detect two orthogonal quadrature components, effectively resolves the problem of polarization drift that occurs for optical signals when passing through the channel without reducing detection efficiency. We experimentally demonstrated a system using the proposed detection scheme with the Gaussian-modulated coherent-states (GMCS) protocol over a 20.06 km optical fiber channel, verified that the system is able to achieve an expected secret key rate of up to 0.187 Mbps. In the second part, we conducted research on side-channel attacks by eavesdroppers. Theoretically, CVQKD systems provide a high level of security, but various side-channel attacks exist that using vulnerabilities present in really implemented systems. Understanding these vulnerabilities is crucial for enhancing the overall security of CVQKD systems, and here, we focused on attacks exploiting the security loopholes that arise from the direct transmission of the LO. Especially, we analyze an attack utilizing the technical limitations of the dynamic polarization controller commonly used in CVQKD receivers and the wavelength dependence of the receiver beam splitter. In this attack, the eavesdropper manipulates the power of the LO arriving at the receiver by rotating LO's polarization and inserts deceptive pulses. This allows the eavesdropper to acquire the secret key without being detected in a system that includes monitoring the power of the LO. Furthermore, considering the characteristics of actual devices, we perform a security analysis of the proposed attack and examine its effectiveness in heterodyne detection systems. Based on the research findings, we propose two countermeasures. Firstly, we suggest adding an optical wavelength filter to the monitoring of the power of LO, which can defend against the proposed attack. However, in CVQKD, careful selection of wavelength filters advantageous to both the transmitter and receiver is required due to the use of narrow-bandwidth quantum signals. Secondly, we propose using the estimated transmittance value from the parameter estimation to determine the abortion of the exchanged key. Through research findings, it is observed that adjusting the power of the local oscillator can significantly reduce the estimated transmittance within middle-range transmission. Determining the presence of eavesdropping attacks by comparing the actual transmittance determined in the calibration step prior to key exchange with the transmittance calculated by the receiver in the parameter estimation step can serve as an efficient countermeasure within the mid-range transmission.