Experimental Coherent One-Way Quantum Key Distribution with Simplicity and Practical Security

TL;DR

The paper tackles the security of coherent one-way QKD (COW-QKD) against zero-error and source-side-channel attacks. It proposes an information-theoretically secure COW-QKD protocol that uses vacuum and nonvacuum states with finite-key security guarantees and no reliance on interference visibility as a security metric. The authors experimentally demonstrate secure key distribution over fiber links up to 100 km, achieving up to 29 bps at 100 km (and higher rates at shorter distances) and encrypt a 6.13 kB logo with information-theoretic security. The work shows that COW-QKD can achieve practical security with simple state preparation and is compatible with photonic integration and real-world quantum networks.

Abstract

Coherent one-way quantum key distribution (COW-QKD) has been widely investigated, and even been deployed in real-world quantum network. However, the proposal of the zero-error attack has critically undermined its security guarantees, and existing experimental implementations have not yet established security against coherent attacks. In this work, we propose and experimentally demonstrate an information-theoretically secure COW-QKD protocol that can resist source side-channel attacks, with secure transmission distances up to 100 km. Our system achieves a secure key rate on the order of kilobits per second over 50 km in the finite-size regime, sufficient for real-time secure voice communication across metropolitan networks. Furthermore, we demonstrate the encrypted transmission of a logo with information-theoretic security over 100 km of optical fiber. These results confirm that COW-QKD can simultaneously provide simplicity and security, establishing it as a strong candidate for deployment in small-scale quantum networks.

Figures (3)

• Figure 1: Overview of the COW-QKD network architecture and experimental implementation.a. Schematic of a quantum network. The network consists of senders, receivers, and intermediate relays. The sender prepares quantum states, while the receiver performs measurements. Relays, capable of both state preparation and measurement, act as intermediate nodes to extend the communication distance between users. b. Conceptual demonstration of COW protocol between two users. Alice, acting as the sender, encodes logic bits onto optical pulses. This scheme requires only the generation of vacuum and non-vacuum states, enabling a simple modulation requirement. Bob serves as the receiver and can also function as a relay, thereby extending the achievable distance for secure key distribution. Owing to its simple experimental requirements for quantum state preparation, COW-QKD is suitable for photonic chip integration, enabling the development of portable transmitter modules. c. Experimental implementation of the proposed COW-QKD. This scheme employs the existing structure of COW protocol without any additional experimental requirement, preserving the simplicity of the implementation. IM, intensity modulation; BS, beam splitter. We gratefully acknowledge the icons designed by kalstud from www.flaticon.com, which are used in this figure.
• Figure 2: Experimental setup and performance of COW-QKD system.a. Schematic of the experimental setup. IM: intensity modulator; ATT: attenuator; BS: beam splitter; Cir: circulator; FM: Faraday mirror; PS: phase shifter; PC: polarization controller; SPD: superconducting nanowire single-photon detector; MZI: Mach-Zehnder interferometer, consisting of two beam splitters; MSI: Michelson interferometer, consisting of one beam splitter, two Faraday mirrors, and one phase shifter. After passing through MZI, each pulse is split into two subpulses, with a time delay of 800 ps. The arm length differences of these two interferometers are matched. b. Interference visibility and the error rate of $Z$ basis over time. Each data point is derived from detection events accumulated for 40 seconds, with a total of 50 points collected. The interference visibility is calculated from the detection results of the $\mathinner{|{\alpha}\rangle}\mathinner{|{\alpha}\rangle}$ in the $X$ basis. A 100-km optical fiber is inserted during this measurement. The quantum bit error rate is calculated from the detection results of the $\mathinner{|{0}\rangle}\mathinner{|{\alpha}\rangle}$ and $\mathinner{|{\alpha}\rangle}\mathinner{|{0}\rangle}$, prepared in the $Z$ basis without fiber. c. Key rates under different transmission distances. The blue curve represents the simulated key rates based on the actual experimental parameters. The red crosses represent the key rate values directly calculated from the experimental measurements, while the five-pointed stars represent the refined key rates.
• Figure 3: Demonstration of the encryption process. The logo of the International Year of Quantum Science and Technology is used to demonstrate the process of protecting information confidentiality. The logo is first compressed to a size of 6.13 kByte and then converted into a 50,296-bit binary string. Alice performs an XOR operation between her secret key $k_a$ and the binary string to generate the encrypted message, which is transmitted to Bob over a public channel. Upon receiving the string, Bob decrypts it using his corresponding secret key $k_b$, successfully recovering the original binary string and reconstructing the logo.