Discrete-modulated coherent-state quantum key distribution with basis-encoding

Background

Continuous-variable quantum cryptography has emerged as a significant branch of quantum cryptography, with continuous-variable quantum key distribution (CVQKD) at its core. However, compared to discrete-variable quantum key distribution protocols, the secure transmission distance of continuous-variable quantum key distribution protocols is significantly shorter. There are two main reasons for this: first, existing CVQKD protocols are overly sensitive to excess noise. Second, CVQKD requires more complex and unique error correction algorithms, which further limits the secure transmission distance of CVQKD. To enhance the secure transmission distance of CVQKD, researchers have proposed a discrete-modulated coherent state CVQKD (DMCS-CVQKD) scheme, which can directly generate discrete raw keys, thereby reducing post-processing complexity, improving reconciliation efficiency, and extending the secure transmission distance. However, since the measurement results are still continuous values for DMCS-CVQKD, the reconciliation process remains complex.

Research Progress

The quantum key distribution protocol with basis-encoding (BE-QKD) differs from traditional CVQKD protocols in that it encodes key information in the selection of measurement bases of two conjugate quadratures. This protocol requires only simple binary reconciliation and has the potential to tolerate greater channel loss compared to traditional CVQKD. Additionally, the physical optical path of this protocol is same with that of traditional CVQKD, with only the post-processing stage differing, enabling seamless compatibility with existing CVQKD equipment. However, the security analysis of BE-QKD is currently not fully complete, and there are no related experimental verifications.

The research team proposed a novel discrete-modulated coherent-state quantum key distribution with basis encoding. In this scheme, Alice performs discrete modulation on the coherent state and sends it to Bob. Bob performs coherent detection and encodes the key in the base selection, ultimately publishing the measurement results. Meanwhile, Alice decodes the raw key according to the decoding principle shown in Fig. 1.

The research team analyzed the security of the proposed scheme against individual and collective attacks in the linear Gaussian channel. Based on this analysis, numerical simulations were conducted, and the results indicate that the proposed protocol significantly enhances the tolerance to channel loss. Under the realistic excess noise level, compared to the original DMCS-CVQKD protocol, it can tolerate approximately 40 dB more channel loss (as shown in Fig. 2).

The researchers set up an experimental optical path to conduct a principle verification experiment of the proposed protocol on a 50.5 km standard single-mode fiber, as shown in Fig. 3. The experimental results show that the coherent detection results and bit error rate are consistent with the theoretical analysis results. Additionally, a key rate of 13.12 kbps was achieved under 11 dB of channel loss, and under the same experimental parameters, the theoretical maximum tolerable channel loss is 33 dB, as shown in Fig. 4.

Future Prospects

Researchers have proved the security of the discrete-modulated coherent-state quantum key distribution protocol with basis-encoding in the linear Gaussian channel, achieving the first experimental verification of the discrete-modulated coherent-state quantum key distribution protocol with basis-encoding. This provides a new, efficient solution for realizing quantum secure communication in complex real-world channel environments.

Sources：https://spj.science.org/doi/10.34133/research.0691