Analyzing the performance of CV-MDI QKD under continuous-mode scenarios

TL;DR

This paper investigates how mode mismatches in continuous-variable MDI QKD affect performance and security, emphasizing the importance of mode calibration for practical large-scale quantum networks.

Contribution

It introduces a temporal mode analysis for CV-MDI QKD under continuous-mode scenarios and highlights the impact of mode mismatches on system performance and security.

Findings

Mode mismatch significantly reduces transmission distance.

Bob's mode mismatch has a greater impact than Alice's.

Secret key rate drops by 83% at 15 km with 5% mismatch.

Abstract

Continuous-variable measurement-device-independent quantum key distribution (CV-MDI QKD) can address vulnerabilities on the detection side of a QKD system. The core of this protocol involves continuous-variable Bell measurements performed by an untrusted third party. However, in high-speed systems, spectrum broadening causes Bell measurements to deviate from the ideal single-mode scenario, resulting in mode mismatches, reduced performance, and compromised security. Here, we introduce temporal modes (TMs) to analyze the performance of CV-MDI QKD under continuous-mode scenarios. The mismatch between Bob's transmitting mode and Bell measurement mode has a more significant effect on system performance compared to that on Alice's side. When the Bell receiver is close to Bob and the mismatch is set to just 5%, the transmission distance drastically decreases from 87.96 km to 18.50 km. In comparison, the same mismatch for Alice reduces the distance to 86.83 km. This greater degradation on Bob's side can be attributed to the asymmetry in the data modification step. Furthermore, the mismatch in TM characteristics leads to a significant reduction in the secret key rate by 83% when the transmission distance is set to 15 km, which severely limits the practical usability of the protocol over specific distances. These results indicate that in scenarios involving continuous-mode interference, such as large-scale MDI network setups, careful consideration of each user's TM characteristics is crucial. Rigorous pre-calibration of these modes is essential to ensure the system's reliability and efficiency.