Security of practical phase-coding quantum key distribution

TL;DR

This paper analyzes the security of practical phase-coding quantum key distribution using UMZI, revealing that active loss compensation reduces the secret key rate, which is counterintuitive.

Contribution

It provides a security analysis for phase-coding QKD with unbalanced interferometers, addressing real-world loss effects not covered in standard models.

Findings

Active loss compensation lowers secret key rate

Unbalanced UMZI affects emitted quantum states

Security proof adapted for practical loss conditions

Abstract

Security proof of practical quantum key distribution (QKD) has attracted a lot of attentions in recent years. Most of real-life QKD implementations are based on phase-coding BB84 protocol, which usually uses Unbalanced Mach-Zehnder Interferometer (UMZI) as the information coder and decoder. However, the long arm and short arm of UMZI will introduce different loss in practical experimental realizations, the state emitted by Alice's side is nolonger standard BB84 states. In this paper, we will give a security analysis in this situation. Counterintuitively, active compensation for this different loss will only lower the secret key bit rate.