Passive decoy state quantum key distribution with practical light sources

TL;DR

This paper analyzes passive decoy state quantum key distribution using various practical light sources and detectors, demonstrating its feasibility and robustness in real-world scenarios with noise and finite data effects.

Contribution

It extends previous work by analyzing passive decoy state QKD with diverse light sources and detectors, including effects of noise and finite data fluctuations.

Findings

Passive decoy state QKD is feasible with thermal, phase randomized WCP, and strong coherent sources.

Detection inefficiencies and noise impact the secret key rate but remain manageable.

Finite data size causes statistical fluctuations that can be estimated and mitigated.

Abstract

Decoy states have been proven to be a very useful method for significantly enhancing the performance of quantum key distribution systems with practical light sources. While active modulation of the intensity of the laser pulses is an effective way of preparing decoy states in principle, in practice passive preparation might be desirable in some scenarios. Typical passive schemes involve parametric down-conversion. More recently, it has been shown that phase randomized weak coherent pulses (WCP) can also be used for the same purpose [M. Curty {\it et al.}, Opt. Lett. {\bf 34}, 3238 (2009).] This proposal requires only linear optics together with a simple threshold photon detector, which shows the practical feasibility of the method. Most importantly, the resulting secret key rate is comparable to the one delivered by an active decoy state setup with an infinite number of decoy settings. In this paper we extend these results, now showing specifically the analysis for other practical scenarios with different light sources and photo-detectors. In particular, we consider sources emitting thermal states, phase randomized WCP, and strong coherent light in combination with several types of photo-detectors, like, for instance, threshold photon detectors, photon number resolving detectors, and classical photo-detectors. Our analysis includes as well the effect that detection inefficiencies and noise in the form of dark counts shown by current threshold detectors might have on the final secret ket rate. Moreover, we provide estimations on the effects that statistical fluctuations due to a finite data size can have in practical implementations.