Passive sources for the Bennett-Brassard 1984 quantum key distribution protocol with practical signals

TL;DR

This paper demonstrates that passive sources, using only linear optics and photodetectors, can effectively generate BB84 quantum key distribution signals, offering potential advantages in robustness and transmission distance over active sources.

Contribution

It introduces a passive method for BB84 signal generation using coherent light and single photon sources without external driving, expanding practical options for quantum key distribution implementations.

Findings

Passive schemes are feasible with coherent light and single photon sources.

Passive transmitters can achieve comparable or longer distances than active ones.

Passive sources may enhance robustness against side-channel attacks.

Abstract

Most experimental realizations of quantum key distribution are based on the Bennett-Brassard 1984 (so-called BB84) protocol. In a typical optical implementation of this scheme, the sender uses an active source to produce the required BB84 signal states. While active state preparation of BB84 signals is a simple and elegant solution in principle, in practice passive state preparation might be desirable in some scenarios, for instance, in those experimental setups operating at high transmission rates. Passive schemes might also be more robust against side-channel attacks than active sources. Typical passive devices involve parametric down-conversion. In this paper, we show that both coherent light and practical single photon sources are also suitable for passive generation of BB84 signal states. Our method does not require any external-driven element, but only linear optical components and photodetectors. In the case of coherent light, the resulting key rate is similar to the one delivered by an active source. When the sender uses practical single photon sources, however, the distance covered by a passive transmitter might be longer than the one of an active configuration.