A passive transmitter for quantum key distribution with coherent light

TL;DR

This paper introduces a passive transmitter for quantum key distribution that uses coherent light and sum-frequency generation, enabling secure key exchange without active modulation, with performance comparable to active setups in ideal conditions.

Contribution

It presents a novel passive transmitter design combining sum-frequency generation and linear optics for decoy-state BB84, simplifying implementation and potentially increasing transmission rates.

Findings

Secret key rate comparable to active decoy-state BB84 with infinite decoy settings

Passive setup eliminates need for external modulation elements

Effective in high transmission rate scenarios

Abstract

Signal state preparation in quantum key distribution schemes can be realized using either an active or a passive source. Passive sources might be valuable in some scenarios; for instance, in those experimental setups operating at high transmission rates, since no externally driven element is required. Typical passive transmitters involve parametric down-conversion. More recently, it has been shown that phase-randomized coherent pulses also allow passive generation of decoy states and Bennett-Brassard 1984 (BB84) polarization signals, though the combination of both setups in a single passive source is cumbersome. In this paper, we present a complete passive transmitter that prepares decoy-state BB84 signals using coherent light. Our method employs sum-frequency generation together with linear optical components and classical photodetectors. In the asymptotic limit of an infinite long experiment, the resulting secret key rate (per pulse) is comparable to the one delivered by an active decoy-state BB84 setup with an infinite number of decoy settings.