Passive state preparation in the Gaussian-modulated coherent states quantum key distribution

TL;DR

This paper introduces a passive state preparation method for Gaussian-modulated coherent states quantum key distribution using a thermal source, potentially simplifying implementation and increasing noise tolerance.

Contribution

The authors propose a passive state preparation scheme for GMCS QKD utilizing a thermal source and homodyne detection, offering advantages over active schemes.

Findings

Scheme can tolerate high detector noise at Alice's side.

Does not require single-mode output from the thermal source.

Preliminary experiments suggest feasible implementation with existing sources.

Abstract

In the Gaussian-modulated coherent states (GMCS) quantum key distribution (QKD) protocol, Alice prepares quantum states \emph{actively}: for each transmission, Alice generates a pair of Gaussian-distributed random numbers, encodes them on a weak coherent pulse using optical amplitude and phase modulators, and then transmits the Gaussian-modulated weak coherent pulse to Bob. Here we propose a \emph{passive} state preparation scheme using a thermal source. In our scheme, Alice splits the output of a thermal source into two spatial modes using a beam splitter. She measures one mode locally using conjugate optical homodyne detectors, and transmits the other mode to Bob after applying appropriate optical attenuation. Under normal conditions, Alice's measurement results are correlated to Bob's, and they can work out a secure key, as in the active state preparation scheme. Given the initial thermal state generated by the source is strong enough, this scheme can tolerate high detector noise at Alice's side. Furthermore, the output of the source does not need to be single mode, since an optical homodyne detector can selectively measure a single mode determined by the local oscillator. Preliminary experimental results suggest that the proposed scheme could be implemented using an off-the-shelf amplified spontaneous emission source.