Design and test of optical payload for polarization encoded QKD for Nanosatellites

TL;DR

This paper presents the design and testing of a compact optical payload for polarization-encoded QKD on nanosatellites, enabling secure quantum communication over thousands of kilometers in space.

Contribution

It introduces a dual-wavelength, polarization-encoded QKD source tailored for nanosatellite deployment, with detailed design, security considerations, and performance evaluation.

Findings

The QKD source operates at 200 MHz with a mean photon number of 0.3 and 0.5.

Achieved a low intrinsic QBER of approximately 0.82%.

Secure key rates of a few kbps are feasible despite high free-space losses.

Abstract

Satellite based Quantum Key Distribution (QKD) in Low Earth Orbit (LEO) is currently the only viable technology to span thousands of kilometres. Since the typical overhead pass of a satellite lasts for a few minutes, it is crucial to increase the the signal rate to maximise the secret key length. For the QUARC CubeSat mission due to be launched within two years, we are designing a dual wavelength, weak-coherent-pulse decoy-state Bennett-Brassard '84 (WCP DS BB84) QKD source. The optical payload is designed in a $12{\times}9{\times}5 cm^3$ bespoke aluminium casing. The Discrete Variable QKD Source consists of two symmetric sources operating at 785 nm and 808 nm. The laser diodes are fixed to produce Horizontal,Vertical, Diagonal, and Anti-diagonal (H,V,D,A) polarisation respectively, which are combined and attenuated to a mean photon number of 0.3 and 0.5 photons/pulse. We ensure that the source is secure against most side channel attacks by spatially mode filtering the output beam and characterising their spectral and temporal characterstics. The extinction ratio of the source contributes to the intrinsic Qubit Error Rate(QBER) with $0.817 \pm 0.001\%$. This source operates at 200MHz, which is enough to provide secure key rates of a few kilo bits per second despite 40 dB of estimated loss in the free space channel