Design and test of optical payload for polarization encoded QKD for Nanosatellites Jaya Sagar a,b , Elliott Hastings a,b , Piede Zhang b , Milan Stefko b , David Lowndes b , Daniel Oi c , John Rarity b , and Siddarth K. Joshi b a Quantum Engineering Centre for Doctoral Training, University of Bristol, BS8 1FD, United Kingdom b Quantum Engineering Technology Labs, NSQI, University of Bristol, BS8 1FD, United Kingdom c Computational Nonlinear and Quantum Optics (CNQO) group, University of Strathclyde, Glasgow, G4 0NG, United Kingdom ABSTRACT Satellite based Quantum Key Distribution (QKD) in Low Earth Orbit (LEO) is currently the only viable tech- nology 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 12×9×5cm 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 ± 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. 1 Keywords: Satellite Quantum Key Distribution, Space optics, Nanosatellites, Quantum Communication, BB84 1. INTRODUCTION Quantum Communication is emerging as a feasible method of sharing a secure key between two parties 2 based on the laws of physics. The secret information contained in the quantum state cannot be cloned or measured without detectable disturbance. This attribute creates a challenge of loss intolerance of Quantum Communication channels. State-of-the-art Quantum Key Distribution (QKD) protocols have been limited to 1000 km in optical fibres due to the exponential losses . 3–5 It is not yet realistic to use quantum repeaters for global extension of quantum networks due to the exhaustive requirements of a stable fibre optic network. 6 The comparatively lower loss media of free space or satellite communication can offer a more flexible, scalable and less expensive platform for communication systems. 7 Various studies have shown the feasibility of satellite quantum communication extending thousands of kilometres. 8, 9 Satellite based quantum communication can join various fibre based quantum channels towards global network . 10 In 2017, the Micius mission (weighing 635 kg) successfully demonstrated intercontinental QKD from the LEO . 11 The reliable communication between LEO satellite and the ground station can be established when the satellite is 10 ◦ above the horizon in 8-10 minutes. 1 This makes it important to achieve high data transmission rates to get a significant key. Thus, efforts are being made to optimise the connection and improve transmission rates. With current advancements in space Further author information: (Send correspondence to J. Sagar) E-mail: jaya.sagar@bristol.ac.uk arXiv:2211.10814v1 [quant-ph] 19 Nov 2022