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Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC)

Photonic Integrated Phase Encoding Transmitter for Satellite QKD

Joost Vermeer, Jonas Pudelko, Ömer Bayraktar, Kevin Günthner, Christoph Marquardt

Published Jun 23, 2025• 1 citation
Publisher

Abstract

In 2017, the Micius mission [1] showed that satellite quantum key distribution (QKD) can surpass the range limit of fiber-based QKD [2]. Due to the high launch cost of satellites the cost of these systems would be reduced significantly if the size, weight and power of the satellites could be reduced. This requires the use of new integration technologies, like the miniaturized modules based on photonic integrated circuits (PIC) tested in the QUBE mission [3], [4]. Using the knowledge gained there, the QUBE-II mission aims to launch an 8U CubeSat$(10\times 20\times 40\text{cm}^{3})$with the goal of performing QKD between the cubesat and an optical ground station [5].

DOI
10.1109/cleo/europe-eqec65582.2025.11109539

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In 2017, the Micius mission [1] showed that satellite quantum key distribution (QKD) can surpass the range limit of fiber-based QKD [2]. Due to the high launch cost of satellites the cost of these systems would be reduced significantly if the size, weight and power of the satellites could be reduced. This requires the use of new integration technologies, like the miniaturized modules based on photonic integrated circuits (PIC) tested in the QUBE mission [3], [4]. Using the knowledge gained there, the QUBE-II mission aims to launch an 8U CubeSat(10×20×40cm3)(10\times 20\times 40\text{cm}^{3})(10×20×40cm3)with the goal of performing QKD between the cubesat and an optical ground station [5].
The limited range of quantum key distribution (QKD) in fiber based systems led to several projects aiming for the development of a satellite based QKD infrastructure, like the conduction of quantum limited measurements from a geostationary satellite [1] or the MICIUS mission, which impressively demonstrated QKD on a global scale [2]. However, due to the high costs of satellite launches the profitability of commercial systems would benefit dramatically from further reduction in size, weight and power. Photonic integration [3] addresses these points by allowing the integration of all optical components of a QKD system on a photonic integrated circuit (PIC). In this work, we are going to demonstrate building blocks for quantum communication from space based on photonic integration technology by building a payload for the CubeSat mission QUBE [4] consisting of an integrated sender for weak coherent states as well as an integrated Quantum Random Number Generator (QRNG).
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Satellite-based Quantum Key Distribution (QKD) can be used to deliver information-theoretically secure keys for cryptographic applications, no matter where end-users are located. Several space missions using this concept have shown feasibility of systems and component technologies. The QUBE-II mission, with a planned launch in Spring 2026, uses an economic CubeSat platform to show the possibility of a global secure communication by simple means. This paper presents the optical system design of the QUBE-II laser communication terminal (LCT). It is based on the previous developments for the CubeL mission (launched in 2021), for CubeISL, and for the precursor mission QUBE-I (launched in 2024). The 2.5U LCT is integrated into an 8U CubeSat and serves as transmit system for two Alice modules operating at 850 nm and in optical L-band1, 2 respectively. At the same time, bi-directional classical links are used for link acquisition, tracking, and data transmission. With its 85 mm aperture and an 800 mm receiver telescope on the ground station, the antenna gain is sufficiently high enough to perform QKD links. The LCT’s optical system is designed based on a commercially available reflective telescope in combination with custom-designed lenses. It integrates several fiber-optical and electrical components for wavelength combination, wavelength splitting, tracking, and uplink data reception into a compact design. The Alice payloads are connected to the LCT by optical fibres allowing a flexible placement of the modules within the satellite bus. Collimators couple the light into free-space beams which are co-aligned and interfaced to the transmit telescope. In addition to the optical system design, this paper provides further information about the integration of the optical into the mechanical system, and the performed functional and qualification tests.
The QUBE-I mission is a technology demonstrator mission for novel building-blocks for satellite-based quantum-key-distribution (QKD). The mission aims to test three highly integrated optical payloads, a quantum random number generator and two transmitters for polarization and phase encoded quantum states as a precursor mission for the first QKD demonstration from a CubeSat. The QKD sources are connected to, and are integrated together with the Optical Space Infrared Downlink System (OSIRIS) for QUBE (O4Q) in a 3U CubeSat. One of the mission's primary challenges is achieving a precise and accurate pointing of the O4Q with an error margin of less than 1° towards the optical ground station (OGS). Additionally, conducting the experiments presents several operational challenges, such as the need for precise orbital elements to track the satellite with the OGS and safety mechanisms to protect the Sunsensitive components of the satellite. Following the launch on 16th August 2024, we conducted an extensive commissioning phase to test all subsystems, payload components, and operational procedures. This paper provides insights and results from QUBE-I's first year in orbit, highlighting lessons learned from integrating multiple complex payload systems into a 3U CubeSat platform. Starting with communication establishment, we present the ground segment and operations capabilities. We elaborate on the use of flexible on-board scripting and recording mechanisms provided by the satellite's flight software stack for operations. An effective review, test, and deployment procedure was established to balance risk mitigation and agile operation planning. We present results of extensive tests conducted for the crucial attitude determination and control system (ADCS), including actuator verification, spin-up and detumbling tests, in-orbit sensor calibration procedures, and the evaluation of various pointing modes, such as the fine pointing mode required for the experiments. Besides that, we give insights in the actual performance of the experimental and scientific payloads after their commissioning. A big milestone marked the successful test of the operational procedure for the QKD experiments. This includes the timely provision of precise orbit determination to the OGS for tracking. In addition, the ADCS needs to prepare the fine pointing mode accordingly and the experiment needs to be started autonomously before actual contact. Flexibly selectable on-board recordings can be downloaded after the experiments for evaluation. As expected, the technical complexity of the QUBE-I mission resulted in a challenging first year of operation. With this paper, we aim to share our experiences and contribute our insights to the space community.