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Objectives
The project aims to design, specify, and validate compact optical modules critical for spaceborne quantum key distribution (QKD) systems. Key objectives include setting the preliminary design, finalizing technical specifications, confirming the detailed design, and defining relevant environmental conditions for prototype validation and functional testing. The focus lies on developing space-compatible Entangled Photon Sources (EPS) and Faint Pulse Sources (FPS), optimizing for Size, Weight, Power, and Cost (SWaP-C). Particular attention is given to balancing EPS unit size and photon pair brightness by adjusting the length and form of the periodically poled potassium titanyl phosphate (ppKTP) crystal. This approach enables the possibility of using multiple compact EPS units on a single satellite to improve performance and redundancy.
Challenges
A key challenge of the project lies in ultimate miniaturisation of single photon sources in order to showcase them in practical QKD applications.
System Architecture
The EPS employs a high-power (>100 mW) SLM pump laser at 405 nm and uses periodically poled nonlinear crystals, for efficient spontaneous parametric down-conversion. Robotic manipulators are used to position and bond small optical components (≤3 mm), ensuring precise alignment. Critical to performance are stable thermoelectric cooling (±0.001 K), high-fidelity entanglement generation, and reliable polarization-maintaining fibre coupling with high polarization extinction ratio and angular stability. The FPS utilises compact, switched gain-chips and a common seed to emit pulses at up to 1 GHz with stable amplitude and precise timing. A two-channel switching driver controls emission. Similar robotic optical assembly techniques are applied to align and bond small optical components, ensuring consistent PM fibre coupling. Both modules use commercial off-the-shelf components and robust assembly methods, supporting potential space qualification. The overall architecture reflects a high degree of integration and precision, enabling stable and scalable photon generation for quantum key distribution applications.
Plan
The project follows a structured plan divided into key phases: initial requirements analysis, preliminary and detailed design, component procurement and assembly, and functional testing. Major milestones include a meeting to define the characterization strategy, a Preliminary Design Review, a Test Readiness Review, and a Critical Design Review. The project concludes with a final presentation to ESA.
Current Status
The project has progressed from concept development toward prototype definition. Detailed technical specifications for key components have been initiated based on system requirements. A market survey identified suitable EU-based suppliers, and initial orders have been placed. A literature review of electrical and fluid power systems is ongoing to support design decisions. Risk identification and mitigation planning have also begun.
