Objective:The objective of the activity is to develop and qualify a faint pulse laser source for space-based quantum key distribution systems.Targeted Improvements:Increase the technical readiness level of faint pulse laser sources suitable for quantum key distribution applications from 5 to 7. Description:Quantum key distribution systems using protocols based on faint laser pulses require a photon source emitting less than one photon per pulse on average. The information required for quantum key distribution can be encoded in the physical properties of the photons, such as their polarisation, phase or timing. This activity shall design, buildand qualify a faint pulse source for space-based quantum key distribution applications. This shall include a first conceptual system design to define the interfaces and requirements for a technology demonstration mission. The faint pulse source hardware shall include the laser source itself, as well as the required optics and electronics to control the photon states. The performance of the weak pulse source (especially the pulse repetition rate and the precision of amplitude and photon state control) shall be suitable forestablishing a quantum link between either a low earth orbit satellite or a geostationary satellite using existing optical terminals and a typical optical ground station. A qualification test campaign shall demonstrate the robustness of the source against mechanical, thermal and radiation loads.