Objective: The objective of the activity is to develop, manufacture and test technologies for high dynamic extinction ratio (ER) optical amplitude modulators operating at GHz modulation frequencies and their associated electronics for space applications. TargetedImprovements: 10 dB increase of extinction ratio compared with current baseline products, which translates as an improvement of theachievable Quantum Bit Error rate (and consequently key rate performance). Description: Practical implementation of faint photon sources for Quantum Key Distribution (QKD) rely on the modulation and polarisation encoding of a continuous wave laser using amplitudemodulators. The modulation speed will determine the data rate transmitted; hence fast switching in at several GHz is a pre-requisite for their suitability. Nevertheless, the final key rate achieved will be limited by the QBER measured at the receiver side. The background from the photons leaking through the AMs in the three nominally closed polarisation channels increases the detected background noise and hence the QBER. In general, the QBER improves quadratically with the ER, while it improves only linearly with the pulse rate. In addition, polarisation-encoded protocols such as the BB84 require modulation of the weak coherent laser pulses at two different mean-photon-number pulses, called signal and decoy. The signal and decoy pulses must be perfectly indistinguishable in all degrees of freedom, including time, frequency, spatial modes, and polarization modes. This perfect indistinguishability is technicallydemanding to achieve, imposing stringent requirements on the stability of the switching amplitude and the dispersion characteristics of different modulators in the operational environment (especially against thermal fluctuations). In this activity, we propose thedevelopment of an optical amplitude modulator demonstrator for QKD applications, with associated driving electronics and its validation in the relevant space environment. An ER of -35dB or better at 850nm wavelength while driven at >4GHz would be needed to limit the QBER to a level enabling viable key rates in a space-to-ground QKD link. It will implement four channels in order to test the consistency across devices of the performance achieved, switching stability and indistinguishability properties of the generated photons. The activity is envisioned to consist of the following phases: State-of-the-art technology review and trade-off analysis. This will include a critical analysis of COTs components, integration platforms, packaging options and their viability for space applications. Methods to measure relevant properties (extinction ratio, pulse timing, polarisation purity) during operation at several GHz shall also be proposed. Demonstrator implementation and performance analysis, Evaluation in representative environment. Procurement Policy: C(1) = Activity restricted to non-prime contractors (incl. SMEs). For additional information please go to: http://www.esa.int/About_Us/Business_with_ESA/Small_and_Medium_Sized_En…

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