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Development of a Receiver System for Passive Polarization-Encoded BB84 Protocol

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Màster Oficial de Ciència i Tecnologia Quàntiques / Quantum Science and Technology, Facultat de Física, Universitat de Barcelona. Curs: 2024-2025. Tutor:José María Gómez Cama

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As the development of quantum computing threatens classical cryptographic methods, Quantum Key Distribution (QKD) provides a pathway to physically secure communication.
The practical implementation of robust and efficient receiver modules, however, remains a key challenge for real world QKD systems. This thesis presents the complete design, implementation, and characterization of a high fidelity optical receiver (Bob) for the BB84 QKD protocol. The architecture is distinguished by its use of a passive random basis selection mechanism, employing a non-polarizing beam splitter to enhance system simplicity, robustness, and cost-effectiveness. Furthermore, the modular design features a fiber coupled input, ensuring flexibility for easy integration into both free-space and fiberbased quantum communication links. For polarization analysis, Wollaston prisms were selected over other alternatives due to their high extinction ratio, which is critical for minimizing measurement errors. The system was designed and optimized for an operational wavelength of 810nm for compatibility with a single photon source based on a beta barium borate (BBO) crystal. The receiver’s performance was experimentally validated by measuring its response to a full range of input polarization states. The successful realization of this project provides a robust, flexible, and high performance receiver module suitable for deployment in further QKD experiments

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Màster Oficial - Ciència i Tecnologia Quàntiques / Quantum Science and Technology

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Development of a Receiver System for Passive Polarization-Encoded BB84 Protocol