Photonic entanglement: unleashing colour

Quantum information is transforming modern society with concepts like entanglement, crucial for communication, computing and sensing. To mainstream these technologies, improved efficiency in encoding, transmission and information processing is required, especially in quantum key distribution (QKD). This method aims to securely distribute encryption keys while improving key rates and overcoming propagation loss and noise. Photons serve as ideal carriers due to their various properties for encoding information and minimal environmental interaction, making them vital for the global internet. High-dimensional and hyper-entanglement offer advantages over traditional two-mode encoding methods, including increase noise tolerance and information capacity. However, practical implementation is often limited by measurement device complexities and errors. Frequencies, or photon colours, present a promising approach to boost quantum communication thanks to their compatibility with existing telecommunications infrastructure and naturally occurs in photon pair generation. This thesis explores the frequency degree of freedom for applications in QKD, utilizing efficient all-fiber photon-pair sources to generate hyper-entangled states in polarization and frequency. The incorporation of fast modulators allows for high-speed operation while maintaining high entanglement fidelity. Reconfigurable spectral filters enable tailored spectro-temporal properties to fulfil requirements in multi-user networks and protocols beyond QKD, such as quantum secret sharing. To maximize quantum information capacity, methods for efficiently characterizing frequency entanglement are developed, achieving a certification of 35 modes, a record for the time and frequency degrees of freedom. Finally, estimates of secure key rates and a discussion of noise effects is shown. This foundation supports broadband and flexible quantum networks that effectively utilize frequency encoding in the future quantum internet.