Towards Hybrid Quantum Key Distribution

We present a scheme for quantum key distribution based on hybrid entanglement. The idea is to couple a free-space link with an optical fibre link by generating polarization qubits in the visible spectrum entangled with time-bin qubits in the telecom window. We discuss two ways to generate this type of entanglement: Using parametric downconversion in a periodically-poled crystal or using four-wave mixing in optical fibres. We also discuss how hybrid entanglement is an interesting way to extend the range of quantum key distribution.