Hyperentangled Time-bin and Polarization Quantum Key Distribution*

Fiber-based quantum key distribution (QKD) networks are currently limited to metropolitan distances without quantum repeaters. To reach longer distances, satellite-based QKD links have been proposed to extend the network domain. We have developed a quantum communication system suitable for a satellite-to-ground link. With this system, using polarization entangled photons, we have executed the QKD protocol developed by Bennett, Brassard, and Mermin in 1992 (BBM92), achieving quantum bit error rates (QBER) below 2$\%$. More importantly, we demonstrate low QBER execution of a higher dimensional hyperentanglement-based QKD protocol (HEQKD) using photons simultaneously entangled in polarization and time-bin. We verify the security of our protocol using a rigorous, modern finite-key analysis, and show it is suitable for a space-to-ground link, after incorporating a Doppler shift compensation scheme. Finally, we demonstrate a distinct advantage to using HEQKD over BBM92.