Nonlinear Nanophotonics for High-Dimensional Quantum States

Quantum nanophotonics merges the precision of nanoscale light manipulation with the capabilities of quantum technologies, offering a pathway for enhanced light-matter interaction and compact realization of quantum devices. Here, we show how a recently-demonstrated nonlinear nanophotonic process can be employed to selectively create photonic high-dimensional quantum states (qudits). We utilize the nonlinearity on the surface of the nanophotonic device to dress, through the polarization of the pump field, the near-field modes carrying angular momentum and their superpositions. We then use this approach for the realization of a multilevel quantum key distribution protocol, which doubles the key rate compared to standard schemes. This idea is an important step towards experimental realizations of quantum state generation and manipulation through nonlinearity within nanophotonic platforms, and enables new capabilities for on-chip quantum devices.