Optimizing path-polarization interconversion on a photonic circuit for secure quantum communication

In this work, we consider an on-chip silicon device that prepares photonic qubits in mutually unbiased bases (MUBs), which can be used to realize quantum key distribution protocols. We present a realistic approach to dealing with generic imperfections in on-chip devices for generating MUBs, in particular, imperfections originating from fabrication. We show how to control the conversion of path to polarization qubits for sending MUBs, and conversely from polarization to path qubits when receiving them: we identify the so-called tilt angle at the conversion station as the parameter to control and improve the preparation of MUBs. We investigate the optimization of a tilt angle for various figures of merit, such as fidelities of MUBs, photon losses, and secret key rates in a practical realization. Our results suggest a realistic, cost-effective strategy for exploiting chip-based fiber-optic quantum communication.