Passive optical scheme for BB84 protocol with polarization encoding on a silicon nitride platform

Quantum key distribution is a technology that promises unconditional security for protecting data. However, despite being based on the laws of quantum physics, its practical implementation may have critical vulnerabilities. One way to address this is to passively prepare quantum states. In our work, we demonstrate a passive optical scheme for the BB84 protocol with polarization encoding. We use a finite-difference time-domain method to simulate the elements of this scheme on the silicon nitride platform. Our simulations suggest that the polarization extinction ratio will be more than 20 dB, which will allow for the generation of quantum states with a QBER (quantum bit error rate) of less than 1%.