High extinction ratio multi-polarization states preparation based on SOI integrated chips

Quantum key distribution (QKD) leverages the principles of quantum mechanics to provide an unconditionally secure public-key cryptographic system. Polarization encoding is a commonly employed method in QKD. However, the low extinction ratio of polarization states significantly impacts the error rate in QKD systems, thereby threatening the security of communication. Here, we propose a structure and method for the preparation of high-extinction-ratio polarization states based on silicon photonic integrated chips. Our chip integrates thermo-optic modulators and silicon carrier-depletion modulators to achieve the transformation of information from path encoding to polarization encoding through a 2-D grating. We have successfully prepared six polarization states using only thermo-optic modulators, with an average polarization extinction ratio of 35.8 dB. Furthermore, we have successfully prepared six polarization states using a combination of thermo-optic and electro-optic modulators, with an average polarization extinction ratio exceeding 30.6 dB. Notably, our system demonstrated remarkable stability over a 2-h testing period, with the polarization extinction ratio remaining essentially unchanged.