Quantum state preparation of time-bin encoding based on SOI integrated chips.

Quantum key distribution (QKD) ensures unconditional security for public key encryption by utilizing the principles of quantum mechanics. This study designs and fabricates a QKD chip based on the silicon-on-insulator (SOI) platform, employing a time-bin encoding scheme with decoy states. In the design, we integrate a slow thermo-optic phase modulator with a carrier-depletion modulator to ensure high fidelity of quantum states and high-speed encoding capabilities, enabling precise and flexible time-bin encoding. We achieve the encoding and decoding of four BB84 quantum states at a repetition rate of 100 MHz. In the experiment, the visibility of the interference fringes for the phase state |+⟩ is 93.66%, and for the phase state |-⟩, it is 92.36%. The extinction ratios for the time states |0⟩ and |1⟩ are 19.33 and 18.72 dB, respectively. The experimental results demonstrate that the chip has efficient quantum state preparation capability, providing significant support for the practical implementation of quantum key distribution technology. Additionally, we propose a new chip structure, to our knowledge, using SOI and Si₃N₄ heterogeneous integration. The SOI waveguide is used for high-speed modulation encoding, while the delay line is composed of Si₃N₄ waveguides. This structure is designed to address the stability issues of the chip caused by temperature variations.