Low-voltage silicon photonics modulator with CMOS-compatible driving for compact quantum key distribution transmitters

Most quantum key distribution (QKD) silicon photonic chips rely on carrier depletion modulators (CDM), which typically require driving voltages beyond 5 V due to their limited modulation efficiency. In this work, we employ a carrier injection modulator (CIM) for quantum bit encoding, which offers a fundamentally higher modulation efficiency and thus a path to significantly lower operating voltages. Through structural optimization, the CIM achieves a half-wave voltage of 1.1 V (DC) and 1.28 V (at 100 MHz), with a modulation depth exceeding 24 dB under both conditions. Successful demonstrations of intensity state preparation as well as polarization state modulation were also conducted. These results confirm the suitability of the CIM for compact and energy-efficient QKD transmitters operating at CMOS-compatible voltages.