Free-Space Reference Frame Independent Quantum Key Distribution System With Photonic Integrated Circuit-Based Transmitter

Deployable quantum key distribution (QKD) for free-space link must be both reference frame tolerant and hardware compact, yet existing free-space implementations still rely on bulk optics and active alignment. To address this gap, we develop the first reference frame independent (RFI) QKD transmitter fully integrated on a photonic-integrated circuit (PIC) and packaged in a CFP2 optical-transceiver form factor. The PIC incorporates a six-channel laser diode array, a six-channel variable optical attenuator array, and a polarization beam combiner, providing wavelength indistinguishability and high polarization fidelity. Combined with a micro-optics based polarization decoder and a bidirectional two-stage pointing, acquisition, and tracking (PAT) system, the PIC transmitter forms a complete free-space RFI QKD system. Based on the system, RFI QKD is performed with 10 m free-space channel, providing that QBERs measured in the same basis are below 3.81 % and the security parameter is$1.77\pm 0.02$. This leads a stable secret key rate of$77.33\pm 1.65 \text{kbps}$for 100 s. These results verify the feasibility of PIC-based RFI QKD transmitter and establish a clear pathway toward fully deployable PIC-based QKD systems.