Passive waveguide-based decoy-state quantum key distribution transmitter

Polarization-encoded photonic qubits are a key resource for quantum communication, particularly for space-based quantum key distribution (QKD). Practical polarization-based QKD transmitters require the combination of multiple polarization states into a single spatial mode, a task that is typically implemented using bulk or fiber-based optics. Both demand precise alignment and are highly sensitive to motion and temperature. Here we demonstrate an ultra-low-birefringence, silica waveguide, which generates different polarization states at the output depending on the input channel. This is achieved by aligning an array of polarization-maintaining (PM) fibers at the interface with the waveguide, with the slow axis of the fiber angled to produce H, V, D, and A polarization states. The eight input channels are combined into one single-mode output via a cascade of adiabatic Y-junction couplers. We demonstrate real-time free-space decoy-state QKD across a 3 m link, at a clock rate of 1 GHz. We also analyze the performance of the waveguide under various temperature conditions (15°C–60°C). The photonic integrated circuit (PIC)-based design is readily suitable for satellite implementation, due to low-SWaP, ease of alignment, and minimal effects from temperature changes.