High-fidelity CNOT gates in photonic integrated circuits using composite segmented directional couplers

Directional couplers are fundamental building blocks for integrated quantum photonic circuits but are highly sensitive to fabrication imperfections. We demonstrate that composite segmented directional couplers (CSDCs) enhance gate fidelity without active tuning. We design and fabricate fully integrated photonic controlled-NOT (CNOT) gates using both uniform and composite coupler variants and compare their performance via simulation, classical characterization, and quantum two-photon interference. The composite design reduces the average computational-basis error probability by nearly a factor of two and variability by a factor of five, with residual error primarily limited by photon indistinguishability. These results establish CSDCs as passive, foundry-compatible building blocks for scalable quantum photonic circuits.