  Photonic integrated circuits play a central role in current and future
applications such as communications, sensing, ranging, and information
processing. Photonic quantum computing will also likely require an integrated
optics architecture for improved stability, scalability, and performance.
Fault-tolerant quantum computing mandates very accurate and robust quantum
gates. In this work, we demonstrate high-fidelity directional couplers for
single-qubit gates in photonic integrated waveguides, utilizing a novel scheme
of detuning-modulated composite segments. Specific designs for reduced
sensitivity to wavelength variations and real-world geometrical fabrication
errors in waveguides width and depth are presented. Enhanced wavelength
tolerance is demonstrated experimentally. The concept shows great promise for
scaling high fidelity gates as part of integrated quantum optics architectures.
