  Integrated nonlinear photonic circuits received rapid development in recent
years, providing all-optical functionalities enabled by cavity-enhanced
photon-photon interaction for classical and quantum applications. A
high-efficiency fiber-to-chip interface is key to the use of these integrated
photonic circuits for quantum information tasks, as photon loss is a major
source that weakens quantum protocols. Here, overcoming material and
fabrication limitation of thin-film aluminum nitride by adopting a stepwise
waveguiding scheme, we demonstrate low-loss adiabatic fiber-optic couplers in
aluminum nitride films with a substantial thickness (600 nm) for optimized
nonlinear photon interaction. For telecom (1550 nm) and near-visible (780 nm)
transverse magnetic-polarized light, the measured insertion loss of the
fiber-optic coupler is -0.97 dB and -2.6 dB, respectively. Our results will
facilitate the use of aluminum nitride integrated photonic circuits as
efficient quantum resources for generation of entangled photons and squeezed
light on microchips.
