  In recent years, the computational demands of deep learning applications have
necessitated the introduction of energy-efficient hardware accelerators.
Optical neural networks are a promising option; however, thus far they have
been largely limited by the lack of energy-efficient nonlinear optical
functions. Here, we experimentally demonstrate an all-optical Rectified Linear
Unit (ReLU), which is the most widely used nonlinear activation function for
deep learning, using a periodically-poled thin-film lithium niobate
nanophotonic waveguide and achieve ultra-low energies in the regime of
femtojoules per activation with near-instantaneous operation. Our results
provide a clear and practical path towards truly all-optical, energy-efficient
nanophotonic deep learning.
