  Twisted light carrying orbital angular momentum (OAM) provides an additional
degree of freedom for modern optics and an emerging resource for both classical
and quantum information technologies. Its inherently infinite dimensions can
potentially be exploited by using mode multiplexing to enhance data capacity
for sustaining the unprecedented growth in big data and internet traffic, and
can be encoded to build large-scale quantum computing machines in
high-dimensional Hilbert space. While the emission of twisted light from the
surface of integrated devices to free space has been widely investigated, the
transmission and processing inside a photonic chip remain to be addressed.
Here, we present the first laser-direct-written waveguide being capable of
supporting OAM modes and experimentally demonstrate a faithful mapping of
twisted light into and out of a photonic chip. The states OAM$_{0}$,
OAM$_{-1}$, OAM$_{+1}$ and their superpositions can transmit through the
photonic chip with a total efficiency up to 60% with minimal crosstalk. In
addition, we present the transmission of quantum twisted light states of single
photons and measure the output states with single-photon imaging. Our results
may add OAM as a new degree of freedom to be transmitted and manipulated in a
photonic chip for high-capacity communication and high-dimensional quantum
information processing.
