  Topological photonics has revolutionized our understanding of light
propagation, but most of current studies are focused on designing a static
photonic structure. Developing a dynamic photonic topological platform to
switch multiple topological functionalities at ultrafast speed is still a great
challenge. Here we demonstrate an ultrafast reprogrammable plasmonic
topological insulator, where the topological propagation route can be
dynamically steered at nanosecond-level switching time, namely more than 10^7
times faster than the current state-of-the-art. This orders-of-magnitude
improvement is achieved by using ultrafast electronic switches in an innovative
way to implement the programmability. Due to the flexible programmability, many
existing photonic topological functionalities can be integrated into this agile
topological platform. Our work brings the current studies of photonic
topological insulators to a digital and intelligent era, which could boost the
development of intelligent and ultrafast photoelectric devices with built-in
topological protection.
