  Adaptable, reconfigurable and programmable are key functionalities for the
next generation of silicon-based photonic processors, neural and quantum
networks. Phase change technology offers proven non-volatile electronic
programmability, however the materials used to date have shown prohibitively
high optical losses which are incompatible with integrated photonic platforms.
Here, we demonstrate the capability of the previously unexplored material
Sb$_2$Se$_3$ for ultralow-loss programmable silicon photonics. The favorable
combination of large refractive index contrast and ultralow losses seen in
Sb$_2$Se$_3$ facilitates an unprecedented optical phase control exceeding
10$\pi$ radians in a Mach-Zehnder interferometer. To demonstrate full control
over the flow of light, we introduce nanophotonic digital patterning as a
conceptually new approach at a footprint orders of magnitude smaller than state
of the art interferometer meshes. Our approach enables a wealth of
possibilities in high-density reconfiguration of optical functionalities on
silicon chip.
