  Topological phases enable protected transport along the edges of materials,
offering immunity against scattering from disorder and imperfections. These
phases were suggested and demonstrated not only for electronic systems, but
also for electromagnetic waves, cold atoms, acoustics, and even mechanics.
Their potential applications range from spintronics and quantum computing to
highly efficient lasers. Traditionally, the underlying model of these systems
is a spatial lattice in two or three dimensions. However, it recently became
clear that many lattice systems can exist also in synthetic dimensions which
are not spatial but extend over a different degree of freedom. Thus far,
topological insulators in synthetic dimensions were demonstrated only in cold
atoms, where synthetic dimensions have now become a useful tool for
demonstrating a variety of lattice models that are not available in spatial
lattices. Subsequently, efforts have been directed towards realizing
topological lattices with synthetic dimensions in photonics, where they are
connected to physical phenomena in high-dimensions, interacting photons, and
more. Here we demonstrate experimentally the first photonic topological
insulator in synthetic dimensions. The ability to study experimentally photonic
systems in synthetic dimensions opens the door for a plethora of unexplored
physical phenomena ranging from PT-symmetry, exceptional points and
unidirectional invisibility to Anderson localization in high dimensions and
high-dimensional lattice solitons, topological insulator lasers in synthetic
dimensions and more. Our study here paves the way to these exciting phenomena,
which are extremely hard (if not impossible) to observe in other physical
systems.
