  Topology manifesting in many branches of physics deepens our understanding on
state of matters. Topological photonics has recently become a rapidly growing
field since artificial photonic structures can be well designed and constructed
to support topological states, especially a promising large-scale
implementation of these states using photonic chips. Meanwhile, due to the
inapplicability of Hall conductance to photons, it is still an elusive problem
to directly measure the integer topological invariants and topological phase
transitions for photons. Here, we present a direct observation of topological
winding numbers by using bulk-state photon dynamics on a chip. Furthermore, we
for the first time experimentally observe the topological phase transition
points via single-photon dynamics. The integrated topological structures,
direct measurement in the single-photon regime and strong robustness against
disorder add the key elements into the toolbox of `quantum topological
photonics' and may enable topologically protected quantum information
processing in large scale.
