  A hallmark feature of topological physics is the presence of one-way
propagating chiral modes at the system boundary. The chirality of edge modes is
a consequence of the topological character of the bulk. For example, in a
non-interacting quantum Hall model, edge modes manifest as mid-gap states
between two topologically distinct bulk bands. The bulk-boundary correspondence
dictates that the number of chiral edge modes, a topological invariant called
the winding number, is completely determined by the bulk topological invariant,
the Chern number. Here, for the first time, we measure the winding number in a
2D photonic system. By inserting a unit flux quantum at the edge, we show that
the edge spectrum resonances shift by the winding number. This experiment
provides a new approach for unambiguous measurement of topological invariants,
independent of the microscopic details, and could possibly be extended to probe
strongly correlated topological orders.
