  Topological matter exhibits exotic properties yet phases characterized by
large topological invariants are difficult to implement, despite rapid
experimental progress. A promising route toward higher topological invariants
is via engineered Floquet systems, particularly in photonics, where flexible
control holds the potential of extending the study of conventional topological
matter to novel regimes. Here we implement a one-dimensional photonic quantum
walk to explore large winding numbers. By introducing partial measurements and
hence loss into the system, we detect winding numbers of three and four in
multi-step non-unitary quantum walks, which agree well with theoretical
predictions. Moreover, by probing statistical moments of the walker, we
identify locations of topological phase transitions in the system, and reveal
the breaking of pseudo-unitary near topological phase boundaries. As the
winding numbers are associated with non-unitary time evolution, our
investigation enriches understanding of topological phenomena in non-unitary
settings.
