  We present a method of locally inverting the sign of the coupling term in
tight-binding systems, by means of inserting a judiciously designed ancillary
site and eigenmode matching of the resulting vertex triplet. Our technique can
be universally applied to all lattice configurations, as long as the individual
sites can be detuned. We experimentally verify this method in laser-written
photonic lattices and confirm both the magnitude and the sign of the coupling
by interferometric measurements. Based on these findings, we demonstrate how
such universal sign-flipped coupling links can be embedded into extended
lattice structures to impose a $\mathbb{Z}_2$-gauge transformation. This opens
a new avenue for investigations on topological effects arising from magnetic
fields with aperiodic flux patterns or in disordered systems.

Optics;Quantum Physics