Universal Wilson Loop Bound of Quantum Geometry

We define the absolute Wilson loop winding and prove that it bounds the (integrated) quantum metric from below. This Wilson loop lower bound naturally reproduces the known Chern and Euler bounds of the integrated quantum metric and provides an explicit lower bound of the integrated quantum metric due to the time-reversal protected Z2 index, answering a hitherto open question. In general, the Wilson loop lower bound can be applied to any other topological invariants characterized by Wilson loop winding, such as the particle-hole Z2 index. As physical consequences of the Z2 bound, we show that the time-reversal Z2 index bounds superfluid weight and optical conductivity from below and bounds the direct gap of a band insulator from above.