Universal Wilson Loop Bound of Quantum Geometry

TL;DR

This paper introduces a universal Wilson loop bound that constrains the quantum metric and related topological invariants, with implications for physical properties like superfluidity and optical responses.

Contribution

It establishes a universal lower bound on the quantum metric via Wilson loop winding, linking topological invariants to physical bounds and solving an open problem.

Findings

Wilson loop bounds the quantum metric from below.

The bound reproduces known Chern and Euler bounds.

It constrains superfluid weight and optical conductivity.

Abstract

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 $Z_2$ 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 $Z_2$ index. As physical consequences of the $Z_2$ bound, we show that the time-reversal $Z_2$ index bounds superfluid weight and optical conductivity from below and bounds the direct gap of a band insulator from above.