  This pedagogical piece provides a surprisingly simple demonstration that the
quantized Hall conductivity of correlated insulators is given by the many-body
Chern number, a topological invariant defined in the space of twisted boundary
conditions. In contrast to conventional proofs, generally based on the Kubo
formula, our approach entirely relies on combining Kramers-Kronig relations and
Fermi's golden rule within a circular-dichroism framework. This pedagogical
derivation illustrates how the Hall conductivity of correlated insulators can
be determined by monitoring single-particle excitations upon a circular drive,
a conceptually simple picture with direct implications for quantum-engineered
systems, where excitation rates can be directly monitored.
