  The Kagome lattice exhibits rich quantum phenomena owing to its unique
geometric properties. Appealing realizations are the Kagome metals AV$_3$Sb$_5$
(A = K, Rb, Cs), where unconventional charge density wave (CDW) is intertwined
with superconductivity and non-trivial band topology. Several experiments
suggest that this CDW is a rare occurrence of chiral CDW characterized by
orbital loop current. However, key evidences of loop current, spontaneous
time-reversal symmetry-breaking (TRSB) and the coupling of its order parameter
with the magnetic field remain elusive. Here, we investigate the CDW in CsV3Sb5
by magneto-optic polar Kerr effect with sub-microradian resolution. Under
magnetic field, we observed a jump of the Kerr angle at the CDW transition.
This jump is magnetic-field switchable and scales with field, indicating
magneto--chirality coupling related to non-trivial band topology. At zero
field, we found non-zero and field-trainable Kerr angle below CDW transition
temperature, signaling spontaneous TRSB. Our results provide a crucial step to
unveil quantum phenomena in correlated Kagome materials.
