Quantum metric induced nonlinear transport in the hidden loop-current phase of kagome metal RbV${}_3$Sb${}_5$

A hidden low-temperature phase with possible loop-current order has been proposed in the kagome metals AV${}_3$Sb${}_5$ (A=K, Rb or Cs), but its experimental signatures remain subtle and indirect. Here, we use third-order nonlinear transport to probe this hidden phase in RbV${}_3$Sb${}_5$. At ~35 K, the longitudinal cubic response develops a strong kink and pronounced directional anisotropy with a strong departure from common relaxation time scaling, while the transverse cubic response acquires a magnetic field-odd component at the same temperature. Their coincident onset identifies the third-harmonic response as a sensitive marker of the low-temperature electronic reconstruction with time-reversal broken symmetry. Crucially, we find that the quantum metric quadrupole contributes directly to the longitudinal third-order response. In a loop-current charge-density-wave model, its direction selective enhancement captures the observed angular reconstruction. Our work shows how higher-order nonlinear transport translates subtle changes in electronic symmetry and quantum metric into measurable electrical signatures in quantum materials that is important for studying complex electronic phases of matter.