Planck isocurvature constraint on primordial black holes lighter than a kiloton

We demonstrate that primordial black holes (PBHs) lighter than $10^9 \, \text{g}$, which evaporated before the Big Bang nucleosynthesis, can induce significant isocurvature perturbations due to their biased clustering amplitude and the branching ratio of the Hawking radiation differing from the abundance ratio. By leveraging the upper bound on the isocurvature perturbations from the cosmic microwave background anisotropies reported by the Planck collaboration, we derive a new upper bound on the abundance of these light PBHs.