Universal Diffusion in Incoherent Black Holes

We study charge and energy diffusion in simple holographic theories with broken translational symmetry. We find that when the effects of momentum relaxation are very strong the diffusion constants take universal values $D_{c} \sim D_{e} \sim \hbar v_B^2/(k_B T)$\mathrm{.}Here$v_B$ is the velocity of the butterfly effect and the coefficients of proportionality depend only on the scaling exponents of the infra-red fixed point. Our results suggest that diffusion in these incoherent black holes is controlled by $\tau \sim {\hbar}/(k_B T)$ independently of the mechanism of momentum relaxation.