  We calculate chromo-electric and chromo-magnetic correlators in quenched QCD
at $1.5T_c$ and $10^4 T_c$ with the aim to estimate the heavy quark diffusion
coefficient at leading order in the inverse heavy quark mass expansion,
$\kappa_E$, as well as the coefficient of first mass suppressed correction,
$\kappa_B$. We use gradient flow for noise reduction. At $1.5T_c$ we obtain:
$1.70 \le \kappa_E/T^3 \le 3.12$ and $1.03< \kappa_B/T^3 < 2.61$. The latter
implies that the mass suppressed effects in the heavy quark diffusion
coefficient are 20% for bottom quarks and 34% for charm quark at this
temperature.
