  Using a single gravitational lens system observed at $\lesssim5$
milli-arcsecond resolution with very long baseline interferometry (VLBI), we
place a lower bound on the mass of the fuzzy dark matter (FDM) particle, ruling
out $m_\chi \leq 4.4\times10^{-21}~\mathrm{eV}$ with a 20:1 posterior odds
ratio relative to a smooth lens model. We generalize our result to non-scalar
and multiple-field models, such as vector FDM, with $m_{\chi,\mathrm{vec}} >
1.4 \times 10^{-21}~\mathrm{eV}$. Due to the extended source structure and high
angular resolution of the observation, our analysis is directly sensitive to
the presence of granule structures in the main dark matter halo of the lens,
which is the most generic prediction of FDM theories. A model based on
well-understood physics of ultra-light dark matter fields in a gravitational
potential well makes our result robust to a wide range of assumed dark matter
fractions and velocity dispersions in the lens galaxy. Our result is
competitive with other lower bounds on $m_\chi$ from past analyses, which rely
on intermediate modelling of structure formation and/or baryonic effects.
Higher resolution observations taken at 10 to 100 GHz could improve our
constraints by up to 2 orders of magnitude in the future.
