  Graphene quantum dots (GQDs) have recently attracted considerable attention,
with appealing properties for terahertz (THz) technology. This includes the
demonstration of large thermal bolometric effects in GQDs when illuminated by
THz radiation. However, the interaction of THz photons with GQDs in the Coulomb
blockade regime - single electron transport regime - remains unexplored. Here,
we demonstrate the ultrasensitive photoresponse to THz radiation (from <0.1 to
10 THz) of a hBN-encapsulated GQD in the Coulomb blockade regime at low
temperature (170 mK). We show that THz radiation of $\sim$10 pW provides a
photocurrent response in the nanoampere range, resulting from a renormalization
of the chemical potential of the GQD of $\sim$0.15 meV. We attribute this
photoresponse to an interfacial photogating effect. Furthermore, our analysis
reveals the absence of thermal effects, opening new directions in the study of
coherent quantum effects at THz frequencies in GQDs.
