  We study the optical response of a suspended graphene field-effect transistor
in magnetic fields of up to 9 T (quantum Hall regime). With an illumination
power of only 3 {\mu}W, we measure a photocurrent of up to 400 nA,
corresponding to a photo-responsivity of 0.14 A/W, which we believe to be the
highest value ever measured in single-layer graphene. We estimate that every
absorbed photon creates more than 8 electron-hole pairs, which demonstrates
highly effective carrier multiplication. As suggested by the dependence of the
photocurrent on gate voltage and magnetic field, we propose a ballistic
two-stage mechanism where the incident photons create primary charge carriers
which then excite secondary charge carriers in the chiral edge states via
Auger-type inelastic Coulomb scattering processes at the graphene edge.
