  The measurement of the Yukawa coupling of the top quark is one important goal
of particle physics after the discovery of the Higgs boson. There are two
processes that offer the possibility to directly measure the Yukawa coupling of
the top quark at the LHC. These are the production of a top-antitop-quark pair
in association with a Higgs boson via the strong interaction ($t\bar{t}H$) and
the electroweak production of a single top quark and a Higgs boson ($tH$). In
the Standard Model, the cross section for $tH$ production is much smaller than
the cross section for $t\bar{t}H$ production as there is a destructive
interference between diagrams where the Higgs boson is radiated off a top quark
and diagrams where it is radiated off a $W$ boson. Despite the reduced cross
section, this interference provides additional sensitivity to the sign of the
Yukawa coupling.
  The considered Higgs boson decay into photons is promising due to the good
energy resolution of the photons and the relatively small backgrounds. But it
is also challenging due to the small branching ratio.
  The analysis strategy and the latest results on the search for $t\bar{t}H/tH$
production in the diphoton channel at the ATLAS experiment are presented.
