  The identification of jets originating from quarks and gluons, often referred
to as quark/gluon tagging, plays an important role in various analyses
performed at the Large Hadron Collider, as Standard Model measurements and
searches for new particles decaying to quarks often rely on suppressing a large
gluon-induced background. This paper describes the measurement of the
efficiencies of quark/gluon taggers developed within the ATLAS Collaboration,
using $\sqrt{s} = 13$ TeV proton-proton collision data with an integrated
luminosity of 140 fb$^{-1}$ collected by the ATLAS experiment. Two taggers with
high performances in rejecting jets from gluon over jets from quarks are
studied: one tagger is based on requirements on the number of inner-detector
tracks associated with the jet, and the other combines several jet substructure
observables using a boosted decision tree. A method is established to determine
the quark/gluon fraction in data, by using quark/gluon-enriched subsamples
defined by the jet pseudorapidity. Differences in tagging efficiency between
data and simulation are provided for jets with transverse momentum between 500
GeV and 2 TeV and for multiple tagger working points.
