  A study of the polarisation and CP properties in $ZZ$ production is
presented. The used data set corresponds to an integrated luminosity of 140
fb$^{-1}$ of proton-proton collisions at a centre-of-mass energy of $13$ TeV
recorded by the ATLAS detector at the Large Hadron Collider. The $ZZ$ candidate
events are reconstructed using two same-flavour opposite-charge electron or
muon pairs. The production of two longitudinally polarised $Z$ bosons is
measured with a significance of 4.3 standard deviations, and its cross-section
is measured in a fiducial phase space to be $2.45 \pm 0.60$ fb, consistent with
the next-to-leading-order Standard Model prediction. The inclusive differential
cross-section as a function of a CP-sensitive angular observable is also
measured. The results are used to constrain anomalous CP-odd neutral triple
gauge couplings.
