  Heavy-ion collisions are a unique tool for testing the behaviour of matter
under extreme conditions. The momentum correlations of charm and bottom hadrons
have been considered for testing heavy quarks' thermalization in the hot, dense
medium produced by the collisions. In this respect, two effects have been
considered: the decrease of the initial back-to-back correlations and the
increase of correlations due to heavy-quark interactions with collectively
flowing medium.
  Here, we show that, in the case of a single charm and anti-charm hadron pair
production, the collective flow allows for testing heavy-quark production
locality and spatial diffusion. Using an example of central Pb+Pb collisions at
the CERN SPS energies, we demonstrate that the azimuthal correlations of charm
and anti-charm hadrons are particularly sensitive to their spatial
correlations. We argue that the existing experimental technology and beam
intensities at the CERN SPS should allow for the corresponding measurements
soon. The correlation measurements in collisions with a single heavy-quark pair
produced will provide a unique input constraining the diffusion of charm quarks
and verifying assumptions concerning production locality of a charm and
anti-charm quark pair.
