  Above the pseudocritical temperature T_c of chiral symmetry restoration a
chiral spin symmetry (a symmetry of the color charge and of electric
confinement) emerges in QCD. This implies that QCD is in a confining mode and
there are no free quarks. At the same time correlators of operators constrained
by a conserved current behave as if quarks were free. This explains observed
fluctuations of conserved charges and the absence of the rho-like structures
seen via dileptons. An independent evidence that one is in a confining mode is
very welcome. Here we suggest a new tool how to distinguish free quarks from a
confining mode. If we put the system into a finite box, then if the quarks are
free one necessarily obtains a remarkable diffractive pattern in the propagator
of a conserved current. This pattern is clearly seen in a lattice calculation
in a finite box and it vanishes in the infinite volume limit as well as in the
continuum. In contrast, the full QCD calculations in a finite box show the
absence of the diffractive pattern implying that the quarks are confined.
