The use of correlation between two open-charm mesons is suggested to g
ive information about the nature of the medium created in heavy-ion co
llisions. insensitivity to the charm production rate is achieved by me
asuring normalized cumulant. The acollinearity of the D momenta in the
transverse plane is a measure of the medium effect. Its dependence on
nuclear size or E(T) provides a signature for the formation of quark
matter.