  We study time evolution of critical fluctuations of conserved charges near
the QCD critical point in the context of relativistic heavy ion collisions. A
stochastic diffusion equation is employed in order to describe the diffusion
property of the critical fluctuation arising from the coupling of the order
parameter field to conserved charges. We show that the diffusion property gives
rise to a possibility of probing the early time fluctuations through the
rapidity window dependence of the second-order cumulant and correlation
function of conserved charges. It is pointed out that their non-monotonic
behaviors as functions of the rapidity interval are robust experimental signals
for the existence of the critical enhancement around the QCD critical point.
