  In this paper, we report an experiment, which demonstrates computation of
topological charges of two optical vortices via non-degenerate four-wave-mixing
process. We show that the output signal beam carries orbital angular momentum
which equals to the subtraction of the orbital angular momenta of the probe
light and the backward pump light. The &#8312;&#8309;Rb atoms are used as the
nonlinear medium, which transfer the orbital angular momenta of lights.
