Measurement of the angular dependence of the dipole-dipole interaction between two individual Rydberg atoms at a Forster resonance

We measure the angular dependence of the resonant dipole-dipole interaction between two individual Rydberg atoms with controlled relative positions. By applying a combination of static electric and magnetic fields on the atoms, we demonstrate the possibility to isolate a single interaction channel at a Forster resonance, that shows a well-defined angular dependence. We first identify spectroscopically the Forster resonance of choice and we then perform a direct measurement of the interaction strength between the two atoms as a function of the angle between the internuclear axis and the quantization axis. Our results show good agreement with the angular dependence alpha(1 - 3 cos(2)theta) expected for this resonance. Understanding in detail the angular dependence of resonant interactions is important in view of using Forster resonances for quantum state engineering with Rydberg atoms.