Spatially resolved observation of dipole-dipole interaction between Rydberg atoms

We have observed resonant energy transfer between cold Rydberg atoms in separate volumes. Two pulsed laser beams are focused in a $^{85}$Rb MOT, with waists of $\sim$~10~$\mu$m. In one laser focus $\sim$~10 atoms are excited to the $49\mathrm{s}$ state and in the other $\sim$~60 atoms are excited to the $41\mathrm{d}$ state. At a field of 0.4~V/cm a resonant dipole-dipole coupling occurs with a wavelength of 1~cm: $41\mathrm{d} + 49\mathrm{s} \rightarrow 42\mathrm{p} + 49\mathrm{p}$. We have measured the production of the $49\mathrm{p}$ state as a function of laser focus separation (0~-~80~$\mu$m) and interaction time (0~-~50~$\mu$s). The procedure can be understood as writing a 0 bit ($49\mathrm{s}$) in volume A and a 1 bit ($41\mathrm{d}$) in volume B. The bit in A goes from 0 to 1 ($49\mathrm{p}$) and the bit in B from 1 to 0 ($42\mathrm{p}$), due to the dipole-dipole energy transfer from B to A.