  Cold atoms in highly excited Rydberg states are promising candidates to
implement quantum logic gates of a quantum computer via long-range
dipole-dipole interaction. Two-qubit gates require a controlled interaction of
only two close Rydberg atoms. We report on the first spectroscopic observation
of the resonant dipole-dipole interaction between two cold rubidium Rydberg
atoms confined in a small laser excitation volume. The interaction strength was
controlled by fine-tuning of the Rydberg levels into a Forster resonance using
the Stark effect. The observed resonance line shapes are in good agreement with
numerical Monte Carlo simulations.
