  Resonant electric dipole-dipole interactions between cold Rydberg atoms were
observed using microwave spectroscopy. Laser-cooled Rb atoms in a
magneto-optical trap were optically excited to 45d Rydberg states using a
pulsed laser. A microwave pulse transferred a fraction of these Rydberg atoms
to the 46p state. A second microwave pulse then drove atoms in the 45d state to
the 46d state, and was used as a probe of interatomic interactions. The
spectral width of this two-photon probe transition was found to depend on the
presence of the 46p atoms, and is due to the resonant electric dipole-dipole
interaction between 45d and 46p Rydberg atoms.
