Spectroscopic Observation of Resonant Electric Dipole-Dipole Interactions between Cold Rydberg Atoms

Resonant electric dipole-dipole interactions between cold Rydberg atoms were observed using microwave spectroscopy. Laser-cooled ${}^{85}\mathrm{R}\mathrm{b}$ atoms in a magneto-optical trap were optically excited to $45d_{5/2}$ Rydberg states using a pulsed laser. A microwave pulse transferred a fraction of these Rydberg atoms to the $46p_{3/2}$ state. A second microwave pulse then drove atoms in the $45d_{5/2}$ state to the $46d_{5/2}$ 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 $46p_{3/2}$ atoms, and is due to the resonant electric dipole-dipole interaction between $45d_{5/2}$ and $46p_{3/2}$ Rydberg atoms.

Figure 1

(a) Timing for experiment and (b) energy levels of relevant states. The spectroscopic data are from Ref. 17. The matrix elements magnitudes $|⟨n{\ell }_j|r|n^{\prime }{\ell }_{j^{\prime }}^{\prime }⟩|$ are shown between states (in atomic units).

Figure 2

Observation of the two-photon $45d_{5/2}–46d_{5/2}$ microwave transition with (a) (○) all Rydberg atoms initially in the $45d_{5/2}$ state at a density of $2\times {10}^6{\mathrm{c}\mathrm{m}}^{-3}$, (b) (□) all Rydberg atoms initially in the $45d_{5/2}$ state at a density of $1\times {10}^7{\mathrm{c}\mathrm{m}}^{-3}$, and (c) (+) one-half of the Rydberg atoms initially in the $45d_{5/2}$ state and the other half in the $46p_{3/2}$ state with a total density of $1\times {10}^7{\mathrm{c}\mathrm{m}}^{-3}$. Also shown with solid lines are (a) the ${\mathrm{s}\mathrm{i}\mathrm{n}\mathrm{c}}^2(\pi fT)$ line shape ($T=6\mu \mathrm{s}$), and (b) and (c) the same line shape convolved with a Lorentzian of variable width to give the best least-squares fits (see text). Spectra are offset vertically for clarity. The probe frequency shown on the horizontal axis is twice the applied frequency, offset by 76.4543 GHz.

Figure 3

Broadening of the $45d_{5/2}–46d_{5/2}$ probe transition as a function of average Rydberg density with and without the transfer pulses (see text for definition of $\delta \nu$).