Resonant electric dipole-dipole interactions between cold Rydberg atoms were observed using microwave spectroscopy. Laser-cooled 85 Rb 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.PACS numbers: 32.80. Rm, 34.20.Cf, 32.80.Pj The vast separation of the electron and ion-core in high-n Rydberg atoms is responsible for their large transition dipole moments [1]. These dipole moments dictate the strength of the dipole-dipole interaction between pairs of atoms. Therefore, excitation to Rydberg states allows one to turn on strong interactions between atoms which would otherwise be negligible. This has recently received considerable attention in the context of quantum information processing with cold neutral atoms [2,3,4,5,6,7]. For example, it has been proposed that a single excited Rydberg atom in a cloud may block further resonant excitation due to the dipole-dipole interaction -a process known as "dipole blockade" [3]. This would allow clouds of cold atoms to store qubits without the addressing of individual atoms and may also be useful for constructing single-atom and single-photon sources [5].Non-resonant dipole-dipole (van der Waals) interactions between Rydberg atoms were first observed by Raimond et al.[8] using spectral line broadening. Recently it has been shown that Rydberg excitation densities in a magneto-optical trap (MOT) are limited by these interactions [9,10]. Dipole-dipole interactions between Rydberg atoms have also been studied in the context of resonant energy transfer [1]. Of particular relevance to this work is the observation of resonant energy transfer between cold Rydberg atoms [11,12], where the Rydberg atoms behave more like an amorphous solid than a gas, and one cannot solely consider binary interactions to explain the transfer process [13,14]. However, use of the resonant dipole-dipole interaction between cold Rydberg atoms to influence radiative transitions -as presented in this work -is an unexplored area.We excite Rydberg states using a pulsed laser with no stringent demands on linewidth or stability. Dipoledipole interactions are then introduced and probed using microwave transitions between Rydberg states. This is advantageous since commercial microwave synthesizers are readily tunable, highly stable, and have easily adjustable powers and pulse-widths, as compared to lasers. Using this approach we have made the first spectroscopic observation of the resonant dipole-dipole interaction between cold Rydberg atoms using radiative transitions.To observe interactions between atoms that are effectiv...

The ac Stark effect was used to induce resonant energy transfer between translationally cold 85Rb Rydberg atoms. When a 28.5 GHz dressing field was set at specific field strengths, the two-atom dipole-dipole process 43d5/2+43d5/2-->45p3/2+41f was dramatically enhanced, due to induced degeneracy of the initial and final states. This method for enhancing interactions is complementary to dc electric-field-induced resonant energy transfer, but has more flexibility due to the possibility of varying the applied frequency.

Laser-cooled 85 Rb atoms were optically excited to 46d 5/2 Rydberg states. A microwave pulse transferred a fraction of the atoms to the 47p 3/2 Rydberg state. The resonant electric dipole-dipole interactions between atoms in these two states were probed using the linewidth of the two-photon microwave transition 46d 5/2 -47d 5/2 . The presence of a weak magnetic field Ϸ0.5 G reduced the observed line broadening, indicating that the interaction is suppressed by the field. The field removes some of the energy degeneracies responsible for the resonant interaction, and this is the basis for a quantitative model of the resulting suppression. A technique for the calibration of magnetic field strengths using the 34s 1/2 -34p 1/2 one-photon transition is also presented.

It is demonstrated that RF current modulation of a frequency stabilized injection-locked diode laser allows the stabilization of an optical cavity to adjustable lengths, by variation of the RF frequency. This transfer cavity may be used to stabilize another laser at an arbitrary wavelength, in the absence of atomic or molecular transitions suitable for stabilization. Implementation involves equipment and techniques commonly used in laser cooling and trapping laboratories, and does not require electro-or acousto-optic modulators. With this technique we stabilize a transfer cavity using a RF current-modulated diode laser which is injection locked to a 780 nm reference diode laser. The reference laser is stabilized using polarization spectroscopy in a Rb cell. A Ti:sapphire ring laser at 960 nm is locked to this transfer cavity and may be precisely scanned by varying the RF modulation frequency. We demonstrate the suitability of this system for the excitation of laser cooled Rb atoms to Rydberg states.

Resonant energy transfer between cold Rydberg atoms was used to determine Rydberg atom energy levels, at precisions approaching those obtainable in microwave spectroscopy. Laser cooled 85 Rb atoms from a magneto-optical trap were optically excited to 32d 5/2 Rydberg states. The two-atom process 32d 5/2 + 32d 5/2 → 34p 3/2 + 30g is resonant at an electric field of approximately 0.3 V/cm. This process is driven by the electric dipole-dipole interaction, which is allowed due to the partial f character that the g state acquires in an electric field. The experimentally observed resonant field, together with the Stark map calculation is used to make a determination of the 85 Rb ng-series quantum defect: δg(n = 30) = 0.00405(6).

An oscillating electric field at 1.356 GHz was used to promote the resonant energy transfer process: 43d 5/2 +43d 5/2 → 45p 3/2 +41f between translationally cold 85 Rb Rydberg atoms. The ac Stark shifts due to this dressing field created degeneracies between the initial and final two-atom states of this process. The ac field strength was scanned to collect spectra which are analogous to dc electric-fieldinduced resonant energy transfer spectra. Different resonances were observed for different magnetic sublevels involved in the process. Compared to earlier work performed at higher frequencies, the choice of dressing frequency and structure of the spectra may be intuitively understood, by analogy with the dc field case.