We report on the observation of ultralong range interactions in a gas of cold Rubidium Rydberg atoms. The van-der-Waals interaction between a pair of Rydberg atoms separated as far as 100,000 Bohr radii features two important effects: Spectral broadening of the resonance lines and suppression of excitation with increasing density. The density dependence of these effects is investigated in detail for the S-and P-Rydberg states with main quantum numbers n ∼ 60 and n ∼ 80 excited by narrow-band continuous-wave laser light. The density-dependent suppression of excitation can be interpreted as the onset of an interaction-induced local blockade.PACS numbers: 32.80. Rm,32.80.Pj,34.20.Cf,03.67.Lx With the advances in laser cooling and trapping, new perspectives for the investigation of Rydberg atoms [1] have been opening. When cooled to very low temperatures, the core motion can be neglected for the timescales of excitation ("frozen Rydberg gas"). Unexpected effects have been discovered, such as the many-body diffusion of excitation [2,3], the population of high-angularmomentum states through free charges [4], or the spontaneous formation and recombination of ultracold plasmas [5,6]. Other fascinating features of cold, interacting Rydberg atoms have been proposed but not been observed so far, e.g. the creation of ultralong range molecules [7,8], whereas molecular crossover resonances have already been found experimentally [9]. One outstanding property of Rydberg atoms is their high polarizability, caused by the large distance between the outer electron and the core. This leads to strong electric field sensitivity and strong long-range dipole-dipole and vander-Waals (vdW) interactions are expected. First indications of interaction effects between Rydberg gases at high densities have been found in an atomic beam experiment [10] and, more recently, collisional evidence for ultralong range interactions in a cold Rydberg gas has been reported [11]. In a frozen gas these interactions make Rydberg atoms possible candidates for quantum information processing [12,13]. One promising approach is based on the concept of a dipole blockade [13], i.e. the inhibition of multiple Rydberg excitations in a confined volume due to interaction-induced energy shifts.In this Letter we report on experimental evidence for ultralong range interactions in a frozen Rydberg gas and we present high-resolution spectroscopic signatures of these interactions. citation from a cold gas [14]. Different to these findings, our experiment makes use of a tunable narrow-bandwidth continuous-wave (cw) laser for Rydberg excitation and thus allows for high-resolution spectroscopy of the resonance lines. By varying the density of Rydberg atoms in a controlled way, the influence of interactions on the strength and the shape of these lines is investigated in detail. Signatures of ultralong range interactions appear as spectral broadening of the excitation lines and saturation of the resonance peak height, the latter being an indication of the dipole blockade.To re...
Microwave spectroscopy was used to probe the superfluid-Mott Insulator transition of a Bose-Einstein condensate in a 3D optical lattice. Using density dependent transition frequency shifts we were able to spectroscopically distinguish sites with different occupation numbers, and to directly image sites with occupation number n = 1 to n = 5 revealing the shell structure of the Mott Insulator phase. We use this spectroscopy to determine the onsite interaction and lifetime for individual shells.The Mott insulator transition is a paradigm of condensed matter physics. It describes how electron correlations can lead to insulating behavior even for partially filled conduction bands.However, this behavior requires a commensurable ratio between electrons and sites. If this condition on the density is not exactly fulfilled, the system will still be conductive. For neutral bosonic particles, the equivalent phenomenon is the transition from a superfluid to an insulator for commensurable densities. In inhomogeneous systems, as in atom traps, the condition 1
The stability of superfluid currents in a system of ultracold bosons was studied using a moving optical lattice. Superfluid currents in a very weak lattice become unstable when their momentum exceeds 0.5 recoil momentum. Superfluidity vanishes already for zero momentum as the lattice deep reaches the Mott insulator (MI) phase transition. We study the phase diagram for the disappearance of superfluidity as a function of momentum and lattice depth between these two limits. Our phase boundary extrapolates to the critical lattice depth for the superfluid-to-MI transition with 2% precision. When a one-dimensional gas was loaded into a moving optical lattice a sudden broadening of the transition between stable and unstable phases was observed.
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