Mechanical Effect of van der Waals Interactions Observed in Real Time in an Ultracold Rydberg Gas

Abstract

We present time-resolved spectroscopic measurements of Rydberg-Rydberg interactions in an ultracold gas, revealing the pair dynamics induced by long-range van der Waals interactions between the atoms. By detuning the excitation laser, a specific pair distribution is prepared. Penning ionization on a microsecond timescale serves as a probe for the pair dynamics under the influence of the attractive long-range forces. Comparison with a Monte Carlo model not only explains all spectroscopic features but also gives quantitative information about the interaction potentials. The results imply that the interaction-induced ionization rate can be influenced by the excitation laser. Surprisingly, interaction-induced ionization is also observed for Rydberg states with purely repulsive interactions.PACS numbers: 32.80. Rm, 34.20.Cf, 34.10.+x, 34.60.+z Long-range dipolar interactions ubiquitously appear in nature as the cause for binding forces ranging from atomic and molecular gases [1] all the way to large biological systems [2]. Rydberg atoms have attracted much interest in this context, as they represent an ideal system to study quantum dynamics under the influence of dipolar interactions. As a prominent example from cavity quantum electrodynamics, attractive forces between Rydberg atoms and conducting surfaces have been observed as level shifts in atomic beam experiments [3]. RydbergRydberg interactions leading to resonant energy transfer have been studied in thermal beam experiments [4] and later in ultracold Rydberg gases [5,6,7]. The longrange dipolar interactions can be used to block multiple Rydberg excitation [8,9] and to create many-particle entangled states, which may be employed for quantum information processing [10,11]. So far, investigations of Rydberg-Rydberg interactions have mainly focused on the electronic degrees of freedom neglecting the center-ofmass motion ("frozen Rydberg gas" [7]). In this Letter, we present real-time measurements of the motion of interacting pairs of Rydberg atoms revealing the character and strength of the long-range interparticle interactions.In most molecular and atomic systems the relevant timescales of interparticle dynamics are in the sub-ns regime calling for very fast probes. Ultracold atoms bridge to the ns regime as the thermal energy (T <1 mK) is negligible which allows one to study systems with much weaker interactions or large interatomic distances. Due to the negligible kinetic energy, the dynamics of the gas is fully determined by the interatomic interactions. Interactions in a cold atomic sample have been studied time-resolved in the case of ground state atoms using a pump-probe scheme [12] allowing for coherent control of the collision process [13]. Evidence for interactioninduced motion in cold Rydberg gases was recently found spectroscopically as the cause for Penning ionization [14]. By combining time-resolved and spectroscopic measure- ments, we quantitatively examine the van der Waals (vdW) interactions between two Rydberg atoms. Our measurements can be dir...

Imaging the evolution of an ultracold strontium Rydberg gas

“…This is quantified in Fig. 6 Rydberg atoms with repulsive interactions are known to Penning ionize and produce plasma [68,72,73], but at a much slower rate than for attractive potentials [73]. However, because this growth is slow, other processes must be considered that might contribute to the growth in the number of visible ion cores after 1 D 2 excitations, such as population redistribution to states with attractive potentials by blackbody radiation (BBR) and BBR-induced photoionization.…”

Optical non-linearity in a dynamical Rydberg gas

“…However, even at 20 µK the motional effects from the attractive interactions can be significant on timescales of tens of microseconds [19,20]. Figure 1 (b) shows the dipole-dipole interaction potential V (R) for the 58D 5/2 pair state as a function of interatomic separation.…”

Three-Body Bound States in Dipole-Dipole Interacting Rydberg Atoms

“…Second, advanced imaging techniques [46,47] allow one to observe spatially resolved patterns of Rydberg excitations. While Rydberg atoms in attractive or repulsive three-atom states will quickly leave their initial spatial configuration [35][36][37][38], stable trimer configurations will stay put. Thus the direct observation of the triangular configuration of Rydberg atoms after a time delay following their excitation would provide further evidence for the preparation of a trimer state.…”