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

We present time-resolved spectroscopic measurements of Rydberg-Rydberg interactions between two Rydberg atoms 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 time scale 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.

Figure 1

Excitation and interaction-induced ionization of a pair of atoms on an attractive vdW potential. The atoms are initially excited to state $|r⟩$ at a distance $R_0$ by a slightly red-detuned laser. After excitation they are accelerated towards each other and collide, leading to ionization of one of the atoms. The region where this Penning ionization can occur is indicated in gray. The calculated number of pairs excited at specific distances is shown in the lower graph for different detunings.

Figure 2

(a) $60D_{5/2}$ Rydberg excitation line (solid), with fitted Lorentzian (dotted), (b) development of the ion signal for different interaction times (solid) compared to Monte Carlo simulation (dotted). No ions were detected directly after excitation ($\Delta t=0$). The laser frequency is given relative to the atomic resonance.

Figure 3

Signal growth at different detunings: 0 MHz (solid), $-6\mathrm{MHz}$ (triangles, dotted), $+6\mathrm{MHz}$ (open circles, dashed). Upper graph: measured data (data points represent bins over 3 MHz), lower graph: model. The scale of the measured data contains a systematic error of a factor of 2 due to different MCP efficiencies for the ion and Rydberg signals.

Figure 4

$60S$ Rydberg excitation spectrum (upper graph) and preionized atoms after $25\mu \mathrm{s}$ interaction time (lower graph). The ions are produced on the blue-detuned side of the resonance. Excitation time is 300 ns, and excitation laser power 22 mW. The graphs have different signal height scales. The fit in the upper graph (dotted) yields a line width of 11.3 MHz, similar to the $60D_{5/2}$ line.