Spontaneous Avalanche Ionization of a Strongly Blockaded Rydberg Gas

We report the sudden and spontaneous evolution of an initially correlated gas of repulsively interacting Rydberg atoms to an ultracold plasma. Under continuous laser coupling we create a Rydberg ensemble in the strong blockade regime, which at longer times undergoes an ionization avalanche. By combining optical imaging and ion detection, we access the full information on the dynamical evolution of the system, including the rapid increase in the number of ions and a sudden depletion of the Rydberg and ground state densities. Rydberg-Rydberg interactions are observed to strongly affect the dynamics of plasma formation. Using a coupled rate-equation model to describe our data, we extract the average energy of electrons trapped in the plasma, and an effective cross section for ionizing collisions between Rydberg atoms and atoms in low-lying states. Our results suggest that the initial correlations of the Rydberg ensemble should persist through the avalanche. This would provide the means to overcome disorder-induced heating, and offer a route to enter new strongly coupled regimes.

Figure 1

Generation of an ultracold plasma from a Rydberg-blockaded gas. (a) An initially uncorrelated gas of ultracold atoms is prepared in an optical dipole trap (I). By coherent laser coupling we excite some atoms to a Rydberg state. The strong Rydberg-Rydberg interactions prevent the excitation of close-by pairs, leading to spatial correlations (II). After a time $t_{\mathrm{crit}}$ the Rydberg gas is observed to spontaneously ionize into an ultracold plasma (III). (b) Qualitative dynamics of the involved populations: ground state atoms $N_g$ (solid line), Rydberg atoms $N_r$ (dashed line), and ions $N_i$ (dash-dotted line).

Figure 2

Density dependence of the Rydberg number for short times. The atoms in $|55S⟩$ are field ionized after $3.7\mu \mathrm{s}$ of continuous laser coupling. Strong interactions lead to a Rydberg excitation suppression for increasing ground state densities, in contrast to the interaction-free case (dashed line). The data are in good agreement with a hard sphere model (solid line) with a sphere diameter corresponding to the blockade radius $R_c\approx 5\mu \mathrm{m}$. The inset shows the spatial correlation function typical for a Rydberg blockaded gas [18].

Figure 3

Measurement of the complete avalanche ionization dynamics for $N_g$, $N_r$, and $N_i$. (a) Absorption images taken after different durations of continuous laser coupling on a cloud of initial density $n_0=2.8\times {10}^{10}{\mathrm{cm}}^{-3}$. (b) Number of ground-state atoms $N_g$ in the excitation volume. The data are compared to a rate equation model (see text), accounting for (solid line) or neglecting (dashed line) the Rydberg blockade mechanism. Vertical lines highlight the avalanche onset time. (c) Simultaneously recorded Rydberg atom population $N_r$ (circles) and ion population $N_i$ (triangles).

Figure 4

Density dependence of the ionization avalanche onset time $t_{\mathrm{crit}}$ (log-log scale). Dark and light symbols correspond to experiments with excitation volumes $V$ and $V_{\mathrm{reduced}}\approx V/2$, respectively. Circles and triangles correspond to the $|55S_{1/2}⟩$ and $|40S_{1/2}⟩$ states, respectively. Continuous lines are the model predictions for $|55S_{1/2}⟩$ for both volumes, using the same parameters fitted on experiments in $V$. The dashed line shows the model outcome in $V$ when neglecting the blockade effect.