Local Blockade of Rydberg Excitation in an Ultracold Gas

In the laser excitation of ultracold atoms to Rydberg states, we observe a dramatic suppression caused by van der Waals interactions. This behavior is interpreted as a local excitation blockade: Rydberg atoms strongly inhibit excitation of their neighbors. We measure suppression, relative to isolated atom excitation, by up to a factor of 6.4. The dependences of this suppression on both laser irradiance and atomic density are in good agreement with a mean-field model. These results are an important step towards using ultracold Rydberg atoms in quantum information processing.

Figure 1

Comparison of Rydberg excitation for the unblockaded (isolated-atom) $30p$ state and the blockaded $70p$ and $80p$ states at a peak density of $6.5\times {10}^{10}\mathrm{c}{\mathrm{m}}^{\mathrm{-}\mathrm{3}}$. Irradiances are scaled by $(n^{*}/{30}^{*}{)}^3$ to account for the $n$ dependence of the electric dipole transition probability. Insets show the region near the origin with an expanded scale. The dashed line for $n=30$ is a least-squares fit to the data, while the solid curves for $n=70$ and $n=80$ are theoretical predictions, using a single adjustable scaling parameter $\alpha$ (see text). We measure the MOT density profile for each run and correct for its effect on the isolated-atom signals by multiplying the $n=30$ results by 1.29 ($n=70$ inset), 1.19 ($n=80$ inset) and 1.24 (main figure, using the average for $n=70,80$).

Figure 2

Mean-field model of excitation suppression. Within a given domain, any one of the numerous ground-state atoms (small spheres) may be excited into a Rydberg state (light sphere). Because of strong Rydberg-Rydberg vdW interactions, the mean-field energy shift $\epsilon$ depends on the particular location, as does the excitation probability $|c_e{|}^2$. Atoms near the domain center are less shifted and their excitation is more probable than those near the periphery. The graphs correspond to $n=80$ atoms at $\rho =6.5\times {10}^{10}\mathrm{c}{\mathrm{m}}^{\mathrm{-}\mathrm{3}}$ and scaled irradiance $I=0.187\mathrm{M}\mathrm{W}/\mathrm{c}{\mathrm{m}}^{\mathrm{2}}$.

Figure 3

Density dependence of the $n=80$ Rydberg signal. Relative densities are accurate to 5%–10%, but the absolute peak density, nominally $6.5\times {10}^{10}\mathrm{c}{\mathrm{m}}^{\mathrm{-}\mathrm{3}}$ at a relative density of 1.0, is uncertain by as much as a factor of 2. MOT fractions are expressed in terms of the available ground-state population in the selected hyperfine level (see text). The solid curve shows the mean-field theoretical prediction, using the same scaling parameter as for Fig. 1. The inset shows the low-density behavior.