Quantifying the impact of state mixing on the Rydberg excitation blockade

The Rydberg excitation blockade has been at the heart of an impressive array of recent achievements; however, state-mixing interactions can compromise its efficiency. When ultracold atoms are excited to Rydberg states near Förster resonance, up to $\sim 50\%$ of atoms can be found in dipole coupled product states within tens of ns after excitation. There has been disagreement in the literature regarding the mechanism by which this mixing occurs. We use state-selective field ionization spectroscopy to measure, on a shot-by-shot basis, the distribution of Rydberg states populated during narrow-band laser excitation. Our method allows us to both determine the number of additional Rydberg excitations added by each mixing event, and to quantify the extent to which state mixing “breaks” the blockade. For excitation of ultracold rubidium atoms to $n{D}_{5/2}$ states, we find that the mixing is consistent with a three-body process, except near exact Förster resonance.

Figure 1

(a) Average SSFI spectrum for excitation to $43D_{5/2}$, the T (green) and P (red) gates which have been offset and scaled for clarity, and the SSFI ramp. (b) An example “sorted” graph: the total number of excitations ($N_T$) as a function of the number in $(n+2)P$ ($N_P$). The false color shows how many of each $\{N_P,N_T\}$ were detected. The green line is a fit, from which we extract the slope and intercept.

Figure 2

Top panels: Slopes of the fits to the sorted data as a function of ground-state atom density. The error bars are the uncertainties in the fitted slopes. Horizontal lines are the results of a Monte Carlo model. Red solid line: two-body model; blue dashed line: three-body model. Bottom panels: Mandel $Q$ parameter for each data set. For $n=41–43$, the horizontal lines represent the average $Q$ value. For $n=44$ and 45, the horizonal lines represent the average $Q$ value for low and high densities, respectively.

Figure 3

Pair potential curves for two-atom $|n{D}_{5/2},m_j=3/2\rangle +|n{D}_{5/2},m_j=5/2\rangle$ states for (a) $n=43$ and (b) $n=44$. The angle between the atomic dipoles is zero. The weight of each curve indicates the overlap with the unperturbed state.

Figure 4

(a) and (c) Total number of excitations (black squares) and intercepts of the sorted graphs (red circles), as a function of density for $41D_{5/2}$ and as a function of $n$ at high density, respectively. (b) Fraction of the averaged SSFI signal in $(n-2)F_{7/2}$ and $(n+2)P_{3/2}$ states as a function of principal quantum number at high density. (d) The average number of excitations “added” by state mixing events.