Rabi Oscillations and Excitation Trapping in the Coherent Excitation of a Mesoscopic Frozen Rydberg Gas

M. Reetz-Lamour, T. Amthor, J. Deiglmayr, and M. Weidemüller

Phys. Rev. Lett. 100, 253001 – Published 26 June 2008

Abstract

We demonstrate the coherent excitation of a mesoscopic ensemble of about 100 ultracold atoms to Rydberg states by driving Rabi oscillations from the atomic ground state. We employ a dedicated beam shaping and optical pumping scheme to compensate for the small transition matrix element. We study the excitation in a weakly interacting regime and in the regime of strong interactions. When increasing the interaction strength by pair state resonances, we observe an increased excitation rate through coupling to high angular momentum states. This effect is in contrast to the proposed and previously observed interaction-induced suppression of excitation, the so-called dipole blockade.

Received 13 November 2007

DOI:https://doi.org/10.1103/PhysRevLett.100.253001

Authors & Affiliations

M. Reetz-Lamour, T. Amthor, J. Deiglmayr, and M. Weidemüller^*

Physikalisches Institut der Universität Freiburg, Hermann-Herder-Strasse 3, D-79104 Freiburg, Germany

^*weidemueller@physik.uni-freiburg.de

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Vol. 100, Iss. 25 — 27 June 2008

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Images

Figure 1
(a) Relevant level scheme for two-photon excitation of rubidium. (b) Preparation of a mesoscopic subensemble. All atoms are pumped to the $F = 1$ hyperfine component of the ground state. Only a tight tube of atoms is pumped to $F = 2$ whence the excitation originates. Within this tube, a mesoscopic subensemble containing about 100 atoms is excited to Rydberg states with two counterpropagating laser beams at 780 and 480 nm. The latter has a flattop intensity profile shown in the inset, to ensure a constant Rabi frequency.Reuse & Permissions
Figure 2
Rabi oscillations between the $5 S_{1 / 2}$ and $31 D_{5 / 2}$ states of ${}^{87}{Rb}$ . Each dot is an average of measurements over 28 experimental repetition cycles. The solid line shows the simulated excitation probability taking the measured intensity distribution, the residual $5 P_{3 / 2}$ admixture, and our finite laser linewidth into account. The scaling between the detector signal and simulated excitation probability is a free parameter that represents the detector efficiency.Reuse & Permissions
Figure 3
Stark shift of dipole-coupled pair states around the $47 D_{5 / 2} + 47 D_{5 / 2}$ asymptote. The two arrows mark the total electric field at which the two spectra of Fig. 5 are taken. The inset shows a simplified level scheme for this system involving the coherent excitation of an atom pair from $| g D ⟩$ to $| D D ⟩$ with a Rabi frequency $Ω_{R}$ , a laser detuning $δ$ , and an effective linewidth $γ$ . $| D D ⟩$ is dipole coupled to an almost degenerate pair $| P F ⟩$ with an interaction energy $ℏ Ω_{F}$ and a field-dependent energy difference $ℏ Δ (E)$ . The dephasing of the dipole coupling is phenomenologically described by a decay rate $Γ$ (see text).Reuse & Permissions
Figure 4
Temporal evolution of the excitation probability for the $47 D_{5 / 2}$ state at an electric field $E = 160 mV$ . For the smaller density of $4.7 \times 10^{9} {cm}^{- 3}$ (gray dots), we see a damped Rabi oscillation similar to Fig. 2. For the higher density of $10^{10} {cm}^{- 3}$ (black dots), the stimulated emission is suppressed leading to a constantly increasing Rydberg population. The lines are the solution of the model schematically shown in Fig. 3 for weak (dashed line) and strong (solid line) interactions. The excitation probability is derived from the detector signal with the scaling factor determined for Fig. 2 [18].Reuse & Permissions
Figure 5
(a) The $47 D_{5 / 2}$ line at an electric field of $E = 160 mV / cm$ at densities of $10^{10}$ (solid blue line) and $4.7 \times 10^{9} {cm}^{- 3}$ (dashed red line). For the higher density, we observe a line shift and broadening and an increased excitation probability. The smooth lines are the result of model calculations. The line shift is a result of an energy difference between $| D D ⟩$ and $| P F ⟩$ . (b) The same for $E = 260 mV / cm$ . The line shift is reduced to zero, while broadening and increased excitation remain. All spectra are taken by scanning the 480 nm laser (fixed detuning from the intermediate level) with an excitation time of 400 ns.Reuse & Permissions

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