Session R2: Rydberg Gases

Generation of localized ``Bohr-like'' wavepackets in near-circular orbit about the nucleus

Atoms in high-lying (n $\sim $ 300) Rydberg states provide a valuable laboratory in which to explore the engineering of electronic wavefunctions using carefully-tailored sequences of short electric field pulses whose characteristic times (duration and/or rise/fall times) are less than the classical electron orbital period. The level of control that can be exercised is illustrated with reference to the generation of localized wavepackets in ``Bohr-like'' near circular orbits. While such wavepackets slowly dephase and undergo dispersion, their localization can be maintained for extended periods (many hundreds of orbits) through external driving using a periodic train of pulses. The wavepackets can be further manipulated by slowly varying, or ``chirping,'' the pulse repetition frequency. The physics underlying these control protocols is explained using classical trajectory Monte Carlo simulations. Even in the absence of external driving, however, wavepacket relocalization is expected at late times due to quantum revivals. The observation of such relocalization is described and demonstrates that quantum phenomena can be seen even in mesoscopic very-high-n atoms. Research undertaken in collaboration with J. J. Mestayer, B. Wyker, C. O. Reinhold, S. Yoshida and J. Burgd\"{o}rfer.   

Deceleration and electrostatic trapping of hydrogen Rydberg molecules

Recent progress in the development of methods by which to decelerate and manipulate the translational motion of Rydberg atoms in the gas phase using static and time-varying inhomogeneous electric fields~[1] has led to the experimental realization of Rydberg atom optics elements including a lens~[2], a mirror~[3] and two- and three-dimensional traps~[4,5]. These experiments exploit the very large electric dipole moments associated with Rydberg Stark states, and have demonstrated the possibility to stop a seeded, pulsed, supersonic beam of atomic hydrogen traveling with an initial velocity of 700~ms$^{-1}$ within 2~mm and only $\sim 5~\mu$s using electric fields of a few kVcm$^{-1}$. We have now extended these techniques to manipulate the translational motion of molecular hydrogen, for applications in precision spectroscopy and in studies of molecular collisions at low temperature or with a high degree of control over collision energies. The results of recent experiments in which we have been able to load hydrogen Rydberg molecules into a three-dimensional electrostatic traps will be summarized. These experiments have relied upon the preparation of nonpenetrating ($\ell\geq3$) Rydberg-Stark states, with principal quantum number in the range $n=20-30$, using circularly polarized laser radiation. The rate of decay of these states in the trap has been determined providing, for the first time, experimental information on the predissociation of nonpenetrating molecular Rydberg states.\\[4pt] [1] S. R. Procter et al., \emph{Chem. Phys. Lett.}, \textbf{374}, 667 (2003).\\[0pt] [2] E. Vliegen et al., \emph{Eur. Phys. J. D}, \textbf{40}, 73 (2006).\\[0pt] [3] E. Vliegen and F. Merkt, \emph{Phys. Rev. Lett.}, \textbf{97}, 033002 (2006).\\[0pt] [4] E. Vliegen et al., \emph{Phys. Rev. A}, \textbf{76}, 023405 (2007).\\[0pt] [5] S. D. Hogan and F. Merkt, \emph{Phys. Rev. Lett.}, \textbf{100}, 043001 (2008).   

Dipole-dipole interaction between cold Rydberg atoms in RF fields

Already for some time, dipole-dipole interaction between cold Rydberg atoms is promising as application in quantum information. This promise drives the research on this interaction. In this context we report two aspects: \newline \newline 1) Coherence. When a quantum-mechanical state energetically shifts under the influence of some external perturbation, the effect of a periodic perturbation will show up as side bands. When this shift is linear (e.g. a dipole in an electric field) the population of this side band is described by Bessel-functions. Here we treat the situation that the shift of the state is quadratic under an external perturbation (a polarizable state in an electric field). Now not only the population of the sidebands is a (known but complicated) function of the perturbation, but also the energy of the state and its sidebands is no longer constant. Both the energy and the population of the sidebands are probed by introduction a second state, which is insensitive for the perturbation and that is weakly couple to the first state. This weak coupling is dipole-dipole interaction between two Rydberg atoms. This coupling leads to an avoided crossing between the externally perturbed and the unperturbed states. This particular realization allows for an alternative interpretation of the population of sidebands, in particular of the minima, in terms of St\"uckelberg oscillations. These oscillations are measured in the sidebands of the resonant interaction between Rb 49s and 41d states with 49p and 42p states and are used to obtain information about the coherent nature of the interaction. \newline \newline 2) Surfaces. In addition we report on the effect of a conducting surface nearby a Rydberg atoms. This issue is particularly relevant in case the conducting surface is an atom chip. The Rydberg atoms are probed by means of electromagnetically induced transparency (EIT) induced in Rb 5s-5p-Rydberg ladder schemes.   

Simulations of a strongly interacting Rydberg gas

We have performed simulations of a gas of Rydberg atoms where the atom-atom interaction is strong enough to generate interesting correlations between atoms. Results will be presented on different aspects of this system. As an example, we simplified the Rydberg-Rydberg interaction so that the interaction was so strong that the probability for finding pairs of Rydberg atoms was 0 for distances less than R and the interaction between atoms was 0 for distances greater than R. We found correlation between atoms separated by several multiples of R even though there was no direct interaction between the atoms, but that the correlation appeared to be classical. In calculations with realistic Rydberg-Rydberg interactions, we found that the line width of photo-excitation from the ground state into the Rydberg state more strongly depended on density fluctuations in the gas than on diffusion of the Rydberg excitation; this result is in contrast to previous interpretations of experiments. We also performed calculations where the laser transition from ground to Rydberg state mimics a spin-echo arrangement.