Session G8: Quantum Gases with Rydberg Atoms

Absence of Collective Decay in a Cold Rydberg Gas

We have studied the decay of Rydberg excitations in a cold Rb gas. A 10 ns, pulsed dye-amplified diode laser drives Rb 5$p$ atoms at 70 $\mu$K in a MOT to $ns$ or $np$ Rydberg states. Excitation of $np$ states is facilitated by Stark mixing in a small static electric field. Time-delayed state-selective field ionization (SSFI) is used to directly monitor the population in the initial and neighboring Rydberg levels. We find that the time-dependence of the Rydberg population is well described by numerical simulations which consider only spontaneous emission and stimulated emission and absorption of black-body radiation. No signature of collective decay phenomena is observed. In contrast, previous studies (T. Wang et al., Phys. Rev. A 75, 033802 (2007)) performed at similar atom density and laser focal volume conditions, but at slightly higher principal quantum number and without state resolved population detection, reported evidence of very rapid Rydberg decay that was attributed to superradiance.   

Rydberg Blockade Effects on Autler-Townes Spectra in a Dense Gas of $^{84}$Sr

We study two-photon spectroscopy of the $5s^2$ $^1S_0 - 5s5p ^{3}P_1 - ns ^{3}S_1$ transition for $n = 24$ in the Autler-Townes regime in an ultracold gas of $^{84}$Sr. These studies are performed on thermal samples ($\sim10^{12} cm^{-3}$) and on a Bose-Einstein condensate (BEC) ($\sim 5 \times 10^{13} cm^{-3}$). Both cases exhibit spectra that are significantly modified from the usual Autler-Townes picture. In the case of thermal atoms, we observe a broadening and shift of the two loss features consistent with Rydberg-Rydberg interactions. However in the case of a BEC, a third feature develops between the two peaks due to light scatter from the $5s^2$ $^1S_0 - 5s5p^3P_1$ transition for heavily blockaded atoms. Implications of these effects for the creation of Rydberg dressed BEC will be discussed. This research was supported by the AFOSR under grant no. FA9550-12-1-0267, the NSF under grants nos. 1301773 and 1205946, and the Robert A. Welch Foundation under grants nos. C-0734 and C-1844.   

Mesoscopic Rydberg-blockaded ensembles in the superatom regime and beyond

We report on the controlled creation and characterization of an isolated mesoscopic superatom by means of accurate density engineering and excitation to Rydberg P-states. \footnote{T.M.~Weber, M.~H\"oning, T.~Niederpr\"um, T.~Manthey, O. Thomas, M. Fleischhauer, G. Barontini and H.~Ott, \textbf{Nature Physics} Mesoscopic Rydberg-blockaded ensembles in the superatom regime and beyond, doi:10.1038/nphys3214} By monitoring continuous laser-induced ionization we are able to determine the $g^2(\tau)$ correlation function and observe anti bunching for resonant excitation, as well as bunching for off resonant coupling. We further investigate the transition from a strongly confined effective two-level to an extended many body system by manipulation of the spatial dimension of the atomic sample with the help of a focused electron beam. The observed amplitudes and timescales can be described with an effective rate-equation model.   

Effect of random positions for coherent dipole transport

We calculate the effect of two kinds of randomness on the coherent motion of an exciton whose transport is governed by the dipole-dipole interaction. As our example, we use the idealized case of stationary Rydberg atoms on a lattice. We present calculations for how fast the excitation can move away from its starting position for different dimensional lattices and for different levels of randomness. We also examine the asymptotic in time final position of the excitation to determine whether or not the excitation can be localized. The one-dimensional system is an example of Anderson localization where the randomness is in the off-diagonal elements although the long-range nature of the interaction leads to nonexponential decay with distance. The two-dimensional square lattice shows a mixture of extended and localized states for large randomness, while there is no visible sign of localized states for weak randomness. The three-dimensional cubic lattice has few localized states even for strong randomness.   

Ultracold chemistry of a single Rydberg atom in a BEC

A single Rydberg excitation in the high density and low temperature environment of a Bose-Einstein condensate (BEC) leads to a fascinating testbed of low-energy electron-neutral and ion-neutral scattering.~~For a Rydberg state with a principal quantum number of 100, there are thousands of ground-state atoms with which the Rydberg electron interacts.~ In a BEC the interparticle spacing is at approximately the same length scale as the~Langevin impact parameter, making it possible to study the effect of ion-neutral collisions on time scales much faster than the Rydberg lifetime.~~Collisions between the Rydberg electron and the ground state atoms cause a mean field density shift of the Rydberg line.~~We present results on how this effect can be used to monitor phase transitions of the BEC and probe thin density shells of the BEC to monitor density-dependent, ultracold chemical reactions.~~We report on experimental findings, of Rydberg state-changing collisions on~$\mu $s timescales, due to collisions of the Rydberg ionic core with neutral ground state atoms.~~We compare our findings to simulations based on classical trajectory calculations for the motion of the ionic core and neutral atoms, whereas the dynamics of the electron is treated quantum mechanically.   

Rydberg Electrons in a Bose-Einstein Condensate

We investigate a hybrid system composed of ultracold Rydberg atoms immersed in an atomic Bose-Einstein condensate (BEC). The coupling between the Rydberg electrons and BEC atoms leads to the excitation of phonons, the exchange of which induces Yukawa interaction between Rydberg atoms. The range of such interaction is equal to the healing length of the BEC, which can be tuned by adjusting the scattering length of the BEC atoms. For a BEC with a small healing length, the Yukawa potential is short-ranged, and distorts the BEC locally, ``mapping'' the electron density onto the BEC density. For large healing lengths, the Yukawa potential is long-ranged and can bind Rydberg atoms and form a new type of ultra-long-range molecule.   

Study of Rydberg lifetimes in BEC

Recent experiments are probing the behavior of Rydberg atoms immersed in an atomic Bose-Einstein condensate (BEC). One of the surprising result is the large shortening of the lifetime of the Rydberg state, by orders of magnitude compared to Rydberg atoms in vacuum. In this presentation, we explore possible processes that might explain these observations. In particular, we investigate the $\ell$-mixing arising from the electron scattering with many perturbers, and reactions involving the positive Rydberg core. We will compare our results to the experimental values.   

Decoherence and absorption spectra of impurities in ultracold quantum gases

We study the finite-mass corrections to the non-equilibrium dynamics of an impurity in a gas of lattice fermions after an interaction quench. Our emphasize is on the question under which conditions a heavy impurity immersed in a Fermi liquid is subject to the orthogonality catastrophe and complete loss of coherence. We calculate the time-dependent Ramsey interference signal using a novel hybrid approach which combines path integral and functional determinant methods. We find that a finite quasiparticle peak persists in d ? 2 in agreement with previous work and explore the influence of finite temperature and system size. Using our microscopic approach we study also the time evolution of a Bose-Einstein-Condensate after the excitation of a Rydberg atom and the formation of a sequence of giant molecular bound states. Our calculation reveals the emergence of a novel type of orthogonality catastrophe present in coupled BEC-Rydberg systems. We determine the absorption spectra of the system and we predict the non-equilibrium time evolution of various experimentally measurable observables following the sudden excitation of the Rydberg state and discuss possible experimental implementations.   

Nonclassical millihertz linewidth light source with Rydberg polaritons

A superradiant laser could be used to generate light with extremely high spectral purity, with important applications to quantum information and precision measurements. These applications can be further extended when the photon statistics becomes nonclassical. Here we propose a practical way to generate arbitrarily strong nonlinearity in a superradiant laser by introducing a Rydberg gas into the cavity. The output photons can be made perfectly anti-bunched, while inheriting the millihertz linewidth of the lasing atoms. Our method can also be used to generate a photon blockade in various cavity QED setups, and has implications for the simulation of quantum many body physics with photons.   

Single-Photon Transistor Using a F\"{o}rster Resonance

An all-optical transistor is a device in which a gate light pulse switches the transmission of a target light pulse with a gain above unity. The gain quantifies the change of the transmitted target photon number per incoming gate photon. We study the quantum limit of one incoming gate photon and observe a gain of 20 [1]. The gate pulse is stored as a Rydberg excitation in an ultracold gas. The transmission of the subsequent target pulse is suppressed by Rydberg blockade which is enhanced by a F\"{o}rster resonance. The detected target photons reveal in a single shot with a fidelity above 0.86 whether a Rydberg excitation was created during the gate pulse. The gain offers the possibility to distribute the transistor output to the inputs of many transistors, thus making complex computational tasks possible.\\[4pt] [1] D. Tiarks et al. PRL 113, 053602 (2014)