C. S. Hofmann

Observing the Dynamics of Dipole-Mediated Energy Transport by Interaction-Enhanced Imaging

Imaging Excitations Complex processes such as chemical reactions and photosynthesis involve the transport of energy. The mechanisms of how the energy migrates, the influence of the surrounding environment, or the extent to which quantum mechanics affects the process remain unclear. Günter et al. (p. 954, published online 7 November; see the Perspective by Donley) found that a cloud of cold atoms suitably prepared and decorated with “impurity” Rydberg atoms could be used to image the transport of excitations between excited Rydberg atoms directly. This ability to tune the influence of the background environment may help in the study of the coherent transport of energy in complex many-body systems. An imaging technique based on a cloud of cold atoms provides a model system to study the coherent transport of energy. [Also see Perspective by Donley] Electronically highly excited (Rydberg) atoms experience quantum state–changing interactions similar to Förster processes found in complex molecules, offering a model system to study the nature of dipole-mediated energy transport under the influence of a controlled environment. We demonstrate a nondestructive imaging method to monitor the migration of electronic excitations with high time and spatial resolution, using electromagnetically induced transparency on a background gas acting as an amplifier. The continuous spatial projection of the electronic quantum state under observation determines the many-body dynamics of the energy transport.

Coherent population trapping with controlled interparticle interactions

Coherent population trapping (CPT) and the related phenomenon of Electromagnetically Induced Transparency (EIT) are paradigms for quantuminterference effects and have been studied intensely over the last decade [1]. Only recently, EIT involving Rydberg states has attracted much interest, with regard to nonlinear optics [2] as well as in the context of quantum information processing [3]. Whereas CPT and EIT are generally described within a singleatom framework, the situation becomes more involved when interparticle interactions have to be considered. To address the effect of interactions, we investigate CPT in a strongly interacting ultracold Rydberg gas [4]. In our experiment we tune the interaction strength by choosing different Rydberg states and control interactions effects by varying the ground state atom density. Even in the blockade regime we observe a resonance with sub-natural linewidth at the single-particle resonance frequency despite the strong van der Waals interactions among Rydberg atoms. Due to the correlations among the atoms the experimental observations cannot be explained within a meanfield model. A theoretical model that includes interparticle correlations is presented and nicely reproduces the observed features.

Evidence of Antiblockade in an Ultracold Rydberg Gas

We present the experimental observation of the antiblockade in an ultracold Rydberg gas recently proposed by Ates et al. [Phys. Rev. Lett. 98, 023002 (2007)]. Our approach allows the control of the pair distribution in the gas and is based on a strong coupling of one transition in an atomic three-level system, while introducing specific detunings of the other transition. When the coupling energy matches the interaction energy of the Rydberg long-range interactions, the otherwise blocked excitation of close pairs becomes possible. A time-resolved spectroscopic measurement of the Penning ionization signal is used to identify slight variations in the Rydberg pair distribution of a random arrangement of atoms. A model based on a pair interaction Hamiltonian is presented which well reproduces our experimental observations and allows one to deduce the distribution of nearest-neighbor distances.

Full counting statistics of laser excited Rydberg aggregates in a one-dimensional geometry

We experimentally study the full counting statistics of few-body Rydberg aggregates excited from a quasi-one-dimensional atomic gas. We measure asymmetric excitation spectra and increased second and third order statistical moments of the Rydberg number distribution, from which we determine the average aggregate size. Estimating rates for different excitation processes we conclude that the aggregates grow sequentially around an initial grain. Direct comparison with numerical simulations confirms this conclusion and reveals the presence of liquidlike spatial correlations. Our findings demonstrate the importance of dephasing in strongly correlated Rydberg gases and introduce a way to study spatial correlations in interacting many-body quantum systems without imaging.

Interaction enhanced imaging of individual Rydberg atoms in dense gases.

We propose a new all-optical method to image individual Rydberg atoms embedded within dense gases of ground state atoms. The scheme exploits interaction-induced shifts on highly polarizable excited states of probe atoms, which can be spatially resolved via an electromagnetically induced transparency resonance. Using a realistic model, we show that it is possible to image individual Rydberg atoms with enhanced sensitivity and high resolution despite photon-shot noise and atomic density fluctuations. This new imaging scheme could be extended to other impurities such as ions, and is ideally suited to equilibrium and dynamical studies of complex many-body phenomena involving strongly interacting particles. As an example we study blockade effects and correlations in the distribution of Rydberg atoms optically excited from a dense gas.

Spontaneous Avalanche Ionization of a Strongly Blockaded Rydberg Gas

We report the sudden and spontaneous evolution of an initially correlated gas of repulsively interacting Rydberg atoms to an ultracold plasma. Under continuous laser coupling we create a Rydberg ensemble in the strong blockade regime, which at longer times undergoes an ionization avalanche. By combining optical imaging and ion detection, we access the full information on the dynamical evolution of the system, including the rapid increase in the number of ions and a sudden depletion of the Rydberg and ground state densities. Rydberg-Rydberg interactions are observed to strongly affect the dynamics of plasma formation. Using a coupled rate-equation model to describe our data, we extract the average energy of electrons trapped in the plasma, and an effective cross section for ionizing collisions between Rydberg atoms and atoms in low-lying states. Our results suggest that the initial correlations of the Rydberg ensemble should persist through the avalanche. This would provide the means to overcome disorder-induced heating, and offer a route to enter new strongly coupled regimes.

Quantum interference in interacting three-level Rydberg gases : coherent population trapping and electromagnetically induced transparency

In this paper, we consider the effects of strong dipole-dipole interactions on three-level interference phenomena such as coherent population trapping and electromagnetically induced transparency. Experiments are performed on laser cooled rubidium atoms and the results compared to a many-body theory based on either a reduced many-body density matrix expansion or Monte Carlo simulations of many-body rate equations. We show that these approaches permit quantitative predictions of the experimentally observed excitation and transmission spectra. Based on the calculations, we moreover predict a universal scaling of the nonlinear response of cold Rydberg gases.

Sub-Poissonian statistics of Rydberg-interacting dark-state polaritons

We observe individual dark-state polaritons as they propagate through an ultracold atomic gas involving Rydberg states coupled via an electromagnetically induced transparency resonance. Strong long-range interactions between Rydberg excitations give rise to a blockade between polaritons, resulting in large optical nonlinearities and modified polariton number statistics. By combining optical imaging and high-fidelity detection of the Rydberg polaritons we investigate both aspects of this coupled atom-light system. We map out the full nonlinear optical response as a function of atomic density and follow the temporal evolution of polaritons through the atomic cloud. In the blockade regime, the statistical fluctuations of the polariton number drop well below the quantum noise limit. The low level of fluctuations indicates that photon correlations modified by the strong interactions have a significant backaction on the Rydberg atom statistics.

Versatile cold atom target apparatus.

We report on a compact and transportable apparatus that consists of a cold atomic target at the center of a high resolution recoil ion momentum spectrometer. Cold rubidium atoms serve as a target which can be operated in three different modes: in continuous mode, consisting of a cold atom beam generated by a two-dimensional magneto-optical trap, in normal mode in which the atoms from the beam are trapped in a three-dimensional magneto-optical trap (3D MOT), and in high density mode in which the 3D MOT is operated in dark spontaneous optical trap configuration. The targets are characterized using photoionization.

High-precision semiconductor wavelength sensor based on a double-layer photo diode

We present a wavelength sensor setup for monochromatic visible light, based on the double-layer photo diode WS-7.56. Employing high-precision electronics and automatic compensation of different error sources, we achieve a measurement accuracy of ±0.025 nm with a resolution below 0.01 nm. The described apparatus is particularly suited for the determination of small laser frequency deviations in atomic physics experiments. Various design issues as well as error sources and diode characteristics are discussed.