COHERENCE:
Cooperativity in Highly Excited Rydberg
Ensembles — Control and Entanglement
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SA I — Many body phenomena and coherence

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Full counting statistics and phase diagram of a dissipative Rydberg gasJune 2013
Pisa, Italy

Dynamical phase transitions are expected to occur in strongly interacting ultra-cold gases under dissipative conditions. Among other characteristics, these systems exhibit intermittency and multi-modal counting distributions. In our experiment we realize a strongly interacting dissipative gas of rubidium Rydberg atoms and measure its phase diagram. Through the analysis of the full counting statistics we show that the regions of strongly super-Poissonian fluctuations in that phase diagram correspond to bimodal counting-distributions compatible with intermittency due to phase coexistence.


Spatial tomography of Rydberg excitations in a MOTJune 2013
Pisa, Italy

A hallmark of the strong interactions between Rydberg atoms is the dipole blockade effect, which prevents the excitation of two atoms to the Rydberg state if the distance between them is smaller than the blockade radius. In order to visualize this effect we scan the excitation lasers across the MOT and study the dependence on the local density both of the number of Rydberg atoms detected and the number ions created by two-photon process via the intermediate 6P level. As a consequence of the supression of Rydberg excitation in the high density regions of the MOT, the spatial excitation profile curve appears broader in the case of the Rydberg atoms. We also measure the dynamics of the excitation whose Rabi frequency is proportional to the square root of the atomic density at the beam position.


Sub-Poissonian statistics of interacting dark-state polaritonsMay 2013
Heidelberg, Germany

Electromagnetically-induced transparency (EIT) and the associated appearance of hybrid quasi-particles (dark-state polaritons) in ultracold Rydberg gases opens the intriguing perspective to create atom-light interfaces operating at the quantum level. For the first time we give a complete picture of interacting dark state polaritons by probing both the photonic and atomic degrees of freedom in a single experiment. This field has recently become a hot topic with several groups worldwide recently showing Rydberg EIT can be used to generate non-classical photon correlations. Using our complementary approach, we show that interactions between dark-state polaritons also result in non-classical statistics for the polariton number distribution.

Reference:
C. S. Hofmann, G. Günter, H. Schempp, M. Robert-de-Saint-Vincent, M. Gärttner, J. Evers, S. Whitlock and M. Weidemüller, Sub-Poissonian statistics of Rydberg-interacting dark-state polaritons, Phys. Rev. Lett. 110, 203601 (2013), or see our full list of publications
Spontaneous avalanche ionization of a blockaded Rydberg gasJanuary 2013
Heidelberg, Germany

We have observed the sudden and spontaneous evolution of an initially correlated gas of repulsively interacting Rydberg atoms to an ultracold plasma. By combining optical imaging and ion detection, we access the full information on the dynamical evolution of the system. The Rydberg blockade effect introduces correlations between the particles and strongly affects the dynamics of plasma formation, which may provide a route to enter new strongly-coupled regimes.

Reference:
M. Robert-de-Saint-Vincent, C. S. Hofmann, H. Schempp, G. Günter, S. Whitlock and M. Weidemüller, Spontaneous avalanche ionization of a strongly blockaded Rydberg gas, Phys. Rev. Lett. 110, 045004 (2013), or see our full list of publications
Interacting Fibonacci anyons in a Rydberg gasOctober 2012
Nottingham, UK

Fibonacci anyons are particles which neither obey fermionic nor bosonic exchange statistics. These non-Abelian anyons are conjectured to occur as quasiparticles in certain fractional quantum Hall states. In this paper we have shown that a Rydberg lattice gas constitutes an analogue quantum simulator for Fibonacci anyons. The underlying insight is that the Rydberg blockade and the resulting exclusion principle direct links to the so-called fusion paths that determine the nature of an ensemble of Fibonacci anyons. By this one can show that the Hilbert space structure of a Rydberg lattice gas is equivalent to that of an ensemble of Fibonacci anyons. Interactions between the anyons can be straight-forwardly implemented and anyonic observables can be measured. This demonstrates that a Rydberg gas is a well-suited platform for the study of exotic quantum states of matter such as topological quantum liquids.

Left: Fibonacci anyons occur in two types, 1 and τ. Each of these types can be related to the internal state of atom. The figure shows a so-called fusion path and the corresponding representation by atoms. Along this path two τ's must not appear next to each other which is ensured by the Rydberg blockade, i.e., the fact that consecutive atoms cannot be excited simultaneously to the Rydberg state.

Reference:
I. Lesanovsky and H. Katsura, Interacting Fibonacci anyons in a Rydberg gas, Phys. Rev. A 86, 041601 (2012), or see our full list of publications
Excitation dynamics of one-dimensional Rydberg latticesOctober 2012
Hamburg, Germany

We focus on the excitation dynamics of a one-dimensional lattice of Rydberg atoms. Such systems possess various well-controlled parameters, e.g. lattice size, laser detuning and Rydberg interaction strength, which endows Rydberg system with prospective quantum applications.

Above left: visualizes the ordered propagation pattern of the excitation among the lattice. Above right: illustrates the dependence of the total excitation on the Rydberg interaction strength.


Optically resolving Schrödinger's cat with Rydberg atomsOctober 2012
Dresden, Germany

In his famous thought experiment, Schroedinger contemplated the existence of macroscopic objects (cats) in quantum superposition states. Since then, experiments have created ever larger superposition states, to probe the boundary of the classical and quantum realms. We propose a Schroedinger's cat state made of a pair of ultracold atom clouds with about 20 atoms each. The clouds are briefly subjected to inter-cloud interactions that crucially involve Rydberg atoms. Since Rydberg interactions intimately link the overall quantum state and the character of mechanical motion, this allows an initial spin cat to be converted into a spatial cat. We then have a state where each atom cloud is quantum mechanically at two different positions at once. For the first time, the spatial separation in the superposition would be large enough to be visualized in direct absorption images.


Cooperative excitation and many-body interactionsJuly 2012
Pisa, Italy in collaboration with Orsay, France

A key signature of the dipole blockade originated by the Rydberg interactions is the suppression of fluctuations in the number of Rydberg excitations. We have clearly observed a sub-Poissonian behavior in that number, an evidence of correlations in the excitation. The experimental finding are interpreted on the basis of an original approach based on the Dicke quantum optics model.

Reference:
M. Viteau, P. Huillery, M. G. Bason, N. Malossi, D. Ciampini, O. Morsch, E. Arimondo, D. Comparat, and P. Pillet, Cooperative Excitation and Many-Body Interactions in a Cold Rydberg Gas, Phys. Rev. Lett. 109, 053002 (2012), or see our full list of publications
Entropic enhancement of spatial correlations in a Rydberg gasJuly 2012
Nottingham, UK

Laser-driven Rydberg gases reach a steady state even when the quantum mechanical evolution of this many-body system is described fully coherently. We have studied the correlations between Rydberg atoms in the steady state of a one-dimensional gas whose density is successively increased, but whose length is kept fixed. With increasing density of the ground state atoms the number of ways to arrange Rydberg excitations, and hence the entropy, increases dramatically. However, in contrast to naive expectations, this increase in entropy decreases the disorder in the sense that the steady state shows stronger and stronger spatial correlations between Rydberg atoms. This indicates that in driven closed many-body systems, states with pronounced spatial correlations can spontaneously emerge.

Left: The figure shows pair correlation functions of the Rydberg gas for three different densities and nicely illustrates the build-up of nearly ordered configurations. Here, x is the distance between Rydberg atoms and l_b is the blockade radius.

Reference:
C. Ates and I. Lesanovsky, Entropic enhancement of spatial correlations in a laser-driven Rydberg gas, Phys. Rev. A 86, 013408 (2012), or see our full list of publications
Dipole Interaction Mediated Laser Cooling of Polar Molecules to Ultra-cold TemperaturesMay 2012
Stuttgart, Germany

Motivated by the fast progress in controlling Rydberg atoms, we studied a hybrid setup of polar molecules with a resonant exchange interaction with Rydberg atoms, see Fig.1. We have demonstrated, that such a hybrid setup allows one to design a finite decay rate for excited rotational states in polar molecules. Such a controllable decay rate opens the way to optically pump the hyperfine levels of polar molecules and it enables the application of conventional laser cooling techniques for cooling polar molecules into quantum degeneracy.

Figure 1: (a) Hybrid system: trapped polar molecules are in proximity to a cloud of a cold atomic gas. (b) Relevant level structure: two rotational states for the polar molecule |e> and |g> are coupled via dipole-dipole interaction to two Rydberg levels |S> and |P> of an atom. In addition, the atom is driven from the ground state |G> into a Rydberg level. The rotational level |e> acquires a finite decay time due to the resonant coupling and the finite life-time of Rydberg levels.

Reference:
S. D. Huber and H.P. Büchler, Dipole Interaction Mediated Laser Cooling of Polar Molecules to Ultra-cold Temperatures, Phys. Rev. Lett. 108, 193006 (2012), or see our full list of publications
Atomic Rydberg reservoirs for polar moleculesMay 2012
Innsbruck, Austria

In the field of cold Rydberg atoms and molecules we presented a proposal to directly cool polar molecules by engineering an atomic reservoir for both elastic and inelastic collisions using laser‐dressed Rydberg atoms. Similar to a “collisional Sisyphus effect” a spontaneously emitted photon carries away (kinetic) energy of the collisional partners, leading to a significant energy loss in a single collision.

(Left) Hot molecules (red) collide with cold Rydberg-dressed atoms (blue). A spontaneously emitted photon carries away kinetic energy similar to a collisional Sisyphus effect.

Reference:
B. Zhao, A. Glätzle, G. Pupillo, P. Zoller, Atomic Rydberg Reservoirs for Polar Molecules, Phys. Rev. Lett. 108 193007 (2012), or see our full list of publications
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