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Strongly correlated dipolar quantum gases with tuneable interactions in one-dimensional traps

Final Report Summary - 1DDIPOLARGAS (Strongly correlated dipolar quantum gases with tuneable interactions in one-dimensional traps)

The interaction of many particles, each following basic rules, often produces remarkably rich and complex new behavior. By laser trapping and cooling atomic gases close to absolute zero temperature, it is possible to create new phases in which interatomic interactions determine the behaviour of the system. By tailoring the interactions, we can create new states of matter and shed new light on many-body quantum effects. Until recently, cold atom experiments have dealt with regimes in which interactions between particles had either a point-like character or played a relatively minor role. Now a new frontier is emerging in which interactions can be controlled in completely new ways giving rise to new and more complex many-body effects.

During this project we have explored the effects of long-range interactions induced between laser cooled atoms excited to high lying states (Rydberg states). Of particular interest is the emergence of strong correlations between these atoms and the complex many-body dynamics which ensue. Our work on this general theme has had impact on four major areas of research: Ultracold plasmas, Rydberg aggregates, Rydberg atom-light interfaces, and Dipolar energy transfer.

Ultracold plasmas:
We have observed the sudden and spontaneous evolution of an initially correlated gas of interacting Rydberg atoms to an ultracold plasma. The strong interactions between Rydberg atoms during laser excitation lead to spatially correlated distributions (the Rydberg blockade effect), which strongly affects the dynamics of plasma formation. This study of the spontaneous formation of an ultracold plasma for a Rydberg gas has demonstrated a route towards the realization of so-called strongly-correlated plasma, in which interactions dominate over thermal motion. In particular, we showed that plasma formation from an initially correlated gas may provide the means to overcome disorder-induced-heating which presently limits the lowest temperatures achievable in ultracold plasma research. Future experiments in this new regime could help better understand the physics of plasmas, for example found at the cores of gas giant planets.

Rydberg aggregates:
We have study the formation of correlated systems of a few Rydberg excitations (Rydberg aggregates) which emerge during detuned laser excitation. Applying the technique of full-counting-statistics borrowed from condensed-matter-physics, we found that correlated many-body states arise due to sequential excitations of individual atoms around an initial grain as opposed to a full coherent multi-photon excitation. 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.

Rydberg atom-light interfaces:
Electromagnetically-induced transparency (EIT) in three-level media and the associated appearance of hybrid quasi-particles (dark-state polaritons) have opened intriguing perspectives to create new atom-light interfaces operating at the quantum level. By substituting one of the atomic levels with a strongly-interacting Rydberg state, it has been possible to impart new properties to the light field, resulting in huge optical nonlinearities and modified statistics of the dark-state polaritons. By probing both the optical and atomic degrees of freedom in a single experiment we have investigated the coherent coupling between light and Rydberg states and provided a more complete picture of how atoms and light are affected by the strong-Rydberg interactions.

Dipolar energy transfer:
We have realized a model system for studying the transport of Rydberg excitations evolving under the influence of strong dipolar interactions. To observe the transport dynamics we use a new imaging method based on electromagnetically induced transparency which makes it possible to precisely measure the Rydberg atom distribution as a function of time. The direct observation of dipolar energy transport is an important step towards answering the question how quantum physics can contribute to the efficiency of energy conversion in photosynthetic systems, for example in light-harvesting systems or photovoltaic devices. Our finding that the interaction with the imaging laser fields crucially influences the energy transport dynamics, and the dynamics can be controlled by tuning the atom-light interaction, has created the opportunity to apply Rydberg atoms to the simulation of open quantum systems and to study the quantum-classical crossover in a precisely controlled way.

Contact details: Dr. Shannon Whitlock
Prof. Dr. Matthias Weidemüller

Physikalisches Institute,
University of Heidelberg,
Im Neuenheimer Feld 226,
69120 Heidelberg, Germany

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