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Strongly Correlated Ultracold Rydberg Gases

Final Report Summary - CORYGAS (Strongly Correlated Ultracold Rydberg Gases)

This project aimed at studying self-organisation phenomena, on the basis of the collective effects arising due to strong interactions in ultracold gazes of atoms laser-driven towards highly-excited (Rydberg) states. The main objectives were the following. First, to establish the Rydberg excitation technique, and to characterize the spatial correlations emerging in Rydbergs ensembles via the existing protocol of counting statistics. Second, to develop a new method enabling the imaging of individual Rydberg excitations, which would greatly facilitate the study of spatially correlated Rydberg ensembles. Third, to study the dynamical formation of correlations, with the aim to allow the controlled preparation of ordered structures known as "Rydberg crystals". Fourth, to map the interaction properties of Rydberg states onto long-lived quantum gases, in order to broaden the application range of Rydberg physics, and to create a new platform for studying long-range-interacting many-body systems.

We first established the excitation procedure of strongly interacting Rydberg gases in a two-photon, doubly-resonant configuration. By combining optical imaging and high-fidelity detection of the Rydberg excitations, we observed the phenomenon of electromagnetically-induced transparency (EIT) and a coherent mapping between atomic excitations and propagating photons, associated to hybrid quasiparticles called polaritons. We observed that strong long-range interactions between polaritons result in large optical nonlinearities and modified excitation number statistics. This result is now published in Physical Review Letters 110, 203601 (2013). A second article will be submitted soon, which explores in further details the relation between the optical and atomic response when exciting Rydberg atoms in EIT configuration. Interestingly, these experiments can already be interpreted as Rydberg dressing of photons, where interaction properties of the atomic states are given to photons propagating through the cloud, thus opening the field for studies of quantum fluids of long-range interacting photons.

Not only do these polaritons interact with each other, but they are also sensitive to the presence of electric charges or dipoles in the atomic medium. We demonstrated an “interaction-enhanced imaging” scheme for impurities immersed in a ground-state atomic gas, which we had proposed before the beginning of this fellowship. We obtained single-shot images of small ensembles of Rydberg atoms (down to five), with microsecond timescale resolution. Using Förster resonances in an external electric field, we demonstrated the state-selectivity and tunability of the method. The direct identification of individual impurities should be within reach after optimization of the optical imaging system and tuning of the Rydberg interactions.

This observation method actually presents a second intriguing characteristics. We observed that it triggers an excitation transport dynamics for the impurities, caused by a dipolar state-exchange interaction with the Rydberg excitations involved in the polaritons. This opens a new platform for the study of quantum transport. Due to the impact of the continuous observation on the coherent exchange process, it will allow to emulate the transport dynamics in complex environments where the role and robustness of coherence is questioned, such as light-harvesting complexes. This result is published in Science 342, 954 (2013).

In a different set of experiments, we could study the dynamical effects in the excitation of correlated Rydberg ensembles using counting statistics (see milestone 1). In collaboration with the two theory groups of A. Komnik and J. Evers, we have studied the formation mechanism of correlations in laser-driven Rydberg ensembles. In our system, under detuned excitation, we observed the transient formation of small aggregates of typically three excitations, interacting so as to compensate the detuning. The dominant formation mechanism was diagnocised as a sequential growth, as opposed to the direct coherent excitation of a correlated ensemble. This result is published in Physical Review Letters 112, 013002 (2014).

In parallel to these experiments, a range of parameters was observed to lead to the unexpected, sudden and spontaneous evolution of an initially correlated gas of repulsively interacting Rydberg atoms to an ultracold plasma. Rydberg-Rydberg interactions were observed to strongly affect the dynamics of plasma formation. We characterized the dominant ionisation processes in our system, different from those governing most experiments on ultracold plasmas so far. Our results suggest that the initial correlations of the Rydberg ensemble should persist through the avalanche, thus offering a route to create strongly correlated plasmas. This result has been published in Physical Review Letters 110, 045004 (2013).

This research project opens the basis for several future fundamental works. First, we developed a new tool for the study of strongly correlated systems : the imaging technique here demonstrated can be used to image any impurity immersed in an atomic gas, as long as it is electrically charged or presents a strong electric dipole or polarisability. Furthermore, our project demonstrated that Rydberg physics can be now used as a starting platform for application to other fundamental fields : creation of strongly correlated plasma, creation of strongly correlated quantum light fields, emulation of quantum transport in the presence of a complex environment, open quantum systems.

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