COHERENCE:
Cooperativity in Highly Excited Rydberg
Ensembles — Control and Entanglement
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SA III — Few-body phenomena and exotic molecules

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Direct measurement of the van der Waals interaction between two Rydberg atomsJune 2013
Palaiseau, France

The van der Waals interaction that exists between all polarizable particles underlies many important effects in physics, chemistry and biology. To date, direct measurements of this interaction have only been performed either between macroscopic bodies, or between an atom and a macroscopic body. Such approaches have the drawback of being only indirectly linked to the underlying interatomic interaction.

We have directly measured this interaction U_{vdW} = C_6/R_6 between two Rydberg atoms, excited from a pair of single atoms trapped in two microscopic optical tweezers separated by a controlled distance R (Fig. A below). For that, we use collective coherent dynamics in the atom pair as a probe of the interaction. Indeed, in a regime of partial blockade (i.e. when the van der Waals interaction U_{vdW} and the Rabi coupling are comparable), the dynamics of the population of the doubly excited state is very sensitive to U_{vdW}. Our results (Fig. B below) are in very good agreement with theoretical calculations and show a very fast increase of the C_6 coefficient with the principal quantum number n. This allows us to observe the effect of interactions between two single atoms separated by as much as 20 µm!

Our results show that an unprecedented experimental control over a system of a few Rydberg atoms can be reached. This is a prerequisite in view of the application of such systems for quantum information processing and quantum simulation of long-range interacting systems.

Reference:
L. Béguin, A. Vernier, R. Chicireanu, T. Lahaye, and A. Browaeys, Direct measurement of the van der Waals interaction between two Rydberg atoms, Phys. Rev. Lett. 110, 263201 (2013)
This article was also highlighted as a Physics Viewpoint, M. Weidemüller, Atomic Interactions at a Distance, or see our full list of publications
Electrically Dressed Ultralong-Range Polar Rydberg MoleculesMay 2013
Hamburg, Germany

The impact of an electric field on ultra-long range molecules is shown to convert one of the rotational degrees of freedom of the molecule into a vibration and we encounter two-dimensional oscillatory adiabatic potential energy surfaces with an antiparallel equilibrium configuration. The electric field allows to shift the corresponding potential wells in such a manner that the importance of the p-wave interaction can be controlled and the individual wells are energetically lowered at different rates. As a consequence the equilibrium configuration and corresponding energetically lowest well move to larger internuclear distances for increasing field strength. For strong fields the admixture of non-polar molecular Rydberg states leads to the possibility of exciting the large angular momentum polar states via two-photon processes from the ground state of the atom. The resulting properties of the electric dipole moment and the vibrational spectra are analyzed with varying field strength.

(top left) In this figure a two-dimensional potential energy surfaces for the electrically dressed polar trilobite states for E=150 V/m (aligned in z-direction) is shown using spherical coordinates. We observe a potential minimum at θ=π.

(top right) Here we show the scaled probability density for a rovibrational wavefunction (2nd excitation) belonging to the trilobite potential curve for E=300 V/m (in cylindrical coordinates). The azimuthal quantum number is m=0.

Reference:
M. Kurz, P. Schmelcher, Electrically Dressed Ultralong-Range Polar Rydberg Molecules, arXiv:1305.0391, or see our full list of publications
Wavefunctions of triatomic Rydberg moleculesOctober 2012
Granada, Spain

We are exploiting the long range-interactions between Rydberg atoms to form exotic Rydberg molecules. We have studied the properties of a triatomic Rydberg molecule in which the two ground state perturbers are located in an arbitrary configuration. Including the s-wave interaction, we find a rich and complex structure of energy surfaces and wave functions.


Observation of Blueshifted Ultralong-Range Cs₂ Rydberg MoleculesOctober 2012
Harvard, USA

In a joint experiment and theory work A homonuclear molecule with a permanent dipole moment (Science 2011) we demonstrated for the first time the existence of a homonuclear molecule with a permanent electric dipole moment. In a follow up publication, we demonstrated how exotic ultracold and ultralong-range Rydberg molecules could form above the parent dissociation channels (blue-shifted Cs2 molecular lines). These molecules also have large permanent dipole moments; even large than those reported in the Science article.

Reference:
J. Tallant, S. T. Rittenhouse, D. Booth, H. R. Sadeghpour and J. P. Shaffer, Observation of Blueshifted Ultralong-Range Cs₂ Rydberg Molecules, Phys. Rev. Lett. 109, 173202 (2012), or see our full list of publications
Ultra-long-range giant dipole molecules in crossed fieldsFebruary 2012
Hamburg, Germany

The hydrogen atom in crossed magnetic and electric fields provides bound electronic states which are characterized by a very low kinetic energy and a huge electron-proton separation. These states are known as giant dipole states. In our worked we have shown that a neutral alkali perturber, i.e. a ground state Rubidium atom, can be bound via a low energy s-wave scattering potential leading to an ultra-long giant dipole molecules (below left). Depending on the specific electronic excitation, the Born-Oppenheimer potential surfaces possess a rich topology starting from simple harmonic potential wells via toroidal potentials (below right) up to avoided crossing of neighbored potential curves.

Reference:
M. Kurz, M. Mayle, P. Schmelcher, Ultra-long-range giant dipole molecules in crossed electric and magnetic fields, EPL 97 43001 (2012), or see our full list of publications
A homonuclear molecule with a permanent dipole momentNovember 2011
Stuttgart, Germany, in collaboration with Dresden and Harvard

In classical systems a dipolar molecule forms as a result of a charge separation between the negative charged electron cloud and the positive core, creating a permanent electric dipole moment. Usually this charge separation originates from different electronegativities of different elements. Due to symmetry reasons homonuclear molecules, consisting only of atoms of the same element, hence do not possess dipole moments. But in the case of an ultralong-range Rydberg molecule, consisting of a ground state atom and a Rydberg atom, it is possible to break this rule. The reason for this can be found in the extremely small energy spacing of rotational states, which advances the mixing of opposite parity states.

This publication received press attention in the form of an interview with the Royal Society of Chemistry and two articles in French popular-science magazines, Science and Vie and La Recherche (Feb 2012).

Reference:
W. Li, T. Pohl, J. M. Rost, S. T. Rittenhouse, H. R. Sadeghpour, J. Nipper, B. Butscher, J. B. Balewski, V. Bendkowsky, R. Löw, T. Pfau, A homonuclear molecule with a permanent electric dipole moment, Science 334, 1110 (2011), or see our full list of publications
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