COHERENCE:
Cooperativity in Highly Excited Rydberg
Ensembles — Control and Entanglement
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Storage and retrieval of optical photons in new experimental setupJuly 2015
Durham, UK

Recently, we completed building and optimisation of a new experimental apparatus to study strong and long-ranged interactions between optical photon stored as Rydberg excitations. The setup features two high numerical aperture lenses, allowing focusing of trapping and excitation lasers to less than 1 micrometre (lens assembly designed in collaboration with the Browaeys group, Palaiseau, COHERENCE collaboration). As the length scales are well below the Rydberg blockade, this allows the individual creation and addressing of single photons stored as Rydberg polaritons. We have created two microscopic cold atom clouds side-by-side (see figure 1) that serve as storage sites for individual photons.

We have recently achieved photon storage in this new apparatus (see figure 2) and will now look for interactions between photons in spatially separated channels which will increase the degree of control over the interactions compared to previous experiments. This can pave the way to applications such as a universal quantum gate for photonic qubits (see D. Parades Barato and C. S. Adams, Phys. Rev. Lett. 112 040501, 2014 for the proposed scheme).

Additionally, we were able to speed up the repetition rates in the new apparatus by two orders of magnitude thanks to the inclusion of a 2D MOT as atomic beam source (developed based on a design from the Weidemüller group, Heidelberg, COHERENCE collaboration).


Frequency comb 2015
TOPTICA, Graefelfing, Germany

We demonstrate and characterize an Er:fiber frequency comb with repetition rate of 80 MHz which is passively phase-stabilized via difference frequency generation (DFG). A universal method to measure the phase noise spectra of comb lines at different wavelengths is demonstrated. With repetition-rate stabilization to an RF oscillator a linewidth of below 100 kHz at 1550 nm is achieved. We have also demonstrated further reduction to the Hz level by locking the DFG comb to an optical reference. The measured phase noise spectra, after different nonlinear wavelength conversion stages, allow the analysis of noise properties in the laser system.

Reference:
N. Hoghooghi, A. Sell, T. Puppe, R. Kliese, S. Falke, A. Zach and J. Stuhler, Frequency comb with Hz level line widthsICOLS 2015 22nd International Conference on Laser Spectroscopy June 28 - July 3 2015 Singapore , Centre for Quantum Technologies National University of Singapore, 129, (2015)
R. Kliese, A. Sell, F. Rohde, T. Puppe, A. Zach and W. G. Kaenders, Versatile characterization of a passively carrier envelope phase stable frequency comb sAbstract Book, 2015 Joint Conference of the IEEE IFCS a. EFTF, April 12-16, Denver, Colorado - USA, 248, (2015), ) , or see our full list of publications
From classical to quantum non-equilibrium dynamics of Rydberg excitations in optical lattices2015
Innsbruck, Austria

Ultracold atoms excited to Rydberg states are a powerful tool to study many-body physics. Strong and long-range interactions between Rydberg atoms leads to a spatially correlated and slowed down dynamics of Rydberg excitations: these phenomena are reminiscent of the relaxation of defects in the so-called One-Spin Facilitated Model, the minimal kinetically constrained model (KC) known to display glassy features. In particular, we encode the dynamical rules typical of KCs in the anti-blockade configuration, i.e. setting the laser detuning of the coupling between the ground-state and the Rydberg state to be exactly equal to the nearest-neighbour interaction between Rydberg-excited atoms. If in a certain region, an atom is already in the Rydberg state, a neighbouring atom will be "facilitated" to be also excited to the Rydberg state (see Figure below). Contrarily if, in the same region, no atom is in the Rydberg state, the Rydberg-excitation of any neighbouring atom is strongly suppressed by a large detuning. In the setup we propose, the coupling from the ground-state to an additional intermediate short-lived excited state can tune the regimes of the dynamics of Rydberg excitations. The two extremes are: the classical rate equation limit of the aforementioned One-Spin Facilitated Model and the coherent limit characterised by the rapid "cascaded" creation and destruction of clusters of Rydberg excitations with varying size from a preexisting isolated Rydberg excitation.

Reference:
M. Mattioli, A. W. Glaetzle and W. Lechner, From classical to quantum non-equilibrium dynamics of Rydberg excitations in optical lattices, New Journal of Physics (submitted, 2015), or see our full list of publications
Resonant charge transfer of hydrogen Rydberg atoms2015
Oxford, UK

Hydrogen Rydberg atoms interacting with a surface undergo ionisaton by transferring the electron into the bulk. The electron transfer happens at surface-atom distances less than 5n2a0. For metal surfaces, the Rydberg electron energy is degenerate with the conduction band of the metal and resonant charge transfer can occur. For an electronically structured surface, a ‘band-gap metal’, charge transfer can only take place via discrete image states which form a series akin to the Rydberg series. Resonant enhancement of the surface ionisation is observed only for principal quantum numbers for which the Rydberg energy matches these image states. With this approach Hydrogen Rydberg atoms can be used to resolve high lying image states that were formerly inaccessible with other techniques.

Reference:
J. A. Gibbard, M. Dethlefsen, M. Kohlhoff, C. J. Rennick, E. So, M. Ford and T. P. Softley, Resonant charge transfer of hydrogen Rydberg atoms incident at a Cu(100) projected band-gap surface, arXiv:1504.07191 (2015), or see our full list of publications
A new three-body interaction in cold Rydberg atoms2015
Paris, France

The strong dipole-dipole interaction in ultracold Rydberg atoms can induce non- radiative energy transfers between atom pairs, similarly to Fluorescence Resonance Energy Transfer (FRET) in biological systems. In this work we observe a new three-body FRET process in cold cesium Rydberg atoms. In this new process the energy transfer between atoms is possible thanks to the action of an additional relay atom which sequentially exchange energy with the other two atoms. It can be considered as an assisted two-body resonance where the relay atoms carry away the energy excess preventing the two-body resonance, leading thus to a Borromean type of energy transfer. This three-body FRET is a general process that can be observed in every atomic species which presents a two-body FRET. We also propose the generalization of this process based on a relay atoms energy exchange to n-body interaction paving the way between two-body and many-body physics. This three-body interaction is also a promising tool for the implementation of three-body quantum gate or entanglement. (Illustration by Emmanuelle Pelle)

Reference:
R. Faoro, B. Pelle, A. Zuliani, P. Cheinet, E. Arimondo and P. Pillet, Borromean three-body FRET in frozen Rydberg gases, Nature Comm. (2015) (accepted), or see our full list of publications
Van der Waals explosion of cold Rydberg clusters 2015
Pisa, Italy

The strong van der Waals interaction in cold Rydberg atoms can lead to a non-negligible mechanical effect, especially when the atoms are excited at close range through the facilitation process for off-resonant excitation. In this work we report on the direct measurement in real space of the effect of the van der Waals forces between individual Rydberg atoms on their external degrees of freedom. Clusters of Rydberg atoms with inter-particle distances of around 5 µm are created by first generating a small number of seed excitations in a magneto-optical trap, followed by off-resonant excitation that leads to a chain of facilitated excitation events. After a variable expansion time the Rydberg atoms are field ionized, and from the arrival time distributions the size of the Rydberg cluster after expansion is calculated. Our results demonstrate the intrinsic mechanical instability of off-resonantly excited Rydberg gases, which has implications for proposed applications in quantum computation and quantum simulation.

Reference:
R. Faoro, C. Simonelli, M. Archimi, G. Masella, M. M. Valdo, E. Arimondo, R. Mannella, D. Ciampini and O. Morsch, Van der Waals explosion of cold Rydberg clusters, ArXiv: 1506.08463v1 (2015), or see our full list of publications
Quantum Simulation of Energy Transport with Embedded Rydberg Aggregates March 2015
Heidelberg, Germany

An array of ultracold Rydberg atoms embedded in a laser driven background gas can serve as a system for simulating exciton dynamics and energy transport with a controlled environment. Energetic disorder and decoherence introduced by the interaction with the background gas atoms can be controlled by the laser parameters. This allows for an almost ideal realization of a Haken-Reineker-Strobl-type model for energy transport. The transport can be monitored using the same mechanism that provides control over the environment. The degree of decoherence in the system is related to information gained on the excitation location through the measurement, allowing to study the effects of quantum measurements on the dynamics of a many-body quantum system.

Reference:
D. W. Schönleber, A. Eisfeld, M. Genkin, S. Whitlock and S. Wüster, Quantum Simulation of Energy Transport with Embedded Rydberg Aggregates, Phys. Rev. Lett. 114, 123005 (2015), or see our full list of publications
Collective Excitation of Rydberg-Atom Ensembles beyond the Superatom Model December 2014
Heidelberg, Germany

In an ensemble of laser-driven atoms involving strongly interacting Rydberg states, the steady-state excitation probability is usually substantially suppressed. In contrast, we identify a regime in which the Rydberg excited fraction is enhanced by the interaction through coherent multiphoton excitations between collective states. This excitation enhancement should be observable under currently existing experimental conditions and may serve as a direct probe for the presence of coherent multiphoton dynamics involving many-body collective quantum states.

Reference:
M. Gärttner, S. Whitlock, D. W. Schönleber and J. Evers, Collective Excitation of Rydberg-Atom Ensembles beyond the Superatom Model, Phys. Rev. Lett. 113, 233002 (2014), or see our full list of publications
Semianalytical model for nonlinear absorption in strongly interacting Rydberg gases June 2014
Heidelberg, Germany

The nonlinear optical effects arising in interacting Rydberg gases can be described using a Rate Equation model and we show that the previously reported universal dependence of the susceptibility on the Rydberg excited fraction is an intrinsic property of this description, rooted in one-body properties.. We show that its results can be understood by considering the excitation of individual superatoms and we deduce a simple semianalytical model that accurately describes the Rydberg density and optical susceptibility for different dimensionalities. Benchmarking against exact master equation calculations, we identify regimes in which the semianalytical model is particularly reliable.

Reference:
M. Gärttner, S. Whitlock, D. W. Schönleber and J. Evers, Semianalytical model for nonlinear absorption in strongly interacting Rydberg gases, Phys. Rev. A 89, 063407 (2014), or see our full list of publications
Full Counting Statistics of Laser Excited Rydberg Aggregates in a One-Dimensional Geometry January 2014
Heidelberg, Germany

Full Counting Statistics (FCS) can provide valuable information on many-body systems especially if the underlying correlations cannot be directly imaged. We apply this method to Rydberg excitations to gain information on Rydberg interacting many-body systems. We find asymmetric excitation spectra and enhanced fluctuations of the Rydberg atom number which we attribute to the formation of Rydberg aggregates, i.e. correlated systems comprised of few excitations. In the presence of dephasing these aggregates are formed via sequential excitation around an initial grain.

Reference:
MH. Schempp et al., Full Counting Statistics of Laser Excited Rydberg Aggregates in a One-Dimensional Geometry, Phys.Rev.Lett. 112, 013002 (2014), or see our full list of publications
An experimental approach for investigating many-body phenomena in Rydberg-interacting quantum systems September 2014
Heidelberg, Germany

We describe a tailored experimental system used to produce and study Rydberg-interacting atoms excited from dense ultracold atomic gases. The experiment is optimized for fast duty cycles using a high flux cold atom source and a three beam optical dipole trap. The latter enables tuning of the atomic density and temperature over several orders of magnitude, all the way to the Bose-Einstein condensation transition. An electrode structure surrounding the atoms allows for precise control over electric fields and single-particle sensitive field ionization detection of Rydberg atoms. With this setup we performed two experiments which highlight the influence of strong Rydberg-Rydberg interactions on different many-body systems. First, the Rydberg blockade effect is used to pre-structure an atomic gas prior to its spontaneous evolution into an ultracold plasma. Second, hybrid states of photons and atoms called dark-state polaritons are studied. By looking at the statistical distribution of Rydberg excited atoms we reveal correlations between dark-state polaritons.

Reference:
C. S. Hofmann, G. Günter, H. Schempp, N. M. L. Müller, A. Faber, H. Busche, M. Robert-de-Saint-Vincent, S. Whitlock and M. Weidemüller, An experimental approach for investigating many-body phenomena in Rydberg-interacting quantum systems, Frontiers of Physics 9, 571-586 (2014), or see our full list of publications
A new scattering channel in long-range Rydberg molecules2015
Zurich, Switzerland

The scattering of a quasi-free Rydberg electron, being only weakly bound to its ion core, off a ground-state atom can lead to molecular potentials that support several vibrational states. In this work we report for the first time effects of the internal quantum state of the ground-state atom on these molecular potentials: The binding energies of some of the long-range Cs2 Rydberg molecules, observed closed to atomic cesium np3/2 Rydberg states in our experiments, show a strong dependence on the hyperfine state in which we prepare the ground-state atom. This dependence is explained by a model of the electron--ground-state-atom scattering that includes singlet and triplet scattering channels and the hyperfine structure of the ground state. Our experiments thus yield the first experimental determination of a zero-energy singlet scattering length in electron-alkali collisions.

Reference:
H. Saßmannshausen, F. Merkt and J. Deiglmayr , Experimental Characterization of Singlet Scattering Channels in Long-Range Rydberg Molecules. , Phys. Rev. Lett. 114, 133201 (2015) , or see our full list of publications
Long-range interatomic forces with a twist2014
Zurich, Switzerland

Upon irradiation of a dense gas of ultracold Cs atoms with an intense pulse of UV laser radiation we observe spectral resonances which we attribute to the correlated excitation of two atoms into interacting Rydberg-atom pair states. The underlying mechanism is the long-range interaction of an induced dipole moment on one atom and an induced quadrupole moment on the other atom. In contrast to the interaction between two dipole moments, this interaction does not conserve the electronic parity. The electronic interaction of the two Rydberg atoms thus causes an additional torque on the relative motion of the two atoms. The coupling is facilitated in our system by the quasi-degeneracy of rotational states of the collision complex with opposite parity.

Reference:
J. Deiglmayr, H. Saßmannshausen, P. Pillet and F. Merkt , Observation of Dipole-Quadrupole Interaction in an Ultracold Gas of Rydberg Atoms. , Phys. Rev. Lett. 113, 193001 (2014) , or see our full list of publications
Preparing cold atomic samples in different density regimes for Rydberg studies2015
Pisa, Italy

Ultracold gases excited to strongly interacting Rydberg states offer the possibility to study a wide range of problems of many-body systems. A systematic study of these effects can be done, e.g. by varying the density of the atomic sample. In this work the authors describe two different experimental techniques which allow them to work in different density regimes, from the few atom regime, with typically N≤10 atoms, to the high density regime, as shown in the figure below, where the total number of atoms N in the MOT was varied.

Reference:
M. M. Valado, M. D. Hoogerland, C. Simonelli, E. Arimondo, D. Ciampini and O. Morsch , Preparing cold atomic samples in different density regimes for rydberg studies , Journal of Physics: Conference Series, 012041 (2015) , or see our full list of publications
Rydberg excitation of a bose-einstein condensate2015
Pisa, Italy

In the present work, the Pisa group investigates the excitation to a high-lying Rydberg state in an atomic sample undergoing the Bose-Einstein condensation phase transition, in the limit that the spatial dimensions of the condensed cloud are smaller than the (van-der-Waals) dipole blockade radius. In the Figure below, the measured mean number of Rydberg excitations (full blue diamonds and left scale), calculated normalized volume (to the volume of a superatom, right scale) of the thermal fraction (open red circles) and of the condensate fraction (open gray circles and closed black circles) are shown as a function of the mean trapping frequency. At low trapping frequency, where the thermal fraction is negligible, the total cloud volume coincides with the volume of the condensate (closed black circles). The insert shows the BEC fraction as a function of mean trapping frequency. We observed a distinct decrease in the number of excitations due to the change of the atomic spatial density distribution, from the thermal cloud, through a bimodal distribution, to the condensed cloud. When only the condensed part is present, the average number of excitations measured levels of at around one which is compatible with having a single collective excitation in the condensed cloud.

Reference:
N. Malossi, M. M. Valado, E. Arimondo, O. Morsch and D. Ciampini , Rydberg excitation of a bose-einstein condensate , Journal of Physics: Conference Series 594, 012041 (2015) , or see our full list of publications
Strongly correlated excitation of a quasi-1d rydberg gas2014
Pisa, Italy

In this work the Pisa group presents a study of the dynamics of an efective 1D-Rydberg sample excited on and off-resonance. In both cases the long-range interactions between the excitations lead to strong spatial and temporal correlations in the sample. The resonant excitation dynamics is dominated by the dipole blockade efect, whereas the off-resonant regime is governed by a complex interplay between single-atom and pair excitations mediated by the Rydberg-Rydberg interaction. These differences are visible when the timescales of both excitation processes are compared (see Figure below). Moreover, the analysis of the full counting statistics of the excitation events highlights the main differences between the two excitation processes, showing a strong sub-Poissonian character for the resonant counting distributions, and bimodal character for the off-resonant excitation case.

Reference:
N. Malossi, M. M. Valado, S. Scotto, D. Ciampini, E. Arimondo and O. Morsch, Strongly correlated excitation of a quasi-1d rydberg gas, Journal of Physics: Conference Series, vol. 497, p. 012031, 2014, or see our full list of publications
Full counting statistics and phase diagram of a dissipative rydberg gas2014
Pisa, Italy

Strongly interacting Rydberg systems in cold gases have been shown as promising system for quantum simulations of many-body systems. The behavior of a Rydberg system where the excitations are generated off resonance depends strongly on the detuning and the character (attractive or repulsive) of the interaction. Furthermore, under highly dissipative conditions, novel phases and exotic phenomena have been predicted for this regime. By characterizing their system through a phase diagram (Figure (a)) and the analysis of the full counting statistics of the excitation events, the team in Pisa observed bimodal counting distributions (Figure (b)), which are compatible with intermittency due to the coexistence of dynamical phases. These results pave the way towards detailed studies of many-body effects in Rydberg gases.

Reference:
N. Malossi, M. M. Valado, S. Scotto, P. Huillery, P. Pillet, D. Ciampini, E. Arimondo and O. Morsch, Full counting statistics and phase diagram of a dissipative rydberg gas, Phys. Rev. Lett. 113, 023006 (2014) , or see our full list of publications
Rydberg Tomography of an Ultracold Atomic Cloud2013
Pisa, Italy

By realizing a scan in position across the cold cloud (see Figure below) the team at University of Pisa and CNR-INO demonstrates a technique to visualize one of the hallmarks of Rydberg physics: the dipole blockade efect, whereby only a single excitation is allowed within the volume of a blockade sphere. Comparing the distribution obtained for the Rydberg excitations to the one obtained for the number of ions created by a two-photon ionization process via the intermediate 5P level, they show that the blockade effect modifies the width of the Rydberg excitation profile.

Reference:
M. M. Valado, N. Malossi, S. Scotto, D. Ciampini, E. Arimondo, O. Morsch, Rydberg tomography of an ultracold atomic cloud, Phys. Rev. A 88, 045401 (2013) , or see our full list of publications
Surface ionisation of molecular H2 and atomic H Rydberg states at doped silicon surfaces2014
Oxford, Great Britain

We have investigated the collisions between H atom Rydberg states and a variety of surfaces, including (a) p-type and n-type doped silicon surfaces, (b) surfaces with metallic thin films of variable thickness, and (c) single crystal copper surfaces with a bandgap at the energy. In cases (b) and (c) the ionization of the Rydberg electron is enhanced when there is a co-incidence between the Rydberg energy and a quantized surface state in the one-dimensional well perpendicular to the surface (see figure). For (a) we observe strong variations of the signal with dopant density and dopant type, explicable in terms of the distribution of discrete charges in the surface.

Reference:
G. Sashikesh, M. S. Ford, E. So and T. P. Softley, Surface ionisation of molecular H2 and atomic H Rydberg states at doped silicon surfaces, Molecular Physics, vol. 112, p. 2495-2503, 2014 , or see our full list of publications
Scattering resonances and bound states for strongly interacting Rydberg polaritonsNovember 2014
Stuttgart, Germany

In this work, we demonstrate a theoretical framework describing slow-light polaritons interacting via atomic Rydberg states, Fig. A. The method allows us to analytically derive the scattering properties of two polaritons. We identify new parameter regimes where polariton-polariton interactions are repulsive. Furthermore, in the regime of attractive interactions, we identify multiple two-polariton bound states, calculate their dispersion (Fig. B), and study the resulting scattering resonances (Fig. C). Finally, we discuss the implications of our results to the ongoing experiments and to the effective many body theory for strongly interacting Rydberg polaritons.

Reference:
P. Bienias, S. Choi, O. Firstenberg, M. F. Maghrebi, M. Gullans, M. D. Lukin, A. V. Gorshkov and H. P. Büchler, Scattering resonances and bound states for strongly interacting Rydberg polaritons, PhysRevA, vol. 90, p. 053804, 2014, or see our full list of publications
Magnetic-film atom chipMay 2014
Amsterdam, Netherlands

Excitation of Rydberg atoms in a magnetic trap lattice was observed by measuring trap loss. The localized nature of the excitation gives access to local Stark shifts near the chip surface. In the picture, the blue circle marks the excitation laser beam, the red circle marks the location where Rydberg atoms are excitated.

Reference:
V. Y. F. Leung, D. R. M. Pijn, H. Schlatter, L. Torralbo-Campo, A. L. La Rooij, G. B. Mulder, J. Naber, M. L. Soudijn, A. Tauschinsky, C. Abarbanel, B. Hadad, E. Golan, R. Folman and R. J. C. Spreeuw, Magnetic-film atom chip with 10 μm period lattices of microtraps for quantum information science with Rydberg atoms, Review of Scientific Instruments 85, 053102 (2014), or see our full list of publications
Coupling a single electron to a Bose-Einstein condensate 2013
Stuttgart, Germany

We created a system in which a single electron interacts with a microscopic quantum object – a Bose-Einstein condensate. There are thousands of atoms within a Rydberg electron orbit. A strong coupling between them leads to a collective mechanical motion - oscillations of the whole BEC. In other words, the electron “makes a splash ” onto a BEC.

Reference:
J. B. Balewski, A. T. Krupp, A. Gaj, D. Peter, H. P. Büchler, R. Löw, S. Hofferberth and T. Pfau,, Coupling a single electron to a Bose-Einstein condensate, Nature 502, 664 (2013), or see our full list of publications
Observation of resonant energy transfer in a ‘spin-chain’ made of three atoms 2015
CNRS, France

In this article, we studied, in collaboration with C.S. Adams from the University of Durham (also member of the ITN Coherence) the propagation of a spin excitation along a linear chain of three spins. The spin states are encoded in two different Rydberg states, and due to the dipole-dipole interactions, a spin excitation can “hop” from one site to the other. We directly observed long-range hopping, and obtained an excellent agreement between the data and a model without any adjustable parameter (see figure below).

Reference:
D. Barredo, H. Labuhn, S. Ravets, T. Lahaye, A. Browaeys, and C. S. Adams, Coherent Excitation Transfer in a “Spin Chain” of Three Rydberg Atoms, Phys. Rev. Lett. 114, 113002 (2015), or see our full list of publications
Controlling interactions between two single Rydberg atoms with electric fields2014
CNRS, France

In this work, we used two single atoms, separated by a controlled distance R, and our ability to control the electric fields applied on the atoms, to tune the system to a so-called Förster resonance. The interactions between the two atoms then scale as 1/R³ . We could measure this scaling directly by observing the coherent exchange of excitations between the two atoms as a function of time, which occurs at a frequency proportional to the interaction strength (see figure below).

Reference:
S. Ravets, H. Labuhn, D. Barredo, L. Béguin, T. Lahaye, and A. Browaeys, Coherent dipole-dipole coupling between two single Rydberg atoms at an electrically-tuned Förster resonance, Nature Physics 10, 914 (2014), or see our full list of publications
Single-atom addressing in an array of microtrapsAugust 2014
CNRS, France

Here, we have demonstrated our ability to address selectively an atom in an array of optical tweezers, using a focused addressing beam, controlled by a 2D AOM (figure below), that shifts the levels of the selected atom. We used this tool not only to enable at will Rydberg blockade between two atoms, but also to engineer a two-atom quantum state, transforming a ‘super-radiant’ state to a sub-radiant one. ality arrays, with low optical aberrations and a highly uniformity.

Reference:
H. Labuhn, S. Ravets, D. Barredo, L. Béguin, F. Nogrette, T. Lahaye, and A. Browaeys , Single-atom addressing in microtraps for quantum-state engineering using Rydberg atoms, Phys. Rev. A 90, 023415 (2014), or see our full list of publications
Creation of arbitrary 2D arrays of optical microtraps for single atomsMai 2014
CNRS, France

In this work, we have demonstrated the loading of single atoms in arbitrary, two-dimensional arrays containing up to 100 microtraps separated by a few microns (see figure below). The arrays are generated by imprinting a phase on the trapping beam, prior to focusing, using a spatial-light modulator. We achieved very high quality arrays, with low optical aberrations and a highly uniformity.

Reference:
F. Nogrette, H. Labuhn, S. Ravets, D. Barredo, L. Béguin, A. Vernier, T. Lahaye, and A. Browaeys , Single-Atom Trapping in Holographic 2D Arrays of Microtraps with Arbitrary Geometries, Phys. Rev. X 4, 021034 (2014), or see our full list of publications
Rydberg blockade in a system of three atoms with anisotropic interactionsMai 2014
CNRS, France

In this work, we demonstrated Rydberg blockade and collective excitation in a system of three atoms that interact via anisotropic van der Waals interactions. We showed (see figure below) that in both linear and triangular configurations, we could observe a strong suppression of multiple Rydberg excitations, as well as an enhancement of the Rabi frequency by the expected factor 3.

Reference:
D. Barredo, S. Ravets, H. Labuhn, L. Béguin, A. Vernier, F. Nogrette, T. Lahaye, and A. Browaeys, Demonstration of a Strong Rydberg Blockade in Three-Atom Systems with Anisotropic Interactions, Phys. Rev. Lett. 112, 183002 (2014), or see our full list of publications
Rydberg spectroscopy of laser cooled Holmium2014 / 2015
Wisconsin, USA

The Saffman research group at University of Wisconsin, Madison is studying the Rydberg properties of Holmium atoms for use as qubits. Among all the known elements Ho has the distinction of having the largest number of stable ground states. We are working to use the 128 ground states of Ho to encode a quantum register. In the last two years important progress towards this goal has been made by demonstrating a magneto-optical trap for Holmium atoms[1] and using it to perform high resolution spectroscopy of Holmium[2]. The measured Rydberg quantum defects shown in the Figure will be used for implementing Rydberg mediated interactions in Holmium gases.

Reference:
J. Miao, J. Hostetter, G. Stratis, and M. Saffman, Magneto-optical trapping of Holmium atoms, Phys. Rev. A 89, 041401(R) (2014)
J. Hostetter, J. D. Pritchard, J. E. Lawler, and M. Saffman, Measurement of Holmium Rydberg series through MOT depletion spectroscopy, Phys. Rev. A 91, 012507 (2015), or see our full list of publications
Dipole-mediated energy transport of Rydberg-excitations (glowing balls) in an atomic sea – artist impressionSeptember 2014
Heidelberg, Germany

By realizing an artificial quantum system, physicists at Heidelberg University have simulated key processes of photosynthesis on a quantum level with high spatial and temporal resolution. In their experiment with Rydberg atoms the team of Prof. Dr. Matthias Weidemüller and Dr. Shannon Whitlock discovered new properties of energy transport. This work is an important step towards answering the question of how quantum physics can contribute to the efficiency of energy conversion in synthetic systems, for example in photovoltaics. The new discoveries, which were made at the Center for Quantum Dynamics and the Institute for Physics of Heidelberg University, have now been published in the journal “Science”.

Reference:
G. Günter, H. Schempp, M. Robert-de-Saint-Vincent, V. Gavryusev, S. Helmrich, C.S. Hofmann, S. Whitlock, M. Weidemüller, Observing the Dynamics of Dipole-Mediated Energy Transport by Interaction Enhanced Imaging, Science, 342 (2013), or see our full list of publications
Exciton dynamics in emergent Rydberg latticesOctober 2013
Nottingham, UK in collaboration with Durham, UK

Gases of Rydberg atoms are a flexible tool for the study of many-body phenomena. In this work we exploit the effect that Rydberg excitations can spontaneously “order” when their laser excitation is conducted in a sufficiently dense gas. The emergent lattice of Rydberg atoms is used to theoretically study the dynamics of excitation transfer between different Rydberg states which is triggered by a microwave pulse. We show that the resulting rich non-equilibrium dynamics can be probed and understood from the conversion of Rydberg excitations into photons which are subsequently detected and analysed.

Reference:
S. Bettelli and D. Maxwell and T. Fernholz and C. S. Adams and I. Lesanovsky and C. Ates, Exciton dynamics in emergent Rydberg lattices, To be published in Phys. Rev. A, or see our full list of publications
Nonadiabatic Motional Effects and Dissipative Blockade for Rydberg Atoms Excited from Optical Lattices or MicrotrapsOctober 2013
Nottingham, UK

Motion can matter even in frozen Rydberg gases where the timescale of the excitation dynamics is much fast than that of the thermal motion. When atoms are resonantly excited from low-lying states to interacting Rydberg pair-states (see main Figure) the excitation can change from coherent (bottom left) to incoherent (top left). In this paper we have analysed this behaviour and demonstrated that this effect becomes important for Rydberg atoms excited from atoms held in tight microtraps or optical lattices.

Reference:
W. Li and C. Ates and I. Lesanovsky, Nonadiabatic Motional Effects and Dissipative Blockade for Rydberg Atoms Excited from Optical Lattices or Microtraps, Phys. Rev. Lett. 110, 213005 (2013), or see our full list of publications
Microwave control of the interaction between two optical photonsOctober 2013
Durham, UK

Non-classical states of light are a key resource in photonic quantum technologies. One way to observe non-classical behaviour is to measure the second order correlation function g(2), which classically must always be greater than or equal to unity. In our recent experiment on storage and retrieval of light pulses in Rydberg states, we have shown that the retrieved light field is non-classical (left). Now we have shown that with the addition of a microwave control field, the degree of ``non-classicality '' can be controlled (right).

Reference:
D. Maxwell, D. J. Szwer, D. P. Barato, H. Busche, J. D. Pritchard, A. Gauguet, M. P. A. Jones, C. S. Adams, Microwave control of the interaction between two optical photons, Phys. Rev. Lett. 110, 263201 (2013), or see our full list of publications
Ultracold giant molecules formed from trapped ultracold Rydberg atoms and polar moleculesOctober 2013
Granada, Spain in collaboration with Hamburg, Germany and Harvard, USA

We have investigated ultracold giant molecules formed from trapped ultracold Rydberg atoms and polar molecules. We describe the dimer with the rigid rotor description and take into account all the components of the electric field produced by the Rydberg electron and core. We have demostrated that the KRb molecule is binded to the Rydberg atom, and that the full treatment of its rotational motion leads to pronounced avoided crossings in the Born–Oppenheimer potentials as R is varied. We are currently analyzing the coupling between these potentials by means of an diabatic description. The orientation of KRb strongly depends on the separation from the Rydberg core R, and we are investigating the possibility of coherently control the molecular dipole orientation.


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
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.


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
Trapping atoms on a magnetic lattice atom chipMay 2013
Amsterdam, The Netherlands

Cold rubidium atoms were trapped on a new magnetic-film atom chip. The pattern of microtraps consists of two lattices, of hexagonal and square geometries respectively, joining in an interface. With a lattice spacing of 10 µm this is an excellent starting point to investigate Rydberg dipole blockade in and between these microtraps.


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
Measurement of 87-Rb Rydberg-state hyperfine splitting in a room-temperature vapor cellApril 2013
Amsterdam, The Netherlands

Hyperfine splittings of Rydberg s-states in 87Rb were measured using electromagnetically induced transparency (EIT) spectroscopy in a room-temperature vapor cell. In spite of Doppler broadening, an accuracy of 100 kHz was achieved. The top figure shows the splitting in a Stark map measurement for the 20s state. The bottom figure shows the differential hyperfine splitting (without the common Stark shift) of the F=2 state.

Reference:
A. Tauschinsky, R. Newell, H. B van Linden van den Heuvell, R. J. C Spreeuw, Measurement of 87-Rb Rydberg-state hyperfine splitting in a room-temperature vapor cell, Phys. Rev. A, 87, 042522, 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
Adiabatic passage for quantum gates in mesoscopic ensemblesDecember 2012
Open University, UK in collaboration with Wisconsin, USA

We demonstrated a protocol for adiabatic passage that allows for geometric phase compensation when applied to atomic ensembles of unknown number of atoms. This opens the way to high fidelity logic operation with mesoqubits.

Reference:
I. Beterov, M. Saffman, E. A. Yakshina, D. B. Tretyakov, V. M. Entin,I. I. Ryabtsev, C. W. Mansell, C. MacCormick, S. Bergamini, and V. P. Zhukov, Adiabatic passage for quantum gates in mesoscopic atomic ensembles, Preprint arXiv:1212.1138, or see our full list of publications
Laser cooling of HolmiumDecember 2012
Wisconsin, USA

In recent years laser cooling techniques have been extended to many different atomic species. During the first year of the Coherence project the Saffman research group at University of Wisconsin Madison has demonstrated laser cooling of the rare earth element Holmium (Ho) for the first time. Among all the known elements Ho has the distinction of having the largest number of stable ground states. We are working to use the 128 ground states of Ho to encode a quantum register. The cold sample of Ho atoms shown in the Figure is an important step towards this goal. A publication is in preparation


Generation of entanglement via laser driven Rydberg atomsDecember 2012
Aarhus, Denmark

An atom excited by a laser field decays back to its ground state in a few tens of nanoseconds. The Aarhus COHERENCE team has shown that as a consequence of the short lifetimes of their optically excited states, pairs of atoms with strong Rydberg excited state interactions will quickly decay into “dark states” which do not absorb laser light and which are strongly entangled superposition states. Since the rapidly decaying state is coupled resonantly we achieve 95% overlap with the entangled state within few microseconds. The state preparation relies on the decay of the unwanted state components, and the dark state protection does not crucially depend on the strengths of the Rydberg interaction.

Figure: Population of maximally entangled state, cf., solution of the master equation for two atoms with ground states |0>,|1>, optically excited state |p> and Rydberg excited state |r>). The transition |1> -|p>-|r> is strongly driven, off the intermediate resonance. of Insert shows that even when the blockade interaction Δ is much smaller than the Rabi frequency Ω, one can still obtain the entangled state with above 90% fidelity.


Deterministic quantum computation with Rydberg interactionsOctober 2012
Open University, UK

Deterministic quantum computation with one quantum bit (DQC1) is a model of quantum computation with an exponential speed-up compared to known classical algorithms. Rather than entanglement, quantum discord is thought to be responsible for this speed-up. We have identified a protocol to experimentally implement the DQC1 algorithm and quantify a geometric measure of quantum discord with ensembles of ultracold atoms exploiting Rydberg interactions. This system provides large Hilbert space for realistic numbers of atoms in the ensemble. This will allow to experimentally investigate the physics and the computational power of quantum discord for a specific algorithm and extend the validity of the protocol to high dimension Hilbert spaces.


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
Performance of saturated absorber Er-fibre oscillatorsOctober 2012
TOPTICA, Germany, in collaboration with Durham, UK

Stable femtosecond lasers have greatly simplied optical precision measurements. Current erbium fibre lasers at Toptica are based on saturated absorbers. The advantage of this technology over Kerr model locked lasers is that they can be built to be polarisation maintaining and reliably self starting. We characterise two femtosecond fibre lasers with respect to noise, spectral and temporal properties which are essential for applications in frequency referencing, pump-probe experiments and timing distribution. A balanced optical cross-correlation (BOC) setup is used to optimise radio frequency electronics for tight locking of two lasers and to characterise the intrinsic timing jitter, found to be 22 fs, integrated between 8 Hz and 39.3 MHz.


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.


Excitation of single Rydberg atoms in a new apparatusOctober 2012
Palaiseau, France

We have built a new experimental setup (below, left) dedicated to the trapping and Rydberg excitation of single atoms in an array of micron-sized optical tweezers. Our ability to cancel stray electric fields using eight independent electrodes around the traps has allowed us to obtain high-quality Rabi oscillations between the ground and Rydberg states of a single atom (below, right).

As a first step towards the entanglement of several atoms, we have observed the first signature of Rydberg blockade between two atoms in the new setup (right). When shining excitation lasers on two atoms separated by a small distance, such that they interact strongly, we observe a very strong reduction of the probability to excite both atoms to the Rydberg state (black dots), while the probability to excite only one of the two atoms (blue triangles) oscillates √2 times as fast as in the case of only one atom (orange diamonds).


A new magnetic-film atom chipOctober 2012
Amsterdam, The Netherlands

A new magnetic-film atom chip was fabricated and installed in our vacuum system. The chip hosts square and triangular lattices of magnetic microtraps, spaced by 10 µm. These will be used to trap ensembles of 10-100 atoms and, using the Rydberg dipole blockade, create entangled states.


Quantum state control of stored Rydberg polaritonsOctober 2012
Durham, UK

Using a weak signal and strong control light fields, we have achieved storage of single photons as Rydberg polaritons. Using a microwave field to drive oscillations of the stored polaritons between neighbouring Rydberg states, we can induce and control long-ranged dipole-dipole interactions between the stored photons. For weak microwave powers, the range of the interactions is increased beyond the separation between the polaritons which leads to dephasing observed as a suppression of the retrieved photon number. The controllable interactions between the polaritons could be exploited to perform quantum operations on photonic qubits, e.g. to implement a controlled phase gate.

Above left: The retrieved photon signal shows a signature of non-classicality imposed by the Rydberg blockade mechanism, namely the suppression of photon coincidences at zero time delay (red bar). Above right: collective oscillations of the retrieved photon number.

Reference:
D. Maxwell, D. J. Szwer, D. Paredes-Barato, H. Busche, J. D. Pritchard, A. Gauguet, K. J. Weatherill, M. P. A. Jones, and C. S. Adams, Storage and Control of Optical Photons Using Rydberg Polaritons, Phys. Rev. Lett. 110, 103001 (2013), or see our full list of publications
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.


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
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
Four wave mixing in a thermal gas involving Rydberg statesJune 2012
Stuttgart, Germany

Four-wave mixing is a spectroscopy technique to control the light emission of atoms in a coherent way. The combination of four-wave mixing with the strong interaction of Rydberg atoms allows for the generation of single photons, as it has been shown with a sample of ultracold atoms (Kuzmich et.al., Science 336, 887, 2012). Our plan is to convert this result to a thermal gas of atoms to build a room temperature single photon source. The observation of a four-wave mixing signal is a major milestone towards this goal.

Reference:
A. Koelle, G. Epple, H. Kuebler, R. Löw, and T. Pfau, Four-wave mixing involving Rydberg states in a thermal vapor cell, Phys. Rev A 85, 063821 (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
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
Trapping Rydberg atoms above the surface of a chipFebruary 2012
Zürich, Switzerland

Controlling the motion and the quantum state of atoms and molecules on the immediate vicinity of a surface is a prerequisite for quantum-information transfer between gas-phase particles and quantum dots located at the surface. At ETH Zürich, a new chip-based method has been developed to slow down supersonic beams of Rydberg atoms and molecules and to trap them above the surface of the chip. The method exploits the very large dipole moments of Rydberg atoms and molecules and the large inhomogeneous electric fields that can be generated above the surface of a chip with surface electrodes. In the experiment, a supersonic beam of hydrogen Rydberg atoms was decelerated from an initial velocity of 760 m/s to zero velocity and stored in an electric trap located just above the chip surface [1]. In complementary experiments, the quantum states of cold helium Rydberg atoms have been coherently manipulated with microwave fields emanating from microwave transmission lines mounted onto the chip surface [2], and the Zeeman effect in high Rydberg states of Cs has been studied at ultrahigh resolution using submillimeter-wave radiation and an ultracold Cs-atom sample in a MOT [3].

Reference:
[1] S. D. Hogan, P. Allmendinger, H. Saßmannshausen, H. Schmutz, and F. Merkt, Surface-Electrode Rydberg-Stark Decelerator, Phys. Rev. Lett. 108, 063008 (2012)
[2] S. D. Hogan, J. A. Agner, F. Merkt, T. Thiele, S. Filipp, and A. Wallraff, Driving Rydberg-Rydberg Transitions from a Coplanar Microwave Waveguide, Phys. Rev. Lett. 108, 063004 (2012)
[3] J. Deiglmayr, H. Saßmannshausen and F. Merkt, High-resolution spectroscopy of Rydberg states in an ultracold cesium gas, Phys. Rev. A 87, 032519 (2013), 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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