Latest research highlights:
click titles to expand/contract each highlight
Storage and retrieval of optical photons in new experimental setup  July 2015 
Durham, UK 
Recently, we completed building and optimisation of a new experimental apparatus to study strong and longranged 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 sidebyside (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).


TOPTICA, Graefelfing, Germany 
We demonstrate and characterize an Er:fiber frequency comb with repetition rate of 80 MHz which is passively phasestabilized via difference frequency generation (DFG). A universal method to measure the phase noise spectra of comb lines at different wavelengths is demonstrated. With repetitionrate 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 1216, Denver, Colorado  USA, 248, (2015), )
, or see our full list of publications

From classical to quantum nonequilibrium dynamics of Rydberg excitations in optical lattices  2015 
Innsbruck, Austria 
Ultracold atoms excited to Rydberg states are a powerful tool to study manybody physics. Strong and longrange 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 socalled OneSpin 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 antiblockade configuration, i.e. setting the laser detuning of the coupling between the groundstate and the Rydberg state to be exactly equal to the nearestneighbour interaction between Rydbergexcited 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 Rydbergexcitation of any neighbouring atom is strongly suppressed by a large detuning. In the setup we propose, the coupling from the groundstate to an additional intermediate shortlived excited state can tune the regimes of the dynamics of Rydberg excitations. The two extremes are: the classical rate equation limit of the aforementioned OneSpin 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 nonequilibrium 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 atoms  2015 
Oxford, UK 
Hydrogen Rydberg atoms interacting with a surface undergo ionisaton by transferring the electron into the bulk. The electron transfer happens at surfaceatom 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 ‘bandgap 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 bandgap surface, arXiv:1504.07191 (2015), or see our full list of publications

A new threebody interaction in cold Rydberg atoms  2015 
Paris, France 
The strong dipoledipole 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 threebody 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 twobody resonance where the relay atoms carry away the energy excess preventing the twobody resonance, leading thus to a Borromean type of energy transfer. This threebody FRET is a general process that can be observed in every atomic species which presents a twobody FRET. We also propose the generalization of this process based on a relay atoms energy exchange to nbody interaction paving the way between twobody and manybody physics. This threebody interaction is also a promising tool for the implementation of threebody quantum gate or entanglement. (Illustration by Emmanuelle Pelle)
 Reference: R. Faoro, B. Pelle, A. Zuliani, P. Cheinet, E. Arimondo and P. Pillet, Borromean threebody 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 nonnegligible mechanical effect, especially when the atoms are excited at close range through the facilitation process for offresonant 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 interparticle distances of around 5 µm are created by first generating a small number of seed excitations in a magnetooptical trap, followed by offresonant 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 offresonantly 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 HakenReinekerStrobltype 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 manybody 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 RydbergAtom Ensembles beyond the Superatom Model
 December 2014 
Heidelberg, Germany 
In an ensemble of laserdriven atoms involving strongly interacting Rydberg states, the steadystate 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 manybody collective quantum states.
 Reference: M. Gärttner, S. Whitlock, D. W. Schönleber and J. Evers, Collective Excitation of RydbergAtom 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 onebody 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 OneDimensional Geometry
 January 2014 
Heidelberg, Germany 
Full Counting Statistics (FCS) can provide valuable information on manybody systems especially if the underlying correlations cannot be directly imaged.
We apply this method to Rydberg excitations to gain information on Rydberg interacting manybody 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 OneDimensional Geometry, Phys.Rev.Lett. 112, 013002 (2014), or see our full list of publications

An experimental approach for investigating manybody phenomena in Rydberginteracting quantum systems
 September 2014 
Heidelberg, Germany 
We describe a tailored experimental system used to produce and study Rydberginteracting 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 BoseEinstein condensation transition. An electrode structure surrounding the atoms allows for precise control over electric fields and singleparticle sensitive field ionization detection of Rydberg atoms. With this setup we performed two experiments which highlight the influence of strong RydbergRydberg interactions on different manybody systems. First, the Rydberg blockade effect is used to prestructure an atomic gas prior to its spontaneous evolution into an ultracold plasma. Second, hybrid states of photons and atoms called darkstate polaritons are studied. By looking at the statistical distribution of Rydberg excited atoms we reveal correlations between darkstate polaritons.
 Reference: C. S. Hofmann, G. Günter, H. Schempp, N. M. L. Müller, A. Faber, H. Busche, M. RobertdeSaintVincent, S. Whitlock and M. Weidemüller, An experimental approach for investigating manybody phenomena in Rydberginteracting quantum systems, Frontiers of Physics 9, 571586 (2014), or see our full list of publications

A new scattering channel in longrange Rydberg molecules  2015 
Zurich, Switzerland 
The scattering of a quasifree Rydberg electron, being only weakly bound to its ion core, off a groundstate 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 groundstate atom on these molecular potentials: The binding energies of some of the longrange 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 groundstate atom. This dependence is explained by a model of the electrongroundstateatom 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 zeroenergy singlet scattering length in electronalkali collisions.
 Reference: H. Saßmannshausen, F. Merkt and J. Deiglmayr
, Experimental Characterization of Singlet Scattering Channels in LongRange Rydberg Molecules.
, Phys. Rev. Lett. 114, 133201 (2015)
, or see our full list of publications

Longrange interatomic forces with a twist  2014 
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 Rydbergatom pair states. The underlying mechanism is the longrange 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 quasidegeneracy of rotational states of the collision complex with opposite parity.
 Reference: J. Deiglmayr, H. Saßmannshausen, P. Pillet and F. Merkt
, Observation of DipoleQuadrupole 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 studies  2015 
Pisa, Italy 
Ultracold gases excited to strongly interacting Rydberg states offer the possibility to study a wide range of problems
of manybody 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 boseeinstein condensate  2015 
Pisa, Italy 
In the present work, the Pisa group investigates the excitation to a highlying Rydberg state in an atomic sample
undergoing the BoseEinstein condensation phase transition, in the limit that the spatial dimensions of the condensed
cloud are smaller than the (vanderWaals) 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 boseeinstein condensate
, Journal of Physics: Conference Series 594, 012041 (2015)
, or see our full list of publications

Strongly correlated excitation of a quasi1d rydberg gas  2014 
Pisa, Italy 
In this work the Pisa group presents a study of the dynamics of an efective 1DRydberg sample excited on and
offresonance. In both cases the longrange 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 offresonant regime is governed by a complex interplay between singleatom and pair excitations mediated by
the RydbergRydberg 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 subPoissonian character for the resonant
counting distributions, and bimodal character for the offresonant excitation case.
 Reference: N. Malossi, M. M. Valado, S. Scotto, D. Ciampini, E. Arimondo and O. Morsch, Strongly correlated excitation of a quasi1d 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 gas  2014 
Pisa, Italy 
Strongly interacting Rydberg systems in cold gases have been shown as promising system for quantum simulations
of manybody 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 manybody 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 Cloud  2013 
Pisa, Italy 
By realizing a scan in position across the cold cloud (see Figure below) the team at University of Pisa and CNRINO
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 twophoton 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 surfaces  2014 
Oxford, Great Britain 
We have investigated the collisions between H atom Rydberg states and a variety of surfaces, including (a) ptype and ntype 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 coincidence between the Rydberg energy and a quantized surface state in the onedimensional 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. 24952503, 2014 , or see our full list of publications

Scattering resonances and bound states for strongly interacting Rydberg polaritons  November 2014 
Stuttgart, Germany 
In this work, we demonstrate a theoretical framework describing slowlight 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 polaritonpolariton interactions are repulsive. Furthermore, in the regime of attractive interactions, we identify multiple twopolariton 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

Magneticfilm atom chip  May 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. TorralboCampo, 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, Magneticfilm 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 BoseEinstein condensate  2013 
Stuttgart, Germany 
We created a system in which a single electron interacts with a microscopic quantum object – a BoseEinstein 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 BoseEinstein condensate, Nature 502, 664 (2013), or see our full list of publications

Observation of resonant energy transfer in a ‘spinchain’ 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 dipoledipole interactions, a spin excitation can “hop” from one site to the other. We directly observed longrange 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 fields  2014 
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 socalled 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 dipoledipole coupling between two single Rydberg atoms at an electricallytuned Förster resonance, Nature Physics 10, 914 (2014), or see our full list of publications

Singleatom addressing in an array of microtraps  August 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 twoatom quantum state, transforming a ‘superradiant’ state to a subradiant 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 , Singleatom addressing in microtraps for quantumstate 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 atoms  Mai 2014 
CNRS, France 
In this work, we have demonstrated the loading of single atoms in arbitrary, twodimensional 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 spatiallight 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 , SingleAtom 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 interactions  Mai 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 ThreeAtom Systems with Anisotropic Interactions, Phys. Rev. Lett. 112, 183002 (2014), or see our full list of publications

Rydberg spectroscopy of laser cooled Holmium  2014 / 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 magnetooptical 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, Magnetooptical 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

Dipolemediated energy transport of Rydbergexcitations (glowing balls) in an atomic sea – artist impression  September 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. RobertdeSaintVincent, V. Gavryusev, S. Helmrich, C.S. Hofmann, S. Whitlock, M. Weidemüller, Observing the Dynamics of DipoleMediated Energy Transport by Interaction Enhanced Imaging, Science, 342 (2013), or see our full list of publications

Exciton dynamics in emergent Rydberg lattices  October 2013 
Nottingham, UK in collaboration with Durham, UK 
Gases of Rydberg atoms are a flexible tool for the study of manybody 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 nonequilibrium 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 Microtraps  October 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 lowlying states to interacting Rydberg pairstates (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 photons  October 2013 
Durham, UK 
Nonclassical states of light are a key resource in photonic quantum technologies. One way to observe nonclassical 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 nonclassical (left). Now we have shown that with the addition of a microwave control field, the degree of ``nonclassicality '' 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 molecules  October 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 atoms  June 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 longrange 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 gas  June 2013 
Pisa, Italy 
Dynamical phase transitions are expected to occur in strongly interacting ultracold gases under dissipative conditions. Among other characteristics, these systems exhibit intermittency and multimodal 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 superPoissonian fluctuations in that phase diagram correspond to bimodal countingdistributions compatible with intermittency due to phase coexistence.


Spatial tomography of Rydberg excitations in a MOT  June 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 twophoton 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 UltralongRange Polar Rydberg Molecules  May 2013 
Hamburg, Germany 
The impact of an electric field on ultralong range molecules is shown to convert one of the rotational degrees of freedom of the molecule into a vibration and we encounter twodimensional 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 pwave 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 nonpolar molecular Rydberg states leads to the possibility of exciting the large angular momentum polar states via twophoton 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 twodimensional potential energy surfaces for the electrically dressed polar trilobite
states for E=150 V/m (aligned in zdirection) 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 UltralongRange Polar Rydberg Molecules, arXiv:1305.0391, or see our full list of publications

Trapping atoms on a magnetic lattice atom chip  May 2013 
Amsterdam, The Netherlands 
Cold rubidium atoms were trapped on a new magneticfilm 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.


SubPoissonian statistics of interacting darkstate polaritons  May 2013 
Heidelberg, Germany 
Electromagneticallyinduced transparency (EIT) and the associated appearance of hybrid quasiparticles (darkstate polaritons) in ultracold Rydberg gases opens the intriguing perspective to create atomlight 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 nonclassical photon correlations. Using our complementary approach, we show that interactions between darkstate polaritons also result in nonclassical statistics for the polariton number distribution.
 Reference: C. S. Hofmann, G. Günter, H. Schempp, M. RobertdeSaintVincent, M. Gärttner, J. Evers, S. Whitlock and M. Weidemüller, SubPoissonian statistics of Rydberginteracting darkstate polaritons, Phys. Rev. Lett. 110, 203601 (2013), or see our full list of publications

Measurement of 87Rb Rydbergstate hyperfine splitting in a roomtemperature vapor cell  April 2013 
Amsterdam, The Netherlands 
Hyperfine splittings of Rydberg sstates in 87Rb were measured using electromagnetically induced transparency (EIT) spectroscopy in a roomtemperature 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 87Rb Rydbergstate hyperfine splitting in a roomtemperature vapor cell, Phys. Rev. A, 87, 042522, or see our full list of publications

Spontaneous avalanche ionization of a blockaded Rydberg gas  January 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 stronglycoupled regimes.
 Reference: M. RobertdeSaintVincent, 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 ensembles  December 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 Holmium  December 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 atoms  December 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 interactions  October 2012 
Open University, UK 
Deterministic quantum computation with one quantum bit (DQC1) is a model of quantum computation with an exponential speedup compared to known classical algorithms. Rather than entanglement, quantum discord is thought to be responsible for this speedup. 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 gas  October 2012 
Nottingham, UK 
Fibonacci anyons are particles which neither obey fermionic nor bosonic exchange statistics. These nonAbelian 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 socalled 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 straightforwardly implemented and anyonic observables can be measured. This demonstrates that a Rydberg gas is a wellsuited 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 socalled 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 Erfibre oscillators  October 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, pumpprobe experiments and timing distribution. A balanced optical crosscorrelation (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 onedimensional Rydberg lattices  October 2012 
Hamburg, Germany 
We focus on the excitation dynamics of a onedimensional lattice of Rydberg atoms. Such systems possess various wellcontrolled 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 apparatus  October 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 micronsized optical tweezers. Our ability to cancel stray electric fields using eight independent electrodes around the traps has allowed us to obtain highquality 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 magneticfilm atom chip  October 2012 
Amsterdam, The Netherlands 
A new magneticfilm 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 10100 atoms and, using the Rydberg dipole blockade, create entangled states.


Quantum state control of stored Rydberg polaritons  October 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 longranged dipoledipole 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 nonclassicality 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. ParedesBarato, 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 atoms  October 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 intercloud 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 molecules  October 2012 
Granada, Spain 
We are exploiting the long rangeinteractions 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 swave interaction, we find a rich and complex structure of energy surfaces and wave functions.


Observation of Blueshifted UltralongRange Cs₂ Rydberg Molecules  October 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 ultralongrange Rydberg molecules could form above the parent dissociation channels (blueshifted 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 UltralongRange Cs₂ Rydberg Molecules, Phys. Rev. Lett. 109, 173202 (2012), or see our full list of publications

Cooperative excitation and manybody interactions  July 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 subPoissonian 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 ManyBody 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 gas  July 2012 
Nottingham, UK 
Laserdriven Rydberg gases reach a steady state even when the quantum mechanical evolution of this manybody system is described fully coherently. We have studied the correlations between Rydberg atoms in the steady state of a onedimensional 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 manybody 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 buildup 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 laserdriven Rydberg gas, Phys. Rev. A 86, 013408 (2012), or see our full list of publications

Four wave mixing in a thermal gas involving Rydberg states  June 2012 
Stuttgart, Germany 
Fourwave mixing is a spectroscopy technique to control the light emission of atoms in a coherent way. The combination of fourwave 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 fourwave mixing signal is a major milestone towards this goal.
 Reference: A. Koelle, G. Epple, H. Kuebler, R. Löw, and T. Pfau, Fourwave 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 Ultracold Temperatures  May 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 dipoledipole 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 lifetime of Rydberg levels.
 Reference: S. D. Huber and H.P. Büchler, Dipole Interaction Mediated Laser Cooling of Polar Molecules to Ultracold Temperatures, Phys. Rev. Lett. 108, 193006 (2012), or see our full list of publications

Atomic Rydberg reservoirs for polar molecules  May 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 Rydbergdressed 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

Ultralongrange giant dipole molecules in crossed fields  February 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 electronproton 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 swave scattering potential leading to an ultralong giant dipole molecules (below left). Depending on the specific electronic excitation, the BornOppenheimer 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, Ultralongrange 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 chip  February 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 quantuminformation transfer between gasphase particles and quantum dots located at the surface. At ETH Zürich, a new chipbased 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 submillimeterwave radiation and an ultracold Csatom sample in a MOT [3].
 Reference: [1] S. D. Hogan, P. Allmendinger, H. Saßmannshausen, H. Schmutz, and F. Merkt, SurfaceElectrode RydbergStark Decelerator, Phys. Rev. Lett. 108, 063008 (2012) [2] S. D. Hogan, J. A. Agner, F. Merkt, T. Thiele, S. Filipp, and A. Wallraff, Driving RydbergRydberg Transitions from a Coplanar Microwave Waveguide, Phys. Rev. Lett. 108, 063004 (2012) [3] J. Deiglmayr, H. Saßmannshausen and F. Merkt, Highresolution 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 moment  November 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 ultralongrange 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 popularscience 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

back to the research page
