Physikalisches Institut
Im Neuenheimer Feld 226
69120 Heidelberg
Tel: 06221/ 54 19471
Fax: 06221/ 54 19545
Highlights from the lab
A detailed analysis of the role of the intraspecies scattering length in the Efimov scenario has been published in Phys. Rev. A  07.07.2017 

Reference: Role of the intraspecies scattering length in the Efimov scenario with large mass difference, Phys. Rev. A 95, 062708, or see our full list of publications 
The PhD work of Juris Ulmanis published in the book series Springer Thesis  10.03.2017 
His thesis explores the Efimov scenario, which is one of the prime examples of how fundamental quantum physics universally transpire across seemingly disparate fields of modern science. Initially speculated for nuclear physics more than 40 years ago, the Efimov effect has become a new research paradigm not only in ultracold atom physics but also in molecular, biological and condensed matter systems. In his work, Juris used a heteronuclear mixture of ultracold Li and Cs atoms to measure the scaling factor, which is a hallmark property and sometimes referred to as the “holy grail” of Efimov physics. These results allowed to pioneer experimental understanding of universal properties that unify the description of different threebody systems, as well as to discern microscopic, nonuniversal properties that sets different systems apart. The book features a completely rewritten introduction that is aimed at young scientists just starting in the field of fewbody physics. On top of a light primer on the Efimov effect, it highlights aspects of threebody physics in ultracold quantum gases and places these ideas in a wider context touching nuclear, atomic, and molecular physics. The rest of the work closely follows the original thesis.

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Binh Tran receives Poster Prize at Summer School in São Paulo  10.02.2017 
During the "School on Interaction of Light with Cold Atoms" in São Paulo for young students and researchers, Binh Tran from the Mixtures team presented a poster with the title “Towards creating Bose and Fermi Polarons in an ultracold LiCs Mixture with a large Mass Ratio” which was selected as the best poster. Congratulations!

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A detailed description of our atom trap and imaging system has been published in Applied Physics B  17.01.2017 
In order to precisely measure the density distribution in our dark sponataneous froce optical trap (DarkSPOT), we developed a novel imaging system which is based on saturation absorption imaging. The details of this setup and the characterization of our atom cloud were published as part of the topical collection “Enlightening the World with the Laser”  honoring T. W. Hänsch.

Reference: A dense gas of lasercooled atoms for hybrid atom–ion trapping, Applied Physics B 123.1, or see our full list of publications 
Review of the EU Training Network on Rydberg physics published as a special issue of the EPJ  04.01.2017 

For more information: 
Interaction Enhanced Imaging of Rydberg P states published in Eur. Phys. J. Special Topics!  19.12.2016 
We experimentally realize fast and efficient threephoton excitation of P states, optimized according to the results of a theoretical effective twolevel model. Few Rydberg Pstate atoms, prepared in a small cloud with dimensions comparable to the blockade radius, are detected with a good sensitivity by averaging over 50 shots. The main aspects of the technique are described with a hard sphere model, finding good agreement with experimental data. This work paves the way to a nondestructive optical detection of single Rydberg atoms with high spatial and temporal resolution.

Reference: Interaction Enhanced Imaging of Rydberg P states: Preparation and detection of Rydberg atoms for engineering longrange interactions, Eur. Phys. J. Special Topics 225, 2863–2889 (2016), or see our full list of publications 
Vladislav Gavryusev is now Dr. V. Gavryusev!  19.12.2016 
Afterwards he received his PhD hat (see picture). The hat came with a replica of one of the lasers used in the Rydberg experiment. Vladislav had to lock the laser to a little cavity and align it to the center of a minivacuum chamber (which was actually at room pressure...). Afterwards Vlad had to proof his german and dancing skills. In the end he mastered all the tasks his hat had to offer and now truly deserves his PhD!

Reference: Imaging of Rydberg Impurities in an Ultracold Atomic Gas, PhD thesis, or see our full list of publications 
The dynamics of an ion in a radiofrequency trap has been investigated  07.12.2016 
This paper is a followup on our recent publication in PRL (see news from 10.06.2016) investigating sympathetic cooling of a trapped ion using a neutral buffer gas. We provide a detailed analysis of the ions energy distribution and it dependence on different parameters such as the rffrequency and voltage of the trap as well as the mass and spatial distribution of the neutral buffer gas. We also provide an estimate for the optimal atomtoion mass ratio in order to achive the maximum cooling rate.

Reference: Dynamics of a single trapped ion immersed in a buffer gas, Phys. Rev. A 94, 062703 , or see our full list of publications 
Bastian Höltkemeier finishes his PhD!  27.10.2016 
After the defense there was a little celebration in the CQD lounge where Bastian received his PhD hat (see picture). Part of the hat is an actual ion trap which Bastian can use to also trap ions at home! Definitely monkey approved!

Reference: Sympathetic cooling of ions in a hybrid atom ion trap, University Library Heidelberg, or see our full list of publications 
New flavor of universal threebody physics discovered  10.10.2016 
In our studies we used an ultracold mixture of Li and Cs atoms to investigate the heteronuclear Efimov scenario. In this scenario, three particles, for our system one Li atom and two Cs atoms, bind together in a bound state even though none of the individual pairs can be bound (analogous to Borromean rings). In the experiments and theory we replaced the unbound Cs pair with a bound one. To our surprise, the resulting Efimov scenario was independent of molecular forces that govern chemical binding of atoms into molecules: the binding of the three atoms was purely quantummechanical and the threebody system became truly universal. In this regime it would not matter if one used atoms or nucleons with the same mass ratio and interactions. Furthermore, our experiments showed that the Efimov effect itself is severely modified by the same change of the fundamental nature of the CsCs bond. The peculiar nature of Efimov physics is illustrated in the figure. It shows the probability density distribution of the Li atom in a CsCsLi Efimov molecule. The two red balls along the central symmetry line indicate the two Cs atoms, which are separated by about 8 nm. In comparison, a typical chemist's molecule, such as CsCs, LiCs or LiCsCs, would have a spatial extent smaller than 1 nm, which was roughly the size of the dark spot between the two Cs atoms. Figure courtesy of Yujun Wang, Kansas State University.

Reference: Heteronuclear Efimov Scenario with Positive Intraspecies Scattering Length, Phys. Rev. Lett. 117, 153201 (2016), or see our full list of publications 
Signing ceremony of a Memorandum of Understanding for a Joint GermanSino Institute for Advanced Quantum Science  03.10.2016 

For more information: 
Density matrix reconstruction of threelevel atoms via Rydberg electromagnetically induced transparency published in J. Phys. B: At., Mol. Opt. Phys.!  22.07.2016 
Our method gives a simple explanation to the counterintuitive features observed in the spectra and provides the optical susceptibility and the Rydberg density as a function of spatial position, as well as the spatial profile of Rabi frequencies of the coupling laser. These results help elucidate the interplay of matter and light degrees of freedom in threelevel media and will facilitate new studies of manybody effects in optically driven Rydberg gases.

Reference: Density matrix reconstruction of threelevel atoms via Rydberg electromagnetically induced transparency, J. Phys. B: At., Mol. Opt. Phys. 49, 164002 (2016), or see our full list of publications 
Matthias Weidemüller mit Daniel Kehlmann und Jürgen Neffe auf dem Literaturfestival LIT:potsdam  07.07.2016 
Antworten auf diesen Fragen suchen auch Matthias Weidemüller mit Daniel Kehlmann und Jürgen Neffe auf dem Literaturfestival LIT:potsdam am 10 Juli 2016.

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Sympathetic cooling in multipole rftraps  10.06.2016 
The comprehensive model presented in this paper provides an intuitive picture of collisions in an RF trap based on a favorable frame transformation, where the micromotion is assigned to the neutral buffer gas. Using numerical simulations, we find three distinct dynamical regimes, characterized by analytical expressions for the ion's equilibrium energy distribution. These results not only comprise earlier studies on collisional cooling of ions but also predict a novel regime of stable cooling of ions beyond the critical mass ratio. In this regime one can actively tune the ions temperature by controlling the buffer gas' extension and/or the RFtrap fields (forced sympathetic cooling). Our findings are directly applicable to cooling of ions with laser cooled atoms or He buffer gas in Paul traps (as used in the quantum information and quantum simulation communities) or multipole traps (as used in the chemical reaction and astrochemistry communities). Especially for experiments investigating interactions of ions with an ensemble of ultracold atoms, the prospect of using heavier atom species, makes a whole new range of possible systems available that have not been studied yet.

Reference: BufferGas Cooling of a Single Ion in a Multipole Radio Frequency Trap Beyond the Critical Mass Ratio, Phys. Rev. Lett. 116, 233003 , or see our full list of publications 
GermanJapanese Collaborative Meeting on Rydberg Quantum Matter  09.06.2016 
The Japanese Society for the Promotion of Science, in cooperation with the Heidelberg Center for Quantum Dynamics, organized a joint GermanJapanese Collaborative Meeting on “Rydberg Quantum Matter” with the participation of industry researchers from Hamamatsu. The symposium explored recent developments in Rydberg physics and technology, with special focus on optical imaging with CCD cameras and SLM applications. Leading scientists and industry researchers from Japan and Germany presented and discussed their contributions to the field, opening new perspectives for ongoing or prospective scientific and industrial collaborations between both countries.

German Research Foundation funds the Collaborative Research Center ISOQUANT  02.06.2016 
Our group is embedded in the framework of ISOQUANT with two projects. We aim to employ strong and tunable interactions in ultracold BoseFermi mixtures of lithium and caesium atoms to explore the properties of composite quantum particles that consist of an impurity, which is strongly coupled to a surrounding quantum gas. Potentially, phenomena occurring in solidstate physics can be simulated in this way. The second project addresses quantum systems with longrange interactions, which are ideal for the investigation of nonequilibrium dynamics at strong coupling. Our experiments will employ ultracold Rydberg atoms to identify common characteristics that govern quantum fluctuations and relaxation in stronglycoupled spin systems and quantum fluids.

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Matthias Weidelmüller receives the first Hefei Friendship Award  03.05.2016 

For more information: 
Tests of universality in the heteronuclear Efimov scenario  16.02.2016 
Universal phenomena do not depend on the details of the underlying twobody potential or the type of interaction, and therefore can be found in various areas of modern quantum physics. A prototypical example is the Efimov scenario, where three resonantly interacting particles, be it elementary particles or neutral atoms, can bind into threebody bound states with an energy spectrum that follows an infinite geometrical progression, or in more general terms – the trimers exhibit a discrete scaling symmetry. The analysis of our data, which was performed in close collaboration with colleagues from Paris, France, revealed the existence of such universal scaling. At the same time, strong indications of nonuniversality due to residual van der Waals interaction could also be observed. This knowledge will be crucial for the further investigation of manybody properties of quantum matter in extreme conditions and could shed light on the structure of very different fewbody systems.

Reference: Universal threebody recombination and Efimov resonances in an ultracold LiCs mixture, Phys. Rev. A 93, 022707 (2016), or see our full list of publications 
Juris Ulmanis obtains his PhD  28.11.2015 
After a successfull thesis defense Juris Ulmanis from the Mixtures team obtains his PhD degree. Congratulations!

Reference: Universality and nonuniversality in the heteronuclear Efimov scenario with large mass imbalance, PhD thesis, or see our full list of publications 
We welcome our new PhD student Renato FerraciniAlves to the group!  01.11.2015 
Renato will work at the Rydberg experiment where he will investigate manybody physics involving strongly interacting Rydberg gases. Welcome to the group!

Three photon offresonant excitation of Rybderg nP> states  12.06.2015 
The first step of the excitation is done via a circularly polarized 780 nm beam, red detuned by 100 MHz from the ground to excited transition, then a circularly polarized 480 nm laser is used to get 100 MHz below a Rydberg nS> state. Finally the Rydberg nP> state is excited by applying a microwave radiation pulse, appropriately tuned to compensate for the detuning of the previous two steps. We apply a small magnetic field to remove the Zeeman degeneracy and to address a well defined Zeeman substate.

Reference: Observing the Dynamics of DipoleMediated Energy Transport by Interaction Enhanced Imaging, Science 342, 954 (2013), or see our full list of publications 
Maria Martinez Valado obtains her PhD  19.05.2015 
Congratulations to Maria Martinez Valado for successfully defending her thesis with title "Investigation of correlations between strongly interacting Rydberg excitations in cold gases using Full Counting Statistics" and for obtaining her PhD degree in a cotutelle between the University of Pisa and the University of Heidelberg.

Universality of weakly bound dimers and Efimov trimers close to Li–Cs Feshbach resonances published in New J. Phys.  14.05.2015 

Reference: Universality of weakly bound dimers and Efimov trimers close to Li–Cs Feshbach resonances, New J. Phys. 17, 055009 (2015), or see our full list of publications 
Analyzing Feshbach resonances: A LiCs case study published in Physical Review A  01.08.2014 
Since an exact analytical solution of the Schrödinger equation for the collision of two ultracold alkali atoms is not possible, assumptions have to be implemented in order to facilitate the calculation. One model, namely the ABM, which applies such assumptions, did not agree with our experimental findings (see [Repp et al., Phys. Rev. A 87, 010701(R) (2013)]). Spurred by this discrepancy, together with our collaborators we applied and compared three different methods for the calculation of Feshbach resonances. In the course of this analysis the ABM was extended so that it can also correctly describe the scattering behavior of a system where both a virtual and a bound state play a role, as is the case for LiCs. With this analysis we now have a very accurate characterization of the field dependent scattering length, which is required for our study of few and manybody physics.

Reference: Analyzing Feshbach resonances: A LiCs case study, Phys. Rev. A 90, 012710 (2014), or see our full list of publications 
Rico Pires and Hanna Schempp obtain their PhDs  24.07.2014 

Reference: Efimov Resonances in an Ultracold Mixture with Extreme Mass Imbalance, PhD thesis Formation of Aggregates and Energy Transport in Ultracold Rydberg Interacting Gases, PhD thesis, or see our full list of publications 
Observation of Efimov Resonances in a Mixture with Extreme Mass Imbalance published in Phys. Rev. Lett.!  25.06.2014 
Vitaly Efimov predicted already 40 years ago that there is a universal law for any three resonantly interacting particles and that this law has a discrete scaling behavior, i.e. that the resonances appear on a regular basis. We started with an ultracold gas of Li and Cs at temperatures as low as 400 nK and held the strongly interacting mixture in an optical dipole trap. By precisely changing the external magnetic field near a socalled Feshbach resonance, where the interaction energy between Li and Cs is tunable, we let the atoms interfere with several threebody bound states of the underlying scattering resonance. When the Efimov resonance is hit, the atoms experience an enhanced threebody loss and we record the threebody loss rate by a timeresolved measurement of the atom number. The scaling factor between resonances depends on the mass ratio, for which we measure a value of 5 from the series of resonances in good agreement with the theoretical prediction. While the ground Efimov state is about 50 nm in size, the 2nd excited Efimov state is very large with a length scale of almost one µm.

Reference: Observation of Efimov Resonances in a Mixture with Extreme Mass Imbalance, Phys. Rev. Lett. 112, 250404 (2014), or see our full list of publications For more information: 
Double degeneracy of Li and Cs in the mixtures lab!  01.03.2014 
Li is loaded directly after magnetooptical trapping into the crossed dimple trap at 140 W provided by an Yb fiber laser. Forced evaporation at 690 G leads to about 120000 atoms at 230 nK before the laser power is finally ramped down to values between 80 and 230 mW at which the bimodal distribution is visible. After Cs atoms are captured in a magnetooptical trap they are cooled by degenerate Raman sideband cooling into a reservoir trap. Before the final evaporation about 85000 Cs atoms are transferred into the dimple trap and BoseEinstein condensation is reached for final laser powers between 130 and 150 mW.

GermanJapanese Colloquium on Frontiers of Laser Science organised by Humboldt awardee Kenji Ohmori and Matthias Weidemüller  16.01.2014 

Full Counting Statistics of Laser Excited Rydberg Aggregates in a OneDimensional Geometry published in Phys.Rev.Lett.!  12.01.2014 
We have used the FCS of 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. We conclude that in the presence of dephasing these aggregates are formed via sequential excitation around an initial grain. Our work opens new perspectives for investigating the buildup of correlations in manybody systems.

Reference: 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 
Eva Kuhnle was accepted into the "Eliteprogramm für Postdoktorandinnen und Postdoktoranden" of the BadenWürttemberg Stiftung  20.12.2013 
Eva Kuhnle receives a grant from the BadenWürttemberg Stiftung for the application of Bragg spectroscopy to strongly correlated BoseFermi mixtures of ^{133}Cs and ^{6}Li. This support allows us to perform exciting new experiments with ultracold BoseFermi mixtures and to investigate their interaction properties.

For more information: 
Dynamics of dipolemediated Rydberg energy transport published in Science!  07.11.2013 
From a gas of ground state atoms we excited some atoms to highly excited Rydberg states. Similar to the lightharvesting complexes of photosynthesis, energy is transported from Rydberg atom to Rydberg atom, similar to a radio transmitter. To observe the transport of energy we use an electromagnetically induced transparency resonance, which makes up to 50 atoms absorb laser light within a characteristic radius around each Rydberg atom, making it possible to precisely measure the Rydberg atom distribution as a function of time. We were surprised to see that the Rydberg atoms quickly diffused from their original positions. Aided by a mathematical model we could show that the background gas of atoms crucially influences the energy transport dynamics, and the dynamics can be controlled by tuning the RydbergRydberg interactions or the interaction with the laser fields.

Reference: Observing the Dynamics of DipoleMediated Energy Transport by Interaction Enhanced Imaging, Science 342, 954 (2013), or see our full list of publications For more information: 
We welcome our new PhD students Miguel Ferreira Cao and Luc Couturier to the group!  01.10.2013 
Miguel will work at the Rydberg experiment where he will investigate manybody physics involving strongly interacting Rydberg gases. Luc will set up a new Rydberg experiment at the campus of the USTC in Shanghai involving ultracold Rydberg gases and quantumdegenerate gases of twoelectron systems. Welcome to the group!

Matthias Weidemüller appointed QianRen B Professor at USTC.  04.09.2013 

KlausGeorg und Sigrid HengstbergerPrize 2013 awarded to Christoph Hofmann, Eva Kuhnle, and Shannon Whitlock  01.07.2013 
The interaction of many simple elements, each obeying basic rules, often produces remarkably rich and complex new behavior. In the quantum realm, this gives rise to exotic effects such as superconductivity, superfluidity, magnetism, new types of 'quasi'particles, and new phases of matter. This symposium, to be held in spring 2014 at the Internationales Wissenschaftsforum Heidelberg (IWH) will be among the first to focus on emergent phenomena in ultracold quantum gases. Highlighting cutting edge research from around the world, including the University of Heidelberg, participants will strive to create a unified understanding of manybody phenomena in interacting quantum systems.

For more information: 
SubPoissonian statistics of Rydberginteracting darkstate polaritons published in Physical Review Letters  14.05.2013 
For the first time we give a complete picture of Rydberg interacting dark state polaritons by probing both the photonic and atomic degrees of freedom in a single experiment. Strong longrange interactions between Rydberg atoms give rise to an effective interaction blockade for darkstate polaritons, which results in large optical nonlinearities and modified polariton number statistics. Our work provides a better understanding of stronglyinteracting darkstate polaritons and creates new avenues in the area of single photon nonlinear optics and for the generation of nonclassical states of light and matter.

Reference: SubPoissonian statistics of Rydberginteracting darkstate polaritons, Phys. Rev. Lett. 110, 203601 (2013), or see our full list of publications 
Marc Repp obtains his Phd  08.05.2013 
Marc Repp (Mixtures team) successfully defends his PhD thesis: Congratulations!

Reference: Interspecies Feshbach Resonances in an Ultracold, Optically Trapped BoseFermi Mixture of Cesium and Lithium, PhD thesis, or see our full list of publications 
We have moved into the new Physics Institute!  03.04.2013 

Photoionization of optically trapped ultracold atoms with a LED  18.04.2013 
Light pulses as short as 30 ns have been realized with the simple LED driver circuit. We measure the ionization cross section of Rb atoms in the first excited state, and show how this technique can be used for calibrating efficiencies of ion detector assemblies.

Reference: Photoionization of optically trapped ultracold atoms with a highpower lightemitting diode, Rev. Sci. Instrum. 84, 043107 (2013), or see our full list of publications 
We welcome new PhD student Henry Lopez to the group!  01.04.2013 
Henry will work at the new MOTRIMS experiment where he will investigate the interactions of ions and ultracold atoms. Welcome to the group!

Christoph Hofmann obtains PhD with Summa Cum Laude  06.02.2013 
Christoph Hofmann (Rydberg team) defends his PhD thesis with highest possible distinctions: Congratulations!

Reference: Emergence of correlations in strongly interacting ultracold Rydberg gases, PhD thesis, or see our full list of publications 
Spontaneous plasma formation from a Rydberg blockaded gas  24.01.2013 
By combining optical imaging and ion detection, we access the full information on the dynamical evolution of the system, including the rapid increase in the number of ions and a sudden depletion of the Rydberg and ground state densities. The Rydberg blockade effect is observed to strongly affect the dynamics of plasma formation, and the initial correlations of the Rydberg distribution should persist through the avalanche. This may provide the means to overcome disorderinducedheating, and offer a route to enter new stronglycoupled regimes.

Reference: Spontaneous avalanche ionization of a strongly blockaded Rydberg gas, Phys. Rev. Lett. 110, 045004 (2013), or see our full list of publications 
Simone Götz obtains PhD with Magna Cum Laude  09.05.2012 
Simone Götz defends her PhD thesis with distinction: Magna Cum Laude. Congratulations!

Reference: A high density target of ultracold atoms and momentum resolved measurements of ionatom collisions, PhD thesis, or see our full list of publications 
Combined EIT and ion detection of Rydberg atoms  15.02.2012 

Interaction Enhanced Imaging of Individual Rydberg Atoms in Dense Gases  05.01.2012 
The basic scheme uses an electromagnetically induced transparency resonance to transfer large level shifts of the Rydberg states to the ground state transition of an atomic gas. Interactions alter the properties of a strong optical transition for many probe atoms within a critical radius, thereby providing two mechanisms which greatly enhance the effect of a single Rydberg impurity on the light field. We show this method can be used to directly image strong spatial correlations and crystalline states of Rydberg atoms, but it could also be used to study the effects of impurities and disorder on superfluids, to realize highfidelity readout of atomic quantum registers, or as a precise way to observe individual charges or defects near surfaces.

Reference: Interaction Enhanced Imaging of Individual Rydberg Atoms in Dense Gases, Phys. Rev. Lett. 108, 013002 (2012), or see our full list of publications 