This month, our paper on "Floquet Hamiltonian engineering of an isolated many-body spin system" got published in Science! In this work, by periodically driving an isolated spin system, we changed a naturally given many-body Hamiltonian into a desired target form. Using a sequence of time-periodic microwave pulses, we change the Hamiltonian of our interacting Rydberg-spin system from a Heisenberg XX-model into an effective XYZ-model with tunable symmetry. As a consequence, the magnetization relaxation dynamic of the is drastically modified. The ability to engineering a wide range of Hamiltonians opens vast opportunities for implementing quantum simulation of non-equilibrium dynamics in a single experimental setting.

For more information:
University press release: https://www.uni-heidelberg.de/en/newsroom/programmable-interaction-between-quantum-magnets

A warm welcome to our news PhDs, Micheal and Tobias. Michael starts his PhD after have concluded his master thesis in our group studying the relation between Efimov scenario and Fermi polarons. Tobias joins us from Karlsruhe where he did his master thesis on optical fiber-based micro resonators. Good luck to both!

Our group participated to the Physics Institute’s effort to create an emergency ventilator to support patients suffering from COVID-19. David, Manuel, Eleonora, Binh and Saba gave a large contribution to this project child of this year of pandemy. The design of the HDvent Emergency Ventilator has now been published and full details are available open source on the HDVent website and on github!

For more information:
Website: HDVent
Github: HDVent on Github

Lauriane Chomaz received her nomination as Junior Professor of Heidelberg University and will start setting up her group "Quantum Fluids" and her new lab on quantum dipolar gases of Dy atoms at the Physikalisches Institut. In parallel, she will also join the Mixtures team to explore the physics of quantum atomic mixtures with extreme mass imbalance! Good luck and welcome to our team, Lauriane!

For more information:
Website: Quantum Fluids

Our paper on the " Scattering of two heavy Fermi polarons: Resonances and quasibound states” got published this month in PRA! We theoretically investigate the scattering properties and compute the scattering phase shifts and scattering lengths between two heavy impurities in an ideal Fermi gas at zero temperature. We find that impurities strongly and attractively interacting with the medium exhibit resonances in the induced scattering with a sign change of the induced scattering length and even strong repulsion.

Our paper on the "Fermions Meet Two Bosons—the Heteronuclear Efimov Effect Revisited" got published in Braz. J. Phys., in a special issue for Prof. Mahir S. Hussein! In this paper we theoretically investigate two limiting cases of the Efimov scenario, first, in vacuum, and second, in the presence of a Fermi Sea, focusing on the specific case of two heavy bosons and a light fermion. While the first case reproduces the well-known features of the Efimov effect, the second case provides novel insights serving as a precursor to understand effective interactions of Fermi polarons, i.e., strongly correlated impurities in a Fermi sea.

Our collaborative research centre (CRC 1225) studying “Isolated Quantum Systems and Universality in Extreme Conditions” has been granted and will continue its work in the second funding period, from July 2020 to 2024! Our group will continue to participate in the collaboration to the project A05 “Dynamics of tunable disordered many-body spin systems” and C03 “Fermi-Bose mixtures with large mass ratio”. Congratulation to everybody!

For more information:
Website: IsoQuant (CRC1225)

This month, our paper on a "Blockade-induced resonant enhancement of the optical nonlinearity in a Rydberg medium" got published in Phys. Rev. A! We predict the enhancement of the nonlinear optical response of the Rydberg gas as a consequence of a two-photon process that resonantly couples electronic states of a pair of atoms dressed by a strong control field. Moreover, we propose a realistic experimental scenario to observe the resonance by performing transmission measurements

Our paper on the "Observation of dipolar splittings in high-resolution atom-loss spectroscopy of Li-6 p-wave Feshbach resonances" got published this month in Phys. Rev. A Rapid Communications! We observed dipolar splitting in Li-6 p-wave Feshbach resonances by high-resolution atom-loss spectroscopy. The observed splittings are in very well agreement with coupled-channel calculations.

We collaborate with the molecular systems group of Professor Roland Wester from Innsbruck. Within this framework we met in Innsbruck at the Tiroler Bildungsinstitut Grillhof. Between tense and fruitful discussions about recent developments and future prospects of the project we enjoyed perfectly brewed cappuccino and a beautiful Alps panorama.

From left to right: Prof. Roland Wester, Dr. Milaim Kas, Jonas Tauch, Dr. Robert Wild, Markus Nötzold, Dr. Eric Endres, Saba Zia Hassan, Christine Lochmann, Prof. Matthias Weidemüller

Congratulations to Clément Hainaut who was awarded an Alexander von Humboldt Fellowship! Clément joined the Rydberg team as a postdoc for investigating relaxation dynamics of Rydberg spin systems as well as nonlinear photon interactions in a Rydberg EIT medium!

During her Master thesis Annika Tebben investigated the nonlinear optical response of an interacting Rydberg gas under conditions of electromagnetically induced transparency. In collaboration with the group of Thomas Pohl (Aarhus) she found a method to resonantly enhance the associated nonlinear susceptibility. For her outstanding work Annika was awarded the Otto-Haxel-Prize of the department of physics and astronomy of Heidelberg University. Congratulations!

From 22nd to 24th of October 2018 the PhD-Meeting of the Forschergruppe "FOR2247: From few to many-body physics with dipolar quantum gases" will take place at the Internationales Wissenschaftsforum Heidelberg (IWH) in Heidelberg. During the meeting, we are honoured to have lectures given by Prof. Dr. Tilman Esslinger and Prof. Dr. Olivier Dulieu concerning long-range interactions in systems of cavity-confined ultracold atoms and ultracold polar molecules.

For more information:
Website: FOR2247

Henry Lopez from the HAITRAP team successfully defended his thesis... Congratulations!

After the defense there was a little celebration in the foyer where Henry received his PhD hat (see picture). Definitely monkey approved!

Congratulations to Nithiwadee Thaicharoen who was awarded a Marie Skłodowska-Curie Fellowship! Nithiwadee joined the Rydberg team as a postdoc for investigating relaxation dynamics in disordered spin systems under tunable XYZ-Hamiltonians using microwave pulse sequences.

In a joint experiment and theory work we study the effect of the intraspecies scattering length onto the heteronuclear Efimov scenario, following up on our earlier observation of Efimov resonances in an ultracold Cs-Li mixture for negative and positive Cs-Cs scattering length. Three theoretical models of increasing complexity are employed to quantify its influence on the scaling factor and the three-body parameter: a simple Born-Oppenheimer picture, a zero-range theory, and a spinless van der Waals model. These models are compared to Efimov resonances observed in an ultracold mixture of bosonic 133Cs and fermionic 6Li atoms close to two Cs-Li Feshbach resonances located at 843 G and 889 G, characterized by different sign and magnitude of the Cs-Cs interaction. By changing the sign and magnitude of the intraspecies scattering length different scaling behaviors of the three-body loss rate are identified, in qualitative agreement with theoretical predictions. The three-body loss rate is strongly influenced by the intraspecies scattering length.

Juris Ulmanis from the mixtures team receives the Springer thesis award for his PhD work. Congratulations!

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 three-body systems, as well as to discern microscopic, non-universal 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 few-body physics. On top of a light primer on the Efimov effect, it highlights aspects of three-body 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.

For more information:
Read the book on Springer Theses

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 Li-Cs Mixture with a large Mass Ratio” which was selected as the best poster. Congratulations!

For more information:
Website: School on Interaction of Light with Cold Atoms

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.

The recently published special issue of the European Physical Journal (EPJ) reviews achievements of the EU Integrated Training Network on Rydberg Physics. The training network, which is going under the abbreviation COHERENCE and was funded from 2012 to 2015 by the European Commission, was devoted to promote young researchers in the field of Rydberg gases. The central research topic was the investigation of interactions among highly excited (Rydberg) atoms under extremely controlled conditions. This kind of research has many applications in fundamental science and technology at the crossroads between atomic, molecular and condensed matter physics, quantum optics, quantum simulation, and others. Our research group, representing Heidelberg University, was one of the leading teams among 18 European and US universities and research institutions that were involved in the network. During the project more than 100 articles in high-impact journals were published. This special issue summarizes the major scientific results of COHERENCE in a series of 15 review-style articles.

For more information:
Read the complete special issue here (open access!): Cooperativity and Control in Highly Excited Rydberg Ensembles - Achievements of the European Marie Curie ITN COHERENCE
Contribution from our group: Gavryusev et al., Interaction Enhanced Imaging of Rydberg P states, Eur. Phys. J. Spec. Top. 225, 2863 (2016)

The Interaction Enhanced Imaging technique allows to detect the spatial distribution of strongly interacting impurities embedded within a gas of background atoms used as a contrast medium. Here we present a detailed study of this technique, applied to detect Rydberg P states.

We experimentally realize fast and efficient threephoton excitation of P states, optimized according to the results of a theoretical effective two-level model. Few Rydberg P-state 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 non-destructive optical detection of single Rydberg atoms with high spatial and temporal resolution.

Vladislav Gavryusev from the Rydberg team successfully defended his thesis... Congratulations!

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 mini-vacuum 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!

This paper is a follow-up 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 rf-frequency 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 atom-to-ion mass ratio in order to achive the maximum cooling rate.

Bastian Höltkemeier from the HAITRAP team successfully defended his thesis... Congratulations!

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!

Our recent theory – experiment collaboration between physicists from Purdue, Kansas State and Heidelberg universities has revealed an unexpected universality in the three-body system consisting of two heavy and one light particles. The results for the first time showed that a class of atomic and nuclear few-body systems behaves identically. Since three-body physics is a critical building block towards theories describing strongly interacting many-body systems, these novel findings will be important for further studies of exotic quantum matter in extreme conditions. They will also contribute to the understanding of one of the oldest questions in quantum mechanics, namely, to what extent different physical systems can be described by the same fundamental laws of quantum mechanics.

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 quantum-mechanical and the three-body 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 Cs-Cs 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.

The University of Heidelberg and the University of Science and Technology of China have signed a Memorandum of Understanding to found a Joint German-Sino Institute for Advanced Quantum Science. The ceremony was witnessed by (left to right on the picture) the Chinese Vice Consul General, Mr. Wei-Ping Xing, the Secretary General of Anhui Province, Mr. Ying-Chun Wang, the Excutive Director of the Hefei National Laboratory for Physical Sciences at the Microscale, Prof. Li Yuo, the Vice Rector of the University of Heidelberg, Prof. Stephen Hashmi, the Representative of the Baden-Württemberg Ministery for Science, Education and the Arts, Ms. Martina Diesing, and the Director of the Heidelberg Center for Quantum Dynamics, Prof. Matthias Weidemüller.

For more information:
The press release of USTC (in Chinese): Memorandum of Understanding signed between China University of Science and Technology and Heidelberg University

The full one-body density matrix of an ultracold gas of three-level atoms under electromagnetically induced transparency conditions can be reconstructed through a combined measurement and analysis of the spatially resolved optical spectrum and of the total excited-atom number.

Our method gives a simple explanation to the counter-intuitive 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 three-level media and will facilitate new studies of many-body effects in optically driven Rydberg gases.

Was geschah im Moment des Urknalls? Woraus bestehen Raum und Zeit? Welche Rolle spielten Zufall und Notwendigkeit bei der Entstehung der Welt? Auf diese Fragen findet die Physik unterschiedliche Antworten: Albert Einsteins Relativitätstheorie beschreibt die Struktur von Raum und Zeit und damit die Makrowelt, die Welt im Großen. Mit der Quantenphysik lässt sich die Mikrowelt der Atome und Elementarteilchen erklären. Beide Theorien bringen uns den Antworten auf diese großen Fragen ein Stück näher, und doch lassen sie sich nicht miteinander in Einklang bringen. Seit über einem Jahrhundert fasziniert dieser Widerspruch die Wissenschaft und bis heute sucht sie nach einer “Weltformel”, um die unvollendete Geschichte über den Ursprung der Welt zu Ende zu erzählen. (LIT:potsdam)

Antworten auf diesen Fragen suchen auch Matthias Weidemüller mit Daniel Kehlmann und Jürgen Neffe auf dem Literaturfestival LIT:potsdam am 10 Juli 2016.

For more information:
Weitere Information und Programm: LIT:potsdam

It has been common wisdom, starting from early work of Dehmelt and others, that one cannot cool ions in a radiofrequency (RF) trap with a buffer gas if the mass ratio between the buffer gas atom and ion exceeds a critical value. In this paper, we theoretically show that one can overcome this dogma by using a spatially localized buffer gas and/or a higher multipole order for the radial trapping potential. These approaches make use of the fact, that the principle hindrance of sympathetic cooling inside an RF trap arises from a collisionally induced energy transfer between the RF-driven micromotion and the macromotion (see Figure). Thus, spatially restricting collisions to the volume of minimal micromotion and/or reducing the average micromotion altogether, leads to an increased critical mass ratio, enabling the use of heavier buffer gases.

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 RF-trap 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.

The Japanese Society for the Promotion of Science, in cooperation with the Heidelberg Center for Quantum Dynamics, organized a joint German-Japanese 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 (DFG) funds the Collaborative Research Center on "Isolated quantum systems and universality in extreme conditions" (ISOQUANT). The center will explore universal properties of different quantum systems that range from particle and nuclear to atomic, molecular, and solid state physics. A special focus will lie on conditions, under which the behaviour of such apparently disparate systems can be identical, despite the vastly different temperature, density, or field strength that characterizes them. ISOQUANT comprises research groups and projects from Institute for Theoretical Physics, Kirchoff-Institute for Physics, Physics Institute, and Max-Planck Institute for Nuclear Physics in Heidelberg and Vienna University of Technology.

Our group is embedded in the framework of ISOQUANT with two projects. We aim to employ strong and tunable interactions in ultracold Bose-Fermi 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 solid-state physics can be simulated in this way. The second project addresses quantum systems with long-range interactions, which are ideal for the investigation of non-equilibrium dynamics at strong coupling. Our experiments will employ ultracold Rydberg atoms to identify common characteristics that govern quantum fluctuations and relaxation in strongly-coupled spin systems and quantum fluids.

For more information:
DFG press release (in German): DFG fördert 20 neue Sonderforschungsbereiche
Heidelberg University press release (in German): Universität Heidelberg mit vier Förderanträgen für Sonder­forschungs­bereiche erfolgreich
Collaborative Research Center homepage (in German): SFB 1225 ISOQUANT

Matthias Weidelmüller has received the first Hefei Friendship Award. This year ten foreign experts from seven different countries received the newly established and prestigious prize, which recognizes outstanding contributions to Hefei's economic and social progress. The award ceremony was held on 19th April in University of Science and Technology (USTC), China. Congratulations!

For more information:
USTC press release: Weidemüller wins first Hefei Friendship Award
Hefei local news (in Chinese): Towards better world with quantum gases

We have observed a series of three consecutive Efimov resonances in a three-body recombination spectrum of an ultracold mixture of Li and Cs atoms for the first time. This is an important benchmark for universal zero-range theories, and is considered to be a key ingredient for the fundamental understanding of the transition between few- and many-body physics.

Universal phenomena do not depend on the details of the underlying two-body 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 three-body 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 non-universality due to residual van der Waals interaction could also be observed. This knowledge will be crucial for the further investigation of many-body properties of quantum matter in extreme conditions and could shed light on the structure of very different few-body systems.

After a successfull thesis defense Juris Ulmanis from the Mixtures team obtains his PhD degree. Congratulations!

Renato will work at the Rydberg experiment where he will investigate many-body physics involving strongly interacting Rydberg gases. Welcome to the group!

We can now reliably excite Rydberg atoms in an |nP> state via a three photon off-resonant excitation scheme. This is an important step towards the achievement of single Rydberg atom sensitivity in the Interaction Enhanced Imaging technique that we recently developed and demonstrated. In this method, the interaction between a Rydberg state that we call "impurity" and another one called "probe", coupled via Electromagnetically Induced Transparency (EIT) to the ground state, is leveraged to change the optical properties of the atom cloud, which would be transparent due to EIT, and make it absorptive only in the neighbourhood of the impurity, thus allowing to detect its position. We can now use |nP> states as impurities and |nS> as probing ones to leverage their strong dipole-dipole interactions, which lead to an increase of absorption per each impurity.

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.

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.

We study the interspecies scattering properties of ultracold Li-Cs mixtures in their two energetically lowest spin channels in the magnetic field range between 800 G and 1000 G. Close to two broad Feshbach resonances we create weakly bound LiCs dimers by radio-frequency association and measure the dependence of the binding energy on the external magnetic field strength. Based on the binding energies and complementary atom loss spectroscopy of three other Li-Cs s-wave Feshbach resonances we construct precise molecular singlet and triplet electronic ground state potentials using a coupled-channels calculation. We extract the Li-Cs interspecies scattering length as a function of the external field and obtain almost a ten-fold improvement in the precision of the values for the pole positions and widths of the s-wave Li-Cs Feshbach resonances as compared to our previous work [Pires et al., Phys. Rev. Lett. 112, 250404 (2014)]. We discuss implications on the Efimov scenario and the universal geometric scaling for LiCsCs trimers.

We comprehensively compare three different models for the description of Feshbach resonances: The coupled-channel calculation, the asymptotic bound state model (ABM), and the multichannel quantum defect theory (MQDT). All models describe our previously measured Li-Cs Feshbach resonances accurately. This work demonstrates on the example of Li-Cs, how measured Feshbach resonances can be interpreted by use of simple models.

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 Li-Cs. 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 many-body physics.

This week both Hanna Schempp from the Rydberg team, and Rico Pires from the mixtures team successfully defended their PhD theses: Congratulations!

We observed universal Efimov trimer states in a mixed quantum gas, where they appear with a universal scaling factor that is different from homogeneous systems and - in our case - is small enough to reveal a series of trimer states.

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 so-called Feshbach resonance, where the interaction energy between Li and Cs is tunable, we let the atoms interfere with several three-body bound states of the underlying scattering resonance. When the Efimov resonance is hit, the atoms experience an enhanced three-body loss and we record the three-body loss rate by a time-resolved 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.

For more information:
University press release: www.uni-heidelberg.de/presse/news2014
wired.com: Physicists Prove Surprising Rule of Threes
science20.com: A Universal Solution For A Quantum Three-Body Problem

We successfully produced quantum degenerate gases of bosonic ¹³³Cs and fermionic ⁶Li. This allows us to explore new physics in many exciting experiments with mixed quantum gases.

Li is loaded directly after magneto-optical 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 magneto-optical trap they are cooled by degenerate Raman side-band cooling into a reservoir trap. Before the final evaporation about 85000 Cs atoms are transferred into the dimple trap and Bose-Einstein condensation is reached for final laser powers between 130 and 150 mW.

The Japanese Society for the Promotion of Science, in cooperation with the Heidelberg Center for Quantum Dynamics, organized a joint German-Japanese Colloquium on “Frontiers of Laser Science”. The symposium explored recent developments in modern Atomic, Molecular, Optical Physics and Quantum Optics with special emphasis on advances in fundamental science based on the application of lasers. Leading scientists from Japan and Germany presented and discussed their contributions to the field, opening new perspectives for ongoing or prospective scientific collaborations between both countries. The format of the symposium was similar to the 2011 “Round Table on the Basic Natural Science”, also held at the IWH.

Full Counting Statistics (FCS) can provide valuable information on manybody systems especially if the underlying correlations cannot be directly imaged.

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 build-up of correlations in manybody systems.

Eva Kuhnle receives a grant from the Baden-Württemberg Stiftung for the application of Bragg spectroscopy to strongly correlated Bose-Fermi mixtures of ¹³³Cs and ⁶Li. This support allows us to perform exciting new experiments with ultracold Bose-Fermi mixtures and to investigate their interaction properties.

For more information:
Baden-Württemberg Stiftung: www.bwstiftung.de

By synthesising an artificial quantum system, we have simulated key processes of photosynthesis on a quantum level with high spatial and temporal resolution and discovered new properties of energy transport. This work is an important step towards answering the question how quantum physics can contribute to the efficiency of energy conversion in synthetic systems, for example in photovoltaics.

From a gas of ground state atoms we excited some atoms to highly excited Rydberg states. Similar to the light-harvesting 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 Rydberg-Rydberg interactions or the interaction with the laser fields.

For more information:
University press release: www.uni-heidelberg.de/presse/news2013
pro-physik.de: Photosynthese auf Quantenebene nachgestellt, http://www.pro-physik.de/details/news/

Miguel will work at the Rydberg experiment where he will investigate many-body 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 quantum-degenerate gases of two-electron systems. Welcome to the group!

Matthias Weidemüller was appointed QianRen B Professor holding a Chair in the Division of Quantum Physics and Quantum Information of the National Laboratory for Physical Sciences at the Microscale at the University of Science and Technology of China (USTC), one of China's C9 universities . QianRen B Professorships are part of the 1000Talent Program of the Chinese Government. The activities in China involve setting up a new laboratory at the campus of the USTC in Shanghai for experiments on ultracold Rydberg gases and quantum-degenerate gases of two-electron systems.

With the awarded Hengstberger-Prize for young scientists at the University of Heidelberg, Christoph Hofmann, Eva Kuhnle and Shannon Whitlock will host a symposium on the topic of "Shedding light on emergent quantum phenomena".

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 many-body phenomena in interacting quantum systems.

For more information:
Official announcement from the anniversary celebration of the University of Heidelberg http://www.uni-heidelberg.de/presse/meldungen/2013
Official announcement of the IWH: www.iwh.uni-hd.de
University press release: www.uni-heidelberg.de/presse/news2013
Official conference webpage: emergence2014.uni-hd.de

Electromagnetically-induced transparency (EIT) and the associated appearance of hybrid quasi-particles (dark-state polaritons) in ultracold Rydberg gases have opened intriguing perspectives to create new atom-light interfaces operating at the quantum level and with fully tunable interactions.

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 long-range interactions between Rydberg atoms give rise to an effective interaction blockade for dark-state polaritons, which results in large optical nonlinearities and modified polariton number statistics. Our work provides a better understanding of strongly-interacting dark-state polaritons and creates new avenues in the area of single photon nonlinear optics and for the generation of nonclassical states of light and matter.

Marc Repp (Mixtures team) successfully defends his PhD thesis: Congratulations!

After a big team effort and tremendous support from our mechanical workshop we moved our laboratories from the former Physics Institute, situated at Heidelbergs 'Philosophenweg' to the new Physics Building on the science campus in the 'Neuenheimer Feld'.

We demonstrated how to use a high power LED to create short ion bunches.

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.

Henry will work at the new MOTRIMS experiment where he will investigate the interactions of ions and ultra-cold atoms. Welcome to the group!

Christoph Hofmann (Rydberg team) defends his PhD thesis with highest possible distinctions: Congratulations!

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, 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 disorder-induced-heating, and offer a route to enter new strongly-coupled regimes.

Vladislav (previously from Florence, Italy) is a student of the COHERENCE network and will join the Rydberg project working on atom-light interfaces using strongly-interacting Rydberg atoms

Simone Götz defends her PhD thesis with distinction: Magna Cum Laude. Congratulations!

We have tested our Rydberg detector and made our first simultaneous experiments looking at optical transmission (under electromagnetically induced transparency conditions) and Rydberg atoms detected via field ionisation. The figure shows the optical transmission (red) for a weak probe in the presence of a strong coupling to the Rydberg state, and the number of Rydberg atoms detected directly after the excitation pulse (black). The capability to coherently excite Rydberg atoms, to look at their optical properties, precisely control electric fields and to count Rydberg atoms will give us complete access to the microscopic and macroscopic properties of our system.

We propose a new approach to imaging individual Rydberg atoms, by exploiting their interaction with a bath of easily interrogated probe atoms.

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 high-fidelity readout of atomic quantum registers, or as a precise way to observe individual charges or defects near surfaces.