





Liechtenstein Prize for Florian MeinertFormer PhD student Florian Meinert got awarded the Liechtenstein Prize from the government of Liechtenstein. Every year this prize is given to two scientists from the University of Innsbruck and to two from the Medical University of Innsbruck to acknowledge outstanding research contributions. Florian Meinert received the award for his research on transport phenomena in quantum manybody systems, that he performed within the CsIII project. Photo IQOQI/Knabl. 

Bloch oscillations in the absence of a latticeF. Meinert, M. Knap, E. Kirilov, K. JagLauber, M. B. Zvonarev, E. Demler, H.C. Nägerl The interplay of strong quantum correlations and farfromequilibrium conditions can give rise to striking dynamical phenomena. We experimentally investigated the quantum motion of an impurity atom immersed in a strongly interacting onedimensional Bose liquid and subject to an external force. We found that the momentum distribution of the impurity exhibits characteristic Bragg reflections at the edge of an emergent Brillouin zone. Although Bragg reflections are typically associated with lattice structures, in our strongly correlated quantum liquid they result from the interplay of shortrange crystalline order and kinematic constraints on the manybody scattering processes in the onedimensional system. As a consequence, the impurity exhibits periodic dynamics, reminiscent of Bloch oscillations, although the quantum liquid is translationally invariant. Our observations are supported by largescale numerical simulations. 

SAMOP Thesis Prize and IQOQI Dissertation Prize for Florian MeinertFormer PhD student Florian Meinert received the SAMOP Thesis Prize 2017 from the German Physical Society (DPG), as well as the IQOQI Dissertation Prize 2017 for his outstanding work on the CsIII Project. Photo IQOQI/Knabl. 

Floquet engineering of correlated tunneling in the BoseHubbard model with ultracold atomsF. Meinert, M. J. Mark, K. Lauber, A. J. Daley, and H.C. Nägerl We report on the experimental implementation of tunable occupationdependent tunneling in a BoseHubbard system of ultracold atoms via timeperiodic modulation of the onsite interaction energy. The tunneling rate is inferred from a timeresolved measurement of the lattice site occupation after a quantum quench. We demonstrate coherent control of the tunneling dynamics in the correlated manybody system, including full suppression of tunneling as predicted within the framework of Floquet theory. We find that the tunneling rate explicitly depends on the atom number difference in neighboring lattice sites. Our results may open up ways to realize artificial gauge fields that feature density dependence with ultracold atoms. 

Probing the Excitations of a LiebLiniger Gas from Weak to Strong CouplingF. Meinert, M. Panfil, M.J. Mark, K. Lauber, J.S. Caux, H.C. Nägerl We probe the excitation spectrum of an ultracold onedimensional Bose gas of Cesium atoms with repulsive contact interaction that we tune from the weakly to the strongly interacting regime via a magnetic Feshbach resonance. The dynamical structure factor, experimentally obtained using Bragg spectroscopy, is compared to integrabilitybased calculations valid at arbitrary interactions and finite temperatures. Our results unequivocally underly the fact that holelike excitations, which have no counterpart in higher dimensions, actively shape the dynamical response of the gas. 

Observation of DensityInduced TunnelingO. Jürgensen, F. Meinert, M. J. Mark, H.C. Nägerl, D.S. Lühmann We study the dynamics of bosonic atoms in a tilted onedimensional optical lattice and report on the first direct observation of densityinduced tunneling. We show that the interaction affects the time evolution of the doublon oscillation via densityinduced tunneling and pinpoint its density and interactiondependence. The experimental data for different lattice depths are in good agreement with our theoretical model. Furthermore, resonances caused by secondorder tunneling processes are studied, where the densityinduced tunneling breaks the symmetric behavior for attractive and repulsive interactions predicted by the Hubbard model. 

Observation of manybody dynamics in longrange tunneling after a quantum quenchF. Meinert, M. J. Mark, E. Kirilov, K. Lauber, P. Weinmann, M. Gröbner, A. J. Daley, and H.C. Nägerl Quantum tunneling constitutes one of the most fundamental processes in nature. We observe resonantlyenhanced longrange quantum tunneling in onedimensional Mottinsulating Hubbard chains that are suddenly quenched into a tilted configuration. Higherorder manybody tunneling processes occur over up to five lattice sites when the tilt per site is tuned to integer fractions of the Mott gap. Starting from a oneatompersite Mott state the response of the manybody quantum system is observed as resonances in the number of doubly occupied sites and in the emerging coherence in momentum space. Second and thirdorder tunneling shows up in the transient response after the tilt, from which we extract the characteristic scaling in accordance with perturbation theory and numerical simulations. 

Interactioninduced quantum phase revivals and evidence for the transition to the quantum chaotic regime in 1D atomic Bloch oscillationsF. Meinert, M. J. Mark, E. Kirilov, K. Lauber, P. Weinmann, M. Gröbner, and H.C. Nägerl We study atomic Bloch oscillations in an ensemble of onedimensional tilted superfluids in the BoseHubbard regime. For large values of the tilt, we observe interactioninduced coherent decay and matterwave quantum phase revivals of the Bloch oscillating ensemble. We analyze the revival period dependence on interactions by means of a Feshbach resonance. When reducing the value of the tilt, we observe the disappearance of the quasiperiodic phase revival signature towards an irreversible decay of Bloch oscillations, indicating the transition from regular to quantum chaotic dynamics. 

Quantum quench in an atomic onedimensional Ising chainF. Meinert, M. J. Mark, E. Kirilov, K. Lauber, P. Weinmann, A. J. Daley, and H.C. Nägerl We study nonequilibrium dynamics for an ensemble of tilted onedimensional atomic BoseHubbard chains after a sudden quench to the vicinity of the transition point of the Ising paramagnetic to antiferromagnetic quantum phase transition. The quench results in coherent oscillations for the orientation of effective Ising spins, detected via oscillations in the number of doublyoccupied lattice sites. We characterize the quench by varying the system parameters. We report significant modification of the tunneling rate induced by interactions and show clear evidence for collective effects in the oscillatory response. 

Preparation and spectroscopy of a metastable Mott insulator state with attractive interactionsM. J. Mark, E. Haller, K. Lauber, J. G. Danzl, A. Janisch, H. P. Büchler, A. J. Daley, and H.C. Nägerl We prepare and study a metastable attractive Mott insulator state formed with bosonic atoms in a threedimensional optical lattice. Starting from a Mott insulator with Cs atoms at weak repulsive interactions, we use a magnetic Feshbach resonance to tune the interactions to large attractive values and produce a metastable state pinned by attractive interactions with a lifetime on the order of 10 seconds. We probe the (de)excitation spectrum via lattice modulation spectroscopy, measuring the interaction dependence of two and threebody bound state energies. As a result of increased onsite threebody loss we observe resonance broadening and suppression of tunneling processes that produce threebody occupation. 

Threebody correlation functions and recombination rates for bosons in three and one dimensionsE. Haller, M. Rabie, M.J. Mark, J.G. Danzl, R. Hart, K. Lauber, G. Pupillo, H.C. Nägerl We investigate local threebody correlations for bosonic particles in three and one dimensions as a function of the interaction strength. The threebody correlation function g(3) is determined by measuring the threebody recombination rate in an ultracold gas of Cs atoms. In three dimensions, we measure the dependence of g(3) on the gas parameter in a BEC, finding good agreement with the theoretical prediction accounting for beyondmeanfield effects. In one dimension, we observe a reduction of g(3) by several orders of magnitude upon increasing interactions from the weakly interacting BEC to the strongly interacting TonksGirardeau regime, in good agreement with predictions from the LiebLiniger model for all strengths of interaction. 

Precision Measurements on a Tunable Mott Insulator of Ultracold AtomsM. J. Mark, E. Haller, K. Lauber, J. G. Danzl, A. J. Daley, H.C. Nägerl We perform precision measurements on a Mottinsulator quantum state of ultracold atoms with tunable interactions. We probe the dependence of the superfluidtoMottinsulator transition on the interaction strength and explore the limits of the standard BoseHubbard model description. By tuning the onsite interaction energies to values comparable to the interband separation, we are able to quantitatively measure numberdependent shifts in the excitation spectrum caused by effective multibody interactions. 

Demonstration of the temporal matterwave Talbot effect for trapped matter wavesM. J. Mark, E. Haller, J. G. Danzl, K. Lauber, M. Gustavsson, H.C. Nägerl We demonstrate the temporal Talbot effect for trapped matter waves using ultracold atoms in an optical lattice. We investigate the phase evolution of an array of essentially noninteracting matter waves and observe matterwave collapse and revival in the form of a Talbot interference pattern. By using long expansion times, we image momentum space with subrecoil resolution, allowing us to observe fractional Talbot fringes up to 10^{th} order. 

Pinning quantum phase transition for a Luttinger liquid of strongly interacting bosonsE. Haller, R. Hart, M.J. Mark, J.G. Danzl, L. Reichsöllner, M. Gustavsson, M. Dalmonte, G. Pupillo, H.C. Nägerl One of the most remarkable results of quantum mechanics is the fact that manybody quantum systems may exhibit phase transitions even at zero temperature. Quantum fluctuations, deeply rooted in Heisenberg's uncertainty principle, and not thermal fluctuations, drive the system from one phase to another. Typically, the relative strength of two competing terms in the system's Hamiltonian is changed across a finite critical value. A wellknown example is the MottHubbard quantum phase transition from a superfluid to an insulating phase, which has been observed for weakly interacting bosonic atomic gases. However, for strongly interacting quantum systems confined to lowerdimensional geometry a novel type of quantum phase transition may be induced for which an arbitrarily weak perturbation to the Hamiltonian is sufficient to drive the transition. Here, for a onedimensional (1D) quantum gas of bosonic caesium atoms with tunable interactions, we observe the commensurateincommensurate quantum phase transition from a superfluid Luttinger liquid to a Mottinsulator. For sufficiently strong interactions, the transition is induced by adding an arbitrarily weak optical lattice commensurate with the atomic granularity, which leads to immediate pinning of the atoms. We map out the phase diagram and find that our measurements in the strongly interacting regime agree well with a quantum field description based on the exactly solvable sineGordon model. We trace the phase boundary all the way to the weakly interacting regime where we find good agreement with the predictions of the 1D BoseHubbard model. Our results open up the experimental study of quantum phase transitions, criticality, and transport phenomena beyond Hubbardtype models in the context of ultracold gases. 

Interference of interacting matter wavesM. Gustavsson, E. Haller, M. J. Mark, J. G. Danzl, R. Hart, A. J. Daley, H.C. Nägerl The phenomenon of matter wave interference lies at the heart of quantum physics. It has been observed in various contexts in the limit of noninteracting particles as a single particle effect. Here we observe and control matter wave interference whose evolution is driven by interparticle interactions. In a multipath matter wave interferometer, the macroscopic manybody wave function of an interacting atomic BoseEinstein condensate develops a regular interference pattern, allowing us to detect and directly visualize the effect of interactioninduced phase shifts. We demonstrate control over the phase evolution by inhibiting interactioninduced dephasing and by refocusing a dephased macroscopic matter wave in a spinecho type experiment. Our results show that interactions in a manybody system lead to a surprisingly coherent evolution, possibly enabling narrowband and highbrightness matter wave interferometers based on atom lasers. 

Inducing Transport in a DissipationFree Lattice with Super Bloch OscillationsE. Haller, R. Hart, M. J. Mark, J. G. Danzl, L. Reichsöllner, H.C. Nägerl Particles in a perfect lattice potential perform Bloch oscillations when subject to a constant force, leading to localization and preventing conductivity. For a weaklyinteracting BoseEinstein condensate (BEC) of Cs atoms, we observe giant centerofmass oscillations in position space with a displacement across hundreds of lattice sites when we add a periodic modulation to the force near the Bloch frequency. We study the dependence of these "super" Bloch oscillations on lattice depth, modulation amplitude, and modulation frequency and show that they provide a means to induce linear transport in a dissipationfree lattice. Surprisingly, we find that, for an interacting quantum system, super Bloch oscillations strongly suppress the appearance of dynamical instabilities and, for our parameters, increase the phasecoherence time by more than a factor of hundred. 

ConfinementInduced Resonances in LowDimensional Quantum SystemsE. Haller, M.J. Mark, R. Hart, J.G. Danzl, L. Reichsöllner, V. Melezhik, P. Schmelcher, H.C. Nägerl We report on the observation of confinementinduced resonances in strongly interacting quantumgas systems with tunable interactions for one and twodimensional geometry. Atomatom scattering is substantially modified when the swave scattering length approaches the length scale associated with the tight transversal confinement, leading to characteristic loss and heating signatures. Upon introducing an anisotropy for the transversal confinement we observe a splitting of the confinementinduced resonance. With increasing anisotropy additional resonances appear. In the limit of a twodimensional system we find that one resonance persists. 

An ultracold highdensity sample of rovibronic groundstate molecules in an optical latticeJ. G. Danzl, M. J. Mark, E. Haller, M. Gustavsson, R. Hart, J. Aldegunde, J. M. Hutson, H.C. Nägerl Control over all internal and external degrees of freedom of molecules at the level of single quantum states will enable a series of fundamental studies in physics and chemistry. In particular, samples of groundstate molecules at ultralow temperatures and high number densities will allow novel quantumgas studies and future applications in quantum information science. However, high phasespace densities for molecular samples are not readily attainable as efficient cooling techniques such as laser cooling are lacking. Here we produce an ultracold and dense sample of molecules in a single hyperfine level of the rovibronic ground state with each molecule individually trapped in the motional ground state of an optical lattice well. Starting from a zerotemperature atomic Mottinsulator state with optimized doublesite occupancy, weaklybound dimer molecules are efficiently associated on a Feshbach resonance and subsequently transferred to the rovibronic ground state by a stimulated fourphoton process with >50% efficiency. The molecules are trapped in the lattice and have a lifetime of 8 s. Our results present a crucial step towards BoseEinstein condensation of groundstate molecules and, when suitably generalized to polar heteronuclear molecules, the realization of dipolar quantumgas phases in optical lattices. 

Realization of an Excited, StronglyCorrelated Quantum Gas PhaseE. Haller, M. Gustavsson, M. J. Mark, J. G. Danzl, R. Hart, G. Pupillo, H.C. Nägerl Ultracold atomic physics offers myriad possibilities to study strongly correlated manybody systems in lower dimensions. Typically, only ground state phases are accessible. Using a tunable quantum gas of bosonic cesium atoms, we realize and control in one dimensional geometry a highly excited quantum phase that is stabilized in the presence of attractive interactions by maintaining and strengthening quantum correlations across a confinementinduced resonance. We diagnose the crossover from repulsive to attractive interactions in terms of the stiffness and the energy of the system. Our results open up the experimental study of metastable excited manybody phases with strong correlations and their dynamical properties. 

Deeply bound ultracold molecules in an optical latticeJ. G. Danzl, M. J. Mark, E. Haller, M. Gustavsson, R. Hart, A. Liem, H. Zellmer, H.C. Nägerl We demonstrate efficient transfer of ultracold molecules into a deeply bound rovibrational level of the singlet ground state potential in the presence of an optical lattice. The overall molecule creation efficiency is 25%, and the transfer efficiency to the rovibrational level v=73,J=2> is above 80%. We find that the molecules in v=73,J=2> are trapped in the optical lattice, limited by optical excitation by the lattice light. The molecule trapping time for a lattice depth of 15 atomic recoil energies is about 20 ms. We determine the trapping frequency by the lattice phase and amplitude modulation technique. It will now be possible to transfer the molecules to the rovibrational ground state v=0,J=0> in the presence of the optical lattice. 

Dark resonances for ground state transfer of molecular quantum gasesM. J. Mark, J. G. Danzl, E. Haller, M. Gustavsson, N. Bouloufa, O. Dulieu, H. Salami, T. Bergeman, H. Ritsch, R. Hart, H.C. Nägerl One possible way to produce ultracold, highphasespacedensity quantum gases of molecules in the rovibronic ground state is given by molecule association from quantumdegenerate atomic gases on a Feshbach resonance and subsequent coherent optical multiphoton transfer into the rovibronic ground state. In ultracold samples of Cs_{2} molecules, we observe twophoton dark resonances that connect the intermediate rovibrational level v=73,J=2> with the rovibrational ground state v=0,J=0> of the singlet X^{1}Σ^{+}_{g} ground state potential. For precise dark resonance spectroscopy we exploit the fact that it is possible to efficiently populate the level v=73,J=2> by twophoton transfer from the dissociation threshold with the stimulated Raman adiabatic passage (STIRAP) technique. We find that at least one of the twophoton resonances is sufficiently strong to allow future implementation of coherent STIRAP transfer of a molecular quantum gas to the rovibrational ground state v=0,J=0>. 

Quantum Gas of Deeply Bound Ground State MoleculesJ. G. Danzl, E. Haller, M. Gustavsson, M. J. Mark, R. Hart, N. Bouloufa, O. Dulieu, H. Ritsch, H.C. Nägerl We create an ultracold dense quantum gas of ground state molecules bound by more than 1000 wavenumbers by stimulated twophoton transfer of molecules associated on a Feshbach resonance from a BoseEinstein condensate of cesium atoms. The transfer efficiency exceeds 80%. In the process, the initial loose, longrange electrostatic bond of the Feshbach molecule is coherently transformed into a tight chemical bond. We demonstrate coherence of the transfer in a Ramseytype experiment and show that the molecular sample is not heated during the transfer. Our results show that the preparation of a quantum gas of molecules in arbitrary rovibrational states is possible and that the creation of a BoseEinstein condensate of molecules in their rovibronic ground state is within reach. 

Control of InteractionInduced Dephasing of Bloch OscillationsM. Gustavsson, E. Haller, M. J. Mark, J.G. Danzl, G. RojasKopeinig, H.C. Nägerl We report on the control of interactioninduced dephasing of Bloch oscillations for an atomic BoseEinstein condensate in an optical lattice under the influence of gravity. When tuning the strength of the interaction towards zero by means of a Feshbach resonance, the dephasing time is increased from a few to more than twenty thousand Bloch oscillation periods. We quantify the dephasing in terms of the width of the quasimomentum distribution and measure its dependence on time for different values of the scattering length. Minimizing the dephasing allows us to realize a BECbased atom interferometer in the noninteracting limit. We use it for a precise determination of a zerocrossing for the atomic scattering length and to observe collapse and revivals of Bloch oscillations when the atomic sample is subject to a spatial force gradient. 
2003–2009 START project Y227N02: Tunable quantum Matter for Precision Measurements 

2008 EuroQUASAR project QuDeGPM: Quantum Degenerate Gases for Precision Measurements 

2008–2010 Marie Curie Action by FP7, project MBEC: BoseEinstein Condensation of Ground State Molecules 

2009–2012 Project P 21555: Quantum Gases of Ground State Molecules 

2012–2016 ERC Starting Grant MicroQuant: Microscopy of Tunable ManyBody Quantum Systems 

2014–2016 Project I 1798N20: Control of ultracold quantum gases with shielded interactions Lead agency: ANR 

2016–2019 Project I 2922: Impurity dynamics in tunable 1D quantum gases Lead agency: ANR 

2016– Doktoratskolleg W 1259: Atoms, Light, and Molecules 

2018–2023 Wittgensteinpreis Z 336: exp. quantum physics, quantum gases, lowdimensional quantum systems, ultracold molecules 