## Publications

### Direct control of high magnetic fields for cold atom experiments based on NV centers

2020 PDF-FileIn atomic physics experiments, magnetic fields allow to control the interactions between atoms, eg. near Feshbach resonances, or by employing spin changing collisions. The magnetic field control is typically performed indirectly, by stabilizing the current of Helmholtz coils producing the large bias field. Here, we overcome the limitations of such an indirect control through a direct feedback scheme, which is based on nitrogen-vacancy centers acting as a sensor. This allows us to measure and stabilize magnetic fields of 46.6 G down to 1.2 mG RMS noise, with the potential of reaching much higher field strengths. Because the magnetic field is measured directly, we reach minimum shot-to-shot fluctuations of 0.32(4) ppm on a 22 minute time interval, ensuring high reproducibility of experiments. This approach extends the direct magnetic field control to high magnetic fields, which could enable new precise quantum simulations in this regime.

### A scalable realization of local U(1) gauge invariance in cold atomic mixtures

2020, Science, Vol. 367, Issue 6482 PDF-FileIn the fundamental laws of physics, gauge fields mediate the interaction between charged particles. An example is quantum electrodynamics -- the theory of electrons interacting with the electromagnetic field -- based on U(1) gauge symmetry. Solving such gauge theories is in general a hard problem for classical computational techniques. While quantum computers suggest a way forward, it is difficult to build large-scale digital quantum devices required for complex simulations. Here, we propose a fully scalable analog quantum simulator of a U(1) gauge theory in one spatial dimension. To engineer the local gauge symmetry, we employ inter-species spin-changing collisions in an atomic mixture. We demonstrate the experimental realization of the elementary building block as a key step towards a platform for large-scale quantum simulations of continuous gauge theories.

### Quantized refrigerator for an atomic cloud

2019, Quantum, arXiv:1812.08474 (3) 155 PDF-FileWe propose to implement a quantized thermal machine based on a mixture of two atomic species. One atomic species implements the working medium and the other implements two (cold and hot) baths. We show that such a setup can be employed for the refrigeration of a large bosonic cloud starting above and ending below the condensation threshold. We analyze its operation in a regime conforming to the quantized Otto cycle and discuss the prospects for continuous-cycle operation, addressing the experimental as well as theoretical limitations. Beyond its applicative significance, this setup has a potential for the study of fundamental questions of quantum thermodynamics.

### Elastic Scattering Time of Matter Waves in Disordered Potentials

2019, Physical Review Letters, arXiv:1810.07574 (122) 100403 PDF-FileWe report on the direct measurement of the elastic scattering time τ

_{s}of ultracold atoms propagating in optical disordered potentials. By exploring this fundamental quantity over a large range of experimental parameters, we observe variations of τ_{s}over more than three orders of magnitude, in excellent agreement with numerical calculations. It allows us to study the crossover from the weak to the strong scattering regimes, which are explicitly identified by a comparison to the first order Born approximation. We especially discuss the relevance of the widely used criterion*kl*_{s}∼1 to locate this crossover. While it is validated for a Gaussian disorder, it breaks down for the laser speckle disorders used in the experiments, where large deviations to Born predictions are observed. This result highlights the strong influence of the disorder statistics on the crossover and, more generally, on the behavior of the time τ_{s}in the strong scattering regime.### Dynamical topological transitions in the massive Schwinger model with a θ-term

2019, Phys. Rev. Lett., arXiv:1808.07885 (122) 050403 PDF-FileAiming at a better understanding of anomalous and topological effects in gauge theories out-of-equilibrium, we study the real-time dynamics of a prototype model for CP-violation, the massive Schwinger model with a θ-term. We identify dynamical quantum phase transitions between different topological sectors that appear after sufficiently strong quenches of the θ-parameter. Moreover, we establish a general dynamical topological order parameter, which can be accessed through fermion two-point correlators and, importantly, which can be applied for interacting theories. Enabled by this result, we show that the topological transitions persist beyond the weak-coupling regime. Finally, these effects can be observed with table-top experiments based on existing cold-atom, superconducting-qubit, and trapped-ion technology. Our work, thus, presents a significant step towards quantum simulating topological and anomalous real-time phenomena relevant to nuclear and high-energy physics.

### Quantum simulation of lattice gauge theories using Wilson fermions

2018, Quantum Sci. Technol., arXiv:1802.06704 (3) 034010 PDF-FileQuantum simulators have the exciting prospect of giving access to real-time dynamics of lattice gauge theories, in particular in regimes that are difficult to compute on classical computers. Future progress towards scalable quantum simulation of lattice gauge theories, however, hinges crucially on the efficient use of experimental resources. As we argue in this work, due to the fundamental non-uniqueness of discretizing the relativistic Dirac Hamiltonian, the lattice representation of gauge theories allows for an optimization that up to now has been left unexplored. We exemplify our discussion with lattice quantum electrodynamics in two-dimensional space-time, where we show that the formulation through Wilson fermions provides several advantages over the previously considered staggered fermions. Notably, it enables a strongly simplified optical lattice setup and it reduces the number of degrees of freedom required to simulate dynamical gauge fields. Exploiting the optimal representation, we propose an experiment based on a mixture of ultracold atoms trapped in a tilted optical lattice. Using numerical benchmark simulations, we demonstrate that a state-of-the-art quantum simulator may access the Schwinger mechanism and map out its non-perturbative onset.

### Implementing quantum electrodynamics with ultracold atomic systems

2017, New Journal of Physics, arXiv:1608.03480 (19) 023030 PDF-FileWe discuss the experimental engineering of model systems for the description of QED in one spatial dimension via a mixture of bosonic

^{23}Na and fermionic^{6}Li atoms. The local gauge symmetry is realized in an optical superlattice, using heteronuclear boson-fermion spin-changing interactions which preserve the total spin in every local collision. We consider a large number of bosons residing in the coherent state of a Bose-Einstein condensate on each link between the fermion lattice sites, such that the behavior of lattice QED in the continuum limit can be recovered. The discussion about the range of possible experimental parameters builds, in particular, upon experiences with related setups of fermions interacting with coherent samples of bosonic atoms. We determine the atomic system's parameters required for the description of fundamental QED processes, such as Schwinger pair production and string breaking. This is achieved by benchmark calculations of the atomic system and of QED itself using functional integral techniques. Our results demonstrate that the dynamics of one-dimensional QED may be realized with ultracold atoms using state-of-the-art experimental resources. The experimental setup proposed may provide a unique access to longstanding open questions for which classical computational methods are no longer applicable.### Subwavelength-width optical tunnel junctions for ultracold atoms

2016, Physical Review A, arXiv:1609.01285 (94) 063422 PDF-FileWe propose a new method for creating far-field optical barrier potentials for ultracold atoms with widths that are narrower than the diffraction limit and can approach tens of nanometers. The reduced widths stem from the nonlinear atomic response to control fields that create spatially varying dark resonances. The subwavelenth barrier is the result of the geometric scalar potential experienced by an atom prepared in such a spatially varying dark state. The performance of this technique, as well as its applications to the study of many-body physics and to the implementation of quantum information protocols with ultracold atoms, are discussed, with a focus on the implementation of tunnel junctions.

### Observation of the Phononic Lamb Shift with a Synthetic Vacuum

2016, Physical Review X, arXiv:1605.01874 (6) 041041 PDF-FileIn contrast to classical empty space, the quantum vacuum fundamentally alters the properties of embedded particles. This paradigm shift allows one to explain the discovery of the celebrated Lamb shift in the spectrum of the hydrogen atom. Here, we engineer a synthetic vacuum, building on the unique properties of ultracold atomic gas mixtures, offering the ability to switch between empty space and quantum vacuum. Using high-precision spectroscopy, we observe the phononic Lamb shift, an intriguing many-body effect originally conjectured in the context of solid-state physics. We find good agreement with theoretical predictions based on the Fröhlich model. Our observations establish this experimental platform as a new tool for precision benchmarking of open theoretical challenges, especially in the regime of strong coupling between the particles and the quantum vacuum.

## Previous work

### Temperature induced decay of persistent currents in superfluid ultracold gas

2017, Phys. Rev. A (95) 021602(R)We study how temperature affects the lifetime of a quantized, persistent current state in a toroidal Bose-Einstein condensate. When the temperature is increased, we find a decrease in the persistent current lifetime. Comparing our measured decay rates to simple models of thermal activation and quantum tunneling,wedo not find agreement. We also measured the size of the hysteresis loops in our superfluid ring as a function of temperature, enabling us to extract the critical velocity. The measured critical velocity is found to depend strongly on temperature, approaching the zero-temperature mean-field solution as the temperature is decreased. This indicates that an appropriate definition of critical velocity must incorporate the role of thermalfluctuations, something not explicitly contained in traditional theories.

### Contact resistance and phase slips in mesoscopic superfluid-atom transport

2016, Phys. Rev. A (93) 063619We have experimentally measured transport of superfluid, bosonic atoms in a mesoscopic system: a small channel connecting two large reservoirs. Starting far from equilibrium (superfluid in a single reservoir), we observe first resistive flow transitioning at a critical current into superflow, characterized by long-lived oscillations. We reproduce this full evolution with a simple electronic circuit model. We find that the resistance is consistent with phase slips and the associated creation of vortices, as proposed in [R. P. Feynman, in

*Prog. Low Temp. Phys.*, edited by C. J. Gorter (North Holland Publishing Company, Amsterdam, 1955), Chap. 2]. We also show that the oscillations are consistent with LC oscillations as estimated by the kinetic inductance and effective capacitance in our system. Our system allows only a few single-particle, transverse modes to propagate, a situation that, for fermions, would lead to a conductance of only a few*h*^{−1}. By contrast, in our bosonic system, we observe conductances of thousands of*h*^{−1}, showing the definitive role played by particle statistics.### Subwavelength-width optical tunnel junctions for ultracold atoms

2016, Physical Review A, arXiv:1609.01285 (94) 063422 PDF-FileWe propose a new method for creating far-field optical barrier potentials for ultracold atoms with widths that are narrower than the diffraction limit and can approach tens of nanometers. The reduced widths stem from the nonlinear atomic response to control fields that create spatially varying dark resonances. The subwavelenth barrier is the result of the geometric scalar potential experienced by an atom prepared in such a spatially varying dark state. The performance of this technique, as well as its applications to the study of many-body physics and to the implementation of quantum information protocols with ultracold atoms, are discussed, with a focus on the implementation of tunnel junctions.

### Resonant wavepackets and shock waves in an atomtronic SQUID

2015, New J. Phys. (17) 125012The fundamental dynamics of ultracold atomtronic devices are reflected in their phonon modes of excitation. We probe such a spectrum by applying a harmonically driven potential barrier to a

^{23}Na Bose-Einstein condensate in a ring-shaped trap. This perturbation excites phonon wavepackets. When excited resonantly, these wavepackets display a regular periodic structure. The resonant frequencies depend upon the particular configuration of the barrier, but are commensurate with the orbital frequency of a Bogoliubov sound wave traveling around the ring. Energy transfer to the condensate over many cycles of the periodic wavepacket motion causes enhanced atom loss from the trap at resonant frequencies. Solutions of the time-dependent Gross-Pitaevskii equation exhibit quantitative agreement with the experimental data. We also observe the generation of supersonic shock waves under conditions of strong excitation, and collisions of two shock wavepackets.### Self-heterodyne detection of the in-situ phase of an atomic superconducting quantum interference device

2015, Phys. Rev. A (92) 033602We present theoretical and experimental analysis of an interferometric measurement of the in situ phase drop across and current flow through a rotating barrier in a toroidal Bose-Einstein condensate (BEC). This experiment is the atomic analog of the rf-superconducting quantum interference device (SQUID). The phase drop is extracted from a spiral-shaped density profile created by the spatial interference of the expanding toroidal BEC and a reference BEC after release from all trapping potentials.We characterize the interferometer when it contains a single particle, which is initially in a coherent superposition of a torus and reference state, as well as when it contains a many-body state in the mean-field approximation. The single-particle picture is sufficient to explain the origin of the spirals, to relate the phase-drop across the barrier to the geometry of a spiral, and to bound the expansion times for which the in situ phase can be accurately determined. Mean-field estimates and numerical simulations show that the interatomic interactions shorten the expansion time scales compared to the single-particle case. Finally, we compare themean-field simulations with our experimental data and confirm that the interferometer indeed accurately measures the in situ phase drop.

### Interferometric Measurement of the Current-Phase Relationship of a Superfluid Weak Link

2014, Phys. Rev. X (4) 031052Weak connections between superconductors or superfluids can differ from classical links due to quantum coherence, which allows flow without resistance. Transport properties through such weak links can be described with a single function, the current-phase relationship, which serves as the quantumanalog of the current-voltage relationship. Here, we present a technique for inteferometrically measuring the current-phase relationship of superfluid weak links. We interferometrically measure the phase gradient around a ring-shaped superfluid Bose-Einstein condensate containing a rotating weak link, allowing us to identify the current flowing around the ring. While our Bose-Einstein condensate weak link operates in the hydrodynamic regime, this technique can be extended to all types of weak links (including tunnel junctions) in any phase-coherent quantum gas. Moreover, it can also measure the current-phase relationships of excitations. Such measurements may open new avenues of research in quantum transport.

### Resistive Flow in a Weakly Interacting Bose-Einstein Condensate

2014, Phys. Rev. Lett. (113) 045305We report the direct observation of resistive flow through a weak link in a weakly interacting atomic Bose-Einstein condensate. Two weak links separate our ring-shaped superfluid atomtronic circuit into two distinct regions, a source and a drain. Motion of these weak links allows for creation of controlled flow between the source and the drain. At a critical value of the weak link velocity, we observe a transition from superfluid flow to superfluid plus resistive flow. Working in the hydrodynamic limit, we observe a conductivity that is 4 orders of magnitude larger than previously reported conductivities for a Bose-Einstein condensate with a tunnel junction. Good agreement with zero-temperature Gross-Pitaevskii simulations and a phenomenological model based on phase slips indicate that the creation of excitations plays an important role in the resulting conductivity. Our measurements of resistive flow elucidate the microscopic origin of the dissipation and pave the way for more complex atomtronic devices.

### Hysteresis in a quantized superfluid 'atomtronic' circuit.

2014, Nature (506) 200Atomtronics is an emerging interdisciplinary field that seeks to develop new functional methods by creating devices and circuits where ultracold atoms, often superfluids, have a role analogous to that of electrons in electronics. Hysteresis is widely used in electronic circuits-it is routinely observed in superconducting circuits and is essential in radio-frequency superconducting quantum interference devices. Furthermore, it is as fundamental to superfluidity (and superconductivity) as quantized persistent currents, critical velocity and Josephson effects. Nevertheless, despite multiple theoretical predictions, hysteresis has not been previously observed in any superfluid, atomic-gas Bose-Einstein condensate. Here we directly detect hysteresis between quantized circulation states in an atomtronic circuit formed from a ring of superfluid Bose-Einstein condensate obstructed by a rotating weak link (a region of low atomic density). This contrasts with previous experiments on superfluid liquid helium where hysteresis was observed directly in systems in which the quantization of flow could not be observed, and indirectly in systems that showed quantized flow. Our techniques allow us to tune the size of the hysteresis loop and to consider the fundamental excitations that accompany hysteresis. The results suggest that the relevant excitations involved in hysteresis are vortices, and indicate that dissipation has an important role in the dynamics. Controlled hysteresis in atomtronic circuits may prove to be a crucial feature for the development of practical devices, just as it has in electronic circuits such as memories, digital noise filters (for example Schmitt triggers) and magnetometers (for example superconducting quantum interference devices).

### Coherent Backscattering of Ultracold Atoms

2012, Phys. Rev. Lett. (109) 195302 PDF-FileWe report on the direct observation of coherent backscattering (CBS) of ultracold atoms in a quasi-two- dimensional configuration. Launching atoms with a well-defined momentum in a laser speckle disordered potential, we follow the progressive build up of the momentum scattering pattern, consisting of a ring associated with multiple elastic scattering, and the CBS peak in the backward direction. Monitoring the depletion of the initial momentum component and the formation of the angular ring profile allows us to determine microscopic transport quantities.We also study the time evolution of the CBS peak and find it in fair agreement with predictions, at long times as well as at short times. The observation of CBS can be considered a direct signature of coherence in quantum transport of particles in disordered media. It is responsible for the so calledweaklocalizationphenomenon,which is the precursor ofAnderson localization.

### Three-dimensional localization of ultracold atoms in an optical disordered potential

2012, Nature Physics (8) 398 PDF-FileIn disordered media, quantum interference effects are expected to induce complete suppression of electron conduction. The phenomenon, known as Anderson localization, has a counterpart with classical waves that has been observed in acoustics, electromagnetism and optics, but a direct observation for particles remains elusive. Here, we report the observation of the three-dimensional localization of ultracold atoms in a disordered potential created by a speckle laser field.Aphenomenological analysis of our data distinguishes a localized component of the resulting density profile from a diffusive component. The observed localization cannot be interpreted as the classical trapping of particles with energy below the classical percolation threshold in the disorder, nor can it be understood as quantum trapping in local potential minima. Instead, our data are compatible with the self-consistent theory of Anderson localization tailored to our system, involving a heuristic energy shift that offers scope for future interpretation.

### Quasi-continuous horizontally guided atom laser: coupling spectrum and flux limits

2011, New J. Phys. (13) 065015 PDF-FileWe study in detail the flux properties of a radiofrequency (rf) outcoupled horizontally guided atom laser by following the scheme demonstrated by Guerin W et al (2006 Phys. Rev. Lett. 97 200402). Both the outcoupling spectrum (flux of the atom laser versus rf frequency of the outcoupler) and the flux limitations imposed on operating in the quasi- continuous regime are investigated. These aspects are studied using a quasi-one- dimensional model, whose predictions are shown to be in fair agreement with the experimental observations. This work allows us to identify the operating range of the guided atom laser and to confirm its promises with regard to studying quantum transport phenomena.

## Thesis

- 2019
- Building up a modular Na-K quantum gas
experiment

Lilo Höcker, Masterarbeit, 2019 PDF-File - Starting a Na-K experiment for simulating
quantum many-body phenomena

Jan Kilinc, Masterarbeit, 2019 PDF-File - 2018
- Implementing an Optical Accordion Lattice for the Realization of a Quantized Otto Cycle

Kai-Niklas Schymik, Masterarbeit, 2018 - Nitrogen Vacancy Center Magnetometry as a New Approach for an Active Magnetic Field Control

Kerim Köster, Bachelorarbeit, 2018 PDF-File - Simulation and Measurements of Single and Coupled Coaxial Qubits

Tanja Behrle, Masterarbeit, 2018 PDF-File - 2016
- Design and Implementation of a Versatile Imaging Objective for Imaging of Ultracold Mixtures of Sodium and Lithium

Alexander Mil, Masterarbeit, 2016 PDF-File - Spin Dynamics and Feshbach Resonances in Ultracold Sodium-Lithium Mixtures

Arno Trautmann, Dissertation, 2016 PDF-File - Observation of the Phononic Lamb Shift in a Synthetic Vacuum

Tobias Rentrop, Dissertation, 2016 PDF-File - 2015
- Active Magnetic Field Stabilisation for Ultracold
Sodium Lithium Mixtures

Marcell Gall, Masterarbeit, 2015 PDF-File

- 2012
- Quantum transport of ultracold atoms in disordered potentials

Fred Jendrzejewski,