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The transport properties of gapless edge modes at boundaries between topologically distinct domains are of fundamental and technological importance. Therefore, it is crucial to gain a better understanding of topological edge states and their response to interparticle interactions. Here, we experimentally study long-distance quantised Hall drifts in a harmonically confined topological pump of non-interacting and interacting ultracold fermionic atoms. We find that quantised drifts halt and reverse their direction when the atoms reach a critical slope of the confining potential, revealing the presence of a topological boundary. The drift reversal corresponds to a band transfer between a band with Chern number $C = +1$ and a band with $C = -1$ via a gapless edge mode, in agreement with the bulk-edge correspondence for non-interacting particles. We establish that a non-zero repulsive Hubbard interaction leads to the emergence of an additional edge in the system, relying on a purely interaction-induced mechanism, in which pairs of fermions are split.

We measure superfluid transport of strongly interacting fermionic lithium atoms through a quantum point contact with local, spin-dependent particle loss. We observe that the characteristic non-Ohmic superfluid transport enabled by high-order multiple Andreev reflections transitions into an excess Ohmic current as the dissipation strength exceeds the superfluid gap. We develop a model with mean-field reservoirs connected via tunneling to a dissipative site. Our calculations in the Keldysh formalism reproduce the observed nonequilibrium particle current, yet do not fully explain the observed loss rate or spin current., Physical Review Letters, 130 (20), ISSN:0031-9007, ISSN:1079-7114

Nature, 608, ISSN:0028-0836, ISSN:1476-4687

Pumps are transport mechanisms in which direct currents result from a cyclic evolution of the potential

We observe a structural phase transition between two configurations of a superradiant crystal by coupling a Bose-Einstein condensate to an optical cavity and applying imbalanced transverse pump fields. The transition can be interpreted as a transition between two nonpolar, centro-symmetric structures involving a change in polarization. We find that this first order phase transition is accompanied by transient dynamics of the order parameter which we measure in real time. The phase transition and the excitation spectrum can be derived from a microscopic Hamiltonian, in quantitative agreement with our experimental data. ISSN:2643-1564

We report on the experimental realization and detection of dynamical currents in a spin-textured lattice in momentum space. Collective tunneling is implemented via cavity-assisted Raman scattering of photons by a spinor Bose-Einstein condensate into an optical cavity. The photon field inducing the tunneling processes is subject to cavity dissipation, resulting in effective directional dynamics in a non-Hermitian setting. We observe that the individual tunneling events are superradiant in nature and locally resolve them in the lattice by performing real-time, frequency-resolved measurements of the leaking cavity field. The results can be extended to a regime exhibiting a cascade of currents and simultaneous coherences between multiple lattice sites, where numerical simulations provide further understanding of the dynamics. Our observations showcase dynamical tunneling in momentum-space lattices and provide prospects to realize dynamical gauge fields in driven-dissipative settings.

Physical Review B, 103 (14), ISSN:1098-0121, ISSN:0163-1829, ISSN:1550-235X, ISSN:0556-2805, ISSN:2469-9969, ISSN:1095-3795, ISSN:2469-9950

Achieving high fidelity for the measurement and control of quantum experiments imposes strict requirements on the precision and stability of surrounding electronics. Controlling electronics from a central device is more challenging when they are distributed in a laboratory and require analog signals where effects like ground loops and radiative cross-talk can limit their performance. Here, we present our design to address these challenges with a flexible and scalable analog signal generator. Our design is based on a field-programmable gate array (FPGA) development board, a custom PCB hosting a digital-to-analog converter (DAC) with 20 bit precision at 1 MSPS, and a custom breakout board. The FPGA development board accepts data from a master PC via TCP/IP where a user programs the waveform and sampling rate of each output channel and writes the data to on-board RAM. At runtime, the direct memory access (DMA) and Serial Peripheral Interface (SPI) modules inside the FPGA stream data to the custom DAC board via an Ethernet cable carrying the samples as differential signals along with the supply voltage. We designed the DAC board to be resistant to digital and analog noise by separating ground planes to prevent ground loops and by using high-precision and low-noise power supplies and voltage reference circuits. External trigger and clock inputs can be used to synchronize the DACs and multiple FPGAs. The time resolution and precision of our solution is optimized for experiments on quantum gases though it is flexible and can be adapted for many more applications.

We report on coupled heat and particle transport measurements through a quantum point contact (QPC) connecting two reservoirs of resonantly interacting, finite temperature Fermi gases. After heating one of them, we observe a particle current flowing from cold to hot. We monitor the temperature evolution of the reservoirs and find that the system evolves after an initial response into a non-equilibrium steady state with finite temperature and chemical potential differences across the QPC. In this state any relaxation in the form of heat and particle currents vanishes. From our measurements we extract the transport coefficients of the QPC and deduce a Lorenz number violating the Wiedemann-Franz law by one order of magnitude, a characteristic persisting even for a wide contact. In contrast, the Seebeck coefficient takes a value close to that expected for a non-interacting Fermi gas and shows a smooth decrease as the atom density close to the QPC is increased beyond the superfluid transition. Our work represents a fermionic analog of the fountain effect observed with superfluid helium and poses new challenges for microscopic modeling of the finite temperature dynamics of the unitary Fermi gas., Comment: 14 pages including Appendix, 4 + 3 figures

Physical Review Letters, 125 (6), ISSN:0031-9007, ISSN:1079-7114

We study thermoelectric currents of neutral, fermionic atoms flowing through a mesoscopic channel connecting a hot and a cold reservoir across the superfluid transition. The thermoelectric response results from a competition between density-driven diffusion from the cold to the hot reservoir and the channel favoring transport of energetic particles from hot to cold. We control the relative strength of both contributions to the thermoelectric response using an external optical potential in a nearly non-interacting and a strongly-interacting system. Without interactions, the magnitude of the particle current can be tuned over a broad range but is restricted to flow from hot to cold in our parameter regime. Strikingly, strong interparticle interactions additionally reverse the direction of the current. We quantitatively model ab initio the non-interacting observations and qualitatively explain the interaction-assisted reversal by the reduction of entropy transport due to pairing correlations. Our work paves the way to studying the coupling of spin and heat in strongly correlated matter using spin-dependent optical techniques with cold atoms., Comment: 7 + 11 pages, 4 + 5 figures

Physical Review Letters, 123 (23), ISSN:0031-9007, ISSN:1079-7114

Physical Review A, 100 (5), ISSN:1094-1622, ISSN:0556-2791, ISSN:1050-2947

Gauge fields that appear in models of high-energy and condensed matter physics are dynamical quantum degrees of freedom due to their coupling to matter fields. Since the dynamics of these strongly correlated systems is hard to compute, it was proposed to implement this basic coupling mechanism in quantum simulation platforms with the ultimate goal to emulate lattice gauge theories. Here, we realize the fundamental ingredient for a density-dependent gauge field acting on ultracold fermions in an optical lattice by engineering non-trivial Peierls phases that depend on the site occupations. We propose and implement a Floquet scheme that relies on breaking time-reversal symmetry (TRS) by driving the lattice simultaneously at two frequencies which are resonant with the onsite interactions. This induces density-assisted tunnelling processes that are controllable in amplitude and phase. We demonstrate techniques in a Hubbard dimer to quantify the amplitude and to directly measure the Peierls phase with respect to the single-particle hopping. The tunnel coupling features two distinct regimes as a function of the modulation amplitudes, which can be characterised by a $\mathbb{Z}_2$-invariant. Moreover, we provide a full tomography of the winding structure of the Peierls phase around a Dirac point that appears in the driving parameter space., Comment: 7+17 pages, 4+7 figures

We realize a nondegenerate two-mode Dicke model with competing interactions in a Bose-Einstein condensate (BEC) coupled to two orthogonal polarization modes of a single optical cavity. The BEC is coupled to the cavity modes via the scalar and vectorial part of the atomic polarizability. We can independently change these couplings and determine their effect on a self-organization phase transition. Measuring the phases of the system, we characterize a crossover from a single-mode to a two-mode Dicke model. This work provides perspectives for the realization of coupled phases of spin and density., Physical Review A, 100 (20), ISSN:1094-1622, ISSN:0556-2791, ISSN:1050-2947

This corrects the article DOI: 10.1103/PhysRevLett.117.255302.

Physical Review Letters, 123 (19), ISSN:0031-9007, ISSN:1079-7114

Dissipative and unitary processes define the evolution of a many-body system. Their interplay gives rise to dynamical phase transitions and can lead to instabilities. We discovered a non-stationary state of chiral nature in a synthetic many-body system with independently controllable unitary and dissipative couplings. Our experiment is based on a spinor Bose gas interacting with an optical resonator. Orthogonal quadratures of the resonator field coherently couple the Bose-Einstein condensate to two different atomic spatial modes whereas the dispersive effect of the resonator losses mediates a dissipative coupling between these modes. In a regime of dominant dissipative coupling we observe the chiral evolution and map it to a positional instability., Comment: 13 pages including supplementary information

Physical Review Letters, 123 (19), ISSN:0031-9007, ISSN:1079-7114

Journal of Low Temperature Physics, 138 (3-4), ISSN:0022-2291, ISSN:1573-7357

We experimentally study the stability of a bosonic Mott-insulator against the formation of a density wave induced by long-range interactions, and characterize the intrinsic dynamics between these two states. The Mott-insulator is created in a quantum degenerate gas of 87-Rubidium atoms, trapped in a three-dimensional optical lattice. The gas is located inside and globally coupled to an optical cavity. This causes interactions of global range, mediated by photons dispersively scattered between a transverse lattice and the cavity. The scattering comes with an atomic density modulation, which is measured by the photon flux leaking from the cavity. We initialize the system in a Mott-insulating state and then rapidly increase the global coupling strength. We observe that the system falls into either of two distinct final states. One is characterized by a low photon flux, signaling a Mott insulator, and the other is characterized by a high photon flux, which we associate with a density wave. Ramping the global coupling slowly, we observe a hysteresis loop between the two states - a further signature of metastability. A comparison with a theoretical model confirms that the metastability originates in the competition between short- and global-range interactions. From the increasing photon flux monitored during the switching process, we find that several thousand atoms tunnel to a neighboring site on the time scale of the single particle dynamics. We argue that a density modulation, initially forming in the compressible surface of the trapped gas, triggers an avalanche tunneling process in the Mott-insulating region., Comment: 8 + 8 pages, 6 + 4 figures

We observe cavity mediated spin-dependent interactions in an off-resonantly driven multilevel atomic Bose-Einstein condensate that is strongly coupled to an optical cavity. Applying a driving field with adjustable polarization, we identify the roles of the scalar and the vectorial components of the atomic polarizability tensor for single and multicomponent condensates. Beyond a critical strength of the vectorial coupling, we infer the formation of a spin texture in a condensate of two internal states from the analysis of the cavity output field. Our work provides perspectives for global dynamical gauge fields and self-consistently spin-orbit coupled gases.

We study particle and spin transport in a single mode quantum point contact using a charge neutral, quantum degenerate Fermi gas with tunable, attractive interactions. This yields the spin and particle conductance of the point contact as a function of chemical potential or confinement. The measurements cover a regime from weak attraction, where quantized conductance is observed, to the resonantly interacting superfluid. Spin conductance exhibits a broad maximum when varying the chemical potential at moderate interactions, which signals the emergence of Cooper pairing. In contrast, the particle conductance is unexpectedly enhanced even before the gas is expected to turn into a superfluid, continuously rising from the plateau at 1/h for weak interactions to plateaux-like features at non-universal values as high as 4/h for intermediate interactions. For strong interactions, the particle conductance plateaux disappear and the spin conductance gets suppressed, confirming the spin-insulating character of a superfluid. Our observations document the breakdown of universal conductance quantization as many-body correlations appear. The observed anomalous quantization challenges a Fermi liquid description of the normal phase, shedding new light on the nature of the strongly attractive Fermi gases., Comment: 14 pages, 8 figures

We propose a method by which the quantization of the Hall conductance can be directly measured in the transport of a one-dimensional atomic gas. Our approach builds on two main ingredients: (1) a constriction optical potential, which generates a mesoscopic channel connected to two reservoirs, and (2) a time-periodic modulation of the channel specifically designed to generate motion along an additional synthetic dimension. This fictitious dimension is spanned by the harmonic oscillator modes associated with the tightly confined channel, and hence, the corresponding "lattice sites" are intimately related to the energy of the system. We analyze the quantum-transport properties of this hybrid two-dimensional system, highlighting the appealing features offered by the synthetic dimension. In particular, we demonstrate how the energetic nature of the synthetic dimension combined with the quasienergy spectrum of the periodically driven channel allows for the direct and unambiguous observation of the quantized Hall effect in a two-reservoir geometry. Our work illustrates how topological properties of matter can be accessed in a minimal one-dimensional setup, with direct and practical experimental consequences., SCOPUS: ar.j, info:eu-repo/semantics/published

His arguably most succinct paper Ted Hansch wrote together with Art Schawlow, proposing laser cooling of an atomic gas on a mere two pages of Optics Communications [1]. Once asked why he did not further pursue their idea of laser cooling at Stanford, he gave two reasons. First, he was not so keen on building a complicated ultra-high vacuum apparatus. Second, and perhaps more importantly, he had heard of Chebotayev’s proposal for two-photon spectroscopy, which he thought would be a much more effective approach to circumvent the Doppler shift in laser spectroscopy. Fortunately for many researchers in the field of cold atoms who had learned their profession in Ted’s laboratories in Munich and Garching, he eventually decided to give the field of laser cooling a second chance. This decision was characteristic for his style of running a large research group since coming back to Germany. He supports his PhD students and postdocs in following their own ideas and initiatives, even if the topics have fairly little to do with laser spectroscopy.

Physical Review Letters, 121 (23), ISSN:0031-9007, ISSN:1079-7114