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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

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

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

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

We analyze the recently measured anomalous transport properties of an ultracold gas through a ballistic constriction [S. Krinner et al., PNAS 201601812 (2016)]. The quantized conductance observed at weak interactions increases several-fold as the gas is made strongly interacting, which cannot be explained by the Landauer theory of single-channel transport. We show that this phenomenon is due to the multichannel Andreev reflections at the edges of the constriction, where the interaction and confinement result in a superconducting state. Andreev processes convert atoms of otherwise reflecting channels into the condensate propagating through the constriction, leading to a significant excess conductance. Furthermore, we find the spin conductance being suppressed by superconductivity; the agreement with experiment provides an additional support for our model., 5 pages, 4 figures. Supplementary Material available as an ancillary file

We realize and study the ionic Hubbard model using an interacting two-component gas of fermionic atoms loaded into an optical lattice. The bipartite lattice has honeycomb geometry with a staggered energy-offset that explicitly breaks the inversion symmetry. Distinct density-ordered phases are identified using noise correlation measurements of the atomic momentum distribution. For weak interactions the geometry induces a charge density wave. For strong repulsive interactions we detect a strong suppression of doubly occupied sites, as expected for a Mott insulating state, and the externally broken inversion symmetry is not visible anymore in the density distribution. The local density distributions in different configurations are characterized by measuring the number of doubly occupied lattice sites as a function of interaction and energy-offset. We further probe the excitations of the system using direction dependent modulation spectroscopy and discover a complex spectrum, which we compare with a theoretical model., 5+3 pages

We demonstrate a versatile method for creating state-dependent optical lattices by applying a magnetic field gradient modulated in time. This allows for tuning the relative amplitude and sign of the tunneling for different internal states. We observe substantially different momentum distributions depending on the spin state of fermionic ^{40}K atoms. Using dipole oscillations, we probe the spin-dependent band structure and find good agreement with theory. In situ expansion dynamics demonstrate that one state can be completely localized while others remain itinerant. A systematic study shows negligible heating and lifetimes of several seconds in the Hubbard regime.

We report on the observation of anti-ferromagnetic correlations of ultracold fermions in a variety of optical lattice geometries that are well described by the Hubbard model, including dimers, 1D chains, ladders, isolated and coupled honeycomb planes, as well as square and cubic lattices. The dependence of the strength of spin correlations on the specific geometry is experimentally studied by measuring the correlations along different lattice tunneling links, where a redistribution of correlations between the different lattice links is observed. By measuring the correlations in a crossover between distinct geometries, we demonstrate an effective reduction of the dimensionality for our atom numbers and temperatures. We also investigate the formation and redistribution time of spin correlations by dynamically changing the lattice geometry and studying the time-evolution of the system. Timescales ranging from a sudden quench of the lattice geometry to an adiabatic evolution are probed., Comment: 5 + 5 pages

We experimentally study the motion of atoms interacting with a periodically pulsed near resonant standing wave. For discrete pulse frequencies we observe a comb-like momentum distribution. The peaks have widths of 0.3 recoil momenta and a spacing which is an integer multiple of the recoil momentum. The atomic population is trapped in ground states which periodically evolve to dark states each time the standing wave is switched on., 4 pages, 3 figures. LaTeX/RevTeX (uses psfig). Submitted to Phys. Rev. Lett

We demonstrate a new type of optical lattice which confines atoms in nearly dark states. The lattice is formed by cooperation of an optical standing wave detuned to the blue with respect to an atomic $F\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}1\ensuremath{\rightarrow}F\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}1$ transition and a homogeneous magnetic field. The atoms are predominantly localized where the light is purely $\ensuremath{\pi}$ polarized such that they are optically pumped to the ${m}_{F}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}0$ Zeeman component which is decoupled from the light field. We have observed quantized vibrations of the localized atoms by means of probe transmission spectroscopy. Our ``dark'' optical lattice works in two and three dimensions.

We study symmetry breaking at the Dicke quantum phase transition by coupling a motional degree of freedom of a Bose-Einstein condensate to the field of an optical cavity. Using an optical heterodyne detection scheme we observe symmetry breaking in real-time and distinguish the two superradiant phases. We explore the process of symmetry breaking in the presence of a small symmetry-breaking field, and study its dependence on the rate at which the critical point is crossed. Coherent switching between the two ordered phases is demonstrated., 4 pages, 4 figures

Local density fluctuations and density profiles of a Fermi gas are measured in situ and analyzed. In the quantum degenerate regime, the weakly interacting {sup 6}Li gas shows a suppression of the density fluctuations compared to the nondegenerate case, where atomic shot noise is observed. This manifestation of antibunching is a direct result of the Pauli principle and constitutes a local probe of quantum degeneracy. We analyze our data using the predictions of the fluctuation-dissipation theorem and the local density approximation, demonstrating a fluctuation-based temperature measurement.

We perform a quantitative simulation of the repulsive Fermi-Hubbard model using an ultracold gas trapped in an optical lattice. The entropy of the system is determined by comparing accurate measurements of the equilibrium double occupancy with theoretical calculations over a wide range of parameters. We demonstrate the applicability of both high-temperature series and dynamical mean-field theory to obtain quantitative agreement with the experimental data. The reliability of the entropy determination is confirmed by a comprehensive analysis of all systematic errors. In the center of the Mott insulating cloud we obtain an entropy per atom as low as 0.77kB which is about twice as large as the entropy at the Neel transition. The corresponding temperature depends on the atom number and for small fillings reaches values on the order of the tunneling energy., 5 pages, 3 figures, 1 table

We create molecules from fermionic atoms in a three-dimensional optical lattice using a Feshbach resonance. In the limit of low tunnelling, the individual wells can be regarded as independent three-dimensional harmonic oscillators. The measured binding energies for varying scattering length agree excellently with the theoretical prediction for two interacting atoms in a harmonic oscillator. We demonstrate that the formation of molecules can be used to measure the occupancy of the lattice and perform thermometry., Comment: 4 pages

We demonstrate time-resolved counting of single atoms extracted from a weakly interacting Bose-Einstein condensate of $^{87}$Rb atoms. The atoms are detected with a high-finesse optical cavity and single atom transits are identified. An atom laser beam is formed by continuously output coupling atoms from the Bose-Einstein condensate. We investigate the full counting statistics of this beam and measure its second order correlation function $g^{(2)}(\tau)$ in a Hanbury Brown and Twiss type experiment. For the monoenergetic atom laser we observe a constant correlation function $g^{(2)}(\tau)=1.00\pm0.01$ and an atom number distribution close to a Poissonian statistics. A pseudo-thermal atomic beam shows a bunching behavior and a Bose distributed counting statistics.

We study a spin-polarized degenerate Fermi gas interacting via a $p$-wave Feshbach resonance in an optical lattice. The strong confinement available in this system allows us to realize one- and two-dimensional gases and, therefore, to restrict the asymptotic scattering states of atomic collisions. When aligning the atomic spins along (or perpendicular to) the axis of motion in a one-dimensional gas, scattering into channels with the projection of the angular momentum of $|m|=1$ (or $m=0$) can be inhibited. In two and three dimensions, we observe the doublet structure of the $p$-wave Feshbach resonance. For both the one-dimensional and the two-dimensional gases, we find a shift of the position of the resonance with increasing confinement due to the change in collisional energy. In a three-dimensional optical lattice, the losses on the Feshbach resonance are completely suppressed.

We have studied interacting and noninteracting quantum degenerate Fermi gases in a three-dimensional optical lattice. We directly image the Fermi surface of the atoms in the lattice by turning off the optical lattice adiabatically. Because of the confining potential, gradual filling of the lattice transforms the system from a normal state into a band insulator. The dynamics of the transition from a band insulator to a normal state is studied, and the time scale is measured to be an order of magnitude larger than the tunneling time in the lattice. Using a Feshbach resonance, we increase the interaction between atoms in two different spin states and dynamically induce a coupling between the lowest energy bands. We observe a shift of this coupling with respect to the Feshbach resonance in free space which is anticipated for strongly confined atoms.

We prepare a Bose-Einstein condensed gas in a three-dimensional optical lattice and study the excitation spectrum of the superfluid phase for different interaction strengths. We probe the response of the system by modulating the depth of the optical lattice along one axis. The interactions can be controlled independently by varying the tunnel coupling along the other two lattice axes. In the weakly interacting regime we observe a small susceptibility of the superfluid to excitations, while for stronger interactions an unexpected resonance appears in the excitation spectrum. In addition we measure the coherent fraction of the atomic gas, which determines the depletion of the condensate.

In optical lattices where each site is occupied in its lowest energy state by a superposition of zero, one, and two atoms, one can in a controllable manner convert the atomic pair into a molecule while retaining the vacuum and one-atom amplitudes. The microscopic quantum coherence on each site between the vacuum and the single molecule component leads to a macroscopically populated molecular condensate when the lattice is removed.

We report on a quantitative study of the growth process of $^{87}$Rb Bose-Einstein condensates. By continuous evaporative cooling we directly control the thermal cloud from which the condensate grows. We compare the experimental data with the results of a theoretical model based on quantum kinetic theory. We find quantitative agreement with theory for the situation of strong cooling, whereas in the weak cooling regime a distinctly different behaviour is found in the experiment., Comment: 4 pages, 4 figures

We report on the atom optical manipulation of an atom laser beam. Reflection, focusing, and its storage in a resonator are demonstrated. Precise and versatile mechanical control over an atom laser beam propagating in an inhomogeneous magnetic field is achieved by optically inducing spin flips between atomic ground states with different magnetic moment. The magnetic force acting on the atoms can thereby be effectively switched on and off. The surface of the atom optical element is determined by the resonance condition for the spin flip in the inhomogeneous magnetic field. More than 98% of the incident atom laser beam is reflected specularly.

Bose-Einstein condensates of rubidium atoms are stored in a two-dimensional periodic dipole force potential, formed by a pair of standing wave laser fields. The resulting potential consists of a lattice of tightly confining tubes, each filled with a 1D quantum gas. Tunnel-coupling between neighboring tubes is controlled by the intensity of the laser fields. By observing the interference pattern of atoms released from more than 3000 individual lattice tubes the phase coherence of the coupled quantum gases is studied. The lifetime of the condensate in the lattice and the dependence of the interference pattern on the lattice configuration are investigated., 4 pages, 6 figures

Constructing new topological materials is of vital interest for the development of robust quantum applications. However, engineering such materials often causes technological overhead, such as large magnetic fields, specific lattice geometries, strong spin-orbit coupling, synthetic dimensions, or dynamical superlattice potentials. Simplifying the experimental requirements has been addressed on a conceptual level - by proposing to combine simple lattice structures with Floquet engineering - but there has been no experimental implementation. Here, we demonstrate topological pumping in a Floquet-Bloch band using a plain sinusoidal lattice potential and two-tone driving with frequencies $\omega$ and $2\omega$. We adiabatically prepare a near-insulating Floquet band of ultracold fermions via a frequency chirp, which avoids gap closings en route from trivial to topological bands. Subsequently, we induce topological pumping by slowly cycling the amplitude and the phase of the $2 \omega$ drive. Our system is well described by an effective Shockley model, establishing a novel paradigm to engineer topological matter from simple underlying lattice geometries. This approach could enable the application of quantised pumping in metrology, following recent experimental advances on two-frequency driving in real materials., Comment: 13 pages, 11 figures