tag:www1.physik.uni-hamburg.de,2005:/en/ilp/sengstock/research/bose-fermi/newsScientific achievements2020-04-22T12:07:03ZNAGR-fakmin-18134810-production2018-09-16T22:00:00ZIdentifying quantum phase transitions using artificial neural networks on experimental data<img width="293" height="165" style="float:left" src="https://assets.rrz.uni-hamburg.de/instance_assets/fakmin/19630037/machine-learning-scaled-598feb8a113081cae15715432352421dc59957a9.png" /><p class="bodytext">Machine learning techniques such as artificial neural networks are currently revolutionizing many technological areas and have also proven successful in quantum physics applications. Here we employ an artificial neural network and deep learning techniques to identify quantum phase transitions from single-shot experimental momentum-space density images of ultracold quantum gases and obtain results, which were not feasible with conventional methods. We map out the complete two-dimensional topological phase diagram of the Haldane model and provide an accurate characterization of the superfluid-to-Mott-insulator transition in an inhomogeneous Bose-Hubbard system. Our work points the way to unravel complex phase diagrams of general experimental systems, where the Hamiltonian and the order parameters might not be known.</p>
<p class="bodytext">Nature Physics 15, 917 (2019) </p>
<p class="bodytext">Press Release UHH: "Künstliche Intelligenz erkennt Quantenphasenübergänge"</p><p>Photo: Figure: AG Sengstock</p>NAGR-fakmin-18134824-production2018-05-28T22:00:00ZMeasuring quantized circular dichroism in ultracold topological matter<img width="293" height="165" style="float:left" src="https://assets.rrz.uni-hamburg.de/instance_assets/fakmin/18136810/news2-1e6b286a28e9fa9e80b1928cfe000c64a799a8e4.png" /><p class="bodytext">The topology of two-dimensional materials traditionally manifests itself through the quantization of the Hall conductance, which is revealed in transport measurements. Recently, it was predicted that topology can also give rise to a quantized spectroscopic response upon subjecting a Chern insulator to a circular drive: Comparing the frequency-integrated depletion rates associated with drives of opposite orientations leads to a quantized response dictated by the topological Chern number of the populated Bloch band. Here we experimentally demonstrate this intriguing topological effect for the first time, using ultracold fermionic atoms in topological Floquet bands. In addition, our depletion-rate measurements also provide a first experimental estimation of the Wannier-spread functional, a fundamental geometric property of Bloch bands. Our results establish topological spectroscopic responses as a versatile probe, which could be applied to access the geometry and topology of many-body quantum systems, such as fractional Chern insulators.</p>
<p class="bodytext">arXiv:1805.11077</p><p>Photo: AG Sengstock</p>NAGR-fakmin-18134838-production2018-02-16T23:00:00ZNick Fläschner erhält SAMOP-Dissertationspreis der DPG<img width="293" height="165" style="float:left" src="https://assets.rrz.uni-hamburg.de/instance_assets/fakmin/18136820/news3-4f6eab6b1f0914e6f3e10ad9a854e2cb7eb56c32.png" /><p>Nick Fläschner erhält den Dissertationspreis 2018 für seine Dissertation mit dem Titel "Ultracold Fermions in Tunable Hexagonal Lattices: From High-Precision Spectroscopy to the Measurement of Berry Curvature", die in der Gruppe von Klaus Sengstock angefertigt wurde. <br><br>Der Dissertationspreis der Sektion AMOP der DPG wird jährlich ausgeschrieben und zeichnet herausragende wissenschaftliche Arbeiten und deren exzellente Darstellung in einem Vortrag aus. Aus den vielen Vorschlägen wurden vier Finalisten ausgewählt, die auf der Frühjahrstagung in Erlangen vortrugen. Nick Fläschner konnte dabei die Jury mit seinem Vortrag überzeugen.<br><br>Der renommierte Preis ist mit einem Preisgeld in Höhe von 1500 Euro und einem Reisestipendium in Höhe von 500 Euro verknüpft. Er ging zuletzt im Jahr 2007 an Doktoranden aus der Gruppe Sengstock. Herzlichen Glückwunsch!</p><p>Photo: AG Sengstock</p>NAGR-fakmin-18134852-production2018-01-16T23:00:00ZHigh-precision multiband spectroscopy of ultracold fermions in a nonseparable optical lattice<img width="293" height="165" style="float:left" src="https://assets.rrz.uni-hamburg.de/instance_assets/fakmin/18136830/news4-7eef5858beb988c268c0135fe7e9a5e23ca55051.png" /><p>Spectroscopic tools are fundamental for the understanding of complex quantum systems. Here we demonstrate high-precision multi-band spectroscopy in a graphene-like lattice using ultracold fermionic atoms. From the measured band structure, we characterize the underlying lattice potential with a relative error of 1.2 10^(-3). Such a precise characterization of complex lattice potentials is an important step towards precision measurements of quantum many-body systems. Furthermore, we explain the excitation strengths into the different bands with a model and experimentally study their dependency on the symmetry of the perturbation operator. This insight suggests the excitation strengths as a suitable observable for interaction effects on the eigenstates.</p>
<p class="bodytext">Phys. Rev. A 97, 051601(R) (2018)</p>
<p class="bodytext">(Editor's Suggestion)</p><p>Photo: AG Sengstock</p>NAGR-fakmin-18134867-production2017-09-07T22:00:00ZCharacterizing topology by dynamics: Chern number from linking number<img width="293" height="165" style="float:left" src="https://assets.rrz.uni-hamburg.de/instance_assets/fakmin/18136840/news5-4de03f06a502bd0ce75a1a1371a6ad6f8e5acf95.png" /><p class="bodytext">Topology plays an important role in modern solid state physics describing intriguing quantum states such as topological insulators. It is an intrinsically non-local property and therefore challenging to access, often studied only via the resulting edge states. Here, we measure the topological index directly from the far-from-equilibrium dynamics of the bulk. We use the mapping of the Chern number to the linking number of dynamical vortex trajectories appearing after a quench to the Hamiltonian of interest. We thereby map out the topological phase diagram of quantum gases in optical lattices via a purely dynamical response. Such relations between two topological indices in static and dynamical properties could be also an important approach for exploring topology in the case of interactions.</p>
<p class="bodytext">arXiv:1709.01046 (2017)</p><p>Photo: AG Sengstock</p>NAGR-fakmin-18351702-production2017-03-07T23:00:00ZObservation of topological Bloch-state defects and their merging transition<img width="293" height="165" style="float:left" src="https://assets.rrz.uni-hamburg.de/instance_assets/fakmin/18351636/observation-of-topological-bloch-state-a709b0f203ceedbfdd8cf5128274428cdd18d974.png" /><p class="bodytext">Topological defects in Bloch bands, such as Dirac points in graphene, and their resulting Berry phases play an important role for the electronic dynamics in solid state crystals. Such defects can arise in systems with a two-atomic basis due to the momentum-dependent coupling of the two sublattice states, which gives rise to a pseudo-spin texture. The topological defects appear as vortices in the azimuthal phase of this pseudo-spin texture. Here, we demonstrate a complete measurement of the azimuthal phase in a hexagonal optical lattice employing a versatile method based on time-of-flight imaging after off-resonant lattice modulation. Furthermore we map out the merging transition of the two Dirac points induced by beam imbalance. Our work paves the way to accessing geometric properties in general multi-band systems also with spin-orbit coupling and interactions.</p>
<p class="bodytext">Phys. Rev. Lett. 118, 240403 (2017)</p><p>Photo: AG Sengstock</p>NAGR-fakmin-18351716-production2016-08-21T22:00:00ZObservation of a dynamical topological phase transition<img width="293" height="165" style="float:left" src="https://assets.rrz.uni-hamburg.de/instance_assets/fakmin/18351546/observation-dynamical-topological-4b4978aa14303f95bf6c126a8314b83801cbe66b.jpg" /><p class="bodytext">We have achieved the first experimental observation of a dynamical phase transition - a recently introduced concept for the description of the dynamics of a many-body quantum state far from equilibrium, where order appears and disappears as a function of time rather than temperature. We perform time and momentum-resolved state tomography of our fermionic many-body state after a quench within the Haldane model, realized by lattice shaking, and observe the appearance of dynamical vortices in the phase profile. This dynamical order is sensitive to the change of Chern number across the quench and therefore allows mapping out the Haldane-like Floquet phase diagram, which demonstrates a new approach to the study of phase diagrams.</p>
<p class="bodytext">arxiv:1608.05616</p>
<p class="bodytext">Nature Physics 14, 265 (2018)</p>
<p class="bodytext">Press coverage: UHH, CUI</p>
<p class="bodytext">Siehe auch den Brennpunkt "Quantenphasen, aber dynamisch" von Patrick Windpassinger und Ferdinand Schmidt-Kaler.</p><p>Photo: AG Sengstock</p>NAGR-fakmin-18351748-production2016-05-26T22:00:00ZExperimental Reconstruction of the Berry Curvature in a Floquet Bloch Band<img width="293" height="165" style="float:left" src="https://assets.rrz.uni-hamburg.de/instance_assets/fakmin/18351600/experimental-reconstruction-berry-05b29d38a8b5c7f780d784f4fef00c0108d8f4b7.png" /><p>Topological properties lie at the heart of many fascinating phenomena in solid state systems such as quantum Hall systems or Chern insulators. The topology can be captured by the distribution of Berry curvature, which describes the geometry of the eigenstates across the Brillouin zone. Employing fermionic ultracold atoms in a hexagonal optical lattice, we generate topological bands using resonant driving and show a full momentum-resolved measurement of the ensuing Berry curvature. Our results pave the way to the exploration of intriguing phases of matter with interactions in topological band structures.</p>
<p class="bodytext">Science 352, 1091 (2016)</p>
<p class="bodytext">Press coverage: CUI, UHH</p><p>Photo: AG Sengstock</p>NAGR-fakmin-18351765-production2016-05-24T22:00:00ZMarie Curie Fellowship for Benno Rem<img width="293" height="165" style="float:left" src="https://assets.rrz.uni-hamburg.de/instance_assets/fakmin/18351519/benno-rem-marie-curie-8c988cfaa830a9484dfd1a1269108619bf27f166.png" /><p>Benno Rem has been selected for the prestigious Marie Skłodowska-Curie Individual Fellowship. The fellowship has been awarded for the project ISOTOP, which will focus on studying interactions, spin mixtures and edge-states in topological band structures. During the project, the team will utilise ultra-cold fermions in a shaken hexagonal optical lattice, to engineer the geometry and topology of band structures. Congratulations!</p><p>Photo: AG Sengstock</p>NAGR-fakmin-18351783-production2014-04-15T22:00:00ZRelaxation Dynamics of an Isolated Large-Spin Fermi Gas Far from Equilibrium<img width="293" height="165" style="float:left" src="https://assets.rrz.uni-hamburg.de/instance_assets/fakmin/18351591/relaxation-dynamics-e526acc51462c13747e4e57e9ef1e316a9addb95.png" /><p class="bodytext">We show that the spatial “collisions” of the atoms can flip their internal spins and lead to the relaxation of the spin-state distribution to its equilibrium. In our experiment, we knock an ultracold gas of potassium atoms out of its equilibrium by putting the atoms in two of the ten available internal spin states only. We then monitor how the atoms redistribute themselves among all ten states, and we are able to keep track of that process for time spans of up to 10 seconds by “counting” the number of atoms in each spin state. We have unraveled many aspects of the relaxation process: There are a number of different processes of atomic collision taking place at different time scales that cause spin changes, but the collision-aided spin-redistribution process is the slowest one in the whole system. Our results significantly advance the understanding of relaxation processes in quantum many-body systems and have been awarded as research highlight in Nature Physics.</p>
<p class="bodytext">Phys. Rev. X 4, 021011 (2014)</p>
<p class="bodytext">Research Highlight in Nature Physics</p><p>Photo: AG Sengstock</p>NAGR-fakmin-18351802-production2013-07-30T22:00:00ZGiant spin oscillations in an ultracold Fermi sea<img width="293" height="165" style="float:left" src="https://assets.rrz.uni-hamburg.de/instance_assets/fakmin/18351528/news213-c9c5767d7cc60e8ba97c7d32bf4ad40b078fa964.png" /><p class="bodytext">Collective behavior in many-body systems is the origin of many fascinating phenomena in nature ranging from swarms of birds and modeling of human behavior to fundamental magnetic properties of solids. We report on the first observation of collective spin dynamics in an ultracold Fermi sea with large spin: We observe long-lived and large-amplitude coherent spin oscillations, driven by local spin interactions. At ultralow temperatures, Pauli blocking stabilizes the collective behavior and the Fermi sea behaves as a single entity in spin space. With increasing temperature, we observe a stronger damping associated with particle-hole excitations. As a striking feature, we find a high-density regime where excited spin configurations are collisionally stabilized.</p>
<p class="bodytext">Science 343, 6167 (2014)</p><p>Photo: AG Sengstock</p>NAGR-fakmin-18351818-production2013-02-17T23:00:00ZEngineering spin-waves in a high-spin ultracold Fermi gas<img width="293" height="165" style="float:left" src="https://assets.rrz.uni-hamburg.de/instance_assets/fakmin/18351654/photoconductivity-16571ca655a17f90a9022cc53789b43b25563f66.png" /><p class="bodytext">We have studied in detail multi-component spin-waves in an s=3/2 Fermi gas where the high spin leads to novel tensorial degrees of freedom compared to s = 1/2 systems. The excitations of a spin-nematic state are investigated from the linear to the nonlinear regime, where the tensorial character is particularly pronounced. By tuning the initial state we engineer the tensorial spin-wave character, such that the magnitude and sign of the counterflow spin-currents are effectively controlled. A comparison of our data with numerical and analytical results shows excellent agreement.</p>
<p class="bodytext">Phys. Rev. Lett. 110, 250402 (2013)</p><p>Photo: AG Sengstock</p>NAGR-fakmin-18351837-production2012-09-02T22:00:00ZIntrinsic Photoconductivity of Ultracold Fermions in Optical Lattices<img width="293" height="165" style="float:left" src="https://assets.rrz.uni-hamburg.de/instance_assets/fakmin/18351555/intrinsic-photoconductivity-9231b0935797371a3922dbf5d690545ea75e30f9.png" /><p class="bodytext">We report on the experimental observation of an analog to a persistent alternating photocurrent in an ultracold gas of fermionic atoms in an optical lattice. The dynamics is induced and sustained by an external harmonic confinement. While particles in the excited band exhibit long-lived oscillations with a momentum dependent frequency a strikingly different behavior is observed for holes in the lowest band. An initial fast collapse is followed by subsequent periodic revivals. Both observations are fully explained by mapping the system onto a nonlinear pendulum.</p>
<p class="bodytext">Selected for a Viewpoint in Physics </p>
<p class="bodytext">Phys. Rev. Lett. 110, 085302 (2013)</p><p>Photo: AG Sengstock</p>NAGR-fakmin-18351852-production2012-03-06T23:00:00ZCoherent multi-flavour spin dynamics in a fermionic quantum gas<img width="293" height="165" style="float:left" src="https://assets.rrz.uni-hamburg.de/instance_assets/fakmin/18351582/coherent-multi-flavour-spin-dynamics-01869a4f219ae11a062ee28e34fcf76a18089256.png" /><p class="bodytext">We demonstrate the first experimental realization of a well‐controlled fermionic spinor gas with interaction‐driven spin oscillations. By properly choosing the initial spin states we can change the effective length of the atomic spin from 1/2 to 9/2. The control over the magnetic field allows to initialize and stop spin dynamics and to select the number of involved levels. We extract the microscopic interaction parameters and find excellent agreement with a two particle model including all spin‐dependent interactions. By tuning the depth of the optical lattice, we investigated the transition from on‐site dominated to quantum manybody spin dynamics, where spins diffuse throughout the lattice producing highly entangled states. For this case, we observe a new form of melting of a band insulator.</p>
<p class="bodytext">Nature Physics 8, 813–818 (2012)</p><p>Photo: AG Sengstock</p>NAGR-fakmin-18351871-production2011-07-25T22:00:00ZMulti-band spectroscopy of ultracold fermions: Observation of reduced tunneling in attractive Bose-Fermi mixtures<img width="293" height="165" style="float:left" src="https://assets.rrz.uni-hamburg.de/instance_assets/fakmin/18351618/multi-band-spectroscopy-ultracold-fermions-880a9d61e61b562786885ab1c1b05b2f1854557e.png" /><p class="bodytext">We perform a detailed experimental study of the band excitations and tunneling properties of ultracold fermions in optical lattices. Employing a novel multi-band spectroscopy for fermionic atoms we can measure the full band structure and tunneling energy with high accuracy. In an attractive Bose-Fermi mixture we observe a significant reduction of the fermionic tunneling energy, which depends on the relative atom numbers. We attribute this to an interaction-induced increase of the lattice depth due to self-trapping of the atoms.</p>
<p class="bodytext">Phys. Rev. Lett. 107, 135303 (2011)</p>
<p class="bodytext">News coverage:<br>Article at Physical Review Focus</p><p>Photo: AG Sengstock</p>NAGR-fakmin-18351886-production2010-10-10T22:00:00ZAmplitude Mode in Strongly Correlated Superfluid<img width="293" height="165" style="float:left" src="https://assets.rrz.uni-hamburg.de/instance_assets/fakmin/18351609/amplitude-mode-strongly-correlated-baf07d0ba43a98cc86f031b60fcb107c8122103a.png" /><p class="bodytext">We report the first detection of the Higgs-type amplitude mode using Bragg spectroscopy in a strongly interacting condensate of ultracold atoms in an optical lattice. By the comparison of our experimental data with a spatially resolved, time-dependent dynamic Gutzwiller calculation, we obtain good quantitative agreement. This allows for a clear identification of the amplitude mode, showing that it can be detected with full momentum resolution by going beyond the linear response regime. A systematic shift of the sound and amplitude modes' resonance frequencies due to the finite Bragg beam intensity is observed.</p>
<p class="bodytext">Phys. Rev. Lett. 106, 205303 (2011)</p><p>Photo: AG Sengstock</p>NAGR-fakmin-18351898-production2009-11-28T23:00:00ZMomentum-Resolved Bragg Spectroscopy in Optical Lattices<img width="293" height="165" style="float:left" src="https://assets.rrz.uni-hamburg.de/instance_assets/fakmin/18351645/momentum-resolved-bragg-spectroscopy-6094d97324fc920d6c08f0667d5e8adb6b5970fd.jpg" /><p>We have measured a fully momentum-resolved bandstructure of a bosonic quantum gas in an optical lattice. Many-body interaction effects could be quantified with high resolution and compared to different theoretical models. State preparation and indications for Bogoliubov backscattering are among other results we obtained. <br>arXiv or Nature Physics</p><p>Photo: AG Sengstock</p>NAGR-fakmin-18351965-production2006-07-31T22:00:00ZFirst ultracold long-lived heteronuclear molecules produced<img width="293" height="165" style="float:left" src="https://assets.rrz.uni-hamburg.de/instance_assets/fakmin/18351573/first-ultracold-long-lived-heteronuclear-1d2e5d2ef9d5b345ad950a788b48c4377a6a41fc.jpg" /><p>Using RF association up to 104 heteronuclear molecules in a 3D optical lattice near the Feshbach resonance at 546 G were produced. In a deep lattice lifetimes over 100ms could be realized depending on the interaction strength. Furthermore, we could observe also molecules on the attractive side of the Feshbach resonance which are not stable outside a trapping potential as well as repulsively bound atom pairs. Heteronuclear molecules in their internal ground state open up spectacular possibilities of fundamental experiments concerning for example a permanent electric dipole moment. Phys.Rev.Lett. 97, 120403 (2006)</p><p>Photo: AG Sengstock</p>NAGR-fakmin-18351977-production2006-06-14T22:00:00ZFeshbach Resonances between K and Rb<img width="293" height="165" style="float:left" src="https://assets.rrz.uni-hamburg.de/instance_assets/fakmin/18351564/feshbach-resonances-kandrb-d8e6e9fa85814c56e58631e2d4be9c4ecf1e04ce.jpg" /><p>A current stabilization for the Helmholtz coils was finished which works quite well (1ppm!) s- and p-wave Feshbach resonances are measured and the interaction tuned continously. Strong repulsive interaction leads to phase separation, in the attractive case we can induce a collapse by strengthen the interaction. Phys.Rev.Lett. 97, 120403 (2006)</p><p>Photo: AG Sengstock</p>