arXiv subjects

Tilman Esslinger

Publications and source records attributed to Tilman Esslinger.

At least 55 records · Page 3Linked to original sources

arXiv2019

Quantum Simulation Meets Nonequilibrium Dynamical Mean Field Theory: Exploring the Periodically Driven, Strongly Correlated Fermi-Hubbard Model

We perform an ab-initio comparison between nonequilibrium dynamical mean-field theory and optical lattice experiments by studying the time evolution of double occupations in the periodically driven Fermi-Hubbard model. For off-resonant driving, the range of validity of a description in terms of an effective static Hamiltonian is determined and its breakdown due to energy absorption close to resonance is demonstrated. For near-resonant driving, we investigate the response to a change in driving amplitude and discover an asymmetric excitation spectrum with respect to the detuning. In general, we find good agreement between experiment and theory, which cross-validates the experimental and numerical approaches in a strongly-correlated nonequilibrium system.

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arXiv2019

Continuous feedback on a quantum gas coupled to an optical cavity

We present an active feedback scheme acting continuously on the state of a quantum gas dispersively coupled to a high-finesse optical cavity. The quantum gas is subject to a transverse pump laser field inducing a self-organization phase transition, where the gas acquires a density modulation and photons are scattered into the resonator. Photons leaking from the cavity allow for a real-time and non-destructive readout of the system. We stabilize the mean intra-cavity photon number through a micro-processor controlled feedback architecture acting on the intensity of the transverse pump field. The feedback scheme can keep the mean intra-cavity photon number $n_\text{ph}$ constant, in a range between $n_\text{ph}=0.17\pm 0.04$ and $n_\text{ph}=27.6\pm 0.5$, and for up to 4 s. Thus we can engage the stabilization in a regime where the system is very close to criticality as well as deep in the self-organized phase. The presented scheme allows us to approach the self-organization phase transition in a highly controlled manner and is a first step on the path towards the realization of many-body phases driven by tailored feedback mechanisms.

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arXiv2019

Quantized conductance through a spin-selective atomic point contact

We implement a microscopic spin filter for cold fermionic atoms in a quantum point contact (QPC) and create fully spin-polarized currents while retaining conductance quantization. Key to our scheme is a near-resonant optical tweezer inducing a large effective Zeeman shift inside the QPC while its local character limits dissipation. We observe a renormalization of this shift due to interactions of a few atoms in the QPC. Our work represents the analog of an actual spintronic device and paves the way to studying the interplay between spin-splitting and interactions far from equilibrium.

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arXiv2019

Quantized conductance through a dissipative atomic point contact

Signatures of quantum transport are expected to quickly vanish as dissipation is introduced in a system. This dissipation can take several forms, including that of particle loss, which has the consequence that the total probability current is not conserved. Here, we study the effect of such losses at a quantum point contact (QPC) for ultracold atoms. Experimentally, dissipation is provided by a near-resonant optical tweezer whose power and detuning control the loss rates for the different internal atomic states as well as their effective Zeeman shifts. We theoretically model this situation by including losses in the Landauer-Büttiker formalism over a wide range of dissipative rates. We find good agreement between our measurements and our model, both featuring robust conductance plateaus. Finally, we are able to map out the atomic density by varying the position of the near-resonant tweezer inside the QPC, realizing a dissipative scanning gate microscope for cold atoms.

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arXiv2019

Enhancement and sign change of magnetic correlations in a driven quantum many-body system

Periodic driving can be used to coherently control the properties of a many-body state and to realize new phases which are not accessible in static systems. For example, exposing materials to intense laser pulses enables to provoke metal-insulator transitions, control the magnetic order and induce transient superconducting behaviour well above the static transition temperature. However, pinning down the responsible mechanisms is often difficult, since the response to irradiation is governed by complex many-body dynamics. In contrast to static systems, where extensive calculations have been performed to explain phenomena such as high-temperature superconductivity, theoretical analyses of driven many-body Hamiltonians are more demanding and new theoretical approaches have been inspired by the recent observations. Here, we perform an experimental quantum simulation in a periodically modulated hexagonal lattice and show that anti-ferromagnetic correlations in a fermionic many-body system can be reduced or enhanced or even switched to ferromagnetic correlations. We first demonstrate that in the high frequency regime, the description of the many-body system by an effective Floquet-Hamiltonian with a renormalized tunnelling energy remains valid, by comparing the results to measurements in an equivalent static lattice. For near-resonant driving, the enhancement and sign reversal of correlations is explained by a microscopic model, in which the particle tunnelling and magnetic exchange energies can be controlled independently. In combination with the observed sufficiently long lifetime of correlations, Floquet engineering thus constitutes an alternative route to experimentally investigate unconventional pairing in strongly correlated systems.

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arXiv2019

Two-mode Dicke model from non-degenerate polarization modes

We realize a non-degenerate 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.

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arXiv2019

Dissipation Induced Structural Instability and Chiral Dynamics in a Quantum Gas

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.

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arXiv2018

Floquet dynamics in driven Fermi-Hubbard systems

We study the dynamics and timescales of a periodically driven Fermi-Hubbard model in a three-dimensional hexagonal lattice. The evolution of the Floquet many-body state is analyzed by comparing it to an equivalent implementation in undriven systems. The dynamics of double occupancies for the near- and off-resonant driving regime indicate that the effective Hamiltonian picture is valid for several orders of magnitude in modulation time. Furthermore, we show that driving a hexagonal lattice compared to a simple cubic lattice allows to modulate the system up to 1~s, corresponding to hundreds of tunneling times, with only minor atom loss. Here, driving at a frequency close to the interaction energy does not introduce resonant features to the atom loss.

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arXiv2018

Breakdown of the Wiedemann-Franz law in a unitary Fermi gas

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.

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arXiv2018

Band and correlated insulators of cold fermions in a mesoscopic lattice

We investigate the transport properties of neutral, fermionic atoms passing through a one-dimensional quantum wire containing a mesoscopic lattice. The lattice is realized by projecting individually controlled, thin optical barriers on top of a ballistic conductor. Building an increasingly longer lattice, one site after another, we observe and characterize the emergence of a band insulating phase, demonstrating control over quantum-coherent transport. We explore the influence of atom-atom interactions and show that the insulating state persists as contact interactions are tuned from moderately to strongly attractive. Using bosonization and classical Monte-Carlo simulations we analyze such a model of interacting fermions and find good qualitative agreement with the data. The robustness of the insulating state supports the existence of a Luther-Emery liquid in the one-dimensional wire. Our work realizes a tunable, site-controlled lattice Fermi gas strongly coupled to reservoirs, which is an ideal test bed for non-equilibrium many-body physics.

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arXiv2018

Experimental realisation of the topological Haldane model

The Haldane model on the honeycomb lattice is a paradigmatic example of a Hamiltonian featuring topologically distinct phases of matter. It describes a mechanism through which a quantum Hall effect can appear as an intrinsic property of a band-structure, rather than being caused by an external magnetic field. Although an implementation in a material was considered unlikely, it has provided the conceptual basis for theoretical and experimental research exploring topological insulators and superconductors. Here we report on the experimental realisation of the Haldane model and the characterisation of its topological band-structure, using ultracold fermionic atoms in a periodically modulated optical honeycomb lattice. The model is based on breaking time-reversal symmetry as well as inversion symmetry. The former is achieved through the introduction of complex next-nearest-neighbour tunnelling terms, which we induce through circular modulation of the lattice position. For the latter, we create an energy offset between neighbouring sites. Breaking either of these symmetries opens a gap in the band-structure, which is probed using momentum-resolved interband transitions. We explore the resulting Berry-curvatures of the lowest band by applying a constant force to the atoms and find orthogonal drifts analogous to a Hall current. The competition between both broken symmetries gives rise to a transition between topologically distinct regimes. By identifying the vanishing gap at a single Dirac point, we map out this transition line experimentally and quantitatively compare it to calculations using Floquet theory without free parameters. We verify that our approach, which allows for dynamically tuning topological properties, is suitable even for interacting fermionic systems. Furthermore, we propose a direct extension to realise spin-dependent topological Hamiltonians.

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arXiv2018

Controlling the Floquet state population and observing micromotion in a periodically driven two-body quantum system

Near-resonant periodic driving of quantum systems promises the implementation of a large variety of novel effective Hamiltonians. The challenge of Floquet engineering lies in the preparation and measurement of the desired quantum state. We address these aspects in a model system consisting of interacting fermions in a periodically driven array of double wells created by an optical lattice. The singlet and triplet fractions and the double occupancy of the Floquet states are measured, and their behavior as a function of the interaction strength is analyzed in the high- and low-frequency regimes. We demonstrate full control of the Floquet state population and find suitable ramping protocols and time-scales which adiabatically connect the initial ground state to different targeted Floquet states. The micromotion which exactly describes the time evolution of the system within one driving cycle is observed. Additionally, we provide an analytic description of the model and compare it to numerical simulations.

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arXiv2018

Creating State-Dependent Lattices for Ultracold Fermions by Magnetic Gradient Modulation

We demonstrate a versatile method to create state-dependent optical lattices by applying a magnetic field gradient modulated in time. This allows for tuning the relative amplitude and sign of the tunnelling for different internal states. We observe substantially different momentum distributions depending on the spin-state of fermionic 40K 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 whilst others remain itinerant. A systematic study shows negligible heating and lifetimes of several seconds in the Hubbard regime.

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arXiv2017

Metastability and avalanche dynamics in strongly-correlated gases with long-range interactions

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.

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arXiv2017

Coupling two order parameters in a quantum gas

Controlling matter to simultaneously support multiple coupled properties is of fundamental and technological importance. For example, the simultaneous presence of magnetic and ferroelectric orders in multiferroic materials leads to enhanced functionalities. In high-temperature superconductors, intertwining between charge- and spin-order can form superconducting states at high transition temperatures. However, pinning down the microscopic mechanisms responsible for the simultaneous presence of different orders is difficult, making it hard to predict the phenomenology of a material or to experimentally modify its properties. Here we use a quantum gas to engineer an adjustable interaction at the microscopic level between two orders, and demonstrate scenarios of competition, coexistence and coupling between them. In the latter case, intriguingly, the presence of one order lowers the critical point of the other. Our system is realized by a Bose-Einstein condensate which can undergo self-organization phase transitions in two optical resonators, resulting in two distinct crystalline density orders. We characterize the intertwining between these orders by measuring the composite order parameter and the elementary excitations. We explain our results with a mean-field free energy model, which is derived from a microscopic Hamiltonian. Our system is ideally suited to explore properties of quantum tricritical points as recently realized in and can be extended to study the interplay of spin and density orders also as a function of temperature.

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arXiv2017

Two-terminal transport measurements with cold atoms

In the last years, the ability of cold atoms experiments to explore condensed- matter related questions has dramatically progressed. Transport experiments, in particular, have expanded to the point that conductances and other transport coefficients can now be measured in a way directly analogous to solid state physics, extending cold atoms based quantum simulations into the domain of quantum electronic devices. In this topical review, we describe the transport experiments performed with cold gases in the two terminals configuration, with an emphasis on the specific features of cold atomic gases compared to solid state physics. We present the experimental techniques and the main experimental findings, focusing on but not restricted to the recent experiments performed in our group. We eventually discuss the perspectives opened by this approach, the main technical and conceptual challenges for future developments, and the potential applications as a quantum simulator for transport phenomena and mesoscopic physics problems.

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arXiv2017

Monitoring and manipulating Higgs and Goldstone modes in a supersolid quantum gas

Access to collective excitations lies at the heart of our understanding of quantum many-body systems. We study the Higgs and Goldstone modes in a supersolid quantum gas that is created by coupling a Bose-Einstein condensate symmetrically to two optical cavities. The cavity fields form a U(1)-symmetric order parameter that can be modulated and monitored along both quadratures in real time. This enables us to measure the excitation energies across the superfluid-supersolid phase transition, establish their amplitude and phase nature, as well as characterize their dynamics from an impulse response. Furthermore, we can give a tunable mass to the Goldstone mode at the crossover between continuous and discrete symmetry by changing the coupling of the quantum gas with either cavity.

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arXiv2017

A scanning gate microscope for cold atomic gases

We present a scanning probe microscopy technique for spatially resolving transport in cold atomic gases, in close analogy with scanning gate microscopy in semiconductor physics. The conductance of a quantum point contact connected to two atomic reservoirs is measured in the presence of a tightly focused laser beam acting as a local perturbation that can be precisely positioned in space. By scanning its position and recording the subsequent variations of conductance, we retrieve a high-resolution map of transport through a quantum point contact. We demonstrate a spatial resolution comparable to the extent of the transverse wave function of the atoms inside the channel, and a position sensitivity below 10nm. Our measurements agree well with an analytical model and ab-initio numerical simulations, allowing us to identify a regime in transport where tunneling dominates over thermal effects. Our technique opens new perspectives for the high-resolution observation and manipulation of cold atomic gases.

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