This thesis reports on two experiments employing ultracold atoms to realize many-body physics in the presence of an orbital degree of freedom. In particular, we develop the experimental techniques to probe the Fermi polaron problem across an orbital Feshbach resonance and examine the relaxation dynamics in the one-dimensional mass-imbalanced Fermi-Hubbard model.
Quasiparticles like the Fermi polaron constitute an elementary part of Landau's Fermi liquid theory and find extensive application in the description of interacting electron systems. In the first part of this thesis, we exploit the recently observed orbital Feshbach resonance in ytterbium to produce multiorbital Fermi polarons in mixtures of ground state and clock state atoms. To this end, we employ clock-line spectroscopy and identify the repulsive as well as attractive polaron in the many-body spectrum of the two-dimensional impurity-bath system. We also determine other important quasiparticle properties, namely, the residue and lifetime. Our measurement results agree well with a tailored many-body theory, thereby confirming the relevance of orbital interactions for the Fermi polaron problem.
In the second experiment, we study the mass-imbalanced Fermi-Hubbard model with ytterbium atoms trapped in a state-dependent optical lattice for the ground and metastable clock state. Atomic populations are prepared in both of these states and correspond to heavy and light particles exhibiting vastly different dynamical time scales. Recently, such models have been extensively explored in the context of localization and thermalization in isolated quantum many-body systems. To probe non-equilibrium dynamics in our system, we record the density of the light species after adjusting the external trapping potential. In this way, we identify a strong suppression of transport at early times and slow relaxation at late times, with a significant dependence on the dynamical time scale and interaction strength of both species. Our results demonstrate the emergence of metastability and extremely slow thermalization due to dynamical constraints.
The implementations of multiorbital many-body physics established within this thesis pave the way for quantum simulators that could shed light on open questions in condensed-matter theory.
This thesis reports on two experiments employing ultracold atoms to realize many-body physics in the presence of an orbital degree of freedom. In particular, we develop the experimental techniques to probe the Fermi polaron problem across an orbital Feshbach resonance and examine the relaxation dynamics in the one-dimensional mass-imbalanced Fermi-Hubbard model.
Quasiparticles like the Fermi polaron constitute an elementary part of Landau's Fermi liquid theory and find extensive application in the description of interacting electron systems. In the first part of this thesis, we exploit the recently observed orbital Feshbach resonance in ytterbium to produce multiorbital Fermi polarons in mixtures of ground state and clock state atoms. To this end, we employ clock-line spectroscopy and identify the repulsive as well as attractive polaron in the many-body spectrum of the two-dimensional impurity-bath system. We also determine other important quasiparticle properties, namely, the residue and lifetime. Our measurement results agree well with a tailored many-body theory, thereby confirming the relevance of orbital interactions for the Fermi polaron problem.
In the second experiment, we study the mass-imbalanced Fermi-Hubbard model with ytterbium atoms trapped in a state-dependent optical lattice for the ground and metastable clock state. Atomic populations are prepared in both of these states and correspond to heavy and light particles exhibiting vastly different dynamical time scales. Recently, such models have been extensively explored in the context of localization and thermalization in isolated quantum many-body systems. To probe non-equilibrium dynamics in our system, we record the density of the light species after adjusting the external trapping potential. In this way, we identify a strong suppression of transport at early times and slow relaxation at late times, with a significant dependence on the dynamical time scale and interaction strength of both species. Our results demonstrate the emergence of metastability and extremely slow thermalization due to dynamical constraints.
The implementations of multiorbital many-body physics established within this thesis pave the way for quantum simulators that could shed light on open questions in condensed-matter theory.
We report on the experimental observation of multiorbital polarons in a two-dimensional Fermi gas of ^{173}Yb atoms formed by mobile impurities in the metastable ^{3}P_{0} orbital and a Fermi sea in the ground-state ^{1}S_{0} orbital. We spectroscopically probe the energies of attractive and repulsive polarons close to an orbital Feshbach resonance and characterize their coherence by measuring the quasiparticle residue. For all probed interaction parameters, the repulsive polaron is a long-lived quasiparticle with a decay rate more than 2 orders of magnitude below its energy. We formulate a many-body theory, which accurately treats the interorbital interactions in two dimensions and agrees well with the experimental results. Our work paves the way for the investigation of many-body physics in multiorbital ultracold Fermi gases.
We study the possibility of realizing strong coupled many-body quantum phases in ultracold atomic systems. Motivated by recent experiments, we first analyze the phase diagram of a Bose-Fermi mixture across a Feshbach resonance, and offer an explanation for the collapse of the system observed close to the Feshbach resonance: we find that phase separation leads to a high density phase which causes the collapse. We then focus on the recent attempts to realize the three-dimensional fermionic Hubbard model in an optical lattice. One milestone on the experimentalists' agenda is to access the antiferromagnetic ordered Neél phase, which has so far been hindered by the low ordering temperature. We ask which experimental parameters maximize the antiferromagnetic interactions, which set the scale for the ordering temperature. We find that the maximum is obtained in a regime where the effective Hamiltonian describing the system no longer corresponds to a simple one-band Hubbard model, and we characterize the physics of the system in this regime. The final system we consider is mass imbalanced polarized two-component Fermi gases interacting via a Feshbach resonance. By going to the strongly polarized limit, we use a recently developed method to obtain results which have been shown to be accurate in the mass balanced case, and we find an intriguing set of competing phases in this limit. We discuss what these results imply for the full phase diagram.
The quantum many-body problem is at the heart of some of the most formidable open problems in modern physics, such as high-Tc superconductivity or the behaviour of neutron stars. Ultracold atomic systems can now be used to simulate model Hamiltonians of condensed matter or nuclear physics, in very well-controlled environnement. In this thesis, we have developed a general method to probe the thermodynamics of homogeneous quantum systems using trapped atomic gases. These measurements are directly compared to the predictions of theories of the quantum many-body problem. We have applied this technique to the spin-1/2 Fermi gas and the Bose with short-range interactions. Using fermionic 6Li, we explored a part of the wide parameter space by changing the interaction strength, the spin-population imbalance or the temperature of the gas. This system exhibits remarkably rich physics, such as normal/superfluid phase transitions (that can be of thermal or quantum character) or a Fermi liquid-type behaviour of the normal phase. We have also used this method to probe a Bose gas of 7Li atoms close to a Feshbach resonance. We have measured the Equation of State of the Bose gas as a function of interactions at very low temperature. For the first time, we measured quantitatively the Lee-Huang-Yang beyond mean-field correction to the ground-state energy of the system, first predicted in 1957. We compared the experimental results to Quantum Monte-Carlo calculations. We have extended this study using out-of-equilibrium measurements of the Bose gas in the strongly interacting regime, which gives a first hint on the properties of the hypothetical unitary Bose gas.
Strongly correlated quantum systems display a myriad of complex, fascinating behaviour. The competition between charge, spin, orbital, or lattice degrees of freedom and their respective energy scales gives rise to rich and complex emergent phases. More recently, the non-equilibrium properties of correlated quantum many-body systems have come into the focus of active research, with remarkable advances. While the advent of tensor network based simulations on the theoretical side has allowed access to the full time-evolution of the quantum state as it traverses the Hilbert space, the experimental progress in the ultracold atom community has allowed physicists to enter the paradigm of analog quantum simulation. Their tunability and isolation from the environment makes them ideal platforms to emulate and study open problems in condensed matter physics and go beyond the simulational capabilities of state of the art classical simulations. In this thesis, we explore the dynamics of correlated Fermi gases. We develop and present different facets of applicability of radiofrequency modulation techniques to drive the system out of equilibrium. We begin by considering a homogeneous, three-dimensional Fermi gas in the BCS-BEC crossover and develop an off-resonant radiofrequency transfer scheme to excite the Higgs mode of the superfluid. In a subsequent study we investigate the tunability of the coherence between Cooper pairs by tuning the duration of interaction ramps of the internal interaction strength. These works highlight the possibility to externally tune and stabilise complex quantum many-body states away from equilibrium. Using quasi-exact matrix product state simulations, we study the response of an attractively interacting, one-dimensional Fermi-Hubbard model to weak radiofrequency perturbations. We reveal the emergence of two distinct dynamical regimes in the time evolution and are able to relate it back to the underlying excitation spectrum of the system. Finally, we explore the possibility to perform quantum quenches through radiofrequency π-pulses, achieving nearly complete population transfer between different interacting fermionic states. We reveal non-trivial dynamical effects in the pair correlation of the final state, as well as the excitation of a collective trap mode of the system.
Advances in experiments on dilute ultracold atomic gases have given us access to highly tunable quantum systems. In particular, there have been substantial improvements in achieving different kinds of interaction between atoms. As a result, utracold atomic gases oer an ideal platform to simulate many-body phenomena in condensed matter physics, and engineer other novel phenomena that are a result of the exotic interactions produced between atoms. In this dissertation, I present a series of studies that explore the physics of dilute ultracold atomic gases in different settings. In each setting, I explore a different form of the inter-particle interaction. Motivated by experiments which induce artificial spin-orbit coupling for cold fermions, I explore this system in my first project. In this project, I propose a method to perform universal quantum computation using the excitations of interacting spin-orbit coupled fermions, in which effective p-wave interactions lead to the formation of a topological superfluid. Motivated by experiments which explore the physics of exotic interactions between atoms trapped inside optical cavities, I explore this system in a second project. I calculate the phase diagram of lattice bosons trapped in an optical cavity, where the cavity modes mediates effective global range checkerboard interactions between the atoms. I compare this phase diagram with one that was recently measured experimentally. In two other projects, I explore quantum simulation of condensed matter phenomena due to spin-dependent interactions between particles. I propose a method to produce tunable spin-dependent interactions between atoms, using an optical Feshbach resonance. In one project, I use these spin-dependent interactions in an ultracold Bose-Fermi system, and propose a method to produce the Kondo model. I propose an experiment to directly observe the Kondo effect in this system. In another project, I propose using lattice bosons with a large hyperfine spin, which have Feshbach-induced spin-dependent interactions, to produce a quantum dimer model. I propose an experiment to detect the ground state in this system. In a final project, I develop tools to simulate the dynamics of fermionic superfluids in which fermions interact via a short-range interaction.
The experimental control and observation of quantum many-body systems has become a reality with the advent of ultracold quantum matter. The high level of isolation of these experiments, together with the development of novel measurement techniques, has revived a fundamental debate concerning the thermal equilibration of isolated quantum systems, commonly named "quantum thermalization". In this thesis we make use of a quantum-gas microscope to explore the thermalizing dynamics of highly isolated systems of ultracold bosonic atoms in optical lattices. The ability to prepare and control quantum systems made up of hundreds of atoms makes it possible to explore regimes that represent a challenge for classical numeric simulations. A major part of this dissertation deals with Bose-Hubbard systems in the presence of quenched disorder. We begin by studying the microscopic properties of its phases near equilibrium, where by tuning the strength of the disorder, observe features consistent with the emergence of a so-called Bose-glass phase. We then continue by preparing states far from equilibrium and exploring their quantum many-body dynamics. In particular, we observe signatures of the phenomenon of "many-body localization", which implies a breakdown of quantum thermalization. In addition, we study whether localized systems can be thermalized via the coupling to a bath of few degrees of freedom, i.e. a quantum bath. We do so by preparing a mixture of two interacting atomic species, where one acts as the bath and the other as the localized system. We do observe delocalizing dynamics for large enough baths, though in regimes of weak coupling localization can survive for extremely long times. The second main topic of this thesis is the thermalization of periodically driven many-body systems, so-called Floquet thermalization. In these systems, the absence of energy conservation eventually brings any initial state into a featureless infinite-temperature one. However, for sufficiently high frequencies this thermalization process can take arbitrarily long times, which can enable the engineering of exotic long-lived prethermal states. We use the high isolation of our system, together with the high sensitivity of quantum-gas microscopy, to measure the heating rates for a range of driving frequencies and interaction regimes. Our results show a dramatic suppression of the heating as the frequency of the drive is increased, which is consistent with theoretical expectations.