Constraints in the dynamics of quantum many-body systems can dramatically alter transport properties and relaxation timescales even in the absence of static disorder.Here, we report on the observation of such constrained dynamics arising from the distinct mobility of two species in the one-dimensional massimbalanced Fermi-Hubbard model, realized with ultracold ytterbium atoms in a state-dependent optical lattice.By displacing the trap potential and monitoring the subsequent dynamical response of the system, we identify suppressed transport and slow relaxation with a strong dependence on the mass imbalance and interspecies interaction strength, consistent with eventual thermalization for long times.Our observations demonstrate the potential for quantum simulators to provide insights into unconventional relaxation dynamics arising from constraints.
Constraints in the dynamics of quantum many-body systems can dramatically alter transport properties and relaxation timescales even in the absence of static disorder.Here, we report on the observation of such constrained dynamics arising from the distinct mobility of two species in the one-dimensional massimbalanced Fermi-Hubbard model, realized with ultracold ytterbium atoms in a state-dependent optical lattice.By displacing the trap potential and monitoring the subsequent dynamical response of the system, we identify suppressed transport and slow relaxation with a strong dependence on the mass imbalance and interspecies interaction strength, consistent with eventual thermalization for long times.Our observations demonstrate the potential for quantum simulators to provide insights into unconventional relaxation dynamics arising from constraints.
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 experimentally study localization and dynamics of ultracold fermions in speckle and optical lattice potentials to explore Anderson localization, many-body localization, and relaxation dynamics in strongly correlated systems. Anderson localization is probed by releasing non-interacting, spin-polarized gases into three dimensional, anisotropic disordered potentials produced from optical speckle. A fraction of the atoms are localized by the disorder, and a mobility edge is found separating localized from extended states. The length scale of the speckle is varied, and the localized state is found to scale linearly with the geometric mean of the speckle autocorrelation length. We realize the Fermi Hubbard model by loading atoms in a cubic optical lattice. Non-equilibrium momentum distributions are created via Raman transitions, and the excitation relaxation rate is measured in the lattice. Transport experiments were performed in a disordered optical lattice to explore the disordered Hubbard model. These experiments reveal localization in the presence of strong interactions and an interaction driven metal-to-insulator transition. The localized state is found to be insensitive to a doubling in the temperature of the gas and is consistent with predictions of many-body localization.
We consider two mutually interacting fermionic particle species on a one-dimensional lattice and study how the mass ratio $\ensuremath{\eta}$ between the two species affects the (equilibration) dynamics of the particles. Focusing on the regime of strong interactions and high temperatures, two well-studied points of reference are given by (i) the case of equal masses $\ensuremath{\eta}=1$, i.e., the standard Fermi-Hubbard chain, where initial nonequilibrium density distributions are known to decay, and (ii) the case of one particle species being infinitely heavy, $\ensuremath{\eta}=0$, leading to a localization of the lighter particles in an effective disorder potential. Given these two opposing cases, the dynamics in the case of intermediate mass ratios $0<\ensuremath{\eta}<1$ is of particular interest. To this end, we study the real-time dynamics of pure states featuring a sharp initial nonequilibrium density profile. Relying on the concept of dynamical quantum typicality, the resulting nonequilibrium dynamics can be related to equilibrium correlation functions. Summarizing our main results, we observe that diffusive transport occurs for moderate values of the mass imbalance and manifests itself in a Gaussian spreading of real-space density profiles and an exponential decay of density modes in momentum space. For stronger imbalances, we provide evidence that transport becomes anomalous on intermediate timescales, and in particular, our results are consistent with the absence of strict localization in the long-time limit for any $\ensuremath{\eta}>0$. Based on our numerical analysis, we provide an estimate for the ``lifetime'' of the effective localization as a function of $\ensuremath{\eta}$.
We observe the emergence of a disorder-induced insulating state in a strongly interacting atomic Fermi gas trapped in an optical lattice. This closed quantum system free of a thermal reservoir realizes the disordered Fermi-Hubbard model, which is a minimal model for strongly correlated electronic solids. In measurements of disorder-induced localization obtained via mass transport, we detect interaction-driven delocalization and localization that persists as the temperature of the gas is raised. These behaviors are consistent with many-body localization, which is a novel paradigm for understanding localization in interacting quantum systems at non-zero temperature.
We investigate the non-equilibrium dynamics of the disordered one-dimensional Fermi-Hubbard model with a focus on many-body localization. The system is initialized in a charge-density-wave-state, and its time evolution is analyzed through sublattice imbalance (spin and charge), and bipartite entanglement entropy. A clear crossover from ergodic to non-ergodic behavior is observed with increasing disorder strength. In the weak disorder regime, rapid decay of imbalance and the fast growth of entanglement indicate efficient thermalization. In contrast, a strong disorder leads to persistent imbalance and slow dynamics, signaling the breakdown of ergodicity. The charge and spin sectors exhibit distinct relaxation behavior, providing evidence for partial decoupling between these degrees of freedom. Furthermore, in the interacting regime, the entanglement entropy shows slow logarithmic growth, reflecting the dephasing-driven dynamics characteristic of the many-body localized phase. These results highlight the interplay between disorder and interactions in determining the dynamical properties of the system and establish robust signatures of many-body localization in the Fermi-Hubbard model.
We observe the emergence of a disorder-induced insulating state in a strongly interacting atomic Fermi gas trapped in an optical lattice. This closed quantum system, free of a thermal reservoir, realizes the disordered Fermi-Hubbard model, which is a minimal model for strongly correlated electronic solids. We observe disorder-induced localization of a metallic state through measurements of mass transport. By varying the lattice potential depth, we detect interaction-driven delocalization of the disordered insulating state. We also measure localization that persists as the temperature of the gas is raised. These behaviors are consistent with many-body localization, which is a novel paradigm for understanding localization in interacting quantum systems at nonzero temperature.