Probing quench dynamics across a quantum phase transition into a 2D Ising antiferromagnet

Simulating the real-time evolution of quantum spin systems far out of equilibrium poses a major theoretical challenge, especially in more than one dimension. We experimentally explore the dynamics of a two-dimensional Ising spin system with transverse and longitudinal fields as we quench it across a quantum phase transition from a paramagnet to an antiferromagnet. We realize the system with a near unit-occupancy atomic array of over 200 atoms obtained by loading a spin-polarized band insulator of fermionic lithium into an optical lattice and induce short-range interactions by direct excitation to a low-lying Rydberg state. Using site-resolved microscopy, we probe the correlations in the system after a sudden quench from the paramagnetic state and compare our measurements to exact calculations in the regime where it is possible. We achieve many-body states with longer-range antiferromagnetic correlations by implementing a near-adiabatic quench and study the buildup of correlations as we cross the quantum phase transition at different rates

Motion of a Solitonic Vortex in the BEC-BCS Crossover

We observe a long-lived solitary wave in a superfluid Fermi gas of $^6$Li atoms after phase-imprinting. Tomographic imaging reveals the excitation to be a solitonic vortex, oriented transverse to the long axis of the cigar-shaped atom cloud. The precessional motion of the vortex is directly observed, and its period is measured as a function of the chemical potential in the BEC-BCS crossover. The long period and the correspondingly large ratio of the inertial to the bare mass of the vortex are in good agreement with estimates based on superfluid hydrodynamics that we derive here using the known equation of state in the BEC-BCS crossover

Visualizing Strange Metallic Correlations in the 2D Fermi-Hubbard Model with AI

Strongly correlated phases of matter are often described in terms of straightforward electronic patterns. This has so far been the basis for studying the Fermi-Hubbard model realized with ultracold atoms. Here, we show that artificial intelligence (AI) can provide an unbiased alternative to this paradigm for phases with subtle, or even unknown, patterns. Long- and short-range spin correlations spontaneously emerge in filters of a convolutional neural network trained on snapshots of single atomic species. In the less well-understood strange metallic phase of the model, we find that a more complex network trained on snapshots of local moments produces an effective order parameter for the non-Fermi-liquid behavior. Our technique can be employed to characterize correlations unique to other phases with no obvious order parameters or signatures in projective measurements, and has implications for science discovery through AI beyond strongly correlated systems.Comment: 12 pages, 9 figures; updated in accord with the published versio

Subdiffusion and heat transport in a tilted 2D Fermi-Hubbard system

Using quantum gas microscopy we study the late-time effective hydrodynamics of an isolated cold-atom Fermi-Hubbard system subject to an external linear potential (a "tilt"). The tilt is along one of the principal directions of the two-dimensional (2D) square lattice and couples mass transport to local heating through energy conservation. We study transport and thermalization in our system by observing the decay of prepared initial density waves as a function of wavelength $\lambda$ and tilt strength and find that the associated decay time $\tau$ crosses over as the tilt strength is increased from characteristically diffusive to subdiffusive with $\tau\propto\lambda^4$. In order to explain the underlying physics we develop a hydrodynamic model that exhibits this crossover. For strong tilts, the subdiffusive transport rate is set by a thermal diffusivity, which we are thus able to measure as a function of tilt in this regime. We further support our understanding by probing the local inverse temperature of the system at strong tilts, finding good agreement with our theoretical predictions. Finally, we discuss the relation of the strongly tilted limit of our system to recently studied 1D models which may exhibit nonergodic dynamics.Comment: 7 pages with 5 figures in main text, 5 pages with 3 figures in Supplemental Materia

Visualizing strange metallic correlations in the two-dimensional Fermi-Hubbard model with artificial intelligence

Strongly correlated phases of matter are often described in terms of straightforward electronic patterns. This has so far been the basis for studying the Fermi-Hubbard model realized with ultracold atoms. Here, we show that artificial intelligence (AI) can provide an unbiased alternative to this paradigm for phases with subtle, or even unknown, patterns. Long- A nd short-range spin correlations spontaneously emerge in filters of a convolutional neural network trained on snapshots of single atomic species. In the less well-understood strange metallic phase of the model, we find that a more complex network trained on snapshots of local moments produces an effective order parameter for the non-Fermi-liquid behavior. Our technique can be employed to characterize correlations unique to other phases with no obvious order parameters or signatures in projective measurements, and has implications for science discovery through AI beyond strongly correlated systems

Bad metallic transport in a cold atom Fermi-Hubbard system

Charge transport is a revealing probe of the quantum properties of materials. Strong interactions can blur charge carriers resulting in a poorly understood "quantum soup". Here we study the conductivity of the Fermi-Hubbard model, a testing ground for strong interaction physics, in a clean quantum system - ultracold $^6$Li in a 2D optical lattice. We determine the charge diffusion constant in our system by measuring the relaxation of an imposed density modulation and modeling its decay hydrodynamically. The diffusion constant is converted to a resistivity, which exhibits a linear temperature dependence and exceeds the Mott-Ioffe-Regel limit, two characteristic signatures of a bad metal. The techniques we develop here may be applied to measurements of other transport quantities, including the optical conductivity and thermopower

Exploring many-body quantum dynamics with Rydberg dressed fermions and tilted Hubbard systems

Ultracold atomic gases in optical lattices are an ideal platform for studying quantum many-body physics. The long timescales and isolated nature of these systems makes them particularly suited for exploring the dynamics of nearly closed quantum systems and their relaxation towards thermal equilibrium. In this thesis, we demonstrate the realization of two novel cold atom systems: lattice Fermi gases with non-local interactions and tilted Fermi-Hubbard systems. In both of these systems, we explore the slow relaxation of density perturbations, either due to kinetic constraints or unusual hydrodynamics. The first system we study is a Fermi gas laser coupled to a Rydberg state. For near resonant coupling of a localized gas in a unit-filled lattice, we realize a quantum Ising model with transverse and longitudinal fields. We study the out-of-equilibrium dynamics of antiferromagnetic correlations in this spin system. For far off-resonant Rydberg coupling, we prepare itinerant Fermi gases with strong non-local interactions. In this Rydberg-dressed regime, we introduce a small Rydberg admixture to the ground state of the system which results in a laser-tunable soft-core interaction potential. We use this technique to realize a $t-V$ model with spin-polarized fermions and study the dynamics of imprinted charge density waves. For strong off-site interactions, the number of bonds is approximately conserved, which leads to slow relaxation of these states. More generally, the Rydberg-dressing technique is promising for future studies of extended Hubbard models in multi-component systems. The second system we study is the two-dimensional Fermi-Hubbard model in the presence of a large tilt. When the tilt is aligned with a lattice axis, the system exhibits slow thermalization and subdiffusive charge transport due to modified hydrodynamics where heat transport acts as a bottleneck for charge transport. This work sets the stage for studying the complete breakdown of thermalization expected for more generic tilt angles where Hilbert space fragmentation is expected

A laser system for trapping and cooling of ⁶Li atoms

Thesis: S.B., Massachusetts Institute of Technology, Department of Physics, 2015.Cataloged from PDF version of thesis.Includes bibliographical references (pages 59-60).In this thesis, I designed and built a laser system for the trapping and cooling of ⁶Li atoms. The thesis starts explaining a theoretical background of the necessary laser frequencies for the realization of a Zeeman Slower and a 3D MOT. Next it describes the design of the laser system that makes use of a Raman Fiber Amplifier coupled with a Frequency Doubling Cavity and shows the finalized setup. Finally, the thesis delves into the topic of Modulation Transfer Spectroscopy which was used to lock the laser to the D₂ line transition of ⁶Li and shows the spectroscopy setup built for the laser system.by Elmer Guardado-Sanchez.S.B