Session M3: Dynamics of Cold Atoms in Optical Lattices

Relaxation dynamics in the Fermi-Hubbard model

We report measurements of spin-excitation relaxation rates for ultracold 40K atoms trapped in an optical lattice in the metallic regime of the Hubbard model. A spin-polarized gas is prepared in a well-defined state in a cubic optical lattice. Via a quasimomentum-selective Raman pulse, atoms are transferred into another spin state with non-zero center-of-mass momentum. The timescale for relaxation of this excitation is measured as the temperature and lattice potential depth are varied. Non-Fermi liquid behavior is revealed in both the temperature and interaction-strength dependence.   

Relaxation dynamics of a Fermi gas in an optical superlattice

The question of how a closed quantum system out of equilibrium evolves and relaxes, is still not well understood. A specific setting of coherent quantum dynamics can be provided by quenches when one starts from the ground state of an initial Hamiltonian and suddenly changes the Hamiltonian's parameters. After this change the system is highly excited with respect to the new Hamiltonian and evolves in time. Ultracold quantum gases in optical lattice are good candidates to study such non-equilibrium situations. Here we present an experimental and theoretical investigation of the time evolution of a Fermi gas following fast and slow quenches of a one-dimensional double-well superlattice potential. We probe both the local tunneling in the double wells and the global dynamics towards a steady state. The local observables in the steady state resemble those of a thermal equilibrium state, whereas the global properties indicate a strong non-equilibrium situation. The experimental results are compared to the numerical studies based on the exact diagonalization of the Hamiltonian in the continuum considering the loading procedure of the three-dimentional Fermionic cloud into the one-dimensional optical superlattice and the measurement sequences.   

Raman Sideband Cooling and Lattice Imaging of Lithium

We report on progress towards achieving Raman sideband cooling and site-resolved two-photon fluorescence imaging of fermionic Lithium. This work seeks to extend our nondestructive lattice imaging technique [1] demonstrated in $^{87}$Rb to an atomic species with unresolved hyperfine structure. In addition to enabling in situ studies of a fermionic lattice gas, this technique opens new avenues for the creation and study of non-equilibrium dynamics in strongly correlated many-body systems and measurement-induced quantum control.\\[4pt] [1] Y. S. Patil \em et al.\em\ PRA 90, 033422 (2014)   

Coherence in Modulated Lattices and Strong Gradients

Modulated lattices and strong energy gradients are promising tools in the field of quantum simulation with cold atoms, but they come with heating problems in the form of micromotion and dynamic instabilities. We experimentally study the loss of coherence of a Bose-Einstein Condensate in both one- and two-dimensional lattices under a strong energy gradient, observing the decay of Bloch Oscillations. In addition, we show that coherence is restored when resonant tunneling is allowed via amplitude modulation of the lattices and study its dynamics. This restoration and the subsequent coherence lifetime depend strongly on interaction strength and the turn-on procedure. Finally, we study the effect of an additional spin component on Bloch Oscillations and our simulated Hamiltonian.   

Transport Enhancement of Bose Hubbard Systems via Barrier Modulation

We show that transport characteristics of disordered, lowest band lattices can be greatly-enhanced by modulating the field intensity that generates the lattice structure. We propose two resonant modulation schemes: one requires independent site-by-site control of the tunneling rates, the second involves a global modulation of intensity that generates the lattice structure. We also present effective stationary models for these complicated dynamical systems. For the specific five-site lattices discussed, we numerically predict transport gains ranging from $3\times 10^6$ to roughly $4\times 10^{10}$.   

Microwave Induced Diffraction of Matter Waves from an Optical Lattice

We demonstrate a novel type of Kaptiza-Dirac diffraction of matter waves from a microwave-dressed optical lattice. In our experiment, coherent momentum transfer is driven by internal-state coupling of a BEC to a single orbital of a state-dependent optical lattice. We analyze the internal and external dynamics of the diffraction process and characterize the matter-wave interference using a pump-probe scheme. Possible applications of our scheme will be discussed.   

Cold-atom Realization of the Quantum Kapitza Pendulum

The Kapitza pendulum is perhaps the most famous example of dynamic stabilization in a classical single-particle system. When the pivot of a rigid pendulum is modulated in a certain range of amplitude and frequency, a new stable equilibrium appears in the upward-pointing configuration. Extensions of Kapitza stabilization to the quantum many-body regime remain completely unexplored. We report on progress towards realization of a quantum Kapitza pendulum using lithium atoms in a modulated optical lattice, and discuss prospects for mapping the dynamical phase diagram. Tunable quantum Kapitza pendula could enable a variety of exciting research directions, including investigation of the role of strong interactions, possible applications of controllable stabilization, and advances in our understanding of unconventional dynamical behavior in many-body quantum systems.   

Continuous in situ fluorescence imaging of an ultracold Fermi gas in an optical lattice

We demonstrate continuous in situ fluorescence imaging of ultracold fermionic $^{40}$K atoms held in a three-dimensional optical lattice with 527nm periodicity. Using a 4S-4P$_{1/2}$ grey molasses cooling scheme with a coherent dark state, we obtain a photon scattering rate exceeding 1kHz while measuring a steady-state population of the vibrational ground state of 80\%. Collecting the scattered photons through a 200$\mu$m thin sapphire vacuum window and into a microscope objective allows us to image the in situ density distribution of the lattice gas. Spatially selective state manipulation is used to reduce the number of occupied lattice planes along the imaging direction, as well as to create density patterns along the transverse direction. We characterize the performance of the imaging protocol over a wide range of parameters. For larger-than-unity site occupation we observe efficient removal of atoms due to light-assisted collisions. Singly occupied lattice sites can be continuously imaged for several seconds. This method is suitable for high-resolution imaging of a many-body system in the Fermi-Hubbard regime.   

Composite Quantum phases in a system of tilted dipolar lattice bosons

Dipolar lattice bosons allow for the realization of novel quantum phases due to the long-range nature of the interaction and its inherent anisotropy. We utilize Path Integral Quantum Monte Carlo simulations, using a novel multi-worm extension to the Worm algorithm, to explore the phase diagram of hard-core dipolar bosons in a two-dimensional optical lattice as a function of the tilting angle. This setup allows for the formation of dipolar chains which can form a variety of composite insulating and superfluid phases.~