Session J3: Quantum Gas Microscope and Lattice Techniques

Probing quantum correlations in a one-dimensional bosonic system under a Quantum Gas Microscope

Characterization and direct measurement of quantum coherence in a many-body system are valuable tools to probe quantum states, especially in the context of non-equilibrium dynamics. Several schemes for directly probing entanglement in a system of ultracold atoms in an optical lattice rely on the ability to detect and manipulate atoms at a single site level. Combining the single-site resolution of our quantum gas microscope and the ability to create arbitrary optical potentials using holography, we manipulate a one-dimensional chain of Rb-87 bosons in an optical lattice with a goal to characterize the quantum entanglement. The single site addressability also allows us to access the full counting statistics in a one-dimensional system, which contains valuable information about the quantum coherence in certain systems.   

Fermi Gas Microscope

Site-resolved imaging and control of bosonic 87-Rb atoms in optical lattices have enabled ground-breaking quantum simulations of magnetic and relativistic Hamiltonians. However, many open questions in condensed matter physics arise in strongly correlated many-body systems of fermions. Ultracold Fermi gases of 6-Li are an ideal platform to study these problems because the light atomic mass leads to fast lattice dynamics. We have successfully loaded 6-Li atoms into an optical lattice in the image plane of a microscope with 0.85 numerical aperture. High-fidelity fluorescence imaging inside this trap requires continuous laser cooling. We report on our progress toward using Raman sideband cooling to perform site-resolved imaging in the Hubbard regime and report on the first demonstration of sideband transitions in deep optical lattices with trap frequencies up to 1 MHz.   

A Non-destructive Quantum Gas Microscope for Fermions

We have demonstrated a two photon fluorescence imaging technique which allows in situ imaging of a lattice gas of $Rb$ atoms. We report progress on extending this technique to fermionic species, in our case, $^{6}Li$. In contrast to demonstrated means of quantum gas microscopy using molasses cooling [1,2], our scheme is not restricted to atomic species amenable to polarization gradient cooling. Furthermore, our imaging scheme is nondestructive in the limit of zero duty cycle of using the Raman transition for imaging and cooling. This presents new opportunities for non-equilibrium many-body studies involving the continuous measurement of system dynamics, measurement based many-body control of the lattice gas and quantum zeno physics. We also describe progress towards augmenting our current system with single site resolution imaging.\\[4pt] [1] W. Bakr \em et al.\em, Nature 462, 74-77 (2009)\\[0pt] [2] J. F. Sherson \em et al.\em, Nature 467, 68 (2010)   

Magic Traps for Clock Transitions in Neutral Cesium Atoms

In a system of trapped atoms errors in quantum gates and precision spectroscopy can arise from a differential shift in atomic transitions caused by gradients in the electric and magnetic fields. The thermal motion of an atom in a trap allows it to sample changes in these two fields resulting in a constantly shifting transition frequency. ``Magic'' traps minimize this source of noise by finding experimental conditions where the first-order sensitivity to gradients is nulled. We present refinements to calculations of the Zeeman and A.C. Stark Shift for qubit states in the ground hyperfine state manifold of neutral Cesium atoms. We follow this with a discussion of implications for traps insensitive to electric fields, magnetic fields or both simultaneously.   

Raman Cooling Quasimomentum in an Optical Lattice

In optical lattice experiments, achieving lower temperatures would enable new possibilities for the study of strongly correlated physics, such as quantum magnetism and potentially the analog of d-wave superconductivity in the cuprates. We have developed a novel method for cooling the quasimomentum distribution of an ultracold gas trapped in an optical lattice that can operate with any atomic species. Entropy is removed from the gas by ejecting the most energetic atoms via quasimomentum-selective stimulated Raman transitions. We present evidence of cooling in a proof-of-principle experiment and we discuss prospects for improving this technique.   

Local Measurements of Ultracold Fermions in an Optical Lattice Geometry

Experimental realisations of quantum gases of interacting fermionic atoms confined to reduced dimensionalities, arising from the use of optical lattices, constitute a system which can be employed to investigate a range of phenomena traditionally observed in condensed matter physics, from the low-temperature spin-ordered phases of Hubbard-type models to the physics of the Luttinger liquid model. I will report on our recent experimental efforts to study the physics of such systems by loading a quantum degenerate two-component Fermi gas of $^{40}$K atoms into an optical lattice geometry. By exploiting high-resolution imaging combined with radio-frequency and Raman spectroscopy, we are able to go beyond the standard of global measurement and perform spatially-resolved measurements of the in-situ atom distributions, directly elucidating the local behaviour of the emergent phenomena.   

Veselago lensing with ultracold atoms in an optical lattice

Veselago pointed out that electromagnetic theory allows for materials with a negative index of refraction, in which most known optical phenomena are reversed. A slab of such a material can focus light by negative refraction, an imaging technique strikingly different from conventional positive refractive index optics, where curved surfaces bend the rays to form an image of an object. Here we demonstrate Veselago lensing for matter waves, using ultracold atoms in an optical lattice. A relativistic, i.e. photon-like, dispersion relation for rubidium atoms is realized with a bichromatic optical lattice potential. A Raman pi-pulse technique serves to transfer atoms between two different branches of the dispersion relation, and the relativistic lensing occurs by a backwards propagation of atomic wavepackets on an energetically mirrored branch of the dispersion relation. We observe negative refraction and Veselago lensing both in a one-dimensional geometry and perform a ray-tracing simulation of a two-dimensional Veselago lens.   

Dimerized Mott insulators in hexagonal optical lattices

We numerically study driven optical honeycomb lattices and find dimerized insulator phases with fractional filling. These incompressible insulating phases are characterized by an interaction-driven localization of particles on individual dimers and a coherent superposition within the dimers. We calculate the ground-state phase diagrams and the excitation spectra using an accurate cluster mean-field method as well as perturbation theory employing an effective model. Probing the fundamental excitations of the dimerized Mott insulator allows the distinction from normal Mott insulating phases. By computing finite lattices with large diameters the influence of the experimental confinement is discussed in detail.   

Probing the Bose-Glass---Superfluid Phase Boundary using Quantum Quenches of Disorder in an Optical Lattice

We detect the phase boundary between superfluid (SF) and Bose-glass (BG) states in the disordered Bose Hubbard model using quantum quenches of disorder in an optical lattice. ~The BG phase has not been directly detected in experiments, and there are numerous questions regarding how temperature, density, and an inhomogeneous trap affect the BG---SF transition. ~We use ultracold Rubidium-87 trapped atoms in a disordered optical lattice generated using an optical speckle field to realize the disordered Bose Hubbard model. ~By rapidly quenching the disorder, we detect the BG---SF transition by measuring excitations produced by the quantum Kibble-Zurek mechanism.   

Quantum Walks and Bloch Oscillations of Strongly Interacting Bosons

Microscopy techniques for ultracold quantum gases offer the opportunity to characterize complex bosonic many-body states on a single-particle level. With a novel single-site addressing scheme, we are now able to study the most elemental building blocks of such strongly correlated systems. We initialize Fock states of few bosons in an optical lattice with high fidelity and follow their dynamics in one dimension. Focusing on free quantum walks of two atoms, we directly observe the crossover from bunching to anti-bunching as the bosons fermionize due to strong repulsive interactions. We utilize our control over the initial state to prepare repulsively bound pairs and study their coherent Bloch oscillations in the presence of an externally applied gradient. Our work gives access to interaction effects in the simplest possible setting and allows the assembly of many-body states one particle at a time.