Session M6: Focus Session: In-situ Imaging of Ultracold Atomic Gases

Controlling and Imaging Quantum Gases at the Single Atom Level

Over the past years, ultracold quantum gases in optical lattices have offered remarkable opportunities to investigate static and dynamic properties of strongly correlated bosonic or fermionic quantum many-body systems. In this talk I will show how it has recently not only become possible to image such quantum gases with single atom sensitivity and single site resolution, but also how it is now possible to coherently control single atoms on individual lattice sites within a strongly correlated quantum gas. Using a tightly focused laser beam, atoms on selected lattice sites can be addressed and their spin state fully controlled. Magnetic resonance control techniques were employed to achieve sub-lattice period and sub-diffraction limited resolution in our addressing scheme. The ability to address single atoms on a lattice opens a whole range of novel research opportunities, ranging from quantum information processing over the investigation of quantum spin systems to local entropy control, some of which will be discussed in the talk.   

Probing the Superfluid to Mott Insulator Transition at the Single Atom Level

The Quantum Gas Microscope enables high fidelity detection of single atoms in a Hubbard-regime optical lattice, bringing ultracold atom research to a new, microscopic level. We investigate the Bose-Hubbard model using space- and time-resolved characterization of the number statistics across the superfluid - Mott insulator quantum phase transition. Site-resolved probing of fluctuations provides us with a sensitive local thermometer, allows us to identify microscopic heterostructures of low entropy Mott domains, and enables us to measure local quantum dynamics, revealing surprisingly fast transition timescales. Recently realized $99\%$ fidelity insulator regions will serve as an excellent starting point for studies of quantum magnetism.   

Cavity-aided magnetic-resonance imaging of atoms in optical lattices

Microscopic imaging is a powerful tool for measuring cold-atom systems, enabling the readout of ultracold atomic simulators and registers, the characterization of inhomogeneous environments, and the determination of spatially varying thermodynamic quantities. Single-optical-lattice-site microscopy has recently been demonstrated, using fluorescence and ionization imaging. However, these methods destroy the quantum states being measured and have limited dynamic range. Here we demonstrate single-lattice-site imaging using magnetic-resonance imaging, enabled by using a high-finesse cavity for spin measurement. The images are state sensitive and have 150~nm spatial resolution. We measure atoms with high dynamic range, sensing up to 1000 atoms in single lattice sites with a sensitivity of $\pm 10$ atoms, or a sensitivity of $\pm 2.4$ for up to 70 atoms. We apply this technique to measure the nonequilibrium transport dynamics of the atoms, observing the onset of interaction-inhibited transport in a nondegenerate gas.   

Electron Microscopy of Ultracold Quantum Gases

Scanning electron microscopy is routinely used to study solid objects on a nanometer scale. Applied to ultracold quantum gases it constitutes a powerful imaging and manipulation technique that combines single atom sensitivity with high spatial resolution. We have adapted a scanning electron microscope for the study of Bose-Einstein condensates of rubidium atoms. The focussed electron beam ionizes the atoms which are subsequently detected. The technique allows for high precision density measurements of the trapped gas with a spatial resolution of better than 100 nm. Loading the condensate in a two-dimensional optical lattice with 600 nm period we demonstrate single site addressability and show that one can produce arbitrary patterns of occupied lattice sites. Such micro-structured quantum gases might become a versatile resource for the study of mesoscopic quantum systems and future applications in quantum simulation and quantum information processing. The electron beam can also be used to locally introduce losses, thus paving the way to investigate dissipative processes in quantum gases.   

Imaging spatial correlations of Rydberg excitations in cold atom clouds

Previously, Rydberg excitation blockades have been shown to cause a saturation of Rydberg excitation numbers in atom samples and a narrowing of the excitation number statistics, and they have been employed in quantum information experiments. In the experiment described in this talk, we present measurements of structures in the Rydberg pair correlation function similar to those predicted in.\footnote{F. Robicheaux and J. Hernandez, ``Many-body wave function in a dipole blockade configuration,'' Phys. Rev. {\bf A 72}, 63403, 1-4 (2005).} To achieve sufficient spatial magnification, we use the principle of field ion microscopy. A tungsten tip is placed close to a cold atom cloud in which several Rydberg excitations are prepared using a narrow-linewidth laser. To read out the sample, the tip voltage is suddenly switched to a high value. The Rydberg atoms are field-ionized, and the resultant ions are projected onto a nearby position-sensitive detector. We present the dependence of the pair correlation function on the principle quantum number and other parameters.   

In-situ study of critical behavior in two-dimensional Bose gases

High resolution in-situ imaging of ultracold atoms confined in a two dimensional (2D) trap reveals precise information on the in-trap density distribution and density fluctuations. From density measurements we determine the equation of state through the assumption of the local density approximation; fluctuation measurements reflect the density-density correlation in quantum gases and its growth near a continuous phase transition. The in-situ imaging technique opens up exciting opportunities to study critical behavior in the phase transition region in two dimensions, such as the fluctuation region near the Berezinsky-Kosterlitz-Thouless transition and the quantum critical region near the superfluid-Mott insulator phase boundaries of atoms in a 2D square lattice. In this talk, I will present our study on global scale-invariance and universality in weakly interacting 2D Bose gases [1]. Further investigations of 2D gases near a Feshbach resonance and quantum criticality in optical lattices [2] will be discussed. \\[4pt] [1] Chen-Lung Hung, Xibo Zhang, Nathan Gemelke and Cheng Chin, arXiv:1009.0016v2 (to be published in Nature). \\[0pt] [2] Xibo Zhang, Chen-Lung Hung, Shih-Kuang Tung, Nathan Gemelke and Cheng Chin, arXiv:1101.0284v1.   

Mapping out the quasi-condensate transition through the 1D-3D dimensional crossover

By performing in-situ measurements of density fluctuations in a highly elongated weakly interacting Bose gas, we observe and quantify the transition from the ideal gas to a quasi-condensate regime throughout the dimensional crossover from a purely 1D to an almost 3D gas. We show that that the entire transition region and the dimensional crossover are described surprisingly well by the modified Yang-Yang model. Furthermore, we find that at low temperatures the linear density at the quasi-condensate transition scales according to an \textit{interaction-driven} scenario of a longitudinally uniform 1D Bose gas, whereas at high temperatures it scales according to the \textit{degeneracy-driven} critical scenario of transverse condensation of a 3D ideal gas.