Quantum Gas Magnifier for imaging of ultracold Atoms in optical Lattices

Abstract

Ultracold gases in optical lattice are promising quantum simulation platforms: neutral atoms can be cooled down to quantum degeneracy and loaded into the lattice, thus mimicking the behaviour of electrons in "real" solid state materials. The advantage of optical lattice systems is the high degree of control over the system properties and the possibility to access a wider range of observables.

In this thesis, different experiments with ultracold atoms in optical lattices are performed, addressing both the issue of extending the measurement possibilities and the degree of control over the lattice system.

The first results reported here concern the detection of topological order in a cloud of ultracold fermionic 40K atoms. Topological order can not be captured by a local order parameter as in the description of phase transitions made by Landau. Thus topological properties are often related to elusive quantities, even in a realization with cold atoms.

Following a recent theoretical proposal, we were able to demonstrate that a topological system exhibits circular dichroism, i.e. a dependence on the chirality of a rotating force perturbing the system. We showed also that the dichroic signal can be directly used to detect topological order, which will be of relevance e.g. in cases where novel topological phases are created and no suitable probes exist yet. The experimental techniques developed here also allowed to measure other quantities related to the transport properties and to the geometry of the lattice states.

With the same setup, data were taken in order to train a machine learning algorithm to recognize the topological phase transition; these results are also briefly presented.

In a set of experiments performed with 87Rb, a novel technique for directly imaging the atomic density distribution in the lattice was developed and experimentally demonstrated. This technique is based on a matter-wave dynamics in which the density distribution is magnified up to almost two orders of magnitude and then directly imaged with standard methods, with a single destructive measurement and with sub-lattice resolution. Different experimental cases were studied and are presented, among them the discovery of a density wave which appears when displacing the atomic cloud with respect to the center of the confining potential. Preliminary results and considerations about the possibility of using quantum gas magnification for detecting coherence properties of the matter field with high resolution are also presented.

In the last chapter, we present a novel scheme for the realization of an optical lattice with tunable geometry. This scheme is based on the use of different frequency components controlled by radio-frequencies and we report on how this allowed the geometry of the optical lattice to be tuned fast and with high precision. This allows not only to tune our lattice to simulate different solid state systems, but also to study novel quantum phases which might emerge when changing dynamically the geometry, a scenario without a solid-state counterpart.