Abstract \nThis work is split into two parts, in part one of this thesis we report on the improvement of the lifetime of a quantum memory based on neutral atoms while in part two we will focus on a new setup to perform experiments on neutral atoms within a 2D optical lattice. In the quantum memory experiment we used a blue-detuned optical dipole trap to confine the atoms in a minimum intensity region of the light beam, reducing dephasing due to differential light shift. We saw improvement in comparison to a previous experiment \nusing red-detuned dipole traps. The anticipated lifetime could not be reached, however. We observed oscillations of the retrievability of our stored state which we explain by the evolution of the transversal component of the spinwave using Monte-Carlo simulations. In the second part we report on our new setup, built to investigate a broad range of \nexperiments regarding 2D physics on optical lattices. This field is of special interest as it allows to simulate Hamiltonians of a wide range e.g. from solid state physics and opens possibilities regarding large-scale Entanglement/one-way computing. In particular we discuss 2D Mott-insulators, 2D-Bloch oscillation induced transport phenomena and ring-exchange in a superlattice plaquette. \n
Abstract \nThis work is split into two parts, in part one of this thesis we report on the improvement of the lifetime of a quantum memory based on neutral atoms while in part two we will focus on a new setup to perform experiments on neutral atoms within a 2D optical lattice. In the quantum memory experiment we used a blue-detuned optical dipole trap to confine the atoms in a minimum intensity region of the light beam, reducing dephasing due to differential light shift. We saw improvement in comparison to a previous experiment \nusing red-detuned dipole traps. The anticipated lifetime could not be reached, however. We observed oscillations of the retrievability of our stored state which we explain by the evolution of the transversal component of the spinwave using Monte-Carlo simulations. In the second part we report on our new setup, built to investigate a broad range of \nexperiments regarding 2D physics on optical lattices. This field is of special interest as it allows to simulate Hamiltonians of a wide range e.g. from solid state physics and opens possibilities regarding large-scale Entanglement/one-way computing. In particular we discuss 2D Mott-insulators, 2D-Bloch oscillation induced transport phenomena and ring-exchange in a superlattice plaquette. \n
The tools of atomic physics enable researchers to explore rich quantum mechanical phenomena by way of light-matter interactions. In some applications, light is used to manipulate or modify the properties of a material medium, for instance, laser-cooling and optical trapping of neutral atoms and ions; electromagnetically-induced-transparency (EIT); and Bose- Einstein-Condensation (BEC). Likewise, there are applications where matter is used to manipulate the properties of light, as for example, non-linear optical processes; atomic quantum memories; and quantum-information-processing. This thesis explores two routes towards manipulating light using matter and vice versa in our lab. The work presented in this thesis falls under two categories, one is the experimental side where we have designed and built a laser system to cool a thermal gas of 39K atoms to sub-Doppler temperatures, using the ‘Gray Molasses’ technique. This work is motivated by the long term goal of performing quantum simulation experiments with degenerate quantum gas of K. The second category is on the theoretical (numerical) side, where we introduce a novel quantum memory protocol for coherent storage and processing of broadband light pulses, based on a fundamental physical process in atomic physics, called the ‘Autler-Townes’ effect. Ultracold quantum gases are one of the most versatile systems in the field of cold atom physics; these atomic systems can be experimentally controlled and manipulated with high degree of precision and are thus, widely used to model the behavior of other complex quantum mechanical systems. Laser cooling is the first and indeed a crucial stage towards creating an ultracold gas of atoms. In this thesis, we describe an optical setup for producing a laser-cooled and trapped gas of bosonic 39K atoms, using D1 and D2 atomic transition-lines. The optical setup is built to generate appropriate laser frequencies for implementing various stages of the experiment: red- detuned two and three-dimensional magneto-optical traps (MOT) on the D2-line; a blue-detuned gray molasses phase on the D1-line; as well as optical pumping, imaging and push beams on the D2-line. We also highlight the underlying physics behind the blue-detuned gray molasses cooling and the advantages it offers in cooling species like K and Li. We also discuss in detail, the techniques used for frequency locking our lasers, namely, saturated absorption spectroscopy (SAS) and beatnote locking. The second part of the thesis describes a new atomic quantum-memory scheme for storing broadband pulses of light, which we call the ‘Autler- Townes Splitting (ATS)’ protocol. We show the light storage and retrieval operation in three different configurations, via numerical simulation of Maxwell-Bloch equations. We also discuss the strategies for getting high storage and retrieval efficiency in the ATS memory in terms of only two phenomenological parameters, both of which are experimentally accessible. We have shown that a near-unity efficiency is achievable for a wideband signal, under moderate values of atomic density and laser powers; this significantly relaxes the technical requirements of our protocol when compared to other broadband storage schemes.
We review novel methods for the investigation, control and manipulation of neutral atoms in optical lattices. These setups allow unprecedented quantum control over large numbers of atoms and thus are very promising for applications in quantum information processing. After introducing optical lattices we discuss the superfluid (SF) and Mott insulating (MI) states of neutral atoms trapped in such lattices and investigate the SF-MI transition as observed experimentally recently. In the second part of the paper we give an overview of proposals for quantum information processing and show different ways to entangle the trapped atoms, in particular the usage of cold collisions and Rydberg atoms. Finally, we discuss briefly the implementation of quantum simulators, entanglement enhanced atom interferometers, and ideas for robust quantum memory in optical lattices.
Long-term storage of quantum information has diverse applications in quantum information science. This work presents an experimental realization of quantum memories with lifetimes greater then 0.1 s. The memories are based on cold rubidium atoms confined in one-dimensional optical lattices. First realization of lattice-based quantum memory and entanglement between a light field and a spin wave is presented in Chapter II. Chapter III describes two different methods (two-photon and magnetic) of compensation for inhomogeneous differential light shifts between the memory levels due to optical trapping potentials, and demonstration of entanglement between a telecom-band light field and a light-shift compensated memory qubit. Highly excited Rydberg atoms present a unique platform for study of strongly correlated systems and quantum information, because of their enormous dipole moments and consequent strong, long-range interactions. In the experiment described in Chapter IV single collective Rydberg excitations are created in a cold atomic gas. After a variable storage period the excitations are converted into light. As the principal quantum number n of the Rydberg level is increased beyond ~ 70, no more than a single excitation is retrieved from the entire mesoscopic ensemble of atoms. In Chapter V, by spatially selective conversion of the spin wave into a light field, we demonstrate that Rydberg-level interactions create long-range correlations of collective atomic excitations. These results hold promise for studies of dynamics and disorder in many-body systems with tunable interactions and for scalable quantum information networks. Chapter VI presents initial observations of coherent many-body Rabi oscillations between the ground level and a Rydberg level using several hundred cold rubidium atoms. The strongly pronounced oscillations indicate a nearly complete excitation blockade of the entire mesoscopic ensemble by a single excited atom.
The advances in laser cooling and trapping techniques allow samples of ultra-cold atoms to be created and controlled for the study of quantum physics. In this thesis, we focus on using ensembles of cold atoms to investigate novel quantum effects and their applications in quantum optics. Realization of a scalable intercontinental quantum system, e.g. a global quantum internet, relies on quantum memories with long lifetimes on the order of seconds. We have reduced the differential light shifts of the atomic ground states to a sub-Hz level and achieved a lifetime of 16 seconds. Achieving fast and efficient quantum operations, on the other hand, requires strongly-interacting systems. We explore highly excited Rydberg atoms towards this goal and demonstrate Rydberg excitation blockade, many-body Rabi oscillations, trapping of Rydberg atoms, deterministic single photon source, and atom-photon entanglement. Finally, we integrate the two essential capabilities for quantum information processing, fast quantum state generation and long-term storage, by simultaneously exploiting Rydberg levels for interactions and ground atomic levels for the storage of quantum state. Our work advances the control of coherence properties and interactions in cold atomic ensembles to a new level and opens new opportunities for studies of complex quantum systems.
In this work we present the results of studies of one-dimensional (1D) and twodimensional (2D) optical lattices on a chip.We have developed a method to create 1D and 2D optical lattices above an optical waveguide.The method is based on the destructive interference of the evanescent waves of two modes of the waveguide, resulting in a periodic array of nodes above the waveguide surface.For blue-detuned light, this constitutes an array of optical dipole traps for neutral atoms.One pair of modes coupled into a ridge waveguide creates a 1D array of nodes in this manner, and two orthogonal pairs of modes in a slab waveguide result in a 2D array of nodes.Computational analysis of our method showed that these optical atom chips may be suitable for use as a quantum register, and for studies of atom-surface interactions between the trapped atoms and the waveguide surface.Using realistic waveguide materials and laboratory laser parameters we found that for a laser power of ~100 mW nodes can be created ~100 nm above the waveguide surface.The nodes constitute atom traps that are ~1mK deep, and have trap frequencies ~1MHz and ground state sizes of ~10 nm.The spacing between traps is ~1 µm enabling individual addressing with a focused laser beam.iii With the implementation of two-qubit gates in mind, initial investigations of extensions of the basic 2D optical lattice were conducted.Computational results for a counter-propagating 2D optical lattice with very close trap spacing (~λ/4) in one dimension, and for a 2D optical lattice with adjustable spacing between pairs of traps in one dimension are presented.An experimental setup for the realization of the proposed 2D optical lattices on a chip was designed and built.Several cooling and trapping steps necessary to prepare a cold atom sample for loading of the 2D optical lattice have been implemented successfully.In the course of previous studies of atom-surface interactions, a novel surfaceionization detector for 2D imaging of neutral atom samples was built and characterized.We find a detection efficiency ~50 %, a spatial resolution of 20 µm and a time response < 1/30 s. iv To my family.v ACKNOWLEDGMENTS First and foremost I would like to thank my advisor Greg Lafyatis for everything that he taught me.I also want to thank Frank DeLucia for always being there with his advice when I needed it.I appreciate Eric Herbst and Junko Shigemitsu for being willing to serve on
Ultracold neutral atoms confined in optical dipole traps have important applications in quantum computation and information processing, quantum simulators of interacting-many-body systems and atomic frequency metrology. While optical dipole traps are powerful tools for cold atom experiments, the energy level structures of the trapped atoms are shifted by the trapping field, and it is important to characterize these shifts in order to accurately manipulate and control the quantum state of the system.\n\nIn order to measure the light shifts, we have designed a system that allows us to reliably trap individual 87Rb atoms. A non-destructive detection technique is employed so that the trapped atoms can be continuously observed for over 100 seconds. Single atom spectroscopy, trap frequency measurements, and temperature measurements are performed on single atoms in a single focus trap and small number of atoms in a 1D optical lattice in order to characterize the trapping environment, the perturbed energy level structures, and the probe-induced heating.\n\nIn the second part of the thesis, we demonstrate deterministic delivery of an array of individual atoms to an optical cavity and selective addressability of individual atoms in a 1D optical conveyor, which serves as a potential candidate for scalable quantum information processing. The experiment is extended to a dual lattice system coupled to a single cavity with the capability of independent lattice control and addressability. The mutual interactions of atoms in different lattices mediated by a common cavity field are demonstrated. A semi-classical model in the many-atom regime based on the Jaynes-Cummings model is developed to describe the system that is in good qualitative agreement with the data. This work provides a foundation for developing multi-qubit quantum information experiments with a dual lattice cavity system.