Physics
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Item ENGINEERING OPTICAL LATTICES FOR ULTRACOLD ATOMS WITH SPATIAL FEATURES AND PERIODICITY BELOW THE DIFFRACTION LIMIT and DUAL-SPECIES OPTICAL TWEEZER ARRAYS FOR RUBIDIUM AND YTTERBIUM FOR RYDBERG-INTERACTION-MEDIATED QUANTUM SIMULATIONS(2024) Subhankar, Sarthak; Rolston, Steven; Porto, Trey; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This dissertation is based on two independent projects and is therefore divided into two parts. The first half of this dissertation summarizes a series of investigations, both experimental and theoretical, that culminates in the realization of an optical lattice with a subwavelength spacing of $\lambda/4$, where $\lambda$ is the wavelength of light used to create the lattice. The second half of this thesis presents details on the design andconstruction of an apparatus for dual-species optical tweezer arrays of Rb and Yb for Rydberg-interaction-mediated quantum computation and simulation. Ultracold atoms trapped in optical lattices have proven to be a versatile, highly controllable, and pristine platform for studying quantum many-body physics. However, the characteristic single-particle energy scale in these systems is set by the recoil energy $E_R=h^2 /\left(8 m d^2\right)$. Here, $m$ is the mass of the atom, and $d$, the spatial period of the optical lattice, is limited by diffraction to ${\lambda}/{2}$, where $\lambda$ is the wavelength of light used to create the optical lattice. Although the temperatures in these systems can be exceedingly low, the energy scales relevant for investigating many-body physics phenomena, such as superexchange or magnetic dipole interactions, can be lower yet. This limitation can be overcome by raising the relevant energy scales of the system ($E_R^{\mathrm{eff}}=h^2 /\left(8 m d_{\mathrm{eff}}^2\right)$) by engineering optical lattices with spatial periodicities below the diffraction limit ($d_{\mathrm{eff}} < \lambda/2$). To realize this subwavelength-spaced lattice, we first generated a Kronig-Penney-like optical lattice using the nonlinear optical response of three-level atoms in spatially varying dark states. This conservative Kronig-Penney-like optical potential has strongly subwavelength barriers that can be less than 10 nm ($\equiv\lambda/50$) wide and are spaced $\lambda/2$ apart, where $\lambda$ is the wavelength of light used to generate the optical lattice. Using the same nonlinear optical response, we developed a microscopy technique that allowed the probability density of atoms in optical lattices to be measured with a subwavelength resolution of $\lambda/50$. We theoretically investigated the feasibility of stroboscopically pulsing spatially shifted 1D Kronig-Penney-like optical lattices to create lattices with subwavelength spacings. We applied the lattice pulsing techniques developed in this theoretical investigation to realize a $\lambda/4$-spaced optical lattice. We used the subwavelength resolution microscopy technique to confirm the existence of this $\lambda/4$-spaced optical lattice by measuring the probability density of the atoms in the ground band of the $\lambda/4$-spaced optical lattice. Single neutral atoms trapped in optical tweezer arrays with Rydberg interaction-mediated entangling gate operations have recently emerged as a promising platform for quantum computation and quantum simulation. These systems were first realized using atoms of a single species, with alkali atoms being the first to be trapped in optical tweezers, followed by alkaline-earth (like) atoms, and magnetic lanthanides. Recently, dual-species (alkali-alkali) optical tweezer arrays were also realized. Dual-species Rydberg arrays are a promising candidate for large-scale quantum computation due to their capability for multi-qubit gate operations and crosstalk-free measurements for mid-circuit readouts. However, a dual-species optical tweezer array of an alkali atom and an alkaline-earth (like) atom, which combines the beneficial properties of both types of atoms, has yet to be realized. In this half of the thesis, I present details on the design and construction of an apparatus for dual-species Rydberg tweezer arrays of Rb (alkali) and Yb (alkaline-earth like).Item Rydberg Ensembles for Quantum Networking(2020) Craddock, Alexander Nicholas; Rolston, Steve; Porto, Trey; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Rydberg ensembles, atomic clouds with one or more atoms excited to a Rydberg state, have proven to be a good platform for the study of photon-photon interactions. This is due to the nonlinearities they exhibit at the single photon level arising from Rydberg-Rydberg interactions. As a result, they have shown promise for use in a multitude of applications, among them quantum networking. In this thesis I describe the construction and operation of an apparatus for the purpose of cooling, trapping and probing Rydberg ensemble physics in a cloud of ${}^{87}\textrm{Rb}$ atoms. In addition, I describe a pair of projects undertaken with the apparatus. In the first, I report our demonstration of a Rydberg ensemble based on-demand single photon source. Here, we make use of Rydberg blockade to allow us to prepare a single collective Rydberg excitation in the cloud. The spin wave excitation is then retrieved by coherently mapping it onto a propagating photon. Our source is highly pure and efficient, while producing narrow bandwidth and indistinguishable photons. Such sources are important devices for the purposes of quantum networking, computation and metrology. Following from this, I describe a collaborative project where we show time resolved Hong-Ou-Mandel interference between photons produced by our Rydberg ensemble source, and a collaborators source based on a single trapped barium ion. This demonstration is a critical step in the entanglement, and hybrid quantum networking, of these two disparate systems.Item Magnetic & Electric Field Sensing and Applications Based on Coherent Effects in Neutral Atoms(2018) Meyer, David Henry; Rolston, Steve; Fatemi, Fredrik K; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This work encompasses two projects employing coherent probing of neutral rubidium atom vapors for sensing applications: 1) observing and characterizing new ``twists'' within Nonlinear Magneto-Optical Rotation (NMOR) signals in laser-cooled atoms. Using these features an in-situ, multi-directional characterization of magnetic fields and gradients at the sub-mG (and mG/mm) level in a compact cold-atom system is demonstrated; 2) high-bandwidth, phase-sensitive electrometry via Electromagnetically Induced Transparency (EIT) with a warm vapor of Rydberg atoms is employed as a digital communication receiver of free-space, modulated RF carrier electric fields. Channel capacities in excess of 10 Mbit/s are measured. Performance limitations due to EIT probing and fundamental quantum noise are explored in detail.Item Explorations of Variable Interactions in a Cold Rubidium Rydberg Gas(2012) Robinson, Jennifer Elizabeth; Rolston, Steven L; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)We explore dipole-dipole interactions between cold 87Rb Rydberg atoms, their utility for quantum computing, and their potential role in the development of exotic quantum phases in optical lattice systems. Rydberg atoms can have large dipole-dipole interactions, due to the fact that they are easily polarized. We propose a new atomic state, created by admixing the Rydberg state with the ground state, in order to create an atom with a long lifetime and an intermediate dipole moment, which would be useful for experiments in optical lattices. These states could be used to probe phases of the extended Bose-Hubbard Hamiltonian, as well as create novel R-dependent interactions that are not realizable in conventional condensed matter systems. In addition to the dressed-Rydberg states, we consider the use of external DC electric fields to produce a variable interaction strength. A Stark map of the specific Rydberg levels shows the energy shift of a Rydberg atom in an electric field, as well as the dipole moment, from the slope of the curve. We study Rydberg excitation in an intermediate density regime under the effects of a variable external static electric field. We use superatom analysis and Monte Carlo simulations of a Rydberg system with dipole blockade to determine that our experimental observations are consistent with an increasing dipole-dipole interaction due to an induced dipole moment, with an enhancement due to black-body-induced transitions to nearby higher-angular-momentum states. We also investigate the Van der Waals interaction by considering the zero-field excitation rate for multiple principle quantum numbers.