Physics
Permanent URI for this communityhttp://hdl.handle.net/1903/2269
Browse
5 results
Search Results
Item INTERACTING PHOTONS IN CIRCUIT QUANTUM ELECTRODYNAMICS: DECAY OF THE COLLECTIVE PHASE MODE IN ONE-DIMENSIONAL JOSEPHSON JUNCTION ARRAYS DUE TO QUANTUM PHASE-SLIP FLUCTUATIONS(2020) Grabon, Nicholas Christopher; Manucharyan, Vladimir; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Light does not typically scatter light, as witnessed by the linearity of Maxwell’s equations. In this work, we demonstrate two superconducting circuits, in which microwave photons have well-defined energy and momentum, but their lifetime is finite due to decay into lower energy photons. The circuits we present are formed with Josephson junction arrays where strong quantum phase-slip fluctuations are present either in all of the junctions or in only a single junction. The quantum phase-slip fluctuations are shown to result in the strong inelastic photon-photon interaction observed in both circuits. The phenomenon of a single photon decay provides a new way to study multiple long-standing many-body problems important for condensed matter physics. The examples of such problems, which we cover in this work include superconductor to insulator quantum phase transition in one dimension and a general quantum impurity problem. The photon lifetime data can be treated as a rare example of a verified and useful quantum many-body simulation.Item Generation and Uses of Distributed Entanglement in Quantum Information(2019) Eldredge, Zachary David; Rolston, Steven L; Gorshkov, Alexey V; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)In this thesis, we focus on the questions of how quantum entanglement can be generated between two or more spatially separated systems and, once generated, how it can be applied in quantum technology. First we will discuss a protocol, which we conjecture to be optimal in some regimes, that quickly creates entangled states across long distances in systems with power-law interactions. We will discuss how this protocol compares with currently known bounds on entangled state generation and how it might be implemented in a three-dimensional lattice of Rydberg atoms. Next, we will turn our attention to more general questions of how the Lieb-Robinson bound and other limitations on entanglement can be used to inform the design of quantum computers. Quantum computers will be required to create entanglement if they are to realize significant advantages over classical computers, meaning that the generation of entanglement is an important question. First, we will discuss the implications of the Lieb-Robinson bound on graph descriptions of quantum computer architectures, and how the relevant graph parameter (diameter) compares to likely cost functions for architectures, such as maximum graph degree and total number of necessary connections. We will present a proposed graph architecture, the hierarchical product, which we believe provides excellent balance between these considerations. We will then introduce new methods of evaluating graphs that allow us to include quantum architectures capable of measurement and feedback operations. After doing so, we will show that the generation of entanglement entropy becomes a limit on computation. We will show that, for several possible physical models of computation, the generation of entanglement can be bounded by simple graph properties. We demonstrate a connection between worst-case scenarios for entanglement generation and a graph quantity called the Cheeger constant or isoperimetric number, which we evaluate for several proposed quantum computing architectures. Finally, we will look at the scenario of quantum sensing. In particular, we will examine protocols for quantum function estimation, where quantum sensors are available to measure all of the inputs to the function. We will demonstrate that entangled sensors are more capable than non-entangled ones by first deriving a new lower bound on measurement error and then presenting protocols that saturate these bounds. We will first do so for linear functions of the measured quantities and then extend this to general functions using a two-step linearizing protocol.Item Modeling strong-field laser-atom interactions with nonlocal potentials(2017) Rensink, Thomas C.; Antonsen (Jr.), Thomas M; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Atom-field interactions in the ionization regime give rise to a wide range of physical phenomena, and their study continues to be an active field of research. However, simulation of atom-field dynamics is time-consuming and computationally expensive. In this thesis, a nonlocal model potential is used in place of the Coulomb potential in the time dependent Schrodinger equation, and examined for suitabil- ity of modeling strong field-atom dynamics while offering significant reduction in computation cost. Nonlocal potentials have been used to model many physical systems, from multi-electron molecular configurations to semiconductor theory. Despite their rel- ative success, nonlocal potentials have been largely unexplored for modeling high field laser-gas interactions in the ionizing regime. This work explores the theory and numerical results of a single state gaussian nonlocal model in intense, femtosecond laser pulses, with the main findings: nonlocal potentials are useful for obtaining the photoionization rate in the tunnel and multiphoton regimes, and qualitatively char- acterize the wavefunction dynamics of irradiated atoms. The model is also examined in the context of the two-color technique for producing Terahertz (THz) frequency radiation.Item Experiments with Trapped Ions and Ultrafast Laser Pulses(2016) Johnson, Kale Gifford; Monroe, Christopher; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Since the dawn of quantum information science, laser-cooled trapped atomic ions have been one of the most compelling systems for the physical realization of a quantum computer. By applying qubit state dependent forces to the ions, their collective motional modes can be used as a bus to realize entangling quantum gates. Ultrafast state-dependent kicks [1] can provide a universal set of quantum logic operations, in conjunction with ultrafast single qubit rotations [2], which uses only ultrafast laser pulses. This may present a clearer route to scaling a trapped ion processor [3]. In addition to the role that spin-dependent kicks (SDKs) play in quantum computation, their utility in fundamental quantum mechanics research is also apparent. In this thesis, we present a set of experiments which demonstrate some of the principle properties of SDKs including ion motion independence (we demonstrate single ion thermometry from the ground state to near room temperature and the largest Schrodinger cat state ever created in an oscillator), high speed operations (compared with conventional atom-laser interactions), and multi-qubit entanglement operations with speed that is not fundamentally limited by the trap oscillation frequency. We also present a method to provide higher stability in the radial mode ion oscillation frequencies of a linear radiofrequency (rf) Paul trap--a crucial factor when performing operations on the rf-sensitive modes. Finally, we present the highest atomic position sensitivity measurement of an isolated atom to date of ~0.5 nm Hz^(-1/2) with a minimum uncertainty of 1.7 nm using a 0.6 numerical aperature (NA) lens system, along with a method to correct aberrations and a direct position measurement of ion micromotion (the inherent oscillations of an ion trapped in an oscillating rf field). This development could be used to directly image atom motion in the quantum regime, along with sensing forces at the yoctonewton [10^(-24) N)] scale for gravity sensing, and 3D imaging of atoms from static to higher frequency motion. These ultrafast atomic qubit manipulation tools demonstrate inherent advantages over conventional techniques, offering a fundamentally distinct regime of control and speed not previously achievable.Item Nonequilibrium Quantum Fluctuation Forces(2010) Behunin, Ryan Orson; Hu, Bei-Lok B; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)We study all known and as yet unknown forces between neutral atoms and neutral atoms and surfaces. The forces arise from mutual influences mediated by an attending electromagnetic field and not from direct interaction. We allow as dynamical variables the center of mass motion of the atom (or surface Chapter 5), its internal degrees of freedom, modeled as a three dimensional harmonic oscillator (the internal degrees of freedom of the surface in chapter 4), and the quantum field treated relativistically. We adopt the methods of nonequilibrium quantum field theory (NEqQFT) to study the problem of fluctuation forces beginning from first principles. NEqQFT provides a fully dynamical description of systems far from equilibrium having the advantage of being the synthesis of quantum field theory and nonequilibrium statistical mechanics. The integration of these two paradigms is necessary for a complete study of fluctuation forces; quantum field theory for providing effects such as retardation and quantum field fluctuations, and nonequilbrium statistical mechanics for treating processes involving quantum dissipation and noises. By embarking from first principles we avoid wrong or only partially correct results from inconsistent theories that can be generated from assumptions made at lower levels of accuracy. In thermodynamic equilibrium we reproduce all the effects and forces known in the last century, such as Casimir-Polder-- between neutral atoms, Lifshitz-- between an atom and a surface and Casimir between surfaces (and the generalization of these forces to nonequilibrium stationary-states). More noteworthy is the discovery of the existence of a new type of interatomic force which we call the `entanglement force', originating from the quantum correlations of the internal degrees of freedom of entangled atoms. Fluctuation phenomena associated with quantum fields is a new frontier of future research in atom-field interaction. With NEqQFT we have derived Langevin equations which account for fluctuations of an atom's trajectory about its semi-classical value. These quantum field-induced perturbations of the atom's position could lead to measurable results such as the damping of the center-of-mass oscillations of a trapped Bose-Einstein condensate near a surface or backaction cooling of moving mirror by radiative pressure and quantum viscosity discussed respectively in Chapter 3 and 5 of this thesis. The methods introduced in this thesis for treating atom-field interactions or mirror-field interactions go beyond previous work by providing a fully dynamical description of these forces valid for arbitrary atom and surface motion, indeed the inclusion of self consistent backactions are necessary for the study of phenomena such as quantum decoherence and entanglement dynamics, including non-Markovian processes which invariably will appear when backaction is taken into consideration(especially for strong fields, low temperatures, or fast response).