1,721,015 research outputs found
Quantum Sensing of Photonic Spin Density with Spin Qubits
No full textOptical signals are a necessary tool for quantum technologies to carry information both for long-range and on-chip application. The scope of their use is determined by their ability to effectively interact with qubits. The deep-subwavelength interaction volume demands the understanding of the properties of optical fields in the near-field and light-matter interaction in this regime. Recent studies have unraveled the rich characteristics in the physical quantity known as the near-field photonic spin density (PSD). Photonic spin density is the spatial distribution of light’s spin angular momentum. It is characterized by the degree of circular polarization of an optical field in deep-subwavelength volumes. In this thesis we study the properties of PSD in the near-field regime and demonstrate a platform for coherent light-spinqubit interaction based on PSD. We show that nitrogen-vacancy (NV) centers in diamond can coherently interact with an optical beam where the interaction strength is determined by PSD in the nanoscale. To understand the near-field characteristics of PSD we study the evanescent waves and spin-momentum locking of light.Evanescent electromagnetic waves possess spin-momentum locking, where the direction of propagation (momentum) is locked to the inherent polarization of the wave (transverse spin). We study the optical forces arising from this universal phenomenon and show that the fundamental origin of recently reported optical chiral forces is spin-momentum locking. For evanescent waves, we show that the direction of energy flow, direction of decay, and direction of spin follow a right hand rule for three different cases of total internal reflection, surface plasmon polaritons, and HE11mode of an optical fiber. Furthermore, we explain how the recently reported phenomena of lateral optical force on chiral and achiral particles is caused by the transverse spin of the evanescent field and the spin-momentum locking phenomenon. Our work presents a unified view on spin-momentum locking and how it affects optical forces on chiral and achiral particles.To probe the near-field properties of PSD, we propose and employ a single NV center in diamond as a nanoscale sensor. NV centers have emerged as promising room-temperature quantum sensors for probing condensed matter phenomena ranging from spin liquids, twodimensional (2D) magnetic materials, and magnons to hydrodynamic flow of current. Here,we demonstrate that the NV center in diamond can be used as a quantum sensor for detecting the photonic spin density. We exploit a single spin qubit on an atomic force microscope tip to probe the spinning field of an incident Gaussian light beam. The spinning field of light induces an effective static magnetic field in the single spin qubit probe. We perform room-temperature sensing using Bloch sphere operations driven by a microwave field (XY8 protocol). This nanoscale quantum magnetometer can measure the local polarization of light in ultra-sub-wavelength volumes. We also put forth a rigorous theory of the experimentally measured phase change using the NV center Hamiltonian and perturbation theory involving only virtual photon transitions.In order to study the wavelength dependence of the optically induced magnetic field, we demonstrate this effect for an ensemble of NV centers. We characterize the wavelength dependence of the effective static magnetic field caused by the interaction of PSD and the spin qubit. We show that the strength of the field is inversely dependent on the detuning between the frequency of the optical beam and the optical transition of the NV centers
Quantum Correlations in Nanophotonics: From Long-range Dipole-dipole Interactions to Fundamental Efficiency Limits in Coherent Energy Transfer
Full text linkQuantum properties like coherence and entanglement can lead to enhanced performance characteristics in a wide range of applications including quantum computation, quantum memory storage, optical sensing, and energy harvesting. Entanglement is very sensitive to static and dynamical disorder. Similarly, the generation of highly-entangled states requires strong coupling or strong driving fields. Satisfying all of these requirements is generally quite difficult. In the first part of this thesis, we present an approach to overcome these limitations through the use of exotic light-matter states in hyperbolic media which provide a new approach to control quantum correlations and interatomic interactions. We reveal a class of excited-state, long-range interactions, referred to as Super-Coulombic interactions that are singular along a material-dependent resonance angle. In practical systems, the Super-Coulombic interaction achieves dipole-dipole coupling that is orders of magnitude larger than conventional approaches, while also occurring across a large frequency bandwidth making it robust to static energy-level disorder. This unique hyperbolic response is not only naturally occurring, found in materials like h-BN, BiTe 2, BiSe2, and mono-layered black phosphorus, but can also be designed with artificial nanostructured materials (metamaterials) to create the desired hyperbolic dispersion across different parts of the electromagnetic spectrum. Our theoretical prediction motivated an intense search for the effect and was confirmed by an experimental demonstration at room temperature. To obtain agreement with experimental results, we present a rigorous theoretical framework that takes into account ensemble effects, finite-sized effects, and dimensional effects that arise from confined geometries ultimately modifying the Super-Coulombic spatial scaling law. In the second part of this thesis, we solve an outstanding theoretical problem dealing with the control of resonance energy transfer in nanophotonic environments in both the incoherent and coherent coupling limits. Resonance energy transfer is a fundamental process that is the subject of intense research across all sciences. For example, in chemistry for drug delivery and chemical monitoring, in engineering for photovoltatic and up-conversion devices, and in biology for exciton transport within photosynthetic complexes. First, we consider the disordered and weak coupling limit of resonance energy transfer often encountered in chemistry. We propose new design principles for enhancing and suppressing the energy transfer rate and efficiency quantitatively captured by a simple image dipole model. Our theory explains a wide range of experimental results which have been the subject of an ongoing debate for the past 15 years. Second, we present our recent result aimed at understanding the fundamental role of entanglement and quantum coherence in resonance energy transfer. To uncover the role of these effects, we develop a unified theory of energy transfer valid from the incoherent to quantum coherent coupling regimes. Ultimately, our theory reveals a fundamental bound for energy transfer efficiency arising from the spontaneous emission rates of the donor and acceptor. This bound provides an upper limit to the efficiency of energy transfer regardless of quantum coherence or entanglement, suggesting new design principles for achieving near-unity energy transfer efficiency in coherent systems. The result has important implications for the two-chromophore model found in photosynthetic complexes and paves the way for nanophotonic analogues of efficiency-enhancing environments mimicking biological photosynthetic systems
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Experimental Demonstration of Engineered Dipole-Dipole Interactions in Nanophotonic Environments
Full text linkZero point fluctuations of the electromagnetic radiation field have profound effects on the electronic states of atoms and molecules. For example, vacuum fluctuations of the photonic radiation field stimulate the spontaneous decay of excited states of atoms (spontaneous emission), shift atomic energy levels (Lamb Shift), and allow nearby atoms and molecules to couple via dipole-dipole interactions (van der Waals interactions, Casimir effect, super radiance, F {o}rster resonance energy transfer). The control and modification of vacuum fluctuations has been a long-standing theme in quantum engineering as one can then truly control single photon emission and alter dipole-dipole interactions spatial scaling with distance. The ability to do so would have far reaching impacts in physics (quantum computing and cryptography), engineering (harnessing van der Waals forces), and bio-imaging. In this thesis we leverage the emerging new technology of metamaterials to design and fabricate devices that facilitate strong light matter interactions and allow for long-range dipole-dipole interactions among quantum emitters. Our approach utilizes the unique photonic modes and intrinsically broadband nature of hyperbolic metamaterials, uniaxial media with extreme anisotropy. We show experimentally that hyperbolic media fundamentally extend the non-radiative near-fields of dipole-dipole interactions. In conventional media, these non-radiative near-fields decay dramatically with distance curtailing interactions to only a few nanometers. This first experimental demonstration was achieved through synergistic advances in theory, ultrafast optics, low-light level detection and nanofabrication. We find that dipole-dipole interactions are not directly related to the photonic density of states, but instead are quantified by the two-point spectral density function, a physical quantity distinct from the photonic density of states. We engineer this quantity and construct a metamaterial device that displays dipole-dipole interactions far beyond the range of the conventional Coulombic near-field, achieving Super-Coulombic Dipole Interactions. Our approach is distinct from existing techniques which generally rely on narrow band resonant cavities or band edge photonic crystals to engineer the radiative far-field interactions. We also design a hyperbolic metamaterial device to enhance and direct the spontaneous emission from isolated fluorescent emitters. Stimulating experimental evidence demonstrates that hyperbolic metamaterials are viable candidates for enhancing single photon emission into well defined spatial modes. This thesis also describes rigorous metamaterial fabrication and design principles, and presents experimental isolation of unique Ferrel-Berreman modes in epsilon-near-zero media which are radiative collective charge oscillations in ultrathin films. Finally, we make advancements in nanofabrication of disordered plasmonic media that achieves localized plasmonic resonances to demonstrate giant enhancement in many-body dipole-dipole interactions. From experiments, we infer that the normally spherically symmetric near-field Coulombic potential is anisotropic above disordered gold nano-particle substrates. We envision that controlled dipole-dipole interactions can impact deterministic entanglement creation between remote emitters, quantum coherence in metamaterial mediated photosynthetic energy transfer, lead to many-dipole interactive states in metamaterials, increase the range of biomolecular FRET rulers as well as FRET imaging systems, and accelerate progress towards the long-standing goal of strongly coupled quantum systems at room temperature (Vdd \u3e kBT room)
Foundations of Topological Electrodynamics
No full textOver the last decade, Dirac matter has become one of the most prominent fields of research in contemporary material science due to the incredibly rich physics of the Dirac equation. Notable examples are the Dirac cones in graphene, Weyl points in TaAs, and gapless edge states in Bi2Te3. These unique phases of matter are intimately related to the topological structure of Dirac fermions. However, it remains an open question if the topological structure of Maxwell’s equations predicts yet new phases of matter. This thesis will conclusively answer this question.Topological electrodynamics is concerned with the geometry of electromagnetic waves in condensed matter. At the microscopic level, photons couple to the dipole-carrying excitations of a material, such as plasmons and excitons, which hybridize to form new normal modes of the system. The interaction between these bosonic oscillators is the origin of temporal and spatial dispersion in optical response functions like the conductivity tensor. Our main achievement is motivating a global interpretation of these response functions, over all frequencies and wavevectors. This theory led us to the conclusion that there are topological invariants associated with the conductivity tensor itself. In this thesis, we show exactly how to calculate these electromagnetic invariants, in both continuum and lattice theories, to identify unique Maxwellian phases of matter. Magnetohydrodynamic electron fluids in strongly-correlated 2D materials like graphene are the first candidates of this new class of topological phase. The fundamental physical mechanism that gives rise to a topological electromagnetic classification is Hall viscosity ηH which adds a nonlocal component to the Hall conductivity. To study the topological electrodynamics, we propose viscous Maxwell-ChernSimons theory – a Lagrangian framework that naturally generates the equations of motion, nonlocal Hall response and the boundary conditions. We demonstrate that nonlocal Hall conductivity is the spin-1 photonic equivalent of dispersive mass and induces precession of bulk photonic skyrmions. Nontrivial photonic skyrmions are associated with Dirac monopoles in the bulk momentum space and a singular Berry gauge. A singular gauge occurs when the photonic mass changes sign. Remarkably, the boundary of this medium supports gapless chiral edge states that are spin-1 helically-quantized and satisfy open boundary conditions
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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