463 research outputs found
Effects and detection of quantum noise
No full textThis thesis is about random fluctuations over time (or noise) of electric currents and voltages occuring in small (mesoscopic) electronic devices with typical sizes of micro- to nanometre. Even though the theory presented is of a more general nature, research into such systems has been greatly pushed forward by the prospect of building a quantum computer. There are two important aspects to noise which are addressed in this work. The first can be summarised as detection and refers to the idea, that the fluctuations carry information about the microscopic details (geometric design, scattering, temperature) of transport which causes them. The theoretical problem we investigate is then to relate detector signals to fundamental properties of the sample. Secondly, fluctuations can trigger a variety of processes in the environment. Depending on the system one may wish to enhance or diminish such effects. To achieve this goal we study noise-induced effects and the coupling between noise sources and their environment. The precise way in which these fluctuations occur can be found from the theory of Full Counting Statistics (FCS) which provides a cornerstone for this thesis. In chapter 3 the effect of a weak electromagnetic environment on the Full Counting Statistics of a coherent conductor is investigated. We obtain explicit expressions for the correction to the FCS which are further studied by analytical and numerical means. We also present a reinterpretation of the correction in terms of elementary physical events. The major result in that chapter is a universal relation for Full Counting Statistics which holds at arbitrary voltage, temperature and with no regard to the concrete realization of the contact. For FCS this relation takes the form of detailed balance. In chapter 4 we investigate the detection of finite frequency noise using a quantum tunnelling detector. We focus on a concrete experimental setup consisting of a coherent conductor taking the role of the noise source and a tunnel junction (the detector) which is capacitively coupled to it. We show that the detector rate in a certain parameter range is dominated by a two-photon process and a process involving two interacting electrons in the coherent conductor. We find an explicit analytical expression for the detector signal in terms of system parameters: tunnel coupling, transmissions, environment, voltage over the conductor and coupling parameter. Our results facilitate the detection of many-particle events in the context of quantum transport, particularly electron-electron interactions. The non-Gaussian higher moments of the distribution of current fluctuations in a mesoscopic conductor contain more information than is present in average current and noise. However they are inherently difficult to measure. In order to facilitate such experiments, we propose a completely new way for measuring the Full Counting Statistics in chapter 5. We study threshold detection with a Josephson junction coupled to a mesoscopic conductor. We show that the detailed dependence of the junction's escape rate is sensitive to the distinct FCS of specific conductors (tunnel junction, diffusive, ballistic). We also address issues related to the measurement procedure notably feedback and dispersiveness of the detector. Our theoretical results facilitate a new type of electric noise measurement: direct measurement of the full distribution of transferred charge.Applied Science
Inelastic interaction corrections and universal relations for full counting statistics in a quantum contact
No full textTowards experimental observation of full counting statistics
No full textWe discuss how threshold detectors can be used for a direct measurement of the full counting statistics (FCS) of current fluctuations and how to implement Josephson junctions in this respect. We propose a scheme to characterize the full counting statistics from the current dependence of the escape rate measured. We illustrate the scheme with explicit results for tunnel, diffusive and quasi-ballistic mesoscopic conductors.Kavli Institute of NanoscienceApplied Science
Comparison of a Neutral Density Model With the SET HASDM Density Database
No full textThe EXospheric TEMperatures on a PoLyhedrAl gRid (EXTEMPLAR) method predicts the neutral densities in the thermosphere. The performance of this model has been evaluated through a comparison with the Air Force High Accuracy Satellite Drag Model (HASDM). The Space Environment Technologies (SET) HASDM database that was used for this test spans the 20 years 2000 through 2019, containing densities at 3 hr time intervals at 25 km altitude steps, and a spatial resolution of 10 degrees latitude by 15 degrees longitude. The upgraded EXTEMPLAR that was tested uses the newer Naval Research Laboratory MSIS 2.0 model to convert global exospheric temperature values to neutral density as a function of altitude. The revision also incorporated time delays that varied as a function of location, between the total Poynting flux in the polar regions and the exospheric temperature response. The density values from both models were integrated on spherical shells at altitudes ranging from 200 to 800 km. These sums were compared as a function of time. The results show an excellent agreement at temporal scales ranging from hours to years. The EXTEMPLAR model performs best at altitudes of 400 km and above, where geomagnetic storms produce the largest relative changes in neutral density. In addition to providing an effective method to compare models that have very different spatial resolutions, the use of density totals at various altitudes presents a useful illustration of how the thermosphere behaves at different altitudes, on time scales ranging from hours to complete solar cycles.NASA [80HQTR20T0081]; DARPA/Leidos AtmoSense [HR001121C0081/P0102500070]; NASA interagency [80HQTR20T0081]; Naval Research Laboratory; NSF grant [AGS-2019465]Published versionDaniel Weimer was supported by NASA grant 80NSSC20K1362 to Virginia Tech, through the Space Weather Operations-to-Research Program. Kent Tobiska, Piyush Mehta, and Richard Licata were supported by subcontracts to Space Environment Technologies and West Virginia University. Kent Tobiska and J. Yoshii also acknowledge support from the DARPA/Leidos AtmoSense contracts HR001121C0081/P0102500070 to Space Environment Technologies. Douglas Drob was supported by NASA interagency agreement 80HQTR20T0081 with the Naval Research Laboratory. Daniel Weimer had additional support from NSF grant AGS-2019465
The Martian Thermosphere/Ionosphere at High and Low Solar Activities
No full textWe compare here models of the thermosphere/ionosphere of Mars at low and high solar activities, and we present heating rates and efficiencies due to the absorption of solar radiation in the 18 to 2000 Å range. Using neutral model densities from the NCAR Mars Thermospheric General Circulation Model (MTGCM) of Bougher and co-workers, and solar fluxes from W. K. Tobiska, we have modeled the density profiles of 14 ions and 5 minor neutral species. We predict the variations in the ion densities with solar activity, and describe the sources and sinks of the ions. The major sources and sinks differ in some respects from those for the Venus ionosphere, and these differences are discussed as well. We find that the predicted total electron density profile computed using solar fluxes from Tobiska is somewhat different from that obtained using the fluxes of Hinteregger. One possible conclusion is that, at the time of Mariners 6 and 7, the soft x-ray fluxes were midway between those of the Tobiska and Hinteregger spectra
Computations of flows with interfaces using arbitrary Lagrangian Eulerian method
No full textThis work is devoted to the accurate simulation of incompressible two phase flows. The core of our methodology is the use of interface resolving meshes and the arbitrary Lagrangian-Eulerian (ALE) description of the fluid kinematics. Our numerical scheme is based on second order finite elements, a fractional step ? time discretisation, and a special approximation of the curvature to incorporate surface tension effects. We demonstrate the potential of the proposed numerical method by the simulation of a rising bubble and the Rayleigh-Taylor instability problem
A STREAMLINE DIFFUSION METHOD FOR NONCONFORMING FINITE ELEMENT APPROXIMATIONS APPLIED TO THE LINEARIZED INCOMPRESSIBLE NAVIER-STOKES EQUATION
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