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2005

Dartmouth College

Articles 1 - 11 of 11

Full-Text Articles in Physics

Dynamical Control Of Qubit Coherence: Random Versus Deterministic Schemes, Lea F. Santos, Lorenza Viola Dec 2005

Dynamical Control Of Qubit Coherence: Random Versus Deterministic Schemes, Lea F. Santos, Lorenza Viola

Dartmouth Scholarship

We reexamine the problem of switching off unwanted phase evolution and decoherence in a single two-state quantum system in the light of recent results on random dynamical decoupling methods [L. Viola and E. Knill, Phys. Rev. Lett. 94, 060502 (2005)]. A systematic comparison with standard cyclic decoupling is effected for a variety of dynamical regimes, including the case of both semiclassical and fully quantum decoherence models. In particular, exact analytical expressions are derived for randomized control of decoherence from a bosonic environment. We investigate quantitatively control protocols based on purely deterministic, purely random, as well as hybrid design, and …

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Dynamics Of A Nanomechanical Resonator Coupled To A Superconducting Single-Electron Transistor, M. P. Blencowe, J. Imbers, A. D. Armour Nov 2005

Dynamics Of A Nanomechanical Resonator Coupled To A Superconducting Single-Electron Transistor, M. P. Blencowe, J. Imbers, A. D. Armour

Dartmouth Scholarship

We present an analysis of the dynamics of a nanomechanical resonator coupled to a superconducting single-electron transistor (SSET) in the vicinity of Josephson quasi-particle (JQP) and double Josephson quasi-particle (DJQP) resonances. For weak coupling and wide separation of dynamical timescales, we find that for either superconducting resonances the dynamics of the resonator are given by a Fokker–Planck equation, i.e. the SSET behaves effectively as an equilibrium heat bath, characterized by an effective temperature, which also damps the resonator and renormalizes its frequency. Depending on the gate and drain–source voltage bias points with respect to the superconducting resonance, the SSET can …

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Low Magnetic Prandtl Number Dynamos With Helical Forcing, Pablo D. Mininni, David C. Montgomery Nov 2005

Low Magnetic Prandtl Number Dynamos With Helical Forcing, Pablo D. Mininni, David C. Montgomery

Dartmouth Scholarship

We present direct numerical simulations of dynamo action in a forced Roberts flow. The behavior of the dynamo is followed as the mechanical Reynolds number is increased, starting from the laminar case until a turbulent regime is reached. The critical magnetic Reynolds for dynamo action is found, and in the turbulent flow it is observed to be nearly independent on the magnetic Prandtl number in the range from ∼0.3 to ∼0.1. Also the dependence of this threshold with the amount of mechanical helicity in the flow is studied. For the different regimes found, the configuration of the magnetic and velocity …

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Towards A Precision Measurement Of The Casimir Force In A Cylinder-Plane Geometry, M. Brown-Hayes, D. A.R. Dalvit, F. D. Mazzitelli, W. J. Kim, R. Onofrio Nov 2005

Towards A Precision Measurement Of The Casimir Force In A Cylinder-Plane Geometry, M. Brown-Hayes, D. A.R. Dalvit, F. D. Mazzitelli, W. J. Kim, R. Onofrio

Dartmouth Scholarship

We report on a proposal aimed at measuring the Casimir force in the cylinder-plane configuration. The Casimir force is evaluated including corrections due to finite parallelism, conductivity, and temperature. The range of validity of the proximity force approximation is also discussed. An apparatus to test the feasibility of a precision measurement in this configuration has been developed, and we describe both a procedure to control the parallelism and the results of the electrostatic calibration. Finally we discuss the possibility of measuring the thermal contribution to the Casimir force and deviations from the proximity force approximation, both of which are expected …

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Sympathetic Cooling Route To Bose-Einstein Condensate And Fermi-Liquid Mixtures, Robin Côté, Roberto Onofrio, Eddy Timmermans Oct 2005

Sympathetic Cooling Route To Bose-Einstein Condensate And Fermi-Liquid Mixtures, Robin Côté, Roberto Onofrio, Eddy Timmermans

Dartmouth Scholarship

We discuss a sympathetic cooling strategy that can successfully mitigate fermion-hole heating in a dilute atomic Fermi-Bose mixture and access the temperature regime in which the fermions behave as a Fermi liquid. We introduce an energy-based formalism to describe the temperature dynamics with which we study a specific and promising mixture, composed of 6Li and 87Rb. Analyzing the harmonically trapped mixture, we find that the favorable features of this mixture are further enhanced by using different trapping frequencies for the two species.

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Cosmic Shear Of The Microwave Background: The Curl Diagnostic, Asantha Cooray, Marc Kamionkowski, Robert R. Caldwell Jun 2005

Cosmic Shear Of The Microwave Background: The Curl Diagnostic, Asantha Cooray, Marc Kamionkowski, Robert R. Caldwell

Dartmouth Scholarship

Weak-lensing distortions of the cosmic-microwave-background (CMB) temperature and polarization patterns can reveal important clues to the intervening large-scale structure. The effect of lensing is to deflect the primary temperature and polarization signal to slightly different locations on the sky. Deflections due to density fluctuations, gradient-type for the gradient of the projected gravitational potential, give a direct measure of the mass distribution. Curl-type deflections can be induced by, for example, a primordial background of gravitational waves from inflation or by second-order effects related to lensing by density perturbations. Whereas gradient-type deflections are expected to dominate, we show that curl-type deflections can …

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Dynamo Regimes With A Nonhelical Forcing, Pablo D. Mininni, Yannick Ponty, David C. Montgomery, Jean-Francois Pinton Jun 2005

Dynamo Regimes With A Nonhelical Forcing, Pablo D. Mininni, Yannick Ponty, David C. Montgomery, Jean-Francois Pinton

Dartmouth Scholarship

A three-dimensional numerical computation of magnetohydrodynamic dynamo behavior is described. The dynamo is mechanically forced with a driving term of the Taylor-Green type. The magnetic field development is followed from negligibly small levels to saturated values that occur at magnetic energies comparable to the kinetic energies. Although there is locally a nonzero helicity density, there is no overall integrated helicity in the system. Persistent oscillations are observed in the saturated state for not-too-large mechanical Reynolds numbers, oscillations in which the kinetic and magnetic energies vary out of phase but with no reversal of the magnetic field. The flow pattern exhibits …

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Lattice Thermal Conductance In Nanowires At Low Temperatures: Breakdown And Recovery Of Quantization, Y. Tanaka, F. Yoshida, S. Tamura May 2005

Lattice Thermal Conductance In Nanowires At Low Temperatures: Breakdown And Recovery Of Quantization, Y. Tanaka, F. Yoshida, S. Tamura

Dartmouth Scholarship

The quantization of lattice thermal conductance g normalized by g0=π2k2BT/3h (the universal quantum of thermal conductance) was recently predicted theoretically to take an integer value over a finite range of temperature and then observed experimentally in nanowires with catenoidal contacts. The prediction of this quantization by Rego and Kirczenow [Phys. Rev. Lett. 81, 232 (1998)] relies on a study of only dilatational (longitudinal) vibrational mode in the wires. We study the thermal conductance in catenoidal wires by explicitly calculating the transmission rates of the six distinct vibrational modes (four acoustic and two low-lying optical modes) and applying the Landauer …

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Numerical Solutions Of The Three-Dimensional Magnetohydrodynamic Α Model, Pablo D. Mininni, David C. Montgomery, Annick Pouquet Apr 2005

Numerical Solutions Of The Three-Dimensional Magnetohydrodynamic Α Model, Pablo D. Mininni, David C. Montgomery, Annick Pouquet

Dartmouth Scholarship

We present direct numerical simulations and α-model simulations of four familiar three-dimensional magnetohydrodynamic (MHD) turbulence effects: selective decay, dynamic alignment, inverse cascade of magnetic helicity, and the helical dynamo effect. The MHD α model is shown to capture the long-wavelength spectra in all these problems, allowing for a significant reduction of computer time and memory at the same kinetic and magnetic Reynolds numbers. In the helical dynamo, not only does the α model correctly reproduce the growth rate of magnetic energy during the kinematic regime, it also captures the nonlinear saturation level and the late generation of a large scale …

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Resonant Nucleation, Marcelo Gleiser, Rafael C. Howell Apr 2005

Resonant Nucleation, Marcelo Gleiser, Rafael C. Howell

Dartmouth Scholarship

We investigate the role played by fast quenching on the decay of metastable (or false vacuum) states. Instead of the exponentially slow decay rate per unit volume, ΓHN∼exp[−Eb/kBT] (Eb is the free energy of the critical bubble), predicted by homogeneous nucleation theory, we show that under fast enough quenching the decay rate is a power law ΓRN∼[Eb/kBT]−B, where B is weakly sensitive to the temperature. For a range of parameters, large-amplitude oscillations about the metastable state trigger the resonant emergence of coherent subcritical configurations. Decay mechanisms for different Eb are proposed and illustrated in a (2+1)-dimensional scalar field model.

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Optimal Reconstruction Of Magnetopause Structures From Cluster Data, H Hasegawa, B U. Ö Sonnerup, B Klecker, G Paschmann Mar 2005

Optimal Reconstruction Of Magnetopause Structures From Cluster Data, H Hasegawa, B U. Ö Sonnerup, B Klecker, G Paschmann

Dartmouth Scholarship

The Grad-Shafranov (GS) reconstruction tech- nique, a single-spacecraft based data analysis method for recovering approximately two-dimensional (2-D) magneto- hydrostatic plasma/field structures in space, is improved to become a multi-spacecraft technique that produces a single field map by ingesting data from all four Cluster spacecraft into the calculation. The plasma pressure, required for the technique, is measured in high time resolution by only two of the spacecraft, C1 and C3, but, with the help of spacecraft po- tential measurements available from all four spacecraft, the pressure can be estimated at the other spacecraft as well via a relationship, established from C1 …

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