90,074 research outputs found
Phase-Charge Duality of a Josephson junction in a fluctuating electromagnetic environment
We have measured the current-voltage characteristics of a single Josephson
junction placed in a high impedance environment. The transfer of Cooper pairs
through the junction is governed by overdamped quasicharge dynamics, leading to
Coulomb blockade and Bloch oscillations. Exact duality exists to the standard
overdamped phase dynamics of a Josephson junction, resulting in a dual shape of
the current-voltage characteristic, with current and voltage changing roles. We
demonstrate this duality with experiments which allow for a quantitative
comparison with a theory that includes the effect of fluctuations due to finite
temperature of the electromagnetic environment
TUNNELING SPECTROSCOPY OF QUANTUM CHARGE FLUCTUATIONS IN THE COULOMB BLOCKADE
We present a theory of Coulomb blockade oscillations in tunneling through a
pair of quantum dots connected by a tunable tunneling junction. The positions
and amplitudes of peaks in the linear conductance are directly related,
respectively, to the ground state energy and to the dynamics of charge
fluctuations. We study analytically both strong and weak interdot tunneling. As
the tunneling decreases, the period of the peaks doubles, as observed
experimentally. In the strong tunneling limit, we predict a striking power law
temperature dependence of the peak amplitudes.Comment: 4 pages, revtex3.0, 1 figure uuencode
On the stationarity of linearly forced turbulence in finite domains
A simple scheme of forcing turbulence away from decay was introduced by
Lundgren some time ago, the `linear forcing', which amounts to a force term
linear in the velocity field with a constant coefficient. The evolution of
linearly forced turbulence towards a stationary final state, as indicated by
direct numerical simulations (DNS), is examined from a theoretical point of
view based on symmetry arguments. In order to follow closely the DNS the flow
is assumed to live in a cubic domain with periodic boundary conditions. The
simplicity of the linear forcing scheme allows one to re-write the problem as
one of decaying turbulence with a decreasing viscosity. Scaling symmetry
considerations suggest that the system evolves to a stationary state, evolution
that may be understood as the gradual breaking of a larger approximate symmetry
to a smaller exact symmetry. The same arguments show that the finiteness of the
domain is intimately related to the evolution of the system to a stationary
state at late times, as well as the consistency of this state with a high
degree of isotropy imposed by the symmetries of the domain itself. The
fluctuations observed in the DNS for all quantities in the stationary state can
be associated with deviations from isotropy. Indeed, self-preserving isotropic
turbulence models are used to study evolution from a direct dynamical point of
view, emphasizing the naturalness of the Taylor microscale as a self-similarity
scale in this system. In this context the stationary state emerges as a stable
fixed point. Self-preservation seems to be the reason behind a noted similarity
of the third order structure function between the linearly forced and freely
decaying turbulence, where again the finiteness of the domain plays an
significant role.Comment: 15 pages, 7 figures, changes in the discussion at the end of section
VI, formula (60) correcte
Scaling of the Equilibrium Magnetization in the Mixed State of Type-II Superconductors
We discuss the analysis of mixed-state magnetization data of type-II
superconductors using a recently developed scaling procedure. It is based on
the fact that, if the Ginzburg-Landau parameter kappa does not depend on
temperature, the magnetic susceptibility is a universal function of H/H_c2(T),
leading to a simple relation between magnetizations at different temperatures.
Although this scaling procedure does not provide absolute values of the upper
critical fieldH_c2(T), its temperature variation can be established rather
accurately. This provides an opportunity to validate theoretical models that
are usually employed for the evaluation of H_c2(T) from equilibrium
magnetization data. In the second part of the paper we apply this scaling
procedure for a discussion of the notorious first order phase transition in the
mixed state of high temperature superconductors. Our analysis, based on
experimental magnetization data available in the literature, shows that the
shift of the magnetization accross the transition may adopt either sign,
depending on the particular chosen sample. We argue that this observation is
inconsistent with the interpretation that this transition always represents the
melting transition of the vortex lattice.Comment: 18 pages, 12 figure
Scharnhorst effect at oblique incidence
We consider the Scharnhorst effect (anomalous photon propagation in the
Casimir vacuum) at oblique incidence, calculating both photon speed and
polarization states as functions of angle. The analysis is performed in the
framework of nonlinear electrodynamics and we show that many features of the
situation can be extracted solely on the basis of symmetry considerations.
Although birefringence is common in nonlinear electrodynamics it is not
universal; in particular we verify that the Casimir vacuum is not birefringent
at any incidence angle. On the other hand, group velocity is typically not
equal to phase velocity, though the distinction vanishes for special directions
or if one is only working to second order in the fine structure constant. We
obtain an ``effective metric'' that is subtly different from previous results.
The disagreement is due to the way that ``polarization sums'' are implemented
in the extant literature, and we demonstrate that a fully consistent
polarization sum must be implemented via a bootstrap procedure using the
effective metric one is attempting to define. Furthermore, in the case of
birefringence, we show that the polarization sum technique is intrinsically an
approximation.Comment: 11 pages double-column format, 2 figures, RevTeX 4.0 (beta 2). Final
versio
An optically driven quantum dot quantum computer
We propose a quantum computer structure based on coupled asymmetric
single-electron quantum dots. Adjacent dots are strongly coupled by means of
electric dipole-dipole interactions enabling rapid computation rates. Further,
the asymmetric structures can be tailored for a long coherence time. The result
maximizes the number of computation cycles prior to loss of coherence.Comment: 4 figure
Tomography of Quantum Operations
Quantum operations describe any state change allowed in quantum mechanics,
including the evolution of an open system or the state change due to a
measurement. In this letter we present a general method based on quantum
tomography for measuring experimentally the matrix elements of an arbitrary
quantum operation. As input the method needs only a single entangled state. The
feasibility of the technique for the electromagnetic field is shown, and the
experimental setup is illustrated based on homodyne tomography of a twin-beam.Comment: Submitted to Phys. Rev. Lett. 2 eps + 1 latex figure
Certainty relations between local and nonlocal observables
We demonstrate that for an arbitrary number of identical particles, each
defined on a Hilbert-space of arbitrary dimension, there exists a whole ladder
of relations of complementarity between local, and every conceivable kind of
joint (or nonlocal) measurements. E.g., the more accurate we can know (by a
measurement) some joint property of three qubits (projecting the state onto a
tripartite entangled state), the less accurate some other property, local to
the three qubits, become. We also show that the corresponding complementarity
relations are particularly tight for particles defined on prime dimensional
Hilbert spaces.Comment: 4 pages, no figure
Density fluctuations and single-particle dynamics in liquid lithium
The single-particle and collective dynamical properties of liquid lithium
have been evaluated at several thermodynamic states near the triple point. This
is performed within the framework of mode-coupling theory, using a
self-consistent scheme which, starting from the known static structure of the
liquid, allows the theoretical calculation of several dynamical properties.
Special attention is devoted to several aspects of the single-particle
dynamics, which are discussed as a function of the thermodynamic state. The
results are compared with those of Molecular Dynamics simulations and other
theoretical approaches.Comment: 31 pages (in preprint format), 14 figures. Submitted to Phys. Rev.
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