15,577 research outputs found
Total and partial cloud amount detection during summer 2005 at Westerland (Sylt, Germany)
The detection of cloudiness is investigated by means of partial and total cloud amount estimations from pyrgeometer radiation measurements and visible all-sky imager observations. The measurements have been performed in Westerland, a seaside resort on the North Sea island of Sylt, Germany, during summer 2005.
An improvement to previous studies on this subject resulting in the first time partial cloud amounts (PCAs), defined as cloud amounts without high clouds calculated from longwave downward radiation (LDR) according to the APCADA algorithm (Dürr and Philipona, 2004), are validated against both human observations from the National Meteorological Servive DWD at the nearby airport of Sylt and digital all-sky imaging. The aim is to establish the APCADA scheme at a coastal midlatitude site for longterm observations of cloud cover and to quantify errors resulting from the different methods of detecting cloudiness.
Differences between the resulting total cloud amounts (TCAs), defined as cloud amount for all-cloud situations, derived from the camera images and from human observations are within ±1 octa in 72% and within ±2 octa in 85% of the cases. Compared to human observations, PCA measurements, according to APCADA, underestimate the observed cloud cover in 47% of all cases and the differences are within ±1 octa in 60% and ±2 octa in 74% of all cases. Since high cirrus clouds can not be derived from LDR, separate comparisons for all cases without high clouds have been performed showing an agreement within ±1(2) octa in 73(90)% for PCA and also for camera-derived TCA. For this coastal mid-latitude site under investigation, we find similar though slightly smaller agreements to human observations as reported by Dürr and Philipona (2004). Though limited to daytime, the cloud cover retrievals from the sky imager are not really affected by cirrus clouds and provide a more reliable cloud climatology for all-cloud conditions than APCADA
Is the direct observation of electronic coherence in electron transfer reactions possible?
The observability of electronic coherence in electron transfer reactions is
discussed. We show that under appropriate circumstances large-amplitude
oscillations can be found in the electronic occupation probabilities. The
initial preparation of the system is of crucial importance for this effect, and
we discuss conditions under which experiments detecting electronic coherence
should be feasible. The Feynman-Vernon influence functional formalism is
extended to examine more general and experimentally relevant initial
preparations. Analytical expressions and path integral quantum dynamics
simulations were developed to study the effects of various initial preparations
on the observability of electronic coherence.Comment: 14 pages, 9 figures, to be published in J. Chem. Phy
The private capacity of quantum channels is not additive
Recently there has been considerable activity on the subject of additivity of
various quantum channel capacities. Here, we construct a family of channels
with sharply bounded classical, hence private capacity. On the other hand,
their quantum capacity when combined with a zero private (and zero quantum)
capacity erasure channel, becomes larger than the previous classical capacity.
As a consequence, we can conclude for the first time that the classical
private capacity is non-additive. In fact, in our construction even the quantum
capacity of the tensor product of two channels can be greater than the sum of
their individual classical private capacities.
We show that this violation occurs quite generically: every channel can be
embedded into our construction, and a violation occurs whenever the given
channel has larger entanglement assisted quantum capacity than (unassisted)
classical capacity.Comment: 4+4 pages, 2 eps figures. V2 has title and abstract changed; its new
structure reflects the final version of a main paper plus appendices
containing mathematical detail
Chebyshev approach to quantum systems coupled to a bath
We propose a new concept for the dynamics of a quantum bath, the Chebyshev
space, and a new method based on this concept, the Chebyshev space method. The
Chebyshev space is an abstract vector space that exactly represents the
fermionic or bosonic bath degrees of freedom, without a discretization of the
bath density of states. Relying on Chebyshev expansions the Chebyshev space
representation of a bath has very favorable properties with respect to
extremely precise and efficient calculations of groundstate properties, static
and dynamical correlations, and time-evolution for a great variety of quantum
systems. The aim of the present work is to introduce the Chebyshev space in
detail and to demonstrate the capabilities of the Chebyshev space method.
Although the central idea is derived in full generality the focus is on model
systems coupled to fermionic baths. In particular we address quantum impurity
problems, such as an impurity in a host or a bosonic impurity with a static
barrier, and the motion of a wave packet on a chain coupled to leads. For the
bosonic impurity, the phase transition from a delocalized electron to a
localized polaron in arbitrary dimension is detected. For the wave packet on a
chain, we show how the Chebyshev space method implements different boundary
conditions, including transparent boundary conditions replacing infinite leads.
Furthermore the self-consistent solution of the Holstein model in infinite
dimension is calculated. With the examples we demonstrate how highly accurate
results for system energies, correlation and spectral functions, and
time-dependence of observables are obtained with modest computational effort.Comment: 18 pages, 13 figures, to appear in Phys. Rev.
Nonequilibrium Spin Dynamics in the Ferromagnetic Kondo Model
Motivated by recent experiments on molecular quantum dots we investigate the
relaxation of pure spin states when coupled to metallic leads. Under suitable
conditions these systems are well described by a ferromagnetic Kondo model.
Using two recently developed theoretical approaches, the time-dependent
numerical renormalization group and an extended ow equation method, we
calculate the real-time evolution of a Kondo spin into its partially screened
steady state. We obtain exact analytical results which agree well with
numerical implementations of both methods. Analytical expressions for the
steady state magnetization and the dependence of the long-time relaxation on
microscopic parameters are established. We find the long-time relaxation
process to be much faster in the regime of anisotropic Kondo couplings. The
steady state magnetization is found to deviate significantly from its thermal
equilibrium value.Comment: 4 pages, 3 figures, final version as accepted by Physical Review
Letter
Inelastic Multiple Scattering of Interacting Bosons in Weak Random Potentials
We develop a diagrammatic scattering theory for interacting bosons in a
three-dimensional, weakly disordered potential. We show how collisional energy
transfer between the bosons induces the thermalization of the inelastic
single-particle current which, after only few collision events, dominates over
the elastic contribution described by the Gross-Pitaevskii ansatz.Comment: 5 pages, 3 figures, very close to published versio
On the communication cost of entanglement transformations
We study the amount of communication needed for two parties to transform some
given joint pure state into another one, either exactly or with some fidelity.
Specifically, we present a method to lower bound this communication cost even
when the amount of entanglement does not increase. Moreover, the bound applies
even if the initial state is supplemented with unlimited entanglement in the
form of EPR pairs, and the communication is allowed to be quantum mechanical.
We then apply the method to the determination of the communication cost of
asymptotic entanglement concentration and dilution. While concentration is
known to require no communication whatsoever, the best known protocol for
dilution, discovered by Lo and Popescu [Phys. Rev. Lett. 83(7):1459--1462,
1999], requires a number of bits to be exchanged which is of the order of the
square root of the number of EPR pairs. Here we prove a matching lower bound of
the same asymptotic order, demonstrating the optimality of the Lo-Popescu
protocol up to a constant factor and establishing the existence of a
fundamental asymmetry between the concentration and dilution tasks.
We also discuss states for which the minimal communication cost is
proportional to their entanglement, such as the states recently introduced in
the context of ``embezzling entanglement'' [W. van Dam and P. Hayden,
quant-ph/0201041].Comment: 9 pages, 1 figure. Added a reference and some further explanations.
In v3 some arguments are given in more detai
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