29,951 research outputs found
The Cummins advanced turbocompound diesel engine evaluation
An advanced turbocompound diesel engine program was initiated to improve the tank mileage of the turbocompound engine by 5% over the vehicle test engines. Engine improvements could be realized by increasing the available energy of the exhaust gas at the turbine inlet, incorporating gas turbine techniques into improving the turbomachinery efficiencies, and through refined engine system optimization. The individual and cumulative performance gains achieved with the advanced turbocompound engine improvements are presented
A glance beyond the quantum model
One of the most important problems in Physics is how to reconcile Quantum
Mechanics with General Relativity. Some authors have suggested that this may be
realized at the expense of having to drop the quantum formalism in favor of a
more general theory. However, as the experiments we can perform nowadays are
far away from the range of energies where we may expect to observe non-quantum
effects, it is difficult to theorize at this respect. Here we propose a
fundamental axiom that we believe any reasonable post-quantum theory should
satisfy, namely, that such a theory should recover classical physics in the
macroscopic limit. We use this principle, together with the impossibility of
instantaneous communication, to characterize the set of correlations that can
arise between two distant observers. Although several quantum limits are
recovered, our results suggest that quantum mechanics could be falsified by a
Bell-type experiment if both observers have a sufficient number of detectors
Local Hidden Variable Theories for Quantum States
While all bipartite pure entangled states violate some Bell inequality, the
relationship between entanglement and non-locality for mixed quantum states is
not well understood. We introduce a simple and efficient algorithmic approach
for the problem of constructing local hidden variable theories for quantum
states. The method is based on constructing a so-called symmetric
quasi-extension of the quantum state that gives rise to a local hidden variable
model with a certain number of settings for the observers Alice and Bob.Comment: 8 pages Revtex; v2 contains substantial changes, a strengthened main
theorem and more reference
A universally programmable Quantum Cellular Automaton
We discuss the role of classical control in the context of reversible quantum
cellular automata. Employing the structure theorem for quantum cellular
automata, we give a general construction scheme to turn an arbitrary cellular
automaton with external classical control into an autonomous one, thereby
proving the computational equivalence of these two models. We use this
technique to construct a universally programmable cellular automaton on a
one-dimensional lattice with single cell dimension 12.Comment: 4 pages, 4 figures, minor changes in introduction, fixed typos,
accepted for publication in Physical Review Letter
The virtual observatory service TheoSSA: Establishing a database of synthetic stellar flux standards. I. NLTE spectral analysis of the DA-type white dwarf G 191-B2B
H-rich, DA-type white dwarfs are particularly suited as primary standard
stars for flux calibration. State-of-the-art NLTE models consider opacities of
species up to trans-iron elements and provide reliable synthetic
stellar-atmosphere spectra to compare with observation.
We establish a database of theoretical spectra of stellar flux standards that
are easily accessible via a web interface.
In the framework of the Virtual Observatory, the German Astrophysical Virtual
Observatory developed the registered service TheoSSA. It provides easy access
to stellar spectral energy distributions (SEDs) and is intended to ingest SEDs
calculated by any model-atmosphere code. In case of the DA white dwarf G
191-B2B, we demonstrate that the model reproduces not only its overall
continuum shape but also the numerous metal lines exhibited in its ultraviolet
spectrum.
TheoSSA is in operation and contains presently a variety of SEDs for DA white
dwarfs. It will be extended in the near future and can host SEDs of all primary
and secondary flux standards. The spectral analysis of G 191-B2B has shown that
our hydrostatic models reproduce the observations best at an effective
temperature of 60000 +/- 2000K and a surface gravity of log g = 7.60 +/- 0.05.
We newly identified Fe VI, Ni VI, and Zn IV lines. For the first time, we
determined the photospheric zinc abundance with a logarithmic mass fraction of
-4.89 (7.5 times solar). The abundances of He (upper limit), C, N, O, Al, Si,
O, P, S, Fe, Ni, Ge, and Sn were precisely determined. Upper abundance limits
of 10% solar were derived for Ti, Cr, Mn, and Co.
The TheoSSA database of theoretical SEDs of stellar flux standards guarantees
that the flux calibration of all astronomical data and cross-calibration
between different instruments can be based on the same models and SEDs
calculated with different model-atmosphere codes and are easy to compare.Comment: 42 pages, 27 figure
Drell-Yan Lepton Angular Distribution at Small Transverse Momentum
We investigate the dependence of the Drell-Yan cross section on lepton polar
and azimuthal angles, as generated by the lowest-order QCD annihilation and
Compton processes. We focus in particular on the azimuthal-angular
distributions, which are of the form cos(phi) and cos(2phi). At small
transverse momentum q_T of the lepton pair, q_T << Q, with Q the pair mass,
these terms are known to be suppressed relative to the phi-independent part of
the Drell-Yan cross section by one or two powers of the transverse momentum.
Nonetheless, as we show, like the phi-independent part they are subject to
large logarithmic corrections, whose precise form however depends on the
reference frame chosen. These logarithmic contributions ultimately require
resummation to all orders in the strong coupling. We discuss the potential
effects of resummation on the various angular terms in the cross section and on
the Lam-Tung relation.Comment: 13 pages, 2 figures; reference added, minor other changes, matches
version to be published in PR
Barrier-controlled carrier transport in microcrystalline semiconducting materials: Description within a unified model
A recently developed model that unifies the ballistic and diffusive transport
mechanisms is applied in a theoretical study of carrier transport across
potential barriers at grain boundaries in microcrystalline semiconducting
materials. In the unified model, the conductance depends on the detailed
structure of the band edge profile and in a nonlinear way on the carrier mean
free path. Equilibrium band edge profiles are calculated within the trapping
model for samples made up of a linear chain of identical grains. Quantum
corrections allowing for tunneling are included in the calculation of electron
mobilities. The dependence of the mobilities on carrier mean free path, grain
length, number of grains, and temperature is examined, and appreciable
departures from the results of the thermionic-field-emission model are found.
Specifically, the unified model is applied in an analysis of Hall mobility data
for n-type microcrystalline Si thin films in the range of thermally activated
transport. Owing mainly to the effect of tunneling, potential barrier heights
derived from the data are substantially larger than the activation energies of
the Hall mobilities. The specific features of the unified model, however,
cannot be resolved within the rather large uncertainties of the analysis.Comment: REVTex, 19 pages, 9 figures; to appear in J. Appl. Phy
Lower bounds on the entanglement needed to play XOR non-local games
We give an explicit family of XOR games with O(n)-bit questions requiring 2^n
ebits to play near-optimally. More generally we introduce a new technique for
proving lower bounds on the amount of entanglement required by an XOR game: we
show that near-optimal strategies for an XOR game G correspond to approximate
representations of a certain C^*-algebra associated to G. Our results extend an
earlier theorem of Tsirelson characterising the set of quantum strategies which
implement extremal quantum correlations.Comment: 20 pages, no figures. Corrected abstract, body of paper unchange
Correlative Capacity of Composite Quantum States
We characterize the optimal correlative capacity of entangled, separable, and
classically correlated states. Introducing the notions of the infimum and
supremum within majorization theory, we construct the least disordered
separable state compatible with a set of marginals. The maximum separable
correlation information supportable by the marginals of a multi-qubit pure
state is shown to be an LOCC monotone. The least disordered composite of a pair
of qubits is found for the above classes, with classically correlated states
defined as diagonal in the product of marginal bases.Comment: 4 pages, 1 figur
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