5,127 research outputs found
Unspeakable quantum information
No verbal explanation can indicate a direction in space or the orientation of
a coordinate system. Only material objects can do it. In this article we
consider the use of a set of spin-\half particles in an entangled state for
indicating a direction, or a hydrogen atom in a Rydberg state for transmitting
a Cartesian frame. Optimal strategies are derived for the emission and
detection of the quantum signals.Comment: to appear in "Quantum Theory: Reconsideration of Foundations", ed. by
A. Khrennikov; series ``Math. Modelling in Physics, Engineering and Cognitive
Sciences'' V\"axj\"o Univ. Press (2002) - requires sprocl.st
TRACE-derived temperature and emission measure profiles along long-lived coronal loops: the role of filamentation
In a recent letter (ApJ 517, L155) Lenz et al. have shown the evidence of
uniform temperature along steady long coronal loops observed by TRACE in two
different passbands (171 A and 195 A filters). We propose that such an evidence
can be explained by the sub-arcsecond structuring of the loops across the
magnetic field lines. In this perspective, we present a model of a bundle of
six thin parallel hydrostatic filaments with temperature stratification
dictated by detailed energy balance and with temperatures at their apex ranging
between 0.8 and 5 MK. If analyzed as a single loop, the bundle would appear
isothermal along most of its length.Comment: 9 pages, 4 figs, LaTeX text, PostScript figure
Infinite matrices may violate the associative law
The momentum operator for a particle in a box is represented by an infinite
order Hermitian matrix . Its square is well defined (and diagonal),
but its cube is ill defined, because . Truncating these
matrices to a finite order restores the associative law, but leads to other
curious results.Comment: final version in J. Phys. A28 (1995) 1765-177
The Effects of Symmetries on Quantum Fidelity Decay
We explore the effect of a system's symmetries on fidelity decay behavior.
Chaos-like exponential fidelity decay behavior occurs in non-chaotic systems
when the system possesses symmetries and the applied perturbation is not tied
to a classical parameter. Similar systems without symmetries exhibit
faster-than-exponential decay under the same type of perturbation. This
counter-intuitive result, that extra symmetries cause the system to behave in a
chaotic fashion, may have important ramifications for quantum error correction.Comment: 5 pages, 3 figures, to be published Phys. Rev. E Rapid Communicatio
Hydrodynamic modelling of ejecta shrapnel in the Vela supernova remnant
Many supernova remnants (SNRs) are characterized by a knotty ejecta
structure. The Vela SNR is an excellent example of remnant in which detached
clumps of ejecta are visible as X-ray emitting bullets that have been observed
and studied in great detail. We aim at modelling the evolution of ejecta
shrapnel in the Vela SNR, investigating the role of their initial parameters
(position and density) and addressing the effects of thermal conduction and
radiative losses. We performed a set of 2-D hydrodynamic simulations describing
the evolution of a density inhomogeneity in the ejecta profile. We explored
different initial setups. We found that the final position of the shrapnel is
very sensitive to its initial position within the ejecta, while the dependence
on the initial density contrast is weaker. Our model also shows that moderately
overdense knots can reproduce the detached features observed in the Vela SNR.
Efficient thermal conduction produces detectable effects by determining an
efficient mixing of the ejecta knot with the surrounding medium and shaping a
characteristic elongated morphology in the clump.Comment: Accepted for publication in Monthly Notices of the Royal Astronomical
Societ
Coronal loop hydrodynamics. The solar flare observedon November 12 1980 revisited: the UV line emission
We revisit a well-studied solar flare whose X-ray emission originating from a
simple loop structure was observed by most of the instruments on board SMM on
November 12 1980. The X-ray emission of this flare, as observed with the XRP,
was successfully modeled previously. Here we include a detailed modeling of the
transition region and we compare the hydrodynamic results with the UVSP
observations in two EUV lines, measured in areas smaller than the XRP rasters,
covering only some portions of the flaring loop (the top and the foot-points).
The single loop hydrodynamic model, which fits well the evolution of coronal
lines (those observed with the XRP and the \FeXXI 1354.1 \AA line observed with
the UVSP) fails to model the flux level and evolution of the \OV 1371.3 \AA
line.Comment: A&A, in press, 6 pages, 5 figure
Mass Accretion Processes in Young Stellar Objects: Role of Intense Flaring Activity
According to the magnetospheric accretion scenario, young low-mass stars are
surrounded by circumstellar disks which they interact with through accretion of
mass. The accretion builds up the star to its final mass and is also believed
to power the mass outflows, which may in turn have a significant role in
removing the excess angular momentum from the star-disk system. Although the
process of mass accretion is a critical aspect of star formation, some of its
mechanisms are still to be fully understood. On the other hand, strong flaring
activity is a common feature of young stellar objects (YSOs). In the Sun, such
events give rise to perturbations of the interplanetary medium. Similar but
more energetic phenomena occur in YSOs and may influence the circumstellar
environment. In fact, a recent study has shown that an intense flaring activity
close to the disk may strongly perturb the stability of circumstellar disks,
thus inducing mass accretion episodes (Orlando et al. 2011). Here we review the
main results obtained in the field and the future perspectives.Comment: 4 pages, 2 Figures; accepted for publication on Acta Polytechnica
(Proceedings of the Frascati Workshop 2013
Quantum Fidelity Decay of Quasi-Integrable Systems
We show, via numerical simulations, that the fidelity decay behavior of
quasi-integrable systems is strongly dependent on the location of the initial
coherent state with respect to the underlying classical phase space. In
parallel to classical fidelity, the quantum fidelity generally exhibits
Gaussian decay when the perturbation affects the frequency of periodic phase
space orbits and power-law decay when the perturbation changes the shape of the
orbits. For both behaviors the decay rate also depends on initial state
location. The spectrum of the initial states in the eigenbasis of the system
reflects the different fidelity decay behaviors. In addition, states with
initial Gaussian decay exhibit a stage of exponential decay for strong
perturbations. This elicits a surprising phenomenon: a strong perturbation can
induce a higher fidelity than a weak perturbation of the same type.Comment: 11 pages, 11 figures, to be published Phys. Rev.
Nonlocal effects in Fock space
If a physical system contains a single particle, and if two distant detectors
test the presence of linear superpositions of one-particle and vacuum states, a
violation of classical locality can occur. It is due to the creation of a
two-particle component by the detecting process itself.Comment: final version in PRL 74 (1995) 4571; 76 (1996) 2205 (erratum
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