500 research outputs found
Reexamination of continuous fuzzy measurement on two-level systems
Imposing restrictions on the Feynman paths of the monitored system has in the past been proposed as a universal model-free approach to continuous quantum measurements. Here we revisit this proposition and demonstrate that a Gaussian restriction, resulting in a sequence of many highly inaccurate (weak) von Neumann measurements, is not sufficiently strong to ensure proximity between a readout and the Feynman paths along which the monitored system evolves. Rather, in the continuous limit, the variations of a typical readout become much larger than the separation between the eigenvalues of the measured quantity. Thus, a typical readout is not represented by a nearly constant curve, correlating with one of the eigenvalues of the measured quantity , even when decoherence or Zeno effect is achieved for the observed two-level system, and does not point directly to the system's final state. We show that the decoherence in a ``free'' system can be seen as induced by a Gaussian random walk with a drift, eventually directing the system towards one of the eigenstates of . A similar mechanism appears to be responsible for the Zeno effect in a driven system, when its Rabi oscillations are quenched by monitoring. Alongside the Gaussian case, which can only be studied numerically, we also consider a fully tractable model with a ``hard wall'' restriction and show the results to be similar.MINECO, Fondo Europeo de Desarrollo Regional FEDER, Grant No. FIS2015-67161-P (MINECO/FEDER) (D.S.), MINECO Grant No. SVP-2014-068451 (S.R.), MINECO Grant No. MTM2013-46553-C3-1-P (E.A.), SGI/IZOSGIker UPV/EHU, i2BASQUE academic network
Explicit solution for a Gaussian wave packet impinging on a square barrier
The collision of a quantum Gaussian wave packet with a square barrier is
solved explicitly in terms of known functions. The obtained formula is suitable
for performing fast calculations or asymptotic analysis. It also provides
physical insight since the description of different regimes and collision
phenomena typically requires only some of the terms.Comment: To be published in J. Phys.
Ice-stream flow switching by up-ice propagation of instabilities along glacial marginal troughs
Ice-stream
networks constitute the arteries of ice sheets through which large volumes of
glacial ice are rapidly delivered from the continent to the ocean.
Modifications in ice-stream networks have a major impact on ice sheet mass
balance and global sea level. Reorganizations in the drainage network of ice
streams have been reported in both modern and paleo-ice sheets and usually
result in ice streams switching their trajectory and/or shutting down. While
some hypotheses for the reorganization of ice streams have been proposed, the
mechanisms that control the switching of ice streams remain poorly understood
and documented. Here, we interpret a flow switch in an ice-stream system that
occurred prior to the last glaciation on the northeastern Baffin Island shelf
(Arctic Canada) through glacial erosion of a marginal trough, i.e., deep
parallel-to-coast bedrock moats located up-ice of a cross-shelf trough. Shelf
geomorphology imaged by high-resolution swath bathymetry and
seismo-stratigraphic data in the area indicate the extension of ice streams
from Scott and Hecla & Griper troughs towards the interior of the
Laurentide Ice Sheet. Up-ice propagation of ice streams through a marginal
trough is interpreted to have led to the piracy of the neighboring ice
catchment that in turn induced an adjacent ice-stream flow switch and
shutdown. These results suggest that competition for ice discharge between
the two ice streams, which implies piracy of ice drainage basins via marginal
troughs, was the driving mechanism behind ice flow switching. In turn, the
enlargement of the ice catchment by piracy increased the volume and discharge
of Scott Ice Stream, allowing it to erode deeper and flow farther on the
continental shelf. Similar trough systems observed on many other glaciated
continental shelves may be the product of such competition for ice discharge
between catchments.</p
Time scale of forerunners in quantum tunneling
The forerunners preceding the main tunneling signal of the wave created by a
source with a sharp onset or by a quantum shutter, have been generally
associated with over-the-barrier (non-tunneling) components. We demonstrate
that, while this association is true for distances which are larger than the
penetration lenght, for smaller distances the forerunner is dominated by
under-the-barrier components. We find that its characteristic arrival time is
inversely proportional to the difference between the barrier energy and the
incidence energy, a tunneling time scale different from both the phase time and
the B\"uttiker-Landauer (BL) time.Comment: Revtex4, 14 eps figure
Logos indicating environmental sustainability in wine production: an exploratory study on how do Burgundy wine consumers perceive them.
[possui errata, v. 66, dez. 2014, p. 515]
Quantile Motion and Tunneling
The concepts of quantile position, trajectory, and velocity are defined. For
a tunneling quantum mechanical wave packet, it is proved that its quantile
position always stays behind that of a free wave packet with the same initial
parameters. In quantum mechanics the quantile trajectories are mathematically
identical to Bohm's trajectories. A generalization to three dimensions is
given.Comment: 13 pages, LaTeX, elsart, 3 ps figures, submitted to Phys. Lett.
Action scales for quantum decoherence and their relation to structures in phase space
A characteristic action is defined whose magnitude determines some
properties of the expectation value of a general quantum displacement operator.
These properties are related to the capability of a given environmental
`monitoring' system to induce decoherence in quantum systems coupled to it. We
show that the scale for effective decoherence is given by . We relate this characteristic action with a complementary
quantity, , and analyse their connection with the main features of
the pattern of structures developed by the environmental state in different
phase space representations. The relevance of the -action scale is
illustrated using both a model quantum system solved numerically and a set of
model quantum systems for which analytical expressions for the time-averaged
expectation value of the displacement operator are obtained explicitly.Comment: 12 pages, 3 figure
Composite absorbing potentials
The multiple scattering interferences due to the addition of several
contiguous potential units are used to construct composite absorbing potentials
that absorb at an arbitrary set of incident momenta or for a broad momentum
interval.Comment: 9 pages, Revtex, 2 postscript figures. Accepted in Phys. Rev. Let
Time dependence of evanescent quantum waves
The time dependence of quantum evanescent waves generated by a point source
with an infinite or a limited frequency band is analyzed. The evanescent wave
is characterized by a forerunner (transient) related to the precise way the
source is switched on. It is followed by an asymptotic, monochromatic wave
which at long times reveals the oscillation frequency of the source. For a
source with a sharp onset the forerunner is exponentially larger than the
monochromatic solution and a transition from the transient regime to the
asymtotic regime occurs only at asymptotically large times. In this case, the
traversal time for tunneling plays already a role only in the transient regime.
To enhance the monochromatic solution compared to the forerunner we investigate
(a) frequency band limited sources and (b) the short time Fourier analysis (the
spectrogram) corresponding to a detector which is frequency band limited.
Neither of these two methods leads to a precise determination of the traversal
time. However, if they are limited to determine the traversal time only with a
precision of the traversal time itself both methods are successful: In this
case the transient behavior of the evanescent waves is at a time of the order
of the traversal time followed by a monochromatic wave which reveals the
frequency of the source.Comment: 16 text pages and 9 postscript figure
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