13,280 research outputs found
Decomposability of Linear Maps under Tensor Products
Both completely positive and completely copositive maps stay decomposable
under tensor powers, i.e under tensoring the linear map with itself. But are
there other examples of maps with this property? We show that this is not the
case: Any decomposable map, that is neither completely positive nor completely
copositive, will lose decomposability eventually after taking enough tensor
powers. Moreover, we establish explicit bounds to quantify when this happens.
To prove these results we use a symmetrization technique from the theory of
entanglement distillation, and analyze when certain symmetric maps become
non-decomposable after taking tensor powers. Finally, we apply our results to
construct new examples of non-decomposable positive maps, and establish a
connection to the PPT squared conjecture.Comment: 26 pages, 3 figure
All unital qubit channels are -noisy operations
We show that any unital qubit channel can be implemented by letting the input
system interact unitarily with a -dimensional environment in the maximally
mixed state and then tracing out the environment. We also provide an example
where the dimension of such an environment has to be at least .Comment: 8 pages, no picture
Femtosecond electrons probing currents and atomic structure in nanomaterials
The investigation of ultrafast electronic and structural dynamics in
low-dimensional systems like nanowires and two-dimensional materials requires
femtosecond probes providing high spatial resolution and strong interaction
with small volume samples. Low-energy electrons exhibit large scattering cross
sections and high sensitivity to electric fields, but their pronounced
dispersion during propagation in vacuum so far prevented their use as
femtosecond probe pulses in time-resolved experiments. Employing a
laser-triggered point-like source of either divergent or collimated electron
wave packets, we developed a hybrid approach for femtosecond point projection
microscopy and femtosecond low-energy electron diffraction. We investigate
ultrafast electric currents in nanowires with sub-100 femtosecond temporal and
few 10 nm spatial resolutions and demonstrate the potential of our approach for
studying structural dynamics in crystalline single-layer materials.Comment: 18 pages, 4 figures, includes 8 pages supplementary informatio
Local chain ordering in amorphous polymer melts: Influence of chain stiffness
Molecular dynamics simulation of a generic polymer model is applied to study
melts of polymers with different types of intrinsic stiffness. Important static
observables of the single chain such as gyration radius or persistence length
are determined. Additionally we investigate the overall static melt structure
including pair correlation function, structure function and orientational
correlation function.Comment: 13 pages, 15 figures, PCCP accepte
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