226,982 research outputs found
Excitation and Entanglement Transfer Near Quantum Critical Points
Recently, there has been growing interest in employing condensed matter
systems such as quantum spin or harmonic chains as quantum channels for short
distance communication. Many properties of such chains are determined by the
spectral gap between their ground and excited states. In particular this gap
vanishes at critical points of quantum phase transitions. In this article we
study the relation between the transfer speed and quality of such a system and
the size of its spectral gap. We find that the transfer is almost perfect but
slow for large spectral gaps and fast but rather inefficient for small gaps.Comment: submitted to Optics and Spectroscopy special issue for ICQO'200
Study of a Class of Four Dimensional Nonsingular Cosmological Bounces
We study a novel class of nonsingular time-symmetric cosmological bounces. In
this class of four dimensional models the bounce is induced by a perfect fluid
with a negative energy density. Metric perturbations are solved in an analytic
way all through the bounce. The conditions for generating a scale invariant
spectrum of tensor and scalar metric perturbations are discussed.Comment: 16 pages, 10 figure
Submillimetre-sized dust aggregate collision and growth properties
The collisional and sticking properties of sub-mm-sized aggregates composed
of protoplanetary dust analogue material are measured, including the
statistical threshold velocity between sticking and bouncing, their surface
energy and tensile strength within aggregate clusters. We performed an
experiment on the REXUS 12 suborbital rocket. The protoplanetary dust analogue
materials were micrometre-sized monodisperse and polydisperse SiO2 particles
prepared into aggregates with sizes around 120 m and 330 m,
respectively and volume filling factors around 0.37. During the experimental
run of 150 s under reduced gravity conditions, the sticking of aggregates and
the formation and fragmentation of clusters of up to a few millimetres in size
was observed. The sticking probability of the sub-mm-sized dust aggregates
could be derived for velocities decreasing from 22 to 3 cm/s. The transition
from bouncing to sticking collisions happened at 12.7 cm/s for the smaller
aggregates composed of monodisperse particles and at 11.5 and 11.7 cm/s for the
larger aggregates composed of mono- and polydisperse dust particles,
respectively. Using the pull-off force of sub-mm-sized dust aggregates from the
clusters, the surface energy of the aggregates composed of monodisperse dust
was derived to be 1.6x10-5 J/m2, which can be scaled down to 1.7x10-2 J/m2 for
the micrometre-sized monomer particles and is in good agreement with previous
measurements for silica particles. The tensile strengths of these aggregates
within the clusters were derived to be 1.9 Pa and 1.6 Pa for the small and
large dust aggregates, respectively. These values are in good agreement with
recent tensile strength measurements for mm-sized silica aggregates. Using our
data on the sticking-bouncing threshold, estimates of the maximum aggregate
size can be given. For a minimum mass solar nebula model, aggregates can reach
sizes of 1 cm.Comment: 21 pages (incl. 6 pages of appendix), 23 figure
Enhanced secure key exchange systems based on the Johnson-noise scheme
We introduce seven new versions of the Kirchhoff-Law-Johnson-(like)-Noise
(KLJN) classical physical secure key exchange scheme and a new transient
protocol for practically-perfect security. While these practical improvements
offer progressively enhanced security and/or speed for the non-ideal
conditions, the fundamental physical laws providing the security remain the
same.
In the "intelligent" KLJN (iKLJN) scheme, Alice and Bob utilize the fact that
they exactly know not only their own resistor value but also the stochastic
time function of their own noise, which they generate before feeding it into
the loop.
In the "multiple" KLJN (MKLJN) system, Alice and Bob have publicly known
identical sets of different resistors with a proper, publicly known truth table
about the bit-interpretation of their combination. In the "keyed" KLJN (KKLJN)
system, by using secure communication with a formerly shared key, Alice and Bob
share a proper time-dependent truth table for the bit-interpretation of the
resistor situation for each secure bit exchange step during generating the next
key.
The remaining four KLJN schemes are the combinations of the above protocols
to synergically enhance the security properties. These are: the
"intelligent-multiple" (iMKLJN), the "intelligent-keyed" (iKKLJN), the
"keyed-multiple" (KMKLJN) and the "intelligent-keyed-multiple" (iKMKLJN) KLJN
key exchange systems.
Finally, we introduce a new transient-protocol offering practically-perfect
security without privacy amplification, which is not needed at practical
applications but it is shown for the sake of ongoing discussions.Comment: This version is accepted for publicatio
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