702 research outputs found
Phase retrapping in aφJosephson junction: onset of the butterfly effect
We investigate experimentally the retrapping of the phase in a
φ
Josephson junction upon return of the junction to the zero-voltage state. Since the Josephson energy profile
U
0
(
ψ
)
in
φ
JJ is a
2
π
periodic double-well potential with minima at
ψ
=
±
φ
mod
2
π
, the question is at which of the two minima
−
φ
or
+
φ
the phase will be trapped upon return from a finite voltage state during quasistatic decrease of the bias current (tilt of the potential). By measuring the relative population of two peaks in escape histograms, we determine the probability of phase trapping in the
±
φ
wells for different temperatures. Our experimental results agree qualitatively with theoretical predictions. In particular, we observe an onset of the butterfly effect with an oscillating probability of trapping. Unexpectedly, this probability saturates at a value different from 50% at low temperatures
Analytical calculation of the excess current in the OTBK theory
We present an analytical derivation of the excess current in Josephson
junctions within the Octavio-Tinkham-Blonder-Klapwijk theory for both symmetric
and asymmetric barrier strengths. We confirm the result found numerically by
Flensberg et al. for equal barriers [Phys. Rev. B 38, 8707 (1988)], including
the prediction of negative excess current for low transparencies, and we
generalize it for differing barriers. Our analytical formulae provide for
convenient fitting of experimental data, also in the less studied, but
practically relevant case of the barrier asymmetry.Comment: 13 pages, 3 figures, submitted to Superconductor Science and
Technolog
Multi-Particle Universal Processes
We generalize bipartite universal processes to the subclass of multi-particle universal processes from one to N particles. We show how the general statement for a multi-particle universal process can be constructed. The one-parameter family of processes generating totally anti-symmetric states has been generalized to a multi-particle regime and its entanglement properties have been studied. A view is given on the complete positivity and the possible physical realization of universal processes.
Completely positive covariant two-qubit quantum processes and optimal quantum NOT operations for entangled qubit pairs
The structure of all completely positive quantum operations is investigated
which transform pure two-qubit input states of a given degree of entanglement
in a covariant way. Special cases thereof are quantum NOT operations which
transform entangled pure two-qubit input states of a given degree of
entanglement into orthogonal states in an optimal way. Based on our general
analysis all covariant optimal two-qubit quantum NOT operations are determined.
In particular, it is demonstrated that only in the case of maximally entangled
input states these quantum NOT operations can be performed perfectly.Comment: 14 pages, 2 figure
Electron transport in single wall carbon nanotube weak links in the Fabry-Perot regime
We fabricated reproducible high transparency superconducting contacts
consisting of superconducting Ti/Al/Ti trilayers to gated single-walled carbon
nanotubes (SWCNTs). The reported semiconducting SWCNT have normal state
differential conductance up to and exhibit clear Fabry-Perot
interference patterns in the bias spectroscopy plot. We observed subharmonic
gap structure in the differential conductance and a distinct peak in the
conductance at zero bias which is interpreted as a manifestation of a
supercurrent. The gate dependence of this supercurrent as well as the excess
current are examined and compared to a coherent theory of superconducting point
contacts with good agreement.Comment: 10 pages, 4 figure
Optimal copying of entangled two-qubit states
We investigate the problem of copying pure two-qubit states of a given degree
of entanglement in an optimal way. Completely positive covariant quantum
operations are constructed which maximize the fidelity of the output states
with respect to two separable copies. These optimal copying processes hint at
the intricate relationship between fundamental laws of quantum theory and
entanglement.Comment: 13 pages, 7 figure
On apparent breaking the second law of thermodynamics in quantum transport studies
We consider a model for stationary electronic transport through a
one-dimensional chain of two leads attached to a perturbed central region
(quantum dot) in the regime where the theory proposed recently by Capek for a
similar model of phonon transport predicts the striking phenomenon of a
permanent current between the leads. This result based on a rigorous but
asymptotic Davies theory is at variance with the zero current yielded by direct
transport calculations which can be carried out in the present model. We find
the permanent current to be within the error of the asymptotic expansion for
finite couplings, and identify cancelling terms of the same order.Comment: 5 pages, 3 figure
Defining rural areas of Visegrad countries
The article is focused on the introduction and categorization of various approaches to rurality, and
the identification and delimitation of rural areas in Visegrad countries. Three substantively different groups of
conceptualizations and definitions of rural and rurality are described as follows: functional definitions, rural as
locality (political-economic approaches), and social representation. Latter, basic sorts of methods and appro
-
aches to the delimitation of rural areas in V4 countries are introduced emphasizing its historical development,
differences in spatial level and criteria of delimitation in current research. Due to different nature of rural areas
and even local administrative units (the basic units usually used for delimitation of rural areas), it is not possible
to reach sufficient and reliable identification of rural areas for whole V4 area using any of criteria or definitions
applied in the research at national levels. Therefore, the average population density of entire V4 area was used
as a main criterion for distinguishing between urban and rural LAU 2 at the whole Visegrad area level. Such
approach is also affected by generalization but it captures various conditions in each country relatively well and
moreover, it is comparable with the OECD and European Union methods to some extent
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