2,444 research outputs found
Possible observation of phase separation near a quantum phase transition in doubly connected ultrathin superconducting cylinders of aluminum
The kinetic energy of superconducting electrons in an ultrathin, doubly
connected superconducting cylinder, determined by the applied flux, increases
as the cylinder diameter decreases, leading to a destructive regime around
half-flux quanta and a superconductor to normal metal quantum phase transition
(QPT). Regular step-like features in resistance vs. temperature curves taken at
fixed flux values were observed near the QPT in ultrathin Al cylinders. It is
proposed that these features are most likely resulted from a phase separation
near the QPT in which normal regions nucleate in a homogeneous superconducting
cylinder.Comment: 4 pages, 4 figures, to appear in Phys. Rev. Let
Cloning transformations in spin networks without external control
In this paper we present an approach to quantum cloning with unmodulated spin
networks. The cloner is realized by a proper design of the network and a choice
of the coupling between the qubits. We show that in the case of phase covariant
cloner the XY coupling gives the best results. In the 1->2 cloning we find that
the value for the fidelity of the optimal cloner is achieved, and values
comparable to the optimal ones in the general N->M case can be attained. If a
suitable set of network symmetries are satisfied, the output fidelity of the
clones does not depend on the specific choice of the graph. We show that spin
network cloning is robust against the presence of static imperfections.
Moreover, in the presence of noise, it outperforms the conventional approach.
In this case the fidelity exceeds the corresponding value obtained by quantum
gates even for a very small amount of noise. Furthermore we show how to use
this method to clone qutrits and qudits. By means of the Heisenberg coupling it
is also possible to implement the universal cloner although in this case the
fidelity is 10% off that of the optimal cloner.Comment: 12 pages, 13 figures, published versio
Blockade and Counterflow Supercurrent in exciton-condensate Josephson junctions
We demonstrate that perfect conversion between charged supercurrents in
superconductors and neutral supercurrents in electron-hole pair condensates is
possible via a new Andreev-like scattering mechanism. As a result, when two
superconducting circuits are coupled through a bilayer exciton condensate, the
superflow in both layers is drastically modified. Depending on the phase biases
the supercurrents can be completely blocked or exhibit perfect drag.Comment: 4 pages, 2 figure
Qubits as devices to detect the third moment of current fluctuations
Under appropriate conditions controllable two-level systems can be used to
detect the third moment of current fluctuations. We derive a Master Equation
for a quantum system coupled to a bath valid to the third order in the coupling
between the system and the environment. In this approximation the reduced
dynamics of the quantum system depends on the frequency dependent third moment.
Specializing to the case of a controllable two-level system (a qubit) and in
the limit in which the splitting between the levels is much smaller than the
characteristic frequency of the third moment, it is possible to show that the
decay of the qubit has additional oscillations whose amplitude is directly
proportional to the value of the third moment. We discuss an experimental setup
where this effect can be seen.Comment: 10 pages, 7 figure
Shot noise of a quantum dot with non-Fermi liquid correlations
The shot noise of a one-dimensional wire interrupted by two barriers shows
interesting features related to the interplay between Coulomb blockade effects,
Luttinger correlations and discrete excitations. At small bias the Fano factor
reaches the lowest attainable value, 1/2, irrespective of the ratio of the two
junction resistances. At larger voltages this asymmetry is power-law
renormalized by the interaction strength. We discuss how the measurement of
current and these features of the noise allow to extract the Luttinger liquid
parameter.Comment: 4 pages, 3 figures,to be published in Phys. Rev. B. For high
resolution image of Fig.1 see http://server1.fisica.unige.it/~braggio/doc.ht
A Logical Verification Methodology for Service-Oriented Computing
We introduce a logical verification methodology for checking behavioural properties of service-oriented computing systems. Service properties are described by means of SocL, a branching-time temporal logic that we have specifically designed to express in an effective way distinctive aspects of services, such as, e.g., acceptance of a request, provision of a response, and correlation among service requests and responses. Our approach allows service properties to be expressed in such a way that
they can be independent of service domains and specifications. We show an instantiation of our general methodology that uses the formal language COWS to conveniently specify services and the expressly developed software tool CMC to assist the user in the task of verifying SocL formulae over service specifications. We demonstrate feasibility and effectiveness of our methodology by means of the specification and the analysis of a case study in the automotive domain
Vibrotactile pedals : provision of haptic feedback to support economical driving
The use of haptic feedback is currently an underused modality in the driving environment, especially with respect to vehicle manufacturers. This exploratory study evaluates the effects of a vibrotactile (or haptic) accelerator pedal on car driving performance and perceived workload using a driving simulator. A stimulus was triggered when the driver exceeded a 50% throttle threshold, past which is deemed excessive for economical driving. Results showed significant decreases in mean acceleration values, and maximum and excess throttle use when the haptic pedal was active as compared to a baseline condition. As well as the positive changes to driver behaviour, subjective workload decreased when driving with the haptic pedal as compared to when drivers were simply asked to drive economically. The literature suggests that the haptic processing channel offers a largely untapped resource in the driving environment, and could provide information without overloading the other attentional resource pools used in driving
On the relation of optical obscuration and X-ray absorption in Seyfert galaxies
The optical classification of a Seyfert galaxy and whether it is considered
X-ray absorbed are often used interchangeably. But there are many borderline
cases and also numerous examples where the optical and X-ray classifications
appear to be in conflict. In this article we re-visit the relation between
optical obscuration and X-ray absorption in AGNs. We make use of our "dust
color" method (Burtscher et al. 2015) to derive the optical obscuration A_V and
consistently estimated X-ray absorbing columns using 0.3--150 keV spectral
energy distributions. We also take into account the variable nature of the
neutral gas column N_H and derive the Seyfert sub-classes of all our objects in
a consistent way.
We show in a sample of 25 local, hard-X-ray detected Seyfert galaxies (log
L_X / (erg/s) ~ 41.5 - 43.5) that there can actually be a good agreement
between optical and X-ray classification. If Seyfert types 1.8 and 1.9 are
considered unobscured, the threshold between X-ray unabsorbed and absorbed
should be chosen at a column N_H = 10^22.3 / cm^2 to be consistent with the
optical classification.
We find that N_H is related to A_V and that the N_H/A_V ratio is
approximately Galactic or higher in all sources, as indicated previously. But
in several objects we also see that deviations from the Galactic ratio are only
due to a variable X-ray column, showing that (1) deviations from the Galactic
N_H/A_V can simply be explained by dust-free neutral gas within the broad line
region in some sources, that (2) the dust properties in AGNs can be similar to
Galactic dust and that (3) the dust color method is a robust way to estimate
the optical extinction towards the sublimation radius in all but the most
obscured AGNs.Comment: 7 pages, 3 figures, accepted for publication by A&A; updated PDF to
include abstrac
Decoherence by engineered quantum baths
We introduce, and determine decoherence for, a wide class of non-trivial
quantum spin baths which embrace Ising, XY and Heisenberg universality classes
coupled to a two-level system. For the XY and Ising universality classes we
provide an exact expression for the decay of the loss of coherence beyond the
case of a central spin coupled uniformly to all the spins of the baths which
has been discussed so far in the literature. In the case of the Heisenberg spin
bath we study the decoherence by means of the time-dependent density matrix
renormalization group. We show how these baths can be engineered, by using
atoms in optical lattices.Comment: 4 pages, 4 figure
A novel point mutation in a class IV glucose-6-phosphate dehydrogenase variant (G6PD São Paulo) and polymorphic G6PD variants in São Paulo State, Brazil
In this study, we used red cell glucose-6-phosphate dehydrogenase (G6PD) activity to screen for G6PD-deficient individuals in 373 unrelated asymptomatic adult men who were working with insecticides (organophosphorus and carbamate) in dengue prevention programs in 27 cities in São Paulo State, Brazil. Twenty-one unrelated male children suspected of having erythroenzymopathy who were attended at hospitals in São Paulo city were also studied. Fifteen of the 373 adults and 12 of the 21 children were G6PD deficient. G6PD gene mutations were investigated in these G6PD-deficient individuals by using PCR-RFLP, PCR-SSCP analysis and DNA sequencing. Twelve G6PD A-202A/376G and two G6PD Seattle844C, as well as a new variant identified as G6PD São Paulo, were detected among adults, and 11 G6PD A-202A/376G and one G6PD Seattle844C were found among children. The novel mutation c.660C > G caused the replacement of isoleucine by methionine (I220M) in a region near the dimer interface of the molecule. The conservative nature of this mutation (substitution of a nonpolar aliphatic amino acid for another one) could explain why there was no corresponding change in the loss of G6PD activity (64.5% of normal activity in both cases)
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