435 research outputs found
Elliptical orbits in the Bloch sphere
As is well known, when an SU(2) operation acts on a two-level system, its
Bloch vector rotates without change of magnitude. Considering a system composed
of two two-level systems, it is proven that for a class of nonlocal
interactions of the two subsystems including \sigma_i\otimes\sigma_j (with i,j
\in {x,y,z}) and the Heisenberg interaction, the geometric description of the
motion is particularly simple: each of the two Bloch vectors follows an
elliptical orbit within the Bloch sphere. The utility of this result is
demonstrated in two applications, the first of which bears on quantum control
via quantum interfaces. By employing nonunitary control operations, we extend
the idea of controllability to a set of points which are not necessarily
connected by unitary transformations. The second application shows how the
orbit of the coherence vector can be used to assess the entangling power of
Heisenberg exchange interaction.Comment: 9 pages, 4 figures, few corrections, J. Opt. B: Quantum Semiclass.
Opt. 7 (2005) S1-S
Bloch Equations and Completely Positive Maps
The phenomenological dissipation of the Bloch equations is reexamined in the
context of completely positive maps. Such maps occur if the dissipation arises
from a reduction of a unitary evolution of a system coupled to a reservoir. In
such a case the reduced dynamics for the system alone will always yield
completely positive maps of the density operator. We show that, for Markovian
Bloch maps, the requirement of complete positivity imposes some Bloch
inequalities on the phenomenological damping constants. For non-Markovian Bloch
maps some kind of Bloch inequalities involving eigenvalues of the damping basis
can be established as well. As an illustration of these general properties we
use the depolarizing channel with white and colored stochastic noise.Comment: Talk given at the Conference "Quantum Challenges", Falenty, Poland,
September 4-7, 2003. 21 pages, 3 figure
Completely positive maps with memory
The prevailing description for dissipative quantum dynamics is given by the
Lindblad form of a Markovian master equation, used under the assumption that
memory effects are negligible. However, in certain physical situations, the
master equation is essentially of a non-Markovian nature. This paper examines
master equations that possess a memory kernel, leading to a replacement of
white noise by colored noise. The conditions under which this leads to a
completely positive, trace-preserving map are discussed for an exponential
memory kernel. A physical model that possesses such an exponential memory
kernel is presented. This model contains a classical, fluctuating environment
based on random telegraph signal stochastic variables.Comment: 4 page
New HARPS and FEROS observations of GJ1046
In this paper we present new precise Doppler data of GJ1046 taken between
November 2005 and July 2018 with the HARPS and the FEROS high-resolution
spectographs. In addition, we provide a new stellar mass estimate of GJ1046 and
we update the orbital parameters of the GJ1046 system. These new data and
analysis could be used together with the GAIA epoch astrometry, when available,
for braking the degeneracy and revealing the true mass of the GJ1046
system.Comment: 2 pages, 1 figure, 1 table with RV data (available only in the
Astro-PH version of the paper), Accepted by RNAA
Open Quantum Dynamics: Complete Positivity and Entanglement
We review the standard treatment of open quantum systems in relation to
quantum entanglement, analyzing, in particular, the behaviour of bipartite
systems immersed in a same environment. We first focus upon the notion of
complete positivity, a physically motivated algebraic constraint on the quantum
dynamics, in relation to quantum entanglement, i.e. the existence of
statistical correlations which can not be accounted for by classical
probability. We then study the entanglement power of heat baths versus their
decohering properties, a topic of increasing importance in the framework of the
fast developing fields of quantum information, communication and computation.
The presentation is self contained and, through several examples, it offers a
detailed survey of the physics and of the most relevant and used techniques
relative to both quantum open system dynamics and quantum entanglement.Comment: LaTex, 77 page
A complex network approach to structural inequality of educational deprivation in a Latin American country
To guarantee the human right to education established by the fourth UNESCO’s Sustainable Development Goal, a deep understanding of a big set of non-linear relationships at different scales is need it, as well as to know how they impact on learning outcomes. In doing so, current methods do not provide enough evidence about interactions and, for this reason, some researchers have proposed to model education as a complex system for considering all interactions at individual level, as well as using computer simulation and network analysis to provide a comprehensive look at the educational processes, as well as to predict the outcomes of different public policies.
The highlight of this paper is modeling the structure of the inequality of a national educational system as a complex network from learning outcomes and socio-economic, ethnicity, rurality and type of school funding, for providing a better understanding and measuring of the educational gaps. This new approach might help to integrate insights improving the theoretical framework, as well as to provide valuable information about non-trivial relationships between educational and non-educational variables in order to help policymakers to implement effective solutions for the educational challenge of ensuring inclusive and equitable education.info:eu-repo/semantics/acceptedVersio
NectarCAM : a camera for the medium size telescopes of the Cherenkov Telescope Array
NectarCAM is a camera proposed for the medium-sized telescopes of the
Cherenkov Telescope Array (CTA) covering the central energy range of ~100 GeV
to ~30 TeV. It has a modular design and is based on the NECTAr chip, at the
heart of which is a GHz sampling Switched Capacitor Array and a 12-bit Analog
to Digital converter. The camera will be equipped with 265 7-photomultiplier
modules, covering a field of view of 8 degrees. Each module includes the
photomultiplier bases, high voltage supply, pre-amplifier, trigger, readout and
Ethernet transceiver. The recorded events last between a few nanoseconds and
tens of nanoseconds. The camera trigger will be flexible so as to minimize the
read-out dead-time of the NECTAr chips. NectarCAM is designed to sustain a data
rate of more than 4 kHz with less than 5\% dead time. The camera concept, the
design and tests of the various subcomponents and results of thermal and
electrical prototypes are presented. The design includes the mechanical
structure, cooling of the electronics, read-out, clock distribution, slow
control, data-acquisition, triggering, monitoring and services.Comment: In Proceedings of the 34th International Cosmic Ray Conference
(ICRC2015), The Hague, The Netherlands. All CTA contributions at
arXiv:1508.0589
Frontiers of open quantum system dynamics
We briefly examine recent developments in the field of open quantum system
theory, devoted to the introduction of a satisfactory notion of memory for a
quantum dynamics. In particular, we will consider a possible formalization of
the notion of non-Markovian dynamics, as well as the construction of quantum
evolution equations featuring a memory kernel. Connections will be drawn to the
corresponding notions in the framework of classical stochastic processes, thus
pointing to the key differences between a quantum and classical formalization
of the notion of memory effects.Comment: 15 pages, contribution to "Quantum Physics and Geometry", Lecture
Notes of the Unione Matematica Italiana 25,E. Ballico et al. (eds.
Time Asymmetric Quantum Physics
Mathematical and phenomenological arguments in favor of asymmetric time
evolution of micro-physical states are presented.Comment: Tex file with 2 figure
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