1,278 research outputs found
On open quantum systems, effective Hamiltonians and device characterization
High fidelity models, which support accurate device characterization and
correctly account for environmental effects, are crucial to the engineering of
scalable quantum technologies. As it ensures positivity of the density matrix,
one preferred model for open systems describes the dynamics with a master
equation in Lindblad form. The Linblad operators are rarely derived from first
principles, resulting in dynamical models which miss those additional terms
that must generally be added to bring the master equation into Lindblad form,
together with concomitant other terms that must be assimilated into an
effective Hamiltonian. In first principles derivations such additional terms
are often cancelled (countered), frequently in an ad hoc manner. In the case of
a Superconducting Quantum Interference Device (SQUID) coupled to an Ohmic bath,
the resulting master equation implies the environment has a significant impact
on the system's energy. We discuss the prospect of keeping or cancelling this
impact; and note that, for the SQUID, measuring the magnetic susceptibility
under control of the capacitive coupling strength and the externally applied
flux, results in experimentally measurable differences between models. If this
is not done correctly, device characterization will be prone to systemic
errors.Comment: 5 pages, 3 figure
On the time delay in binary systems
The aim of this paper is to study the time delay on electromagnetic signals
propagating across a binary stellar system. We focus on the antisymmetric
gravitomagnetic contribution due to the angular momentum of one of the stars of
the pair. Considering a pulsar as the source of the signals, the effect would
be manifest both in the arrival times of the pulses and in the frequency shift
of their Fourier spectra. We derive the appropriate formulas and we discuss the
influence of different configurations on the observability of gravitomagnetic
effects. We argue that the recently discovered PSR J0737-3039 binary system
does not permit the detection of the effects because of the large size of the
eclipsed region.Comment: 7 pages, 2 eps figures, RevTex, to appear in Physical Review
Mixed couples in France. Statistical facts, definitions, and social reality
Mixed couples are usually defined as combining national, cultural, racial or religious differences, but these definitions are generally elaborated only empirically. More recently, researchers have proposed the concept of "mixedness" which goes beyond those descriptive factors of difference. The French national Migration, Tradition and Citizenship Act also helps to show why it is not easy to reach a single definition.A detailed look at statistical realities illustrates how complex it is to count mixed couples. The figures differ if we consider mixed marriages as flows (new marriages each year) or as stock (how many people live in a mixed family). They also vary depending on what differences —cultural belonging or nationality— are taken into account. Many French-foreign marriages bring together people who have the same cultural identities. From the sociological point of view, mixed marriage should be defined as combining perceived differences and existing social order and norms. Studies on mixedness look at how socially constructed differences in contact, e.g. in private life, influence social relations and modify social realities. Mixed couples experience specific social constraints due to the migration situation of at least one of the partners, and therefore these couples need time to learn to manage the cultural and social differences in their relationship. Differences between the partners due to their cultural and social affiliations or gender roles are not equal and also influence the transcultural hybridisation process, sometimes even hindering it considerably.Una pareja mixta se define generalmente como la combinación de las diferencias nacionales, culturales, raciales o religiosas. Pero estas definiciones generalmente se elaboran sólo empíricamente. Más recientemente, los investigadores han propuesto el concepto de «mestizaje», que va más allá de los factores descriptivos de la diferencia. La tradición de migración nacional francesa y la Ley de ciudadanía también ayudan a entender por qué no es fácil llegar a una definición única. Una mirada pormenorizada a la realidad estadística ilustra lo complejo que es contar el número de parejas mixtas. Las cifras difieren si se considera el matrimonio mixto como un flujo (los matrimonios nuevos cada año) o como población (número de personas que viven en una familia mixta). También varían en función de qué diferencias se tienen en cuenta —la pertenencia cultural o la nacionalidad. Muchos matrimonios entre franceses y extranjeros unen a personas que tienen la misma identidad cultural. Desde el punto de vista sociológico, el matrimonio mixto debería definirse como la combinación de las diferencias percibidas y el orden social existente y las normas. Los estudios sobre el mestizaje tratan de cómo las diferencias sociales en contacto -por ejemplo en la vida privada- influyen en las relaciones sociales y modifican la realidad social. Las parejas mixtas están involucradas en determinadas limitaciones sociales debido a la situación de migrante de por lo menos uno de los cónyuges. Por lo tanto, estas parejas necesitan tiempo para aprender a gestionar las diferencias culturales y sociales en su relación. Las diferencias entre los cónyuges, debidas a sus filiaciones sociales y culturales y a los roles de género, no están en pie de igualdad y también influyen en el proceso de hibridación transcultural, a veces incluso obstaculizándolo considerablemente
Generalized Phase-Space Techniques to Explore Quantum Phase Transitions in Critical Quantum Spin Systems
We apply the generalized Wigner function formalism to detect and characterize
a range of quantum phase transitions in several cyclic, finite-length,
spin- one-dimensional spin-chain models, viz., the Ising and
anisotropic models in a transverse field, and the anisotropic
Heisenberg model. We make use of the finite system size to provide an
exhaustive exploration of each system's single-site, bipartite and
multi-partite correlation functions. In turn, we are able to demonstrate the
utility of phase-space techniques in witnessing and characterizing first-,
second- and infinite-order quantum phase transitions, while also enabling an
in-depth analysis of the correlations present within critical systems. We also
highlight the method's ability to capture other features of spin systems such
as ground-state factorization and critical system scaling. Finally, we
demonstrate the generalized Wigner function's utility for state verification by
determining the state of each system and their constituent sub-systems at
points of interest across the quantum phase transitions, enabling interesting
features of critical systems to be intuitively analyzed.Comment: 20 pages, 8 figure
The structure and function of complex networks
Inspired by empirical studies of networked systems such as the Internet,
social networks, and biological networks, researchers have in recent years
developed a variety of techniques and models to help us understand or predict
the behavior of these systems. Here we review developments in this field,
including such concepts as the small-world effect, degree distributions,
clustering, network correlations, random graph models, models of network growth
and preferential attachment, and dynamical processes taking place on networks.Comment: Review article, 58 pages, 16 figures, 3 tables, 429 references,
published in SIAM Review (2003
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