10,938 research outputs found
Nonlocality with less Complementarity
In quantum mechanics, nonlocality (a violation of a Bell inequality) is
intimately linked to complementarity, by which we mean that consistently
assigning values to different observables at the same time is not possible.
Nonlocality can only occur when some of the relevant observables do not
commute, and this noncommutativity makes the observables complementary. Beyond
quantum mechanics, the concept of complementarity can be formalized in several
distinct ways. Here we describe some of these possible formalizations and ask
how they relate to nonlocality. We partially answer this question by describing
two toy theories which display nonlocality and obey the no-signaling principle,
although each of them does not display a certain kind of complementarity. The
first toy theory has the property that it maximally violates the CHSH
inequality, although the corresponding local observables are pairwise jointly
measurable. The second toy theory also maximally violates the CHSH inequality,
although its state space is classical and all measurements are mutually
nondisturbing: if a measurement sequence contains some measurement twice with
any number of other measurements in between, then these two measurements give
the same outcome with certainty.Comment: 6 pages, published versio
The Kondo effect in bosonic spin liquids
In a metal, a magnetic impurity is fully screened by the conduction electrons
at low temperature. In contrast, impurity moments coupled to spin-1 bulk
bosons, such as triplet excitations in paramagnets, are only partially
screened, even at the bulk quantum critical point. We argue that this
difference is not due to the quantum statistics of the host particles but
instead related to the structure of the impurity-host coupling, by
demonstrating that frustrated magnets with bosonic spinon excitations can
display a bosonic version of the Kondo effect. However, the Bose statistics of
the bulk implies distinct behavior, such as a weak-coupling impurity quantum
phase transition, and perfect screening for a range of impurity spin values. We
discuss implications of our results for the compound Cs2CuCl4, as well as
possible extensions to multicomponent bosonic gases.Comment: 4 pages, 3 figures. The weak coupling RG flow was corrected and
expanded in last versio
Upper-critical dimension in a quantum impurity model: Critical theory of the asymmetric pseudogap Kondo problem
Impurity moments coupled to fermions with a pseudogap density of states
display a quantum phase transition between a screened and a free moment phase
upon variation of the Kondo coupling. We describe the universal theory of this
transition for the experimentally relevant case of particle-hole asymmetry. The
theory takes the form of a crossing between effective singlet and doublet
levels, interacting with low-energy fermions. Depending on the pseudogap
exponent, this interaction is either relevant or irrelevant under
renormalization group transformations, establishing the existence of an
upper-critical "dimension" in this impurity problem. Using perturbative
renormalization group techniques we compute various critical properties and
compare with numerical results.Comment: 4 pages, 2 figs, (v2) title changed, log corrections for r=1 adde
Correlating the nanostructure of Al-oxide with deposition conditions and dielectric contributions of two-level systems in perspective of superconducting quantum circuits
This work is concerned with Al/Al-oxide(AlO)/Al-layer systems which are
important for Josephson-junction-based superconducting devices such as quantum
bits. The device performance is limited by noise, which has been to a large
degree assigned to the presence and properties of two-level tunneling systems
in the amorphous AlO tunnel barrier. The study is focused on the
correlation of the fabrication conditions, nanostructural and nanochemical
properties and the occurrence of two-level tunneling systems with particular
emphasis on the AlO-layer. Electron-beam evaporation with two different
processes and sputter deposition were used for structure fabrication, and the
effect of illumination by ultraviolet light during Al-oxide formation is
elucidated. Characterization was performed by analytical transmission electron
microscopy and low-temperature dielectric measurements. We show that the
fabrication conditions have a strong impact on the nanostructural and
nanochemical properties of the layer systems and the properties of two-level
tunneling systems. Based on the understanding of the observed structural
characteristics, routes are derived towards the fabrication of
Al/AlO/Al-layers systems with improved properties.Comment: 28 pages, 4 figure
The impact of contributor confidence, expertise and distance on the crowdsourced land cover data quality
There is much interest in the opportunities for formal scientific investigations afforded by crowdsourcing and citizen sensing activities. However, one of the critical research issues relates to the 'quality' of the data collected in this way. This paper uses volunteer data on land cover collected under the Geo-Wiki system, where contributors label the land cover class at a series of locations, with expert labels at the same locations. It examines the statistical relationships between the accuracy of volunteer labels, their self assessed confidence in labeling, their 'experiential distance' to the location under consideration and the level of their domain expertise. The results show that distance has a minor effect on the reliability of land cover labeling, and that generally expertise has a greater effect, but not for all landcover classes
Boundary quantum criticality in models of magnetic impurities coupled to bosonic baths
We investigate quantum impurity problems, where a local magnetic moment is
coupled to the spin density of a bosonic environment, leading to bosonic
versions of the standard Kondo and Anderson impurity models. In a physical
situation, these bosonic environments can correspond either to deconfined
spinons in certain classes of Z_2 frustrated antiferromagnets, or to particles
in a multicomponent Bose gase (in which case the spin degree of freedom is
attributed to hyperfine levels). Using renormalization group techniques, we
establish that our impurity models, which feature an exchange interaction
analogous to Kondo impurities in Fermi liquids, allow the flow towards a stable
strong-coupling state. Since the low-energy bosons live around a single point
in momentum space, and there is no Fermi surface, an impurity quantum phase
transition occurs at intermediate coupling, separating screened and unscreened
phases. This behavior is qualitatively different from previously studied
spin-isotropic variants of the spin-boson model, which display stable
intermediate-coupling fixed points and no screening.Comment: 15 pages, 10 fig
Hydrodynamical simulations of a compact source scenario for G2
The origin of the dense gas cloud G2 discovered in the Galactic Center
(Gillessen et al. 2012) is still a debated puzzle. G2 might be a diffuse cloud
or the result of an outflow from an invisible star embedded in it. We present
here detailed simulations of the evolution of winds on G2's orbit. We find that
the hydrodynamic interaction with the hot atmosphere present in the Galactic
Center and the extreme gravitational field of the supermassive black hole must
be taken in account when modeling such a source scenario. We find that the
hydrodynamic interaction with the hot atmosphere present in the Galactic Center
and the extreme gravitational field of the supermassive black hole must be
taken in account when modeling such a source scenario. We also find that in
this scenario most of the Br\gamma\ luminosity is expected to come from the
highly filamentary densest shocked wind material. G2's observational properties
can be used to constrain the properties of the outflow and our best model has a
mass outflow rate of Mdot,w=8.8 x 10^{-8} Msun/yr and a wind velocity of vw =
50 km/s. These values are compatible with those of a young TTauri star wind, as
already suggested by Scoville & Burkert (2013).Comment: 4 pages, 3 figures; Proceeding of the IAU 303: "The GC: Feeding and
Feedback in a Normal Galactic Nucleus" / September 30 - October 4, 2013,
Santa Fe, New Mexico (USA
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