2,573 research outputs found
Finite-temperature Bell test for quasiparticle entanglement in the Fermi sea
We demonstrate that the Bell test cannot be realized at finite temperatures
in the vast majority of electronic setups proposed previously for quantum
entanglement generation. This fundamental difficulty is shown to originate in a
finite probability of quasiparticle emission from Fermi-sea detectors. In order
to overcome the feedback problem, we suggest a detection strategy, which takes
advantage of a resonant coupling to the quasiparticle drains. Unlike other
proposals, the designed Bell test provides a possibility to determine the
critical temperature for entanglement production in the solid state.Comment: 6 pages, 3 figures, essentially revised and extended versio
Higher-order contributions and non-perturbative effects in the non-degenerate nonlinear optical absorption of direct-gap semiconductors
The semiconductor Bloch equations for a two-band model including inter- and
intraband excitation are used to study the nonlinear absorption of single and
multiple light pulses by direct-gap semiconductors. For a consistent analysis
the contributions to the absorption originating from both the interband
polarization and the intraband current need to be included. In the Bloch
equation approach theses contributions as well as different excitation pathways
in terms of sequences of inter- and intraband excitations can be evaluated
separately which allows for a transparent analysis, the identification of the
dominant terms, and analyzing their dependence on the excitation conditions. In
the perturbative regime, we obtain analytical expressions for the multi-photon
absorption coefficients for continuous-wave excitation. These results are shown
to agree well with numerical results for short pulses and/or finite dephasing
and relaxation times and we confirm the previously predicted strong enhancement
of two-photon absorption for non-degenerate conditions for pulsed excitation.
We discuss the dependencies on the light frequencies, initial band populations,
and the time delay between the pulses. The frequency dependence of the
two-photon absorption coefficient for non-degenerate excitation is evaluated
perturbatively in third-order. The higher-order contributions to the optical
absorption include three- and four-photon absorption and show a rich frequency
dependence including negative regions and dispersive lineshapes.
Non-perturbative solutions of the Bloch equations demonstrate a strongly
non-monotonous behavior of the intensity-dependent optical absorption for a
single incident pulse and in a pump-probe set-up
Quantum Hall criticality and localization in graphene with short-range impurities at the Dirac point
We explore the longitudinal conductivity of graphene at the Dirac point in a
strong magnetic field with two types of short-range scatterers: adatoms that
mix the valleys and "scalar" impurities that do not mix them. A scattering
theory for the Dirac equation is employed to express the conductance of a
graphene sample as a function of impurity coordinates; an averaging over
impurity positions is then performed numerically. The conductivity is
equal to the ballistic value for each disorder realization
provided the number of flux quanta considerably exceeds the number of
impurities. For weaker fields, the conductivity in the presence of scalar
impurities scales to the quantum-Hall critical point with at half filling or to zero away from half filling due to the
onset of Anderson localization. For adatoms, the localization behavior is
obtained also at half filling due to splitting of the critical energy by
intervalley scattering. Our results reveal a complex scaling flow governed by
fixed points of different symmetry classes: remarkably, all key manifestations
of Anderson localization and criticality in two dimensions are observed
numerically in a single setup.Comment: 17 pages, 4 figure
Glycerol does not reduce neuronal damage in experimental streptococcus pneumonia meningitis in rabbits.
Pervasive Phylogenomic Incongruence Underlies Evolutionary Relationships in Eyebrights (Euphrasia, Orobanchaceae)
Disentangling the phylogenetic relationships of taxonomically complex plant groups is often mired by challenges associated with recent speciation, hybridization, complex mating systems, and polyploidy. Here, we perform a phylogenomic analysis of eyebrights (Euphrasia), a group renowned for taxonomic complexity, with the aim of documenting the extent of phylogenetic discordance at both deep and at shallow phylogenetic scales. We generate whole-genome sequencing data and integrate this with prior genomic data to perform a comprehensive analysis of nuclear genomic, nuclear ribosomal (nrDNA), and complete plastid genomes from 57 individuals representing 36 Euphrasia species. The species tree analysis of 3,454 conserved nuclear scaffolds (46 Mb) reveals that at shallow phylogenetic scales postglacial colonization of North Western Europe occurred in multiple waves from discrete source populations, with most species not being monophyletic, and instead combining genomic variants from across clades. At a deeper phylogenetic scale, the Euphrasia phylogeny is structured by geography and ploidy, and partially by taxonomy. Comparative analyses show Southern Hemisphere tetraploids include a distinct subgenome indicative of independent polyploidy events from Northern Hemisphere taxa. In contrast to the nuclear genome analyses, the plastid genome phylogeny reveals limited geographic structure, while the nrDNA phylogeny is informative of some geographic and taxonomic affinities but more thorough phylogenetic inference is impeded by the retention of ancestral polymorphisms in the polyploids. Overall our results reveal extensive phylogenetic discordance at both deeper and shallower nodes, with broad-scale geographic structure of genomic variation but a lack of definitive taxonomic signal. This suggests that Euphrasia species either have polytopic origins or are maintained by narrow genomic regions in the face of extensive homogenizing gene flow. Moreover, these results suggest genome skimming will not be an effective extended barcode to identify species in groups such as Euphrasia, or many other postglacial species groups
Stochastic Description of a Bistable Frustrated Unit
Mixed positive and negative feedback loops are often found in biological
systems which support oscillations. In this work we consider a prototype of
such systems, which has been recently found at the core of many genetic
circuits showing oscillatory behaviour. Our model consists of two interacting
species A and B, where A activates not only its own production, but also that
of its repressor B. While the self-activation of A leads already to a bistable
unit, the coupling with a negative feedback loop via B makes the unit
frustrated. In the deterministic limit of infinitely many molecules, such a
bistable frustrated unit is known to show excitable and oscillatory dynamics,
depending on the maximum production rate of A which acts as a control
parameter. We study this model in its fully stochastic version and we find
oscillations even for parameters which in the deterministic limit are deeply in
the fixed-point regime. The deeper we go into this regime, the more irregular
these oscillations are, becoming finally random excitations whenever
fluctuations allow the system to overcome the barrier for a large excursion in
phase space. The fluctuations can no longer be fully treated as a perturbation.
The smaller the system size (the number of molecules), the more frequent are
these excitations. Therefore, stochasticity caused by demographic noise makes
this unit even more flexible with respect to its oscillatory behaviour.Comment: 28 pages, 17 figure
Ballistic charge transport in chiral-symmetric few-layer graphene
A transfer matrix approach to study ballistic charge transport in few-layer
graphene with chiral-symmetric stacking configurations is developed. We
demonstrate that the chiral symmetry justifies a non-Abelian gauge
transformation at the spectral degeneracy point (zero energy). This
transformation proves the equivalence of zero-energy transport properties of
the multilayer to those of the system of uncoupled monolayers. Similar
transformation can be applied in order to gauge away an arbitrary magnetic
field, weak strain, and hopping disorder in the bulk of the sample. Finally, we
calculate the full-counting statistics at arbitrary energy for different
stacking configurations. The predicted gate-voltage dependence of conductance
and noise can be measured in clean multilayer samples with generic metallic
leads.Comment: 6 pages, 5 figures; EPL published versio
Spin dynamics in the optical cycle of single nitrogen-vacancy centres in diamond
We investigate spin-dependent decay and intersystem crossing in the optical
cycle of single negatively-charged nitrogen-vacancy (NV) centres in diamond. We
use spin control and pulsed optical excitation to extract both the
spin-resolved lifetimes of the excited states and the degree of
optically-induced spin polarization. By optically exciting the centre with a
series of picosecond pulses, we determine the spin-flip probabilities per
optical cycle, as well as the spin-dependent probability for intersystem
crossing. This information, together with the indepedently measured decay rate
of singlet population provides a full description of spin dynamics in the
optical cycle of NV centres. The temperature dependence of the singlet
population decay rate provides information on the number of singlet states
involved in the optical cycle.Comment: 11 pages, 5 figure
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