13,655 research outputs found
Streamlined islands and the English Channel megaflood hypothesis
Recognising ice-age catastrophic megafloods is important because they had significant impact on large-scale drainage evolution and patterns of water and sediment movement to the oceans, and likely induced very rapid, short-term effects on climate. It has been previously proposed that a drainage system on the floor of the English Channel was initiated by catastrophic flooding in the Pleistocene but this suggestion has remained controversial. Here we examine this hypothesis through an analysis of key landform features. We use a new compilation of multi- and single-beam bathymetry together with sub-bottom profiler data to establish the internal structure, planform geometry and hence origin of a set of 36 mid-channel islands. Whilst there is evidence of modern-day surficial sediment processes, the majority of the islands can be clearly demonstrated to be formed of bedrock, and are hence erosional remnants rather than depositional features. The islands display classic lemniscate or tear-drop outlines, with elongated tips pointing downstream, typical of streamlined islands formed during high-magnitude water flow. The length-to-width ratio for the entire island population is 3.4 ± 1.3 and the degree-of-elongation or k-value is 3.7 ± 1.4. These values are comparable to streamlined islands in other proven Pleistocene catastrophic flood terrains and are distinctly different to values found in modern-day rivers. The island geometries show a correlation with bedrock type: with those carved from Upper Cretaceous chalk having larger length-to-width ratios (3.2 ± 1.3) than those carved into more mixed Paleogene terrigenous sandstones, siltstones and mudstones (3.0 ± 1.5). We attribute these differences to the former rock unit having a lower skin friction which allowed longer island growth to achieve minimum drag. The Paleogene islands, although less numerous than the Chalk islands, also assume more perfect lemniscate shapes. These lithologies therefore reached island equilibrium shape more quickly but were also susceptible to total erosion. Our observations support the hypothesis that the islands were initially carved by high-water volume flows via a unique catastrophic drainage of a pro-glacial lake in the southern North Sea at the Dover Strait rather than by fluvial erosion throughout the Pleistocene
Exponential improvement in photon storage fidelities using subradiance and "selective radiance" in atomic arrays
A central goal within quantum optics is to realize efficient interactions
between photons and atoms. A fundamental limit in nearly all applications based
on such systems arises from spontaneous emission, in which photons are absorbed
by atoms and then re-scattered into undesired channels. In typical treatments
of atomic ensembles, it is assumed that this re-scattering occurs
independently, and at a rate given by a single isolated atom, which in turn
gives rise to standard limits of fidelity in applications such as quantum
memories or quantum gates. However, this assumption can be violated. In
particular, spontaneous emission of a collective atomic excitation can be
significantly suppressed through strong interference in emission. Thus far the
physics underlying the phenomenon of subradiance and techniques to exploit it
have not been well-understood. In this work, we provide a comprehensive
treatment of this problem. First, we show that in ordered atomic arrays in free
space, subradiant states acquire an interpretation in terms of optical modes
that are guided by the array, which only emit due to scattering from the ends
of the finite chain. We also elucidate the properties of subradiant states in
the many-excitation limit. Finally, we introduce the new concept of selective
radiance. Whereas subradiant states experience a reduced coupling to all
optical modes, selectively radiant states are tailored to simultaneously
radiate efficiently into a desired channel while scattering into undesired
channels is suppressed, thus enabling an enhanced atom-light interface. We show
that these states naturally appear in chains of atoms coupled to nanophotonic
structures, and we analyze the performance of photon storage exploiting such
states. We find that selectively radiant states allow for a photon storage
error that scales exponentially better with number of atoms than previously
known bounds.Comment: Fixed minor typos, is now analogous to published versio
Entanglement of two qubits mediated by one-dimensional plasmonic waveguides
We investigate qubit-qubit entanglement mediated by plasmons supported by
one-dimensional waveguides. We explore both the situation of spontaneous
formation of entanglement from an unentangled state and the emergence of driven
steady-state entanglement under continuous pumping. In both cases, we show that
large values for the concurrence are attainable for qubit-qubit distances
larger than the operating wavelength by using plasmonic waveguides that are
currently available.Comment: 4 pages, 4 figures. Minor Changes. Journal Reference added.
Highlighted in Physic
Virología, epidemiología y mecanismos de transmisión del VHB
Illness due to the hepatitis B virus is an enormous problem for international public health, affecting over 300 million persons throughout the world, although its prevalence varies considerably between different geographic areas. The universal vaccination of the newly born, promulgated by the World Health Organisation, has made it possible to partially control the spread of the virus in many countries, including Spain. However, the vaccine does not generate protective antibodies in approximately 5% of the population. Besides, infection by the hepatitis B virus can produce few symptoms and the virus is easily transmitted, making its epidemiological control difficult. On the other hand, the growing flow of bi-directional migration of persons between geographical areas with a moderate or high prevalence and Spain also seems to be contributing to the persistence of the disease in our milieu. All of this makes it compulsory to deploy an ensemble of preventive measures based on an increasingly deep understanding of the biological cycle of the virus. In Spain, as in other regions with a low prevalence, the hepatitis B virus infection is considered chiefly to be a disease of sexual transmission, or else contagion takes place between patients who are users of intravenous drugs. On the other hand, blood transmission is of scarce importance in our milieu. Following exposure to the hepatitis B virus, prophylaxis with immunoglobins and also the administration of the vaccine is highly effective, and acquires special interest for all healthcare workers
Geometrically induced modification of surface plasmons in the optical and telecom regimes
We demonstrate that the introduction of a subwavelength periodic modulation
into a metallic structure strongly modifies the guiding characteristics of the
surface plasmon modes supported by the system. Moreover, it is also shown how a
new type of a tightly confined surface plasmon polariton mode can be created by
just milling a periodic corrugation into a metallic ridge placed on top of a
metal surface
Faddeev eigenfunctions for point potentials in two dimensions
We present explicit formulas for the Faddeev eigenfunctions and related
generalized scattering data for point (delta-type) potentials in two
dimensions. In particular, we obtain the first explicit examples of such
eigenfunctions with contour singularity in spectral parameter at a fixed real
energy
Sentiment cascades in the 15M movement
Recent grassroots movements have suggested that online social networks might
play a key role in their organization, as adherents have a fast, many-to-many,
communication channel to help coordinate their mobilization. The structure and
dynamics of the networks constructed from the digital traces of protesters have
been analyzed to some extent recently. However, less effort has been devoted to
the analysis of the semantic content of messages exchanged during the protest.
Using the data obtained from a microblogging service during the brewing and
active phases of the 15M movement in Spain, we perform the first large scale
test of theories on collective emotions and social interaction in collective
actions. Our findings show that activity and information cascades in the
movement are larger in the presence of negative collective emotions and when
users express themselves in terms related to social content. At the level of
individual participants, our results show that their social integration in the
movement, as measured through social network metrics, increases with their
level of engagement and of expression of negativity. Our findings show that
non-rational factors play a role in the formation and activity of social
movements through online media, having important consequences for viral
spreading.Comment: EPJ Data Science vol 4 (2015) (forthcoming
Optimization of photon storage fidelity in ordered atomic arrays
A major application for atomic ensembles consists of a quantum memory for
light, in which an optical state can be reversibly converted to a collective
atomic excitation on demand. There exists a well-known fundamental bound on the
storage error, when the ensemble is describable by a continuous medium governed
by the Maxwell-Bloch equations. The validity of this model can break down,
however, in systems such as dense, ordered atomic arrays, where strong
interference in emission can give rise to phenomena such as subradiance and
"selective" radiance. Here, we develop a general formalism that finds the
maximum storage efficiency for a collection of atoms with discrete, known
positions, and a given spatial mode in which an optical field is sent. As an
example, we apply this technique to study a finite two-dimensional square array
of atoms. We show that such a system enables a storage error that scales with
atom number like ,
and that, remarkably, an array of just atoms in principle allows
for an efficiency comparable to a disordered ensemble with optical depth of
around 600.Comment: paper is now identical to published versio
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