1,305 research outputs found
Coherent Electron-Phonon Coupling in Tailored Quantum Systems
The coupling between a two-level system and its environment leads to
decoherence. Within the context of coherent manipulation of electronic or
quasiparticle states in nanostructures, it is crucial to understand the sources
of decoherence. Here, we study the effect of electron-phonon coupling in a
graphene and an InAs nanowire double quantum dot. Our measurements reveal
oscillations of the double quantum dot current periodic in energy detuning
between the two levels. These periodic peaks are more pronounced in the
nanowire than in graphene, and disappear when the temperature is increased. We
attribute the oscillations to an interference effect between two alternative
inelastic decay paths involving acoustic phonons present in these materials.
This interpretation predicts the oscillations to wash out when temperature is
increased, as observed experimentally.Comment: 11 pages, 4 figure
Explicit Model Checking of Very Large MDP using Partitioning and Secondary Storage
The applicability of model checking is hindered by the state space explosion
problem in combination with limited amounts of main memory. To extend its
reach, the large available capacities of secondary storage such as hard disks
can be exploited. Due to the specific performance characteristics of secondary
storage technologies, specialised algorithms are required. In this paper, we
present a technique to use secondary storage for probabilistic model checking
of Markov decision processes. It combines state space exploration based on
partitioning with a block-iterative variant of value iteration over the same
partitions for the analysis of probabilistic reachability and expected-reward
properties. A sparse matrix-like representation is used to store partitions on
secondary storage in a compact format. All file accesses are sequential, and
compression can be used without affecting runtime. The technique has been
implemented within the Modest Toolset. We evaluate its performance on several
benchmark models of up to 3.5 billion states. In the analysis of time-bounded
properties on real-time models, our method neutralises the state space
explosion induced by the time bound in its entirety.Comment: The final publication is available at Springer via
http://dx.doi.org/10.1007/978-3-319-24953-7_1
How to realize a robust practical Majorana chain in a quantum dot-superconductor linear array
Semiconducting nanowires in proximity to superconductors are promising
experimental systems for Majorana fermions, which may ultimately be used as
building blocks for topological quantum computers. A serious challenge in the
experimental realization of the Majorana fermions is the supression of
topological superconductivity by disorder. We show that Majorana fermions
protected by a robust topological gap can occur at the ends of a chain of
quantum dots connected by s-wave superconductors. In the appropriate parameter
regime, we establish that the quantum dot/superconductor system is equivalent
to a 1D Kitaev chain, which can be tuned to be in a robust topological phase
with Majorana end modes even in the case where the quantum dots and
superconductors are both strongly disordered. Such a spin-orbit coupled quantum
dot - s-wave superconductor array provides an ideal experimental platform for
the observation of non-Abelian Majorana modes.Comment: 8 pages; 3 figures; version 2: Supplementary material updated to
include more general proof for localized Majorana fermion
Two attacks on rank metric code-based schemes: RankSign and an Identity-Based-Encryption scheme
RankSign [GRSZ14a] is a code-based signature scheme proposed to the NIST
competition for quantum-safe cryptography [AGHRZ17] and, moreover, is a
fundamental building block of a new Identity-Based-Encryption (IBE) [GHPT17a].
This signature scheme is based on the rank metric and enjoys remarkably small
key sizes, about 10KBytes for an intended level of security of 128 bits.
Unfortunately we will show that all the parameters proposed for this scheme in
[AGHRZ17] can be broken by an algebraic attack that exploits the fact that the
augmented LRPC codes used in this scheme have very low weight codewords.
Therefore, without RankSign the IBE cannot be instantiated at this time. As a
second contribution we will show that the problem is deeper than finding a new
signature in rank-based cryptography, we also found an attack on the generic
problem upon which its security reduction relies. However, contrarily to the
RankSign scheme, it seems that the parameters of the IBE scheme could be chosen
in order to avoid our attack. Finally, we have also shown that if one replaces
the rank metric in the [GHPT17a] IBE scheme by the Hamming metric, then a
devastating attack can be found
Molecular Characterization of the Gastrula in the Turtle Emys orbicularis: An Evolutionary Perspective on Gastrulation
Due to the presence of a blastopore as in amphibians, the turtle has been suggested to exemplify a transition form from an amphibian- to an avian-type gastrulation pattern. In order to test this hypothesis and gain insight into the emergence of the unique characteristics of amniotes during gastrulation, we have performed the first molecular characterization of the gastrula in a reptile, the turtle Emys orbicularis. The study of Brachyury, Lim1, Otx2 and Otx5 expression patterns points to a highly conserved dynamic of expression with amniote model organisms and makes it possible to identify the site of mesoderm internalization, which is a long-standing issue in reptiles. Analysis of Brachyury expression also highlights the presence of two distinct phases, less easily recognizable in model organisms and respectively characterized by an early ring-shaped and a later bilateral symmetrical territory. Systematic comparisons with tetrapod model organisms lead to new insights into the relationships of the blastopore/blastoporal plate system shared by all reptiles, with the blastopore of amphibians and the primitive streak of birds and mammals. The biphasic Brachyury expression pattern is also consistent with recent models of emergence of bilateral symmetry, which raises the question of its evolutionary significance
Controlling spin relaxation with a cavity
Spontaneous emission of radiation is one of the fundamental mechanisms by
which an excited quantum system returns to equilibrium. For spins, however,
spontaneous emission is generally negligible compared to other non-radiative
relaxation processes because of the weak coupling between the magnetic dipole
and the electromagnetic field. In 1946, Purcell realized that the spontaneous
emission rate can be strongly enhanced by placing the quantum system in a
resonant cavity -an effect which has since been used extensively to control the
lifetime of atoms and semiconducting heterostructures coupled to microwave or
optical cavities, underpinning single-photon sources. Here we report the first
application of these ideas to spins in solids. By coupling donor spins in
silicon to a superconducting microwave cavity of high quality factor and small
mode volume, we reach for the first time the regime where spontaneous emission
constitutes the dominant spin relaxation mechanism. The relaxation rate is
increased by three orders of magnitude when the spins are tuned to the cavity
resonance, showing that energy relaxation can be engineered and controlled
on-demand. Our results provide a novel and general way to initialise spin
systems into their ground state, with applications in magnetic resonance and
quantum information processing. They also demonstrate that, contrary to popular
belief, the coupling between the magnetic dipole of a spin and the
electromagnetic field can be enhanced up to the point where quantum
fluctuations have a dramatic effect on the spin dynamics; as such our work
represents an important step towards the coherent magnetic coupling of
individual spins to microwave photons.Comment: 8 pages, 6 figures, 1 tabl
Sunscreens - Which and what for?
It is well established that sun exposure is the main cause for the development of skin cancer. Chronic continuous UV radiation is believed to induce malignant melanoma, whereas intermittent high-dose UV exposure contributes to the occurrence of actinic keratosis as precursor lesions of squamous cell carcinoma as well as basal cell carcinoma. Not only photocarcinogenesis but also the mechanisms of photoaging have recently become apparent. In this respect the use of sunscreens seemed to prove to be more and more important and popular within the last decades. However, there is still inconsistency about the usefulness of sunscreens. Several studies show that inadequate use and incomplete UV spectrum efficacy may compromise protection more than previously expected. The sunscreen market is crowded by numerous products. Inorganic sunscreens such as zinc oxide and titanium oxide have a wide spectral range of activity compared to most of the organic sunscreen products. It is not uncommon for organic sunscreens to cause photocontact allergy, but their cosmetic acceptability is still superior to the one given by inorganic sunscreens. Recently, modern galenic approaches such as micronization and encapsulation allow the development of high-quality inorganic sunscreens. The potential systemic toxicity of organic sunscreens has lately primarily been discussed controversially in public, and several studies show contradictory results. Although a matter of debate, at present the sun protection factor (SPF) is the most reliable information for the consumer as a measure of sunscreen filter efficacy. In this context additional tests have been introduced for the evaluation of not only the protective effect against erythema but also protection against UV-induced immunological and mutational effects. Recently, combinations of UV filters with agents active in DNA repair have been introduced in order to improve photoprotection. This article reviews the efficacy of sunscreens in the prevention of epithelial and nonepithelial skin cancer, the effect on immunosuppression and the value of the SPF as well as new developments on the sunscreen market. Copyright (C) 2005 S. Karger AG, Basel
Strong Interactions of Single Atoms and Photons near a Dielectric Boundary
Modern research in optical physics has achieved quantum control of strong
interactions between a single atom and one photon within the setting of cavity
quantum electrodynamics (cQED). However, to move beyond current
proof-of-principle experiments involving one or two conventional optical
cavities to more complex scalable systems that employ N >> 1 microscopic
resonators requires the localization of individual atoms on distance scales <
100 nm from a resonator's surface. In this regime an atom can be strongly
coupled to a single intracavity photon while at the same time experiencing
significant radiative interactions with the dielectric boundaries of the
resonator. Here, we report an initial step into this new regime of cQED by way
of real-time detection and high-bandwidth feedback to select and monitor single
Cesium atoms localized ~100 nm from the surface of a micro-toroidal optical
resonator. We employ strong radiative interactions of atom and cavity field to
probe atomic motion through the evanescent field of the resonator. Direct
temporal and spectral measurements reveal both the significant role of
Casimir-Polder attraction and the manifestly quantum nature of the atom-cavity
dynamics. Our work sets the stage for trapping atoms near micro- and
nano-scopic optical resonators for applications in quantum information science,
including the creation of scalable quantum networks composed of many
atom-cavity systems that coherently interact via coherent exchanges of single
photons.Comment: 8 pages, 5 figures, Supplemental Information included as ancillary
fil
The formation of professional identity in medical students: considerations for educators
<b>Context</b> Medical education is about more than acquiring an appropriate level of knowledge and developing relevant skills. To practice medicine students need to develop a professional identity – ways of being and relating in professional contexts.<p></p>
<b>Objectives</b> This article conceptualises the processes underlying the formation and maintenance of medical students’ professional identity drawing on concepts from social psychology.<p></p>
<b>Implications</b> A multi-dimensional model of identity and identity formation, along with the concepts of identity capital and multiple identities, are presented. The implications for educators are discussed.<p></p>
<b>Conclusions</b> Identity formation is mainly social and relational in nature. Educators, and the wider medical society, need to utilise and maximise the opportunities that exist in the various relational settings students experience. Education in its broadest sense is about the transformation of the self into new ways of thinking and relating. Helping students form, and successfully integrate their professional selves into their multiple identities, is a fundamental of medical education
Family coordination in families who have a child with autism spectrum disorder
Little is known about the interactions of families where there is a child with autism spectrum disorder (ASD). The present study applies the Lausanne Trilogue Play (LTP) to explore both its applicability to this population as well as to assess resources and areas of deficit in these families. The sample consisted of 68 families with a child with ASD, and 43 families with a typically developing (TD) child. With respect to the global score for family coordination there were several negative correlations: the more severe the symptoms (based on the child’s ADOS score), the more family coordination was dysfunctional. This correlation was particularly high when parents had to play together with the child. In the parts in which only one of the parents played actively with the child, while the other was simply present, some families did achieve scores in the functional range, despite the child’s symptom severity. The outcomes are discussed in terms of their clinical implications both for assessment and for interventio
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