3,506 research outputs found
Studies in a Random Noise Model of Decoherence
We study the effects of noise and decoherence for a double-potential well
system, suitable for the fabrication of qubits and quantum logic elements. A
random noise term is added to the hamiltonian, the resulting wavefunction found
numerically and the density matrix obtained by averaging over noise signals.
Analytic solutions using the two-state model are obtained and found to be
generally in agreement.
In particular, a simple formula for the decoherence rate in terms of the
noise parameters in the two-state model is reviewed and verified for the full
simulation with the multi-level system. The formalism is extended to describe
multiple sources of noise or different "dephasing" axes at the same time.
Furthermore, the old formula for the "Turing-Watched Pot" effect is generalized
to the case where the environmental interactions do not conserve the "quality"
in question.
Various forms for the noise signal are investigated. An interesting result is
the importance of the noise power at low frequency. If it vanishes there is, in
leading order, no decoherence. This is verified in a numerical simulation where
two apparently similar noise signals, but differing in the power at zero
frequency, give strikingly different decoherence effects. A short discussion of
situations dominated by low frequency noise is given.Comment: 27 pages, 10 figures. New section added on Very Low Frequency Noise,
with two additional figures. Conclusions, Abstract modified accordingly.
Various other small editorial changes and clarification
The transition zone as a host for recycled volatiles: Evidence from nitrogen and carbon isotopes in ultra-deep diamonds from Monastery and Jagersfontein (South Africa)
Sublithospheric (ultra-deep) diamonds provide a unique window into the deepest parts of Earth's mantle, which otherwise remain inaccessible. Here, we report the first combined C- and N-isotopic data for diamonds from the Monastery and Jagersfontein kimberlites that sample the deep asthenosphere and transition zone beneath the Kaapvaal Craton, in the mid Cretaceous, to investigate the nature of mantle fluids at these depths and the constraints they provide on the deep volatile cycle.
Both diamond suites exhibit very light δ13C values (down to − 26‰) and heavy δ15N (up to + 10.3‰), with nitrogen abundances generally below 70 at. ppm but varying up to very high concentrations (2520 at. ppm) in rare cases. Combined, these signatures are consistent with derivation from subducted crustal materials. Both suites exhibit variable nitrogen aggregation states from 25 to 100% B defects. Internal growth structures, revealed in cathodoluminescence (CL) images, vary from faintly layered, through distinct cores to concentric growth patterns with intermittent evidence for dissolution and regular octahedral growth layers in places.
Modelling the internal co-variations in δ13C-δ15N-N revealed that diamonds grew from diverse C-H-O-N fluids involving both oxidised and reduced carbon species. The diversity of the modelled diamond-forming fluids highlights the complexity of the volatile sources and the likely heterogeneity of the deep asthenosphere and transition zone. We propose that the Monastery and Jagersfontein diamonds form in subducted slabs, where carbon is converted into either oxidised or reduced species during fluid-aided dissolution of subducted carbon before being re-precipitated as diamond. The common occurrence of recycled C and N isotopic signatures in super-deep diamonds world-wide indicates that a significant amount of carbon and nitrogen is recycled back to the deep asthenosphere and transition zone via subducting slabs, and that the transition zone may be dominated by recycled C and N
Spatio-temporal properties of motion detectors matched to low image velocities in hovering insects
AbstractOur recent study [O'Carroll et al. (1996). Nature 382, 63–66) described a correlation between the spatio-temporal properties of motion detecting neurons in the optic lobes of flying insects and behaviour. We consider here theoretical properties of insect motion detectors at very low image velocities and measure spatial and temporal sensitivity of neurons in the lobula complex of two specialised hovering insects, the bee-fly Bombylius and the hummingbird hawkmoth, Macroglossum. The spatio-temporal optima of direction-selective neurons in these insects lie at lower velocities than those of other insects which we have studied, including large syrphid flies, which are also excellent hoverers. We argue that spatio-temporal optima reflect a compromise between the demands of diverse behaviour, which can involve prolonged periods of stationary, hovering flight followed by spectacular high speed pursuits of conspecifics. Males of the syrphid Eristalis which engage in such behaviour, have higher temporal frequency optima than females. High contrast sensitivity in these flies nevertheless results in reliable responses at very low image velocities. Neurons of Bombylius have two distinct velocity optima, suggesting that they sum inputs from two classes of motion correlator with different time constants. This also provides sensitivity to a large range of velocities
Identifying Changes in the Synaptic Proteome of Cirrhotic Alcoholic Superior Frontal Gyrus
Hepatic complications are a common side-effect of alcoholism. Without the detoxification capabilities of the liver, alcohol misuse induces changes in gene and protein expression throughout the body. A global proteomics approach was used to identify these protein changes in the brain. We utilised human autopsy tissue from the superior frontal gyrus (SFG) of six cirrhotic alcoholics, six alcoholics without comorbid disease, and six non-alcoholic non-cirrhotic controls. Synaptic proteins were isolated and used in two-dimensional differential in-gel electrophoresis coupled with mass spectrometry. Many expression differences were confined to one or other alcoholic sub-group. Cirrhotic alcoholics showed 99 differences in protein expression levels from controls, of which half also differed from non-comorbid alcoholics. This may reflect differences in disease severity between the sub-groups of alcoholics, or differences in patterns of harmful drinking. Alternatively, the protein profiles may result from differences between cirrhotic and non-comorbid alcoholics in subjects’ responses to alcohol misuse. Ten proteins were identified in at least two spots on the 2D gel; they were involved in basal energy metabolism, synaptic vesicle recycling, and chaperoning. These post-translationally modified isoforms were differentially regulated in cirrhotic alcoholics, indicating a level of epigenetic control not previously observed in this disorder
Carbon and nitrogen systematics in nitrogen-rich ultradeep diamonds from San Luiz, Brazil
Three diamonds from Sao Luiz, Brazil carrying nano- and micro-inclusions of molecular δ-N2 that exsolved at the base of the transition zone were studied for their C and N isotopic composition and the concentration of N utilizing SIMS. The diamonds are individually uniform in their C isotopic composition and most spot analyses yield δ13C values of −3.2 ± 0.1‰ (ON-SLZ-390) and − 4.7 ± 0.1‰ (ON-SLZ-391 and 392). Only a few analyses deviate from these tight ranges and all fall within the main mantle range of −5 ± 3‰. Most of the N isotope analyses also have typical mantle δ15N values (−6.6 ± 0.4‰, −3.6 ± 0.5‰ and − 4.1 ± 0.6‰ for ON-SLZ-390, 391 and 392, respectively) and are associated with high N concentrations of 800–1250 atomic ppm. However, some N isotopic ratios, associated with low N concentrations (<400 ppm) and narrow zones with bright luminescence are distinctly above the average, reaching positive δ15N values. These sharp fluctuations cannot be attributed to fractionation. They may reflect arrival of new small pulses of melt or fluid that evolved under different conditions. Alternatively, they may result from fractionation between different growth directions, so that distinct δ15N values and N concentrations may form during diamond growth from a single melt/fluid. Other more continuous variations, in the core of ON-SLZ-390 or the rim of ON-SLZ-392 may be the result of Rayleigh fractionation or mixing
Imaging oligometastatic cancer before local treatment
The term oligometastases is in common clinical use, but remains poorly defined. As
novel treatment strategies widen the therapeutic window for patients defined as
having oligometastatic cancer, improved biomarkers to reliably define patients who
benefit from these treatments are needed.
Multimodal imaging should be optimized to comprehensively assess the metastatic
sites, disease burden and response to neoadjuvant treatment in each disease setting.
These features will likely remain important prognostic biomarkers, and are critical in
planning multidisciplinary treatment. There are opportunities to extract additional
phenotypic information from conventional imaging, while novel imaging techniques
can also image specific aspects of tumour biology. Imaging can both characterise and
localise the phenotypic heterogeneity of multiple tumour sites. Novel approaches to
existing imaging datasets, and correlation with tumour biology, will be important in
realizing the potential of imaging to guide treatment in the oligometastatic setting.
This article discusses the current status and future directions of imaging in patients
with extracranial oligometastases
The cubic period-distance relation for the Kater reversible pendulum
We describe the correct cubic relation between the mass configuration of a
Kater reversible pendulum and its period of oscillation. From an analysis of
its solutions we conclude that there could be as many as three distinct mass
configurations for which the periods of small oscillations about the two pivots
of the pendulum have the same value. We also discuss a real compound Kater
pendulum that realizes this property.Comment: 25 pages 4figure
Advanced Compton Telescope Designs and SN Science
The Advanced Compton Telescope (ACT) has been suggested to be the optimal
next-generation instrument to study nuclear gamma-ray lines. In this work, we
investigate the potential of three hypothetical designs of the ACT to perform
SN science. We provide estimates of 1) the SN detection rate, 2) the SN Ia
discrimination rate, and 3) which gamma-ray lines would be detected from
specific supernova remnants. We find that the prompt emission from a SN Ia is
such that it is unlikely that one would be within the range that an
INTERMEDIATE ACT would be able to distinguish between explosion scenarios,
although such an instrument would detect a handful of SNRs.
We further find that the SUPERIOR ACT design would be a truly breakthrough
instrument for SN science. By supplying these estimates, we intend to assist
the gamma-ray astrophysics community in deciding the course of the next decade
of gamma-ray SN science.Comment: 10 pages, accepted for publication in New astronomy Reviews
(Astronomy with Radioactivities III
State permutations from manipulation of near level-crossings
We discuss some systematic methods for implementing state manipulations in
systems formally similar to chains of a few spins with nearest-neighbor
interactions, arranged such that there are strong and weak scales of coupling
links. States are permuted by means of bias potentials applied to a few
selected sites. This generic structure is then related to an atoms-in-a-cavity
model that has been proposed in the literature as a way of achieving a
decoherence free subspace. A new method using adiabatically varying laser
detuning to implement a CNOT gate in this model is proposed.Comment: 6 pages, 5 figures. Substantial revision and extension of the
introduction and the atoms-in-a-cavity section
An Analytical Approach to Fluctuations in Showers
We review the problem of fluctuations in particle shower theory. By using a
generalization of Furry equation, we find relations between the --particle
correlation function and the number of particles average or 1--particle
correlation function. Such relations show that the average is the only
independent dynamical variable. We also develop a numerical code to solve the
equation for the correlation functions and compare the results with those from
a Montecarlo simulation which show a perfect agreement between both methods.Comment: 18 pages Latex file and 3 PS figures. Uses Revtex and epsfig.sty
Accepted for publication in Astroparticle Physic
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