5,144 research outputs found
Quantum Computing with an 'Always On' Heisenberg Interaction
Many promising ideas for quantum computing demand the experimental ability to
directly switch 'on' and 'off' a physical coupling between the component
qubits. This is typically the key difficulty in implementation, and precludes
quantum computation in generic solid state systems, where interactions between
the constituents are 'always on'. Here we show that quantum computation is
possible in strongly coupled (Heisenberg) systems even when the interaction
cannot be controlled. The modest ability of 'tuning' the transition energies of
individual qubits proves to be sufficient, with a suitable encoding of the
logical qubits, to generate universal quantum gates. Furthermore, by tuning the
qubits collectively we provide a scheme with exceptional experimental
simplicity: computations are controlled via a single 'switch' of only six
settings. Our schemes are applicable to a wide range of physical
implementations, from excitons and spins in quantum dots through to bulk
magnets.Comment: 4 pages, 3 figs, 2 column format. To appear in PR
A quantum mechanical model of the upper bounds of the cascading contribution to the second hyperpolarizability
Microscopic cascading of second-order nonlinearities between two molecules
has been proposed to yield an enhanced third-order molecular nonlinear-optical
response. In this contribution, we investigate the two-molecule cascaded second
hyperpolarizability and show that it will never exceed the fundamental limit of
a single molecule with the same number of electrons as the two-molecule system.
We show the apparent divergence behavior of the cascading contribution to the
second hyperpolarizability vanishes when properly taking into account the
intermolecular interactions. Although cascading can never lead to a larger
nonlinear-optical response than a single molecule, it provides alternative
molecular design configurations for creating materials with large third-order
susceptibilities that may be difficult to design into a single molecule.Comment: 13 pages, 9 figures, 1 tabl
Indicator patterns of forced change learned by an artificial neural network
Many problems in climate science require the identification of signals
obscured by both the "noise" of internal climate variability and differences
across models. Following previous work, we train an artificial neural network
(ANN) to identify the year of input maps of temperature and precipitation from
forced climate model simulations. This prediction task requires the ANN to
learn forced patterns of change amidst a background of climate noise and model
differences. We then apply a neural network visualization technique (layerwise
relevance propagation) to visualize the spatial patterns that lead the ANN to
successfully predict the year. These spatial patterns thus serve as "reliable
indicators" of the forced change. The architecture of the ANN is chosen such
that these indicators vary in time, thus capturing the evolving nature of
regional signals of change. Results are compared to those of more standard
approaches like signal-to-noise ratios and multi-linear regression in order to
gain intuition about the reliable indicators identified by the ANN. We then
apply an additional visualization tool (backward optimization) to highlight
where disagreements in simulated and observed patterns of change are most
important for the prediction of the year. This work demonstrates that ANNs and
their visualization tools make a powerful pair for extracting climate patterns
of forced change.Comment: The first version of this manuscript has been submitted to the
Journal of Advances in Modeling Earth Systems (JAMES), 202
Single-qubit unitary gates by graph scattering
We consider the effects of plane-wave states scattering off finite graphs, as
an approach to implementing single-qubit unitary operations within the
continuous-time quantum walk framework of universal quantum computation. Four
semi-infinite tails are attached at arbitrary points of a given graph,
representing the input and output registers of a single qubit. For a range of
momentum eigenstates, we enumerate all of the graphs with up to vertices
for which the scattering implements a single-qubit gate. As increases, the
number of new unitary operations increases exponentially, and for the
majority correspond to rotations about axes distributed roughly uniformly
across the Bloch sphere. Rotations by both rational and irrational multiples of
are found.Comment: 8 pages, 7 figure
Effectiveness of a Heads-Up Adaptive Lane Deviation Warning System for Middle-Aged and Older Adults
46 participants (24 younger and 22 older) completed at least one out of four simulated drives designed to test the effectiveness of an Adaptive Lane Deviation Warning (LDW) system, and they drove through both a warnings-on and warnings-off version of each drive. Findings showed that LDW was effective in reducing reaction time for lane deviation corrections for both older (by 1.2 seconds) and younger drivers (by 1.6 seconds). The older and younger drivers did not differ in correction RTs when the warnings were turned off. But older drivers showed slower correction RTs than younger drivers in the warning-on drives. The data indicate that these benefits were specific to LDW rather than general improvement in driving performance. Cognitive processing speed emerged as a particularly robust predictor of benefits from the LDW compared to other domains of cognitive function
Cell-specific activity-dependent fractionation of layer 2/3→5B excitatory signaling in mouse auditory cortex
© The Author(s), 2015. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Journal of Neuroscience 35 (2015): 3112-3123, doi:10.1523/JNEUROSCI.0836-14.2015.Auditory cortex (AC) layer 5B (L5B) contains both corticocollicular neurons, a type of pyramidal-tract neuron projecting to the inferior colliculus, and corticocallosal neurons, a type of intratelencephalic neuron projecting to contralateral AC. Although it is known that these neuronal types have distinct roles in auditory processing and different response properties to sound, the synaptic and intrinsic mechanisms shaping their input–output functions remain less understood. Here, we recorded in brain slices of mouse AC from retrogradely labeled corticocollicular and neighboring corticocallosal neurons in L5B. Corticocollicular neurons had, on average, lower input resistance, greater hyperpolarization-activated current (Ih), depolarized resting membrane potential, faster action potentials, initial spike doublets, and less spike-frequency adaptation. In paired recordings between single L2/3 and labeled L5B neurons, the probabilities of connection, amplitude, latency, rise time, and decay time constant of the unitary EPSC were not different for L2/3→corticocollicular and L2/3→corticocallosal connections. However, short trains of unitary EPSCs showed no synaptic depression in L2/3→corticocollicular connections, but substantial depression in L2/3→corticocallosal connections. Synaptic potentials in L2/3→corticocollicular connections decayed faster and showed less temporal summation, consistent with increased Ih in corticocollicular neurons, whereas synaptic potentials in L2/3→corticocallosal connections showed more temporal summation. Extracellular L2/3 stimulation at two different rates resulted in spiking in L5B neurons; for corticocallosal neurons the spike rate was frequency dependent, but for corticocollicular neurons it was not. Together, these findings identify cell-specific intrinsic and synaptic mechanisms that divide intracortical synaptic excitation from L2/3 to L5B into two functionally distinct pathways with different input–output functions.This work was supported by National Institutes of Health grants DC013272 (T.T. and G.M.G.S.), DC007905 (T.T.), NS061963 (G.M.G.S), R03DC012585 (J.W.M.), T32DC011499 (C.T.A.), and F32DC013734 (C.T.A), and by the Albert and Ellen Grass Faculty Award (T.T. and G.M.G.S.) and Charles Evans Foundation Award (T.T. and G.M.G.S.).2015-08-1
Treatment of Breast Cancer in Countries with Limited Resources
Early and accurate diagnosis of breast cancer is important for optimizing treatment. Local treatment of early stage breast cancer involves either mastectomy or breast-conserving surgery followed by whole-breast irradiation. The pathologic and biologic properties of a woman's breast cancer may be used to estimate her probability for recurrence of and death from breast cancer, as well as the magnitude of benefit she is likely to receive from adjuvant endocrine therapy or cytotoxic chemotherapy. Ovarian ablation or suppression with or without tamoxifen is an effective endocrine therapy in the adjuvant treatment of breast cancer in premenopausal women with estrogen receptor (ER)-positive or ER-unknown breast cancer. In postmenopausal women with ER- and/or progesterone receptor (PR)-positive or PR-unknown breast cancer, the use of tamoxifen or anastrozole is effective adjuvant endocrine therapy. The benefit of tamoxifen is additive to that of chemotherapy. Cytotoxic chemotherapy also improves recurrence rates and survival, with the magnitude of benefit decreasing with increasing age. Substantial support systems are required to optimally and safely use breast-conserving approaches to local therapy or cytotoxic chemotherapy as systemic therapy. Locally advanced breast cancer (LABC) accounts for at least half of all breast cancers in countries with limited resources and has a poor prognosis. Initial treatment of LABC with anthracycline-based chemotherapy is standard and effective. Addition of a sequential, neoadjuvant taxane thereafter increases the rate of pathologic complete responses. Neoadjuvant endocrine therapy may benefit postmenopausal women with hormone receptor-positive LABC. After an initial response to neoadjuvant chemotherapy, the use of local-regional surgery is appropriate. Most women will require a radical or modified radical mastectomy. In those women in whom mastectomy is not possible after neoadjuvant chemotherapy, the use of whole-breast and regional lymph node irradiation alone is appropriate. In those women who cannot receive neoadjuvant chemotherapy because of resource constraints, mastectomy with node dissection, when feasible, may still be considered in an attempt to achieve local-regional control. After local-regional therapy, most women should receive additional systemic chemotherapy. Women with LABC that has a positive or unknown hormone receptor status benefit from endocrine therapy with tamoxifen. The treatment of LABC requires multiple disciplines and is resource intensive. Efforts to reduce the number of breast cancers diagnosed at an advanced stage thus have the potential to improve rates of survival while decreasing the use of limited resources
The Radio Sky at Meter Wavelengths: m-Mode Analysis Imaging with the Owens Valley Long Wavelength Array
A host of new low-frequency radio telescopes seek to measure the 21-cm
transition of neutral hydrogen from the early universe. These telescopes have
the potential to directly probe star and galaxy formation at redshifts , but are limited by the dynamic range they can achieve
against foreground sources of low-frequency radio emission. Consequently, there
is a growing demand for modern, high-fidelity maps of the sky at frequencies
below 200 MHz for use in foreground modeling and removal. We describe a new
widefield imaging technique for drift-scanning interferometers,
Tikhonov-regularized -mode analysis imaging. This technique constructs
images of the entire sky in a single synthesis imaging step with exact
treatment of widefield effects. We describe how the CLEAN algorithm can be
adapted to deconvolve maps generated by -mode analysis imaging. We
demonstrate Tikhonov-regularized -mode analysis imaging using the Owens
Valley Long Wavelength Array (OVRO-LWA) by generating 8 new maps of the sky
north of with 15 arcmin angular resolution, at frequencies
evenly spaced between 36.528 MHz and 73.152 MHz, and 800 mJy/beam thermal
noise. These maps are a 10-fold improvement in angular resolution over existing
full-sky maps at comparable frequencies, which have angular resolutions . Each map is constructed exclusively from interferometric observations
and does not represent the globally averaged sky brightness. Future
improvements will incorporate total power radiometry, improved thermal noise,
and improved angular resolution -- due to the planned expansion of the OVRO-LWA
to 2.6 km baselines. These maps serve as a first step on the path to the use of
more sophisticated foreground filters in 21-cm cosmology incorporating the
measured angular and frequency structure of all foreground contaminants.Comment: 27 pages, 18 figure
Interactional positioning and narrative self-construction in the first session of psychodynamic-interpersonal psychotherapy
The purpose of this study is to identify possible session one indicators of end of treatment psychotherapy outcome using the framework of three types of interactional positioning; client’s self-positioning, client’s positioning between narrated self and different partners, and the positioning between client and therapist. Three successful cases of 8-session psychodynamic-interpersonal (PI) therapy were selected on the basis of client Beck Depression Inventory scores. One unsuccessful case was also selected against which identified patterns could be tested. The successful clients were more descriptive about their problems and demonstrated active rapport-building, while the therapist used positionings expressed by the client in order to explore the positionings developed between them during therapy. The unsuccessful case was characterized by lack of positive self-comment, minimization of agentic self-capacity, and empathy-disrupting narrative confusions. We conclude that the theory of interactional positioning has been useful in identifying patterns worth exploring as early indicators of success in PI therapy
Experimental demonstration of diffusion limitations on resolution and SNR in MR microscopy
Magnetic resonance microscopy images at cellular resolution (< 10 microns)
are limited by diffusion. SNR and spatial resolution suffer from the dephasing
of transverse magnetization caused by diffusion of spins in strong gradients.
Such effects may be reduced by using phase encoding instead of frequency
encoding readout gradients. Demonstration of the benefits of phase encoding are
lacking, and the conditions in which it is preferred are not clearly
established. We quantify when phase encoding outperforms a readout gradient
with emphasis on the detrimental effects of diffusion on SNR and resolution. A
15.2T MRI scanner, with 1 T/m gradients, and micro solenoid RF coils < 1 mm in
diameter, were used to quantify diffusion effects on resolution and SNR of
frequency and phase encoded acquisitions. Frequency and phase encoding
resolution and SNR per square root time were calculated and measured for images
at the diffusion limited resolution. The point-spread-function was measured for
phase and frequency encoding using additional constant time gradients with
voxels 3-15 microns. The effect of diffusion during the readout gradient on SNR
was experimentally demonstrated. The achieved resolutions of frequency and
phase encoded acquisitions were measured via the point-spread-function. SNR per
square root time and actual resolution were calculated for a wide range of
gradient amplitudes, diffusion coefficients, and relaxation properties. The
results provide a practical guide on how to choose between phase and frequency
encoding. Images of excised rat spinal cord at 10 x 10 microns in-plane
demonstrate benefits of phase encoding in the form of higher measured
resolution and SNR vs the same image acquired with a conventional readout. We
demonstrate the extent to which phase encoding outperforms readout gradients in
SNR and resolution over a wide range of voxel sizes, sample, and hardware
properties.Comment: 36 pages, 9 figures, 1 table, and 4 supplemental figures. Submitted
to Journal of Magnetic Resonance; cleaned up metadata, fixed heading typ
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