246 research outputs found
Suppression of electron spin decoherence in a quantum dot
The dominant source of decoherence for an electron spin in a quantum dot is
the hyperfine interaction with the surrounding bath of nuclear spins. The
decoherence process may be slowed down by subjecting the electron spin to
suitable sequences of external control pulses. We investigate the performance
of a variety of dynamical decoupling protocols using exact numerical
simulation. Emphasis is given to realistic pulse delays and the long-time
limit, beyond the domain where available analytical approaches are guaranteed
to work. Our results show that both deterministic and randomized protocols are
capable to significantly prolong the electron coherence time, even when using
control pulse separations substantially larger than what expected from the {\em
upper cutoff} frequency of the coupling spectrum between the electron and the
nuclear spins. In a realistic parameter range, the {\em total width} of such a
coupling spectrum appears to be the physically relevant frequency scale
affecting the overall quality of the decoupling.Comment: 8 pages, 3 figures. Invited talk at the XXXVII Winter Colloquium on
the Physics of Quantum Electronics, Snowbird, Jan 2007. Submitted to J. Mod.
Op
Influence of an external magnetic field on the decoherence of a central spin coupled to an antiferromagnetic environment
Using the spin wave approximation, we study the decoherence dynamics of a
central spin coupled to an antiferromagnetic environment under the application
of an external global magnetic field. The external magnetic field affects the
decoherence process through its effect on the antiferromagnetic environment. It
is shown explicitly that the decoherence factor which displays a Gaussian decay
with time depends on the strength of the external magnetic field and the
crystal anisotropy field in the antiferromagnetic environment. When the values
of the external magnetic field is increased to the critical field point at
which the spin-flop transition (a first-order quantum phase transition) happens
in the antiferromagnetic environment, the decoherence of the central spin
reaches its highest point. This result is consistent with several recent
quantum phase transition witness studies. The influences of the environmental
temperature on the decoherence behavior of the central spin are also
investigated.Comment: 29 preprint pages, 4 figures, to appear in New Journal of Physic
The IDV source J1128+5925, a new candidate for annual modulation?
Short time-scale radio variations of compact extragalactic radio sources,
known as IntraDay Variability, can be explained in at least some sources by a
source-extrinsic effect, in which the variations are interpreted as
scintillation of radio waves caused by the turbulent ISM of the Milky Way. One
of the most convincing observational arguments in favour of propagation-induced
variability is the so called annual modulation of the characteristic
variability time-scale, which is due to the orbital motion of the Earth. Data
for the recently discovered and highly variable IDV source J1128+5925 are
presented. We study the frequency and time dependence of the IDV in this
compact quasar. We measure the characteristic variability time-scale of the IDV
throughout the year, and analyze whether the observed changes in the
variability time-scale are consistent with annual modulation. We monitored the
flux density variability of J1128+5925 with dense time sampling between 2.7 and
10.45GHz with the 100m Effelsberg radio telescope of the MPIfR and with the 25m
Urumqi radio telescope. From ten observing sessions, we determine the
variability characteristics and time-scales. The observed pronounced changes of
the variability time-scale of J1128+5925 are modelled with an anisotropic
annual modulation model. The observed frequency dependence of the variation is
in good agreement with the prediction from interstellar scintillation. Adopting
a simple model for the annual modulation model and using also the frequency
dependence of the IDV, we derive a lower limit to the distance of the
scattering screen and an upper limit to the scintillating source size. The
latter is found to be consistent with the measured core size from VLBI.Comment: 15 pages, 9 figures Accepted for publication in Astronomy and
Astrophysic
Universal quantum control of two-electron spin quantum bits using dynamic nuclear polarization
One fundamental requirement for quantum computation is to perform universal
manipulations of quantum bits at rates much faster than the qubit's rate of
decoherence. Recently, fast gate operations have been demonstrated in logical
spin qubits composed of two electron spins where the rapid exchange of the two
electrons permits electrically controllable rotations around one axis of the
qubit. However, universal control of the qubit requires arbitrary rotations
around at least two axes. Here we show that by subjecting each electron spin to
a magnetic field of different magnitude we achieve full quantum control of the
two-electron logical spin qubit with nanosecond operation times. Using a single
device, a magnetic field gradient of several hundred milliTesla is generated
and sustained using dynamic nuclear polarization of the underlying Ga and As
nuclei. Universal control of the two-electron qubit is then demonstrated using
quantum state tomography. The presented technique provides the basis for single
and potentially multiple qubit operations with gate times that approach the
threshold required for quantum error correction.Comment: 11 pages, 4 figures. Supplementary Material included as ancillary
fil
A neural signature of the unique hues
Since at least the 17th century there has been the idea that there are four simple and perceptually pure “unique” hues: red, yellow, green, and blue, and that all other hues are perceived as mixtures of these four hues. However, sustained scientific investigation has not yet provided solid evidence for a neural representation that separates the unique hues from other colors. We measured event-related potentials elicited from unique hues and the ‘intermediate’ hues in between them. We find a neural signature of the unique hues 230 ms after stimulus onset at a post-perceptual stage of visual processing. Specifically, the posterior P2 component over the parieto-occipital lobe peaked significantly earlier for the unique than for the intermediate hues (Z = -2.9, p = .004). Having identified a neural marker for unique hues, fundamental questions about the contribution of neural hardwiring, language and environment to the unique hues can now be addressed
The Galactic Center Black Hole Laboratory
The super-massive 4 million solar mass black hole Sagittarius~A* (SgrA*)
shows flare emission from the millimeter to the X-ray domain. A detailed
analysis of the infrared light curves allows us to address the accretion
phenomenon in a statistical way. The analysis shows that the near-infrared
flare amplitudes are dominated by a single state power law, with the low states
in SgrA* limited by confusion through the unresolved stellar background. There
are several dusty objects in the immediate vicinity of SgrA*. The source G2/DSO
is one of them. Its nature is unclear. It may be comparable to similar stellar
dusty sources in the region or may consist predominantly of gas and dust. In
this case a particularly enhanced accretion activity onto SgrA* may be expected
in the near future. Here the interpretation of recent data and ongoing
observations are discussed.Comment: 30 pages - 7 figures - accepted for publication by Springer's
"Fundamental Theories of Physics" series; summarizing GC contributions of 2
conferences: 'Equations of Motion in Relativistic Gravity' at the
Physikzentrum Bad Honnef, Bad Honnef, Germany, (Feb. 17-23, 2013) and the
COST MP0905 'The Galactic Center Black Hole Laboratory' Granada, Spain (Nov.
19 - 22, 2013
Serum HER2 Level Measured by Dot Blot: A Valid and Inexpensive Assay for Monitoring Breast Cancer Progression
Human epidermal growth factor receptor 2 (HER2) is one of the most important
prognostic and predictive factors for breast cancer patients. Recently, serum
HER2 ECD level of patients detected by enzyme-linked immunoabsorbent assay
(ELISA) has been shown to predict tumor HER2 status and reveal its association
with tumor progression, recurrence and poor prognosis. In this study, we
established a new method, dot blot assay, to measure the serum HER2 level in
breast cancer patients and further to evaluate the clinical value for monitoring
breast cancer progression. We found that the serum HER2 level measured by dot
blot assay was significantly correlated with tissue HER2 status in breast cancer
patients (P = 0.001), and also
significantly correlated with HER2 level measured by ELISA
(P = 1.06×10−11).
Compared with ELISA method, the specificity and sensitivity of dot blot assay
were 95.3% and 65.0%, respectively. The serum HER2 levels of
patients with grade III or ER-negative were higher than those with grade
I–II (P = 0.004) and ER-positive
(P = 0.033), respectively. Therefore,
the novel dot blot method to detect serum HER2 level is a valid and inexpensive
assay with potential application in monitoring breast cancer progression in
clinical situations
Colour categories are reflected in sensory stages of colour perception when stimulus issues are resolved
Debate exists about the time course of the effect of colour categories on visual processing. We investigated the effect of colour categories for two groups who differed in whether they categorised a blue-green boundary colour as the same- or different-category to a reliably-named blue colour and a reliably-named green colour. Colour differences were equated in just-noticeable differences to be equally discriminable. We analysed event-related potentials for these colours elicited on a passive visual oddball task and investigated the time course of categorical effects on colour processing. Support for category effects was found 100 ms after stimulus onset, and over frontal sites around 250 ms, suggesting that colour naming affects both early sensory and later stages of chromatic processing
Ensemble coding of color and luminance contrast
Ensemble coding has been demonstrated for many attributes including color, but the metrics on which this coding is based remain uncertain. We examined ensemble percepts for stimulus sets that varied in chromatic contrast between complementary hues, or that varied in luminance contrast between increments and decrements, in both cases focusing on the ensemble percepts for the neutral gray stimulus defining the category boundary. Each ensemble was composed of 16 circles with four contrast levels. Observers saw the display for 0.5 s and then judged whether a target contrast was a member of the set. False alarms were high for intermediate contrasts (within the range of the ensemble) and fell for higher or lower values. However, for ensembles with complementary hues, gray was less likely to be reported as a member, even when it represented the mean chromaticity of the set. When the settings were repeated for luminance contrast, false alarms for gray were higher and fell off more gradually for out-of-range contrasts. This difference implies that opposite luminance polarities represent a more continuous perceptual dimension than opponent-color variations, and that “gray” is a stronger category boundary for chromatic than luminance contrasts. For color, our results suggest that ensemble percepts reflect pooling within rather than between large hue differences, perhaps because the visual system represents hue differences more like qualitatively different categories than like quantitative differences within an underlying color “space.” The differences for luminance and color suggest more generally that ensemble coding for different visual attributes might depend on different processes that in turn depend on the format of the visual representation
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