696 research outputs found
Coherent single electron spin control in a slanting Zeeman field
We consider a single electron in a 1D quantum dot with a static slanting
Zeeman field. By combining the spin and orbital degrees of freedom of the
electron, an effective quantum two-level (qubit) system is defined. This
pseudo-spin can be coherently manipulated by the voltage applied to the gate
electrodes, without the need for an external time-dependent magnetic field or
spin-orbit coupling. Single qubit rotations and the C-NOT operation can be
realized. We estimated relaxation () and coherence () times, and
the (tunable) quality factor. This scheme implies important experimental
advantages for single electron spin control.Comment: 4 pages, 3 figure
Atmospheric temperature responses to solar irradiance and geomagnetic activity
The relative effects of solar irradiance and geomagnetic activity on the atmospheric temperature anomalies (Ta) are examined from the monthly to interdecadal timescales. Geomagnetic Ap (Ap) signals are found primarily in the stratosphere, while the solar F10.7-cm radio flux (Fs) signals are found in both the stratosphere and troposphere. In the troposphere, 0.1–0.4 K increases in Ta are associated with Fs. Enhanced Fs signals are found when the stratospheric quasi-biennial oscillation (QBO) is westerly. In the extrapolar region of the stratosphere, 0.1–0.6 and 0.1–0.7 K increases in Ta are associated with solar irradiance and with geomagnetic activity, respectively. In this region, Fs signals are strengthened when either the QBO is easterly, or geomagnetic activity is high, while Ap signals are strengthened when either the QBO is westerly, or solar irradiance is high. High solar irradiance and geomagnetic activity tend to enhance each other's signatures either making the signals stronger and symmetric about the equator or extending the signals to broader areas, or both. Positive Ap signals dominate the middle Arctic stratosphere and are two to five times larger than those of Fs. When solar irradiance is low, the signature of Ap in Ta is asymmetric about the equator, with positive signals in the Arctic stratosphere and negative signals at midlatitudes of the NH stratosphere. Weaker stratospheric QBO signals are associated with high Ap and Fs, suggesting possible disturbances on the QBO. The signals of Ap and Fs are distinct from the positive temperature anomalies resulting from volcanic eruptions
European climate response to tropical volcanic eruptions over the last half millennium
We analyse the winter and summer climatic signal following 15 major tropical volcanic eruptions over the last half millennium based on multi-proxy reconstructions for Europe. During the first and second post-eruption years we find significant continental scale summer cooling and somewhat drier conditions over Central Europe. In the Northern Hemispheric winter the volcanic forcing induces an atmospheric circulation response that significantly follows a positive NAO state connected with a significant overall warm anomaly and wetter conditions over Northern Europe. Our findings compare well with GCM studies as well as observational studies, which mainly cover the substantially shorter instrumental period and thus include a limited set of major eruptions
Evidence for solar cycles in a late Holocene speleothem record from Dongge Cave, China
The association between solar activity and Asian monsoon (AM) remains unclear. Here we evaluate the possible connection between them based on a precisely-dated, high-resolution speleothem oxygen isotope record from Dongge Cave, southwest China during the past 4.2 thousand years (ka). Without being adjusted chronologically to the solar signal, our record shows a distinct peak-to-peak correlation with cosmogenic nuclide 14C, total solar irradiance (TSI) and sunspot number (SN) at multi-decadal to centennial timescales. Further cross-wavelet analyses between our calcite δ18O and atmospheric 14C show statistically strong coherence at three typical periodicities of ~80, 200 and 340 years, suggesting important roles of solar activities in modulating AM changes at those timescales. Our result has further indicated a better correlation between our calcite δ18O record and atmospheric 14C than between our record and TSI. This better correlation may imply that the Sun–monsoon connection is dominated most likely by cosmic rays and oceanic circulation (both associated to atmospheric 14C), instead of the direct solar heating (TSI)
Implication of tropical lower stratospheric cooling in recent trends in tropical circulation and deep convective activity
Large changes in tropical circulation from the mid-to-late 1990s to the
present, in particular changes related to the summer monsoon and cooling of
the sea surface in the equatorial eastern Pacific, are noted. The cause of
such recent decadal variations in the tropics was studied using a
meteorological reanalysis dataset. Cooling of the equatorial southeastern
Pacific Ocean occurred in association with enhanced cross-equatorial
southerlies that were associated with a strengthening of the deep ascending branch
of the boreal summer Hadley circulation over the continental sector connected
to stratospheric circulation. From boreal summer to winter, the anomalous
convective activity center moves southward following the seasonal march to
the equatorial Indian Ocean–Maritime Continent region, which strengthens the
surface easterlies over the equatorial central Pacific. Accordingly, ocean
surface cooling extends over the equatorial central Pacific. We suggest that
the fundamental cause of the recent decadal change in the tropical
troposphere and the ocean is a poleward shift of convective activity that
resulted from a strengthening of extreme deep convection penetrating into the
tropical tropopause layer, particularly over the African and Asian continents
and adjacent oceans. We conjecture that the increase in extreme deep
convection is produced by a combination of land surface warming due to
increased CO2 and a reduction of static stability in the tropical
tropopause layer due to tropical stratospheric cooling.</p
Anisotropic diffusion of point defects in a two-dimensional crystal of streptavidin observed by high-speed atomic force microscopy
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A functional polymorphism in the epidermal growth factor gene predicts hepatocellular carcinoma risk in Japanese hepatitis C patients
Background: A single nucleotide polymorphism (SNP) in the epidermal growth factor (EGF) gene (rs4444903) has been associated with increased risk of cancer, including hepatocellular carcinoma (HCC). The aim of this study was to examine the relationship between the EGF SNP genotype and the development and prognosis of HCC, in a Japanese population. Methods: Restriction fragment-length polymorphism was used to determine the presence of the EGF SNP genotype in 498 patients, including 208 patients with HCC. The level of EGF messenger ribonucleic acid (mRNA) expression in cancerous tissues was measured by quantitative reverse transcription polymerase chain reaction. The correlation between the EGF SNP genotype and prognosis was statistically analyzed in the patients with HCC. Results: The proportion of the A/A, A/G, and G/G genotypes were 5.3%, 42.8%, and 51.9%, respectively, in the patients with HCC, whereas in those without HCC, they were 8.6%, 35.9%, and 55.5%, respectively, revealing that the odds ratio (OR) of developing HCC was higher in patients with a G allele (OR =1.94, P=0.080 for A/G patients and OR =1.52, P=0.261 for G/G patients, as compared with A/A patients). In particular, when the analysis was limited to the 363 patients with hepatitis C, the OR for developing HCC was 3.54 (P=0.014) for A/G patients and was 2.85 (P=0.042) for G/G patients, as compared with A/A patients. Tumoral EGF mRNA expression in G/G patients was significantly higher than that in A/A patients (P=0.033). No statistically significant differences were observed between the EGF SNP genotype and diseasefree or overall survival. Conclusion: The EGF SNP genotype might be associated with a risk for the development of HCC in Japanese patients but not with prognosis. Of note, the association is significantly stronger in patients with hepatitis C, which is the main risk factor for HCC in Japan
Multi-decadal modulations in the Aleutian-Icelandic Low seesaw and the axial symmetry of the Arctic Oscillation signature, as revealed in the 20th century reanalysis
Seesaw relationship in intensity between the surface Aleutian and Icelandic Lows (AIS) is a manifestation of atmospheric teleconnection that bridges the interannual variability over the Pacific and Atlantic in particular winter months. Analysis of the 20th Century Reanalysis data reveals that the strength and timing of AIS have undergone multi-decadal modulations in conjunction with those in structure of the Arctic Oscillation (AO) signature, extracted in the leading mode of interannual sea-level pressure (SLP) variability over the extratropical Northern Hemisphere. Specifically, events of what may be called ‘pure AO’, in which SLP anomalies exhibit a high degree of axial symmetry in association with in-phase SLP variability between the midlatitude Atlantic and Pacific, tended to occur during multi-decadal periods in which the inter-basin teleconnection through AIS was active under the enhanced interannual variability of the Aleutian Low. In contrast, the axial symmetry of the AO pattern was apparently reduced during a multi-decadal period in which the AIS teleconnection was inactive under the weakened interannual variability of the Aleutian Low. In this period, the leading mode of interannual SLP variability represented a meridional seesaw between the Atlantic and Arctic, which resembles SLP anomaly pattern associated with the cold-ocean/warm-land (COWL) temperature pattern. These multi-decadal modulations in interannual AIS signal and the axial symmetry of the interannual AO pattern occurred under multi-decadal changes in the background state that also represented the polarity changes of the COWL-like anomaly pattern
Recipes for spin-based quantum computing
Technological growth in the electronics industry has historically been
measured by the number of transistors that can be crammed onto a single
microchip. Unfortunately, all good things must come to an end; spectacular
growth in the number of transistors on a chip requires spectacular reduction of
the transistor size. For electrons in semiconductors, the laws of quantum
mechanics take over at the nanometre scale, and the conventional wisdom for
progress (transistor cramming) must be abandoned. This realization has
stimulated extensive research on ways to exploit the spin (in addition to the
orbital) degree of freedom of the electron, giving birth to the field of
spintronics. Perhaps the most ambitious goal of spintronics is to realize
complete control over the quantum mechanical nature of the relevant spins. This
prospect has motivated a race to design and build a spintronic device capable
of complete control over its quantum mechanical state, and ultimately,
performing computations: a quantum computer.
In this tutorial we summarize past and very recent developments which point
the way to spin-based quantum computing in the solid-state. After introducing a
set of basic requirements for any quantum computer proposal, we offer a brief
summary of some of the many theoretical proposals for solid-state quantum
computers. We then focus on the Loss-DiVincenzo proposal for quantum computing
with the spins of electrons confined to quantum dots. There are many obstacles
to building such a quantum device. We address these, and survey recent
theoretical, and then experimental progress in the field. To conclude the
tutorial, we list some as-yet unrealized experiments, which would be crucial
for the development of a quantum-dot quantum computer.Comment: 45 pages, 12 figures (low-res in preprint, high-res in journal)
tutorial review for Nanotechnology; v2: references added and updated, final
version to appear in journa
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