Driven coherent oscillations of a single electron spin in a quantum dot

The ability to control the quantum state of a single electron spin in a quantum dot is at the heart of recent developments towards a scalable spin-based quantum computer. In combination with the recently demonstrated exchange gate between two neighbouring spins, driven coherent single spin rotations would permit universal quantum operations. Here, we report the experimental realization of single electron spin rotations in a double quantum dot. First, we apply a continuous-wave oscillating magnetic field, generated on-chip, and observe electron spin resonance in spin-dependent transport measurements through the two dots. Next, we coherently control the quantum state of the electron spin by applying short bursts of the oscillating magnetic field and observe about eight oscillations of the spin state (so-called Rabi oscillations) during a microsecond burst. These results demonstrate the feasibility of operating single-electron spins in a quantum dot as quantum bits.Comment: Total 25 pages. 11 pages main text, 5 figures, 9 pages supplementary materia

Spatio‐temporal patterns of tree growth as related to carbon isotope fractionation in European forests under changing climate

Aim To decipher Europe-wide spatiotemporal patterns of forest growth dynamics and their associations with carbon isotope fractionation processes inferred from tree rings as modulated by climate warming. Location Europe and North Africa (30‒70°N, 10°W‒35°E). Time period 1901‒2003. Major taxa studied Temperate and Euro-Siberian trees. Methods We characterize changes in the relationship between tree growth and carbon isotope fractionation over the 20th century using a European network consisting of 20 site chronologies. Using indexed tree-ring widths (TRWi), we assess shifts in the temporal coherence of radial growth across sites (synchrony) for five forest ecosystems (Atlantic, Boreal, cold continental, Mediterranean and temperate). We also examine whether TRWi shows variable coupling with leaf-level gas exchange, inferred from indexed carbon isotope discrimination of tree-ring cellulose (Δ13Ci). Results We find spatial autocorrelation for TRWi and Δ13Ci extending over up to 1,000 km among forest stands. However, growth synchrony is not uniform across Europe, but increases along a latitudinal gradient concurrent with decreasing temperature and evapotranspiration. Latitudinal relationships between TRWi and Δ13Ci (changing from negative to positive southwards) point to drought impairing carbon uptake via stomatal regulation for water saving occurring at forests below 60°N in continental Europe. A rise in forest growth synchrony over the 20th century together with increasingly positive relationships between TRWi and Δ13Ci indicate intensifying drought impacts on tree performance. These effects are noticeable in drought-prone biomes (Mediterranean, temperate and cold continental). Main conclusions At the turn of this century, convergence in growth synchrony across European forest ecosystems is coupled with coordinated warming-induced drought effects on leaf physiology and tree growth spreading northwards. Such a tendency towards exacerbated moisture-sensitive growth and physiology could override positive effects of enhanced leaf intercellular CO2 concentrations, possibly resulting in Europe-wide declines of forest carbon gain in the coming decades

Consequences of larch budmonth outbreaks on the chlimate significance of ring width and stable isotopes of larch

Tree-ring widths and stable carbon and oxygen isotopes of five European larch trees from Lotschental, Switzerland were investigated for the period 1900-2004. The objective was to test the suitability of each of these parameters for high-frequency climate reconstructions. This is of special interest with regard to the problem of cyclic larch budmoth (LBM) infestations of alpine larch trees. The results clearly demonstrate that tree-ring width chronologies are not suitable for high-frequency reconstructions because infestations lead to variably reduced tree-ring increments, largely suppressing climate signals. On the other hand, the stable isotope chronologies proved less affected by larch budmoth outbreaks, independent of the strength of the infestations. The correlation of the carbon isotopes with summer temperatures was especially high (r = 0.73) and with precipitation lower but nevertheless significant (r = -0.43). Oxygen isotopes were also correlated with summer temperature (r = 0.46); however, a certain perturbation of normal oxygen isotope signatures due to LBM outbreaks was evident. Contrary to tree-ring widths, none of the LBM outbreaks caused a significant disturbance of the current year's isotopic climate signal and, most importantly, there were no delayed effects in the following years. Thus, stable carbon isotopes in tree-ring chronologies of the European larch provide an excellent opportunity for high-frequency temperature reconstructions

Carbon and oxygen isotope dendrochronology in sub-fossil bog oak tree rings - a preliminary study

isotope dendroclimatology is a relatively new field investigating environmental factors that control the radial growth of trees. Tree-ring series of sub-fossil bog oaks can be dated from sites across northwest Europe indicating that the environmental change(s) were regional rather than local. Bog oaks show characteristic periods of suppressed growth thought to have resulted from changes in the hydrological status of bogs towards either wetter or drier conditions. This study investigates relative changes in stable carbon (delta(13)C) and oxygen (delta(18)O) isotope content in phases of suppressed and normal growth in threebog oaks dated as c. 200 BC to 150 AD from Zwolle, eastern Netherlands. Bog oaks show no clear relationship between tree-ring width and isotopic composition although one tree exhibited relatively depleted values of C-13 and O-18 with suppressed growth. Suppressed ring growth is characterised by the formation of earlywood only, possibly as a result of hydrologic alterations that limited the formation of latewood, which would otherwise have locked up a detectable signal in stable isotopic shift

Environmental history of the German Lower Rhine embayment during the middle miocene reflected by carbon isotope signals in brown coal

Stable carbon isotope investigations have been carried out on Miocene brown coal from the Garzweiler Seam of the German Lower Rhine Embayment. Material studied included fossil wood from seven different taxa, and brown coal matrix. Isotope results from macrofossil analysis show variations of more than 6‰ within individual samples and reveal a general isotopic difference between angiosperm and gymnosperm wood specimens. According to mean carbon isotope values found for gymnosperms, angiosperms and brown coal matrix (−23.3‰, −26.0‰ and −25.8‰), the peat-forming vegetation of the Garzweiler Seam was dominated by angiosperm taxa. Results from brown coal matrix establish a continuous high-resolution depth profile of carbon isotope variations during the late Middle Miocene. They show a significant and characteristic isotope pattern with distinct medium- and short-term cycles (high-frequency variations) in the two main units of Garzweiler Seam (locally split into three units). The medium-term δ13C variations are most likely caused by varying proportions of gymnosperms within the peat-forming vegetation while high-frequency oscillations seem to be a direct signal of environmental changes. A long-term decline of carbon isotope values observed within the complete Garzweiler Seam from base to top is presumably due to a cooling trend in the Miocene

Environmental history of the German Lower Rhine Embayment during the Middle Miocene as reflected by carbon isotopes in brown coal

Stable carbon isotope investigations have been carried out on Miocene brown coal from the Garzweiler Seam of the German Lower Rhine Embayment. Material studied included fossil wood from seven different taxa, and brown coal matrix. Isotope results from macrofossil analysis show variations of more than 6‰ within individual samples and reveal a general isotopic difference between angiosperm and gymnosperm wood specimens. According to mean carbon isotope values found for gymnosperms, angiosperms and brown coal matrix (−23.3‰, −26.0‰ and −25.8‰), the peat-forming vegetation of the Garzweiler Seam was dominated by angiosperm taxa. Results from brown coal matrix establish a continuous high-resolution depth profile of carbon isotope variations during the late Middle Miocene. They show a significant and characteristic isotope pattern with distinct medium- and short-term cycles (high-frequency variations) in the two main units of Garzweiler Seam (locally split into three units). The medium-term δ13C variations are most likely caused by varying proportions of gymnosperms within the peat-forming vegetation while high-frequency oscillations seem to be a direct signal of environmental changes. A long-term decline of carbon isotope values observed within the complete Garzweiler Seam from base to top is presumably due to a cooling trend in the Miocene