Efficient C-Phase gate for single-spin qubits in quantum dots

Two-qubit interactions are at the heart of quantum information processing. For single-spin qubits in semiconductor quantum dots, the exchange gate has always been considered the natural two-qubit gate. The recent integration of magnetic field or g-factor gradients in coupled quantum dot systems allows for a one-step, robust realization of the controlled phase (C-Phase) gate instead. We analyze the C-Phase gate durations and fidelities that can be obtained under realistic conditions, including the effects of charge and nuclear field fluctuations, and find gate error probabilities of below 10-4, possibly allowing fault-tolerant quantum computation.Comment: 5 pages, 3 figure

Quantum Coherence at Low Temperatures in Mesoscopic Systems: Effect of Disorder

We study the disorder dependence of the phase coherence time of quasi one-dimensional wires and two-dimensional (2D) Hall bars fabricated from a high mobility GaAs/AlGaAs heterostructure. Using an original ion implantation technique, we can tune the intrinsic disorder felt by the 2D electron gas and continuously vary the system from the semi-ballistic regime to the localized one. In the diffusive regime, the phase coherence time follows a power law as a function of diffusion coefficient as expected in the Fermi liquid theory, without any sign of low temperature saturation. Surprisingly, in the semi-ballistic regime, it becomes independent of the diffusion coefficient. In the strongly localized regime we find a diverging phase coherence time with decreasing temperature, however, with a smaller exponent compared to the weakly localized regime.Comment: 21 pages, 30 figure

The SPHERE data center: a reference for high contrast imaging processing

The objective of the SPHERE Data Center is to optimize the scientific return of SPHERE at the VLT, by providing optimized reduction procedures, services to users and publicly available reduced data. This paper describes our motivation, the implementation of the service (partners, infrastructure and developments), services, description of the on-line data, and future developments. The SPHERE Data Center is operational and has already provided reduced data with a good reactivity to many observers. The first public reduced data have been made available in 2017. The SPHERE Data Center is gathering a strong expertise on SPHERE data and is in a very good position to propose new reduced data in the future, as well as improved reduction procedures.Comment: SF2A proceeding

Effects of zinc supplementation on cognitive function in healthy middle-aged and older adults: the ZENITH study

A randomised double-blind placebo-controlled design was employed to investigate the effects of Zn supplementation on cognitive function in 387 healthy adults aged 55–87 years. Several measures of visual memory, working memory, attention and reaction time were obtained using the Cambridge Automated Neuropsychological Test Battery at baseline and then after 3 and 6 months of 0 (placebo), 15 or 30 mg Zn/d. Younger adults (70 years), and performance improved with practice on some measures. For two out of eight dependent variables, there were significant interactions indicating a beneficial effect (at 3 months only) of both 15 and 30 mg/d on one measure of spatial working memory and a detrimental effect of 15 mg/d on one measure of attention. Further work is required to establish whether these findings generalise to older adults in poorer mental and physical health and with less adequate Zn intake and status than the present sample

Remanence effects in the electrical resistivity of spin glasses

We have measured the low temperature electrical resistivity of Ag : Mn mesoscopic spin glasses prepared by ion implantation with a concentration of 700 ppm. As expected, we observe a clear maximum in the resistivity (T ) at a temperature in good agreement with theoretical predictions. Moreover, we observe remanence effects at very weak magnetic fields for the resistivity below the freezing temperature Tsg: upon Field Cooling (fc), we observe clear deviations of (T ) as compared with the Zero Field Cooling (zfc); such deviations appear even for very small magnetic fields, typically in the Gauss range. This onset of remanence for very weak magnetic fields is reminiscent of the typical signature on magnetic susceptibility measurements of the spin glass transition for this generic glassy system

p75 neurotrophin receptor mediates neuronal cell death by activating GIRK channels through phosphatidylinositol 4,5-bisphosphate

The pan neurotrophin receptor p75(NTR) signals programmed cell death both during nervous system development and after neural trauma and disease in the adult. However, the molecular pathways by which death is mediated remain poorly understood. Here, we show that this cell death is initiated by activation of G-protein-coupled inwardly rectifying potassium (GIRK/Kir3) channels and a consequent potassium efflux. Death signals stimulated by neurotrophin-mediated cleavage of p75(NTR) activate GIRK channels through the generation and binding of phosphatidylinositol 4,5-bisphosphate [PtdIns(4,5)P2/PIP2] to GIRK channels. Both GIRK channel activity and p75(NTR)-mediated neuronal death are inhibited by sequestration of PtdIns(4,5)P2 and application of GIRK channel inhibitors, whereas pertussis toxin treatment has no effect. Thus, p75(NTR) activates GIRK channels without the need for G(i/o)-proteins. Our results demonstrate a novel mode of activation of GIRK channels, representing an early step in the p75(NTR)-mediated cell death pathway and suggesting a function for these channels during nervous system development

Optimally adapted multi-state neural networks trained with noise

The principle of adaptation in a noisy retrieval environment is extended here to a diluted attractor neural network of Q-state neurons trained with noisy data. The network is adapted to an appropriate noisy training overlap and training activity which are determined self-consistently by the optimized retrieval attractor overlap and activity. The optimized storage capacity and the corresponding retriever overlap are considerably enhanced by an adequate threshold in the states. Explicit results for improved optimal performance and new retriever phase diagrams are obtained for Q=3 and Q=4, with coexisting phases over a wide range of thresholds. Most of the interesting results are stable to replica-symmetry-breaking fluctuations.Comment: 22 pages, 5 figures, accepted for publication in PR

The Caviar software package for the astrometric reduction of Cassini ISS images: description and examples

N.J.C. is grateful to the Paris Observatory for funding as an invited researcher at the IMCCE. We thank the FP7-ESPaCE European program for funding under the agreement No. 263466. N.J.C. and C.D.M. thank the Science and Technology Facilities Council (Grant No. ST/P000622/1) for financial support. This work was also supported by the International Space Science Institute (ISSI)

Motion frozen 18F-FDG cardiac PET

BackgroundPET reconstruction incorporating spatially variant 3D Point Spread Function (PSF) improves contrast and image resolution. "Cardiac Motion Frozen" (CMF) processing eliminates the influence of cardiac motion in static summed images. We have evaluated the combined use of CMF- and PSF-based reconstruction for high-resolution cardiac PET.MethodsStatic and 16-bin ECG-gated images of 20 patients referred for (18)F-FDG myocardial viability scans were obtained on a Siemens Biograph-64. CMF was applied to the gated images reconstructed with PSF. Myocardium to blood contrast, maximum left ventricle (LV) counts to defect contrast, contrast-to-noise (CNR) and wall thickness with standard reconstruction (2D-AWOSEM), PSF, ED-gated PSF, and CMF-PSF were compared.ResultsThe measured wall thickness was 18.9 ± 5.2 mm for 2D-AWOSEM, 16.6 ± 4.5 mm for PSF, and 13.8 ± 3.9 mm for CMF-PSF reconstructed images (all P < .05). The CMF-PSF myocardium to blood and maximum LV counts to defect contrasts (5.7 ± 2.7, 10.0 ± 5.7) were higher than for 2D-AWOSEM (3.5 ± 1.4, 6.5 ± 3.1) and for PSF (3.9 ± 1.7, 7.7 ± 3.7) (CMF vs all other, P < .05). The CNR for CMF-PSF (26.3 ± 17.5) was comparable to PSF (29.1 ± 18.3), but higher than for ED-gated dataset (13.7 ± 8.8, P < .05).ConclusionCombined CMF-PSF reconstruction increased myocardium to blood contrast, maximum LV counts to defect contrast and maintained equivalent noise when compared to static summed 2D-AWOSEM and PSF reconstruction