14 research outputs found
Isotope sensitive measurement of the hole-nuclear spin interaction in quantum dots
Decoherence caused by nuclear field fluctuations is a fundamental obstacle to
the realization of quantum information processing using single electron spins.
Alternative proposals have been made to use spin qubits based on valence band
holes having weaker hyperfine coupling. However, it was demonstrated recently
both theoretically and experimentally that the hole hyperfine interaction is
not negligible, although a consistent picture of the mechanism controlling the
magnitude of the hole-nuclear coupling is still lacking. Here we address this
problem by performing isotope selective measurement of the valence band
hyperfine coupling in InGaAs/GaAs, InP/GaInP and GaAs/AlGaAs quantum dots.
Contrary to existing models we find that the hole hyperfine constant along the
growth direction of the structure (normalized by the electron hyperfine
constant) has opposite signs for different isotopes and ranges from -15% to
+15%. We attribute such changes in hole hyperfine constants to the competing
positive contributions of p-symmetry atomic orbitals and the negative
contributions of d-orbitals. Furthermore, we find that the d-symmetry
contribution leads to a new mechanism for hole-nuclear spin flips which may
play an important role in hole spin decoherence. In addition the measured
hyperfine constants enable a fundamentally new approach for verification of the
computed Bloch wavefunctions in the vicinity of nuclei in semiconductor
nanostructures
Reduced microvascular density in omental biopsies of children with chronic kidney disease
Endothelial dysfunction is an early manifestation of cardiovascular disease (CVD) and consistently observed in patients with chronic kidney disease (CKD). We hypothesized that CKD is associated with systemic damage to the microcirculation, preceding macrovascular pathology. To assess the degree of "uremic microangiopathy", we have measured microvascular density in biopsies of the omentum of children with CKD.Omental tissue was collected from 32 healthy children (0-18 years) undergoing elective abdominal surgery and from 23 age-matched cases with stage 5 CKD at the time of catheter insertion for initiation of peritoneal dialysis. Biopsies were analyzed by independent observers using either a manual or an automated imaging system for the assessment of microvascular density. Quantitative immunohistochemistry was performed for markers of autophagy and apoptosis, and for the abundance of the angiogenesis-regulating proteins VEGF-A, VEGF-R2, Angpt1 and Angpt2.Microvascular density was significantly reduced in uremic children compared to healthy controls, both by manual imaging with a digital microscope (median surface area 0.61% vs. 0.95%, p<0.0021 and by automated quantification (total microvascular surface area 0.89% vs. 1.17% p = 0.01). Density measured by manual imaging was significantly associated with age, height, weight and body surface area in CKD patients and healthy controls. In multivariate analysis, age and serum creatinine level were the only independent, significant predictors of microvascular density (r2 = 0.73). There was no immunohistochemical evidence for apoptosis or autophagy. Quantitative staining showed similar expression levels of the angiogenesis regulators VEGF-A, VEGF-receptor 2 and Angpt1 (p = 0.11), but Angpt2 was significantly lower in CKD children (p = 0.01).Microvascular density is profoundly reduced in omental biopsies of children with stage 5 CKD and associated with diminished Angpt2 signaling. Microvascular rarefaction could be an early systemic manifestation of CKD-induced cardiovascular disease
Suppression of nuclear spin bath fluctuations in self-assembled quantum dots induced by inhomogeneous strain
Interaction with nuclear spins leads to decoherence and information loss in solid-state electron-spin qubits. One particular, ineradicable source of electron decoherence arises from decoherence of the nuclear spin bath, driven by nuclear–nuclear dipolar interactions. Owing to its many-body nature nuclear decoherence is difficult to predict, especially for an important class of strained nanostructures where nuclear quadrupolar effects have a significant but largely unknown impact. Here, we report direct measurement of nuclear spin bath coherence in individual self-assembled ​InGaAs/​GaAs quantum dots: spin-echo coherence times in the range 1.2–4.5 ms are found. Based on these values, we demonstrate that strain-induced quadrupolar interactions make nuclear spin fluctuations much slower compared with lattice-matched ​GaAs/​AlGaAs structures. Our findings demonstrate that quadrupolar effects can potentially be used to engineer optically active III-V semiconductor spin-qubits with a nearly noise-free nuclear spin bath, previously achievable only in nuclear spin-0 semiconductors, where qubit network interconnection and scaling are challenging