79 research outputs found
The influence of the long-lived quantum Hall potential on the characteristics of quantum devices
Novel hysteretic effects are reported in magneto-transport experiments on
lateral quantum devices. The effects are characterized by two vastly different
relaxation times (minutes and days). It is shown that the observed phenomena
are related to long-lived eddy currents. This is confirmed by torsion-balance
magnetometry measurements of the same 2-dimensional electron gas (2DEG)
material. These observations show that the induced quantum Hall potential at
the edges of the 2DEG reservoirs influences transport through the devices, and
have important consequences for the magneto-transport of all lateral quantum
devices.Comment: 5 pages, 4 figure
Bipolar spin blockade and coherent state superpositions in a triple quantum dot
Spin qubits based on interacting spins in double quantum dots have been
successfully demonstrated. Readout of the qubit state involves a conversion of
spin to charge information, universally achieved by taking advantage of a spin
blockade phenomenon resulting from Pauli's exclusion principle. The archetypal
spin blockade transport signature in double quantum dots takes the form of a
rectified current. Currently more complex spin qubit circuits including triple
quantum dots are being developed. Here we show both experimentally and
theoretically (a) that in a linear triple quantum dot circuit, the spin
blockade becomes bipolar with current strongly suppressed in both bias
directions and (b) that a new quantum coherent mechanism becomes relevant.
Within this mechanism charge is transferred non-intuitively via coherent states
from one end of the linear triple dot circuit to the other without involving
the centre site. Our results have implications in future complex
nano-spintronic circuits.Comment: 21 pages, 7 figure
Coulomb and Spin blockade of two few-electrons quantum dots in series in the co-tunneling regime
We present Coulomb Blockade measurements of two few-electron quantum dots in
series which are configured such that the electrochemical potential of one of
the two dots is aligned with spin-selective leads. The charge transfer through
the system requires co-tunneling through the second dot which is in
resonance with the leads. The observed amplitude modulation of the resulting
current is found to reflect spin blockade events occurring through either of
the two dots. We also confirm that charge redistribution events occurring in
the off-resonance dot are detected indirectly via changes in the
electrochemical potential of the aligned dot.Comment: 6 pages, 5 figures, submitted to Phys. Rev.
The origin of switching noise in GaAs/AlGaAs lateral gated devices
We have studied the origin of switching (telegraph) noise at low temperature
in lateral quantum structures defined electrostatically in GaAs/AlGaAs
heterostructures by surface gates. The noise was measured by monitoring the
conductance fluctuations around on the first step of a quantum point
contact at around 1.2 K. Cooling with a positive bias on the gates dramatically
reduces this noise, while an asymmetric bias exacerbates it. We propose a model
in which the noise originates from a leakage current of electrons that tunnel
through the Schottky barrier under the gate into the doped layer. The key to
reducing noise is to keep this barrier opaque under experimental conditions.
Bias cooling reduces the density of ionized donors, which builds in an
effective negative gate voltage. A smaller negative bias is therefore needed to
reach the desired operating point. This suppresses tunnelling from the gate and
hence the noise. The reduction in the density of ionized donors also
strengthens the barrier to tunneling at a given applied voltage. Support for
the model comes from our direct observation of the leakage current into a
closed quantum dot, around for this device. The current
was detected by a neighboring quantum point contact, which showed monotonic
steps in time associated with the tunneling of single electrons into the dot.
If asymmetric gate voltages are applied, our model suggests that the noise will
increase as a consequence of the more negative gate voltage applied to one of
the gates to maintain the same device conductance. We observe exactly this
behaviour in our experiments.Comment: 8 pages, 7 figure
Coherent control of three-spin states in a triple quantum dot
Spin qubits involving individual spins in single quantum dots or coupled
spins in double quantum dots have emerged as potential building blocks for
quantum information processing applications. It has been suggested that triple
quantum dots may provide additional tools and functionalities. These include
the encoding of information to either obtain protection from decoherence or to
permit all-electrical operation, efficient spin busing across a quantum
circuit, and to enable quantum error correction utilizing the three-spin
Greenberger-Horn-Zeilinger quantum state. Towards these goals we demonstrate
for the first time coherent manipulation between two interacting three-spin
states. We employ the Landau-Zener-St\"uckelberg approach for creating and
manipulating coherent superpositions of quantum states. We confirm that we are
able to maintain coherence when decreasing the exchange coupling of one spin
with another while simultaneously increasing its coupling with the third. Such
control of pairwise exchange is a requirement of most spin qubit architectures
but has not been previously demonstrated.Comment: 12 pages, 13 figures, and 2 table
A two-stage genome-wide association study of sporadic amyotrophic lateral sclerosis
The cause of sporadic amyotrophic lateral sclerosis (ALS) is largely unknown, but genetic factors are thought to play a significant role in determining susceptibility to motor neuron degeneration. To identify genetic variants altering risk of ALS, we undertook a two-stage genome-wide association study (GWAS): we followed our initial GWAS of 545 066 SNPs in 553 individuals with ALS and 2338 controls by testing the 7600 most associated SNPs from the first stage in three independent cohorts consisting of 2160 cases and 3008 controls. None of the SNPs selected for replication exceeded the Bonferroni threshold for significance. The two most significantly associated SNPs, rs2708909 and rs2708851 [odds ratio (OR) = 1.17 and 1.18, and P-values = 6.98 x 10–7 and 1.16 x 10–6], were located on chromosome 7p13.3 within a 175 kb linkage disequilibrium block containing the SUNC1, HUS1 and C7orf57 genes. These associations did not achieve genome-wide significance in the original cohort and failed to replicate in an additional independent cohort of 989 US cases and 327 controls (OR = 1.18 and 1.19, P-values = 0.08 and 0.06, respectively). Thus, we chose to cautiously interpret our data as hypothesis-generating requiring additional confirmation, especially as all previously reported loci for ALS have failed to replicate successfully. Indeed, the three loci (FGGY, ITPR2 and DPP6) identified in previous GWAS of sporadic ALS were not significantly associated with disease in our study. Our findings suggest that ALS is more genetically and clinically heterogeneous than previously recognized. Genotype data from our study have been made available online to facilitate such future endeavors
Large tunable valley splitting in edge-free graphene quantum dots on boron nitride
Coherent manipulation of binary degrees of freedom is at the heart of modern
quantum technologies. Graphene offers two binary degrees: the electron spin and
the valley. Efficient spin control has been demonstrated in many solid state
systems, while exploitation of the valley has only recently been started, yet
without control on the single electron level. Here, we show that van-der Waals
stacking of graphene onto hexagonal boron nitride offers a natural platform for
valley control. We use a graphene quantum dot induced by the tip of a scanning
tunneling microscope and demonstrate valley splitting that is tunable from -5
to +10 meV (including valley inversion) by sub-10-nm displacements of the
quantum dot position. This boosts the range of controlled valley splitting by
about one order of magnitude. The tunable inversion of spin and valley states
should enable coherent superposition of these degrees of freedom as a first
step towards graphene-based qubits
Performance issues in optical burst/packet switching
The final publication is available at Springer via http://dx.doi.org/10.1007/978-3-642-01524-3_8This chapter summarises the activities on optical packet switching (OPS) and optical burst switching (OBS) carried out by the COST 291 partners in the last 4 years. It consists of an introduction, five sections with contributions on five different specific topics, and a final section dedicated to the conclusions. Each section contains an introductive state-of-the-art description of the specific topic and at least one contribution on that topic. The conclusions give some points on the current situation of the OPS/OBS paradigms
An Inducer of VGF Protects Cells against ER Stress-Induced Cell Death and Prolongs Survival in the Mutant SOD1 Animal Models of Familial ALS
Amyotrophic lateral sclerosis (ALS) is the most frequent adult-onset motor neuron disease, and recent evidence has suggested that endoplasmic reticulum (ER) stress signaling is involved in the pathogenesis of ALS. Here we identified a small molecule, SUN N8075, which has a marked protective effect on ER stress-induced cell death, in an in vitro cell-based screening, and its protective mechanism was mediated by an induction of VGF nerve growth factor inducible (VGF): VGF knockdown with siRNA completely abolished the protective effect of SUN N8075 against ER-induced cell death, and overexpression of VGF inhibited ER-stress-induced cell death. VGF level was lower in the spinal cords of sporadic ALS patients than in the control patients. Furthermore, SUN N8075 slowed disease progression and prolonged survival in mutant SOD1 transgenic mouse and rat models of ALS, preventing the decrease of VGF expression in the spinal cords of ALS mice. These data suggest that VGF plays a critical role in motor neuron survival and may be a potential new therapeutic target for ALS, and SUN N8075 may become a potential therapeutic candidate for treatment of ALS
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