1,873 research outputs found
Impact of g-factors and valleys on spin qubits in a silicon double quantum dot
We define single electron spin qubits in a silicon MOS double quantum dot
system. By mapping the qubit resonance frequency as a function of gate-induced
electric field, the spectrum reveals an anticrossing that is consistent with an
inter-valley spin-orbit coupling. We fit the data from which we extract an
inter-valley coupling strength of 43 MHz. In addition, we observe a narrow
resonance near the primary qubit resonance when we operate the device in the
(1,1) charge configuration. The experimental data is consistent with a
simulation involving two weakly exchanged-coupled spins with a g-factor
difference of 1 MHz, of the same order as the Rabi frequency. We conclude that
the narrow resonance is the result of driven transitions between the T- and T+
triplet states, using an ESR signal of frequency located halfway between the
resonance frequencies of the two individual spins. The findings presented here
offer an alternative method of implementing two-qubit gates, of relevance to
the operation of larger scale spin qubit systems
Effects of cosmic rays on single event upsets
Assistance was provided to the Brookhaven Single Event Upset (SEU) Test Facility. Computer codes were developed for fragmentation and secondary radiation affecting Very Large Scale Integration (VLSI) in space. A computer controlled CV (HP4192) test was developed for Terman analysis. Also developed were high speed parametric tests which are independent of operator judgment and a charge pumping technique for measurement of D(sub it) (E). The X-ray secondary effects, and parametric degradation as a function of dose rate were simulated. The SPICE simulation of static RAMs with various resistor filters was tested
A Silicon Surface Code Architecture Resilient Against Leakage Errors
Spin qubits in silicon quantum dots are one of the most promising building
blocks for large scale quantum computers thanks to their high qubit density and
compatibility with the existing semiconductor technologies. High fidelity
single-qubit gates exceeding the threshold of error correction codes like the
surface code have been demonstrated, while two-qubit gates have reached 98\%
fidelity and are improving rapidly. However, there are other types of error ---
such as charge leakage and propagation --- that may occur in quantum dot arrays
and which cannot be corrected by quantum error correction codes, making them
potentially damaging even when their probability is small. We propose a surface
code architecture for silicon quantum dot spin qubits that is robust against
leakage errors by incorporating multi-electron mediator dots. Charge leakage in
the qubit dots is transferred to the mediator dots via charge relaxation
processes and then removed using charge reservoirs attached to the mediators. A
stabiliser-check cycle, optimised for our hardware, then removes the
correlations between the residual physical errors. Through simulations we
obtain the surface code threshold for the charge leakage errors and show that
in our architecture the damage due to charge leakage errors is reduced to a
similar level to that of the usual depolarising gate noise. Spin leakage errors
in our architecture are constrained to only ancilla qubits and can be removed
during quantum error correction via reinitialisations of ancillae, which ensure
the robustness of our architecture against spin leakage as well. Our use of an
elongated mediator dots creates spaces throughout the quantum dot array for
charge reservoirs, measuring devices and control gates, providing the
scalability in the design
Holomorphic symmetric differentials and a birational characterization of Abelian Varieties
A generically generated vector bundle on a smooth projective variety yields a
rational map to a Grassmannian, called Kodaira map. We answer a previous
question, raised by the asymptotic behaviour of such maps, giving rise to a
birational characterization of abelian varieties.
In particular we prove that, under the conjectures of the Minimal Model
Program, a smooth projective variety is birational to an abelian variety if and
only if it has Kodaira dimension 0 and some symmetric power of its cotangent
sheaf is generically generated by its global sections.Comment: UPDATED: more details added on main proo
Role of electron localisation in H adsorption and hydride formation in the Mg basal plane under aqueous corrosion: a first-principles study
Understanding hydrogen-metal interactions is important in various fields of surface science, including the aqueous corrosion of metals. The interaction between atomic H and a Mg surface is a key process for the formation of sub-surface Mg hydride, which may play an important role in Mg aqueous corrosion. In the present work, we performed first-principles Density Functional Theory (DFT) calculations to study the mechanisms for hydrogen adsorption and crystalline Mg hydride formation under aqueous conditions. The Electron Localisation Function (ELF) is found to be a promising indicator for predicting stable H adsorption in the Mg surface. It is found that H adsorption and hydride layer formation is dominated by high ELF adsorption sites. Our calculations suggest that the on-surface adsorption of atomic H, OH radicals and atomic O could enhance the electron localisation at specific sites in the sub-surface region, thus forming effective H traps locally. This is predicted to result in the formation of a thermodynamically stable sub-surface hydride layer, which is a potential precursor of the crucial hydride corrosion product of magnesium
From LTL and Limit-Deterministic B\"uchi Automata to Deterministic Parity Automata
Controller synthesis for general linear temporal logic (LTL) objectives is a
challenging task. The standard approach involves translating the LTL objective
into a deterministic parity automaton (DPA) by means of the Safra-Piterman
construction. One of the challenges is the size of the DPA, which often grows
very fast in practice, and can reach double exponential size in the length of
the LTL formula. In this paper we describe a single exponential translation
from limit-deterministic B\"uchi automata (LDBA) to DPA, and show that it can
be concatenated with a recent efficient translation from LTL to LDBA to yield a
double exponential, \enquote{Safraless} LTL-to-DPA construction. We also report
on an implementation, a comparison with the SPOT library, and performance on
several sets of formulas, including instances from the 2016 SyntComp
competition
Integrated silicon qubit platform with single-spin addressability, exchange control and robust single-shot singlet-triplet readout
Silicon quantum dot spin qubits provide a promising platform for large-scale
quantum computation because of their compatibility with conventional CMOS
manufacturing and the long coherence times accessible using Si enriched
material. A scalable error-corrected quantum processor, however, will require
control of many qubits in parallel, while performing error detection across the
constituent qubits. Spin resonance techniques are a convenient path to parallel
two-axis control, while Pauli spin blockade can be used to realize local parity
measurements for error detection. Despite this, silicon qubit implementations
have so far focused on either single-spin resonance control, or control and
measurement via voltage-pulse detuning in the two-spin singlet-triplet basis,
but not both simultaneously. Here, we demonstrate an integrated device platform
incorporating a silicon metal-oxide-semiconductor double quantum dot that is
capable of single-spin addressing and control via electron spin resonance,
combined with high-fidelity spin readout in the singlet-triplet basis.Comment: 10 pages, 4 figure
Visual Impairment/Intracranial Pressure Risk Assessment
Since 2006 there have been 6 reported cases of altered visual acuity and intracranial pressure (ICP) in long duration astronauts. In order to document this risk and develop an integrated approach to its mitigation, the NASA Space Life Sciences Directorate (SLSD) and Human Research Program (HRP) have chosen to use the Human System Risk Board (HSRB) and the risk management analysis tool (RMAT). The HSRB is the venue in which the stakeholders and customers discuss and vet the evidence and the RMAT is the tool that facilitates documentation and comparison of the evidence across mission profiles as well as identification of risk factors, and documentation of mitigation strategies. This process allows for information to be brought forward and dispositioned so that it may be properly incorporated into the RMAT and contribute to the design of the research and mitigation plans. The evidence thus far has resulted in the identification of a visual impairment/intracranial pressure (VIIP) project team, updating of both short and long duration medical requirements designed to assess visual acuity, and a research plan to characterize this issue further. In order to understand this issue more completely, a plan to develop an Accelerated Research Collaboration (ARC) has been approved by the HSRB. The ARC is a novel research model pioneered by the Myelin Repair Foundation. It is a patient centered research model that brings together researchers and clinicians, under the guidance of a scientific advisory panel, to collaborate and produce results much quickly than accomplished through traditional research models. The data and evidence from the updated medical requirements and the VIIP ARC will be reviewed at the HSRB on a regular basis. Each review package presented to the HSRB will include an assessment and recommendation with respect to continuation of research, countermeasure development, occupational surveillance modalities, selection criteria, etc. This process will determine the course of the VIIP project and ultimately how SLSD and HRP mitigate this emerging human health and performance risk
Topological properties of punctual Hilbert schemes of almost-complex fourfolds (I)
In this article, we study topological properties of Voisin's punctual Hilbert
schemes of an almost-complex fourfold . In this setting, we compute their
Betti numbers and construct Nakajima operators. We also define tautological
bundles associated with any complex bundle on , which are shown to be
canonical in -theory
Complex diffuse radio emission in the merging PLANCK ESZ cluster Abell 3411
We present VLA radio and Chandra X-ray observations of the merging galaxy
cluster Abell 3411. For the cluster, we find an overall temperature of 6.4 keV
and an X-ray luminosity of 2.8 x 10^{44} erg s^{-1} between 0.5 and 2.0 keV.
The Chandra observation reveals the cluster to be undergoing a merger event.
The VLA observations show the presence of large-scale diffuse emission in the
central region of the cluster, which we classify as a 0.9 Mpc size radio halo.
In addition, a complex region of diffuse, polarized emission is found in the
southeastern outskirts of the cluster, along the projected merger axis of the
system. We classify this region of diffuse emission as a radio relic. The total
extent of this radio relic is 1.9 Mpc. For the combined emission in the cluster
region, we find a radio spectral index of -1.0 \pm 0.1 between 74 MHz and 1.4
GHz. The morphology of the radio relic is peculiar, as the relic is broken up
into five fragments. This suggests that the shock responsible for the relic has
been broken up due to interaction with a large-scale galaxy filament connected
to the cluster or other substructures in the ICM. Alternatively, the complex
morphology reflects the presence of electrons in fossil radio bubbles that are
re-accelerated by a shock.Comment: Accepted for publication in ApJ, 8 pages, 5 figure
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