1,460 research outputs found
Simulation of a non-invasive charge detector for quantum cellular automata
Information in a Quantum Cellular Automata architecture is encoded in the
polarizazion state of a cell, i.e., in the occupation numbers of the quantum
dots of which the cell is made up. Non-invasive charge detectors of single
electrons in a quantum dot are therefore needed, and recent experiments have
shown that a quantum constriction electrostatically coupled to the quantum dot
may be a viable solution. We have performed a numerical simulation of a system
made of a quantum dot and a nearby quantum point contact defined, by means of
depleting metal gates, in a two-dimensional electron gas at a GaAs/AlGaAs
heterointerface. We have computed the occupancy of each dot and the resistance
of the quantum wire as a function of the voltage applied to the plunger gate,
and have derived design criteria for achieving optimal sensitivity.Comment: 8 pages, RevTeX, epsf, 5 figure
Ingestive behaviour and physiology of the medicinal leech
Ingestion lasts 25 min in Hirudo medicinalis and is characterized by pharyngeal peristalsis which fills the crop. This peristalsis has an initial rate of 2.4 Hz which decays smoothly to 1.2 Hz at termination of ingestion. During ingestion, the leech body wall undergoes peristalsis which appears to aid in filling the crop diverticula. Body peristalsis begins at a rate of 10 min^(-1) and decreases linearly to 2 min^(-1) at termination. The body also undergoes dorsoventral flexions when blood flow is occluded. Blood meal size increases slightly with leech size: 8.4 g for 1-g leeches and 9.7 g for 2-g leeches. However, relative meal size decreases markedly with increasing animal size; from 8.15 times body mass for 1-g to 4.80 times for 2-g leeches. When intact leeches were exposed to micromolar concentrations of serotonin, there was an increase in the rate of pharyngeal peristalsis and the size of the blood meals. Leeches excrete the plasma from their ingested blood meals. Excretion is activated during ingestion, which increases feeding efficiency by increasing the proportion of blood cells in the ingestate. Excretion continues for 4–6 days following ingestion, removing all the remaining plasma from the ingestate. Leech ingestion comprises stereotyped muscular movements, secretion of saliva and excretion of plasma. A strikingly similar feeding physiology is seen in the blood-sucking insect Rhodnius, and we suggest that efficient sanguivory may require the convergent evolution of similar ingestive mechanisms
Worthwhile work? Childcare, feminist ethics and cooperative research practices
Interdisciplinary research collaborations are often encouraged within higher education while the practicalities of such collaborations are glossed over. This project specifically addresses the praxis of research collaborations, exploring how feminist academics within different countries and disciplines came together to explore their mutual concern about the perceived worth and well-being of early childhood practitioners. Engaging in a formal methodological dialogue over eight months, seven academics discussed, analysed and dissected their different investments in research methods and intents, with the aim of agreeing to a common methodological framework. Unexpectedly, what emerged was not a product, but a process. We argue that this process offers much to those seeking deep collaboration in and through shared research. Building on a collective research interest, we found ourselves in a process of becoming, germinating the seed of a transnational research cooperative, based on trust and mutual respect, rather than the arid methodological contract originally envisioned
Bias spectroscopy and simultaneous SET charge state detection of Si:P double dots
We report a detailed study of low-temperature (mK) transport properties of a
silicon double-dot system fabricated by phosphorous ion implantation. The
device under study consists of two phosphorous nanoscale islands doped to above
the metal-insulator transition, separated from each other and the source and
drain reservoirs by nominally undoped (intrinsic) silicon tunnel barriers.
Metallic control gates, together with an Al-AlOx single-electron transistor,
were positioned on the substrate surface, capacitively coupled to the buried
dots. The individual double-dot charge states were probed using source-drain
bias spectroscopy combined with non-invasive SET charge sensing. The system was
measured in linear (VSD = 0) and non-linear (VSD 0) regimes allowing
calculations of the relevant capacitances. Simultaneous detection using both
SET sensing and source-drain current measurements was demonstrated, providing a
valuable combination for the analysis of the system. Evolution of the triple
points with applied bias was observed using both charge and current sensing.
Coulomb diamonds, showing the interplay between the Coulomb charging effects of
the two dots, were measured using simultaneous detection and compared with
numerical simulations.Comment: 7 pages, 6 figure
Coherent electronic transfer in quantum dot systems using adiabatic passage
We describe a scheme for using an all-electrical, rapid, adiabatic population
transfer between two spatially separated dots in a triple-quantum dot system.
The electron spends no time in the middle dot and does not change its energy
during the transfer process. Although a coherent population transfer method,
this scheme may well prove useful in incoherent electronic computation (for
example quantum-dot cellular automata) where it may provide a coherent
advantage to an otherwise incoherent device. It can also be thought of as a
limiting case of type II quantum computing, where sufficient coherence exists
for a single gate operation, but not for the preservation of superpositions
after the operation. We extend our analysis to the case of many intervening
dots and address the issue of transporting quantum information through a
multi-dot system.Comment: Replaced with (approximately) the published versio
Entangled Electronic States in Multiple Quantum-Dot Systems
We present an analytically solvable model of colinear, two-dimensional
quantum dots, each containing two electrons. Inter-dot coupling via the
electron-electron interaction gives rise to sets of entangled ground states.
These ground states have crystal-like inter-plane correlations and arise
discontinously with increasing magnetic field. Their ranges and stabilities are
found to depend on dot size ratios, and to increase with .Comment: To appear in Physical Review B (in press). RevTeX file. Figures
available from [email protected]
Bound States and Threshold Resonances in Quantum Wires with Circular Bends
We study the solutions to the wave equation in a two-dimensional tube of unit
width comprised of two straight regions connected by a region of constant
curvature. We introduce a numerical method which permits high accuracy at high
curvature. We determine the bound state energies as well as the transmission
and reflection matrices, and and focus on the nature of
the resonances which occur in the vicinity of channel thresholds. We explore
the dependence of these solutions on the curvature of the tube and angle of the
bend and discuss several limiting cases where our numerical results confirm
analytic predictions.Comment: 24 pages, revtex file, one style file and 17 PostScript figures
include
Silicon Atomic Quantum Dots Enable Beyond-CMOS Electronics
We review our recent efforts in building atom-scale quantum-dot cellular
automata circuits on a silicon surface. Our building block consists of silicon
dangling bond on a H-Si(001) surface, which has been shown to act as a quantum
dot. First the fabrication, experimental imaging, and charging character of the
dangling bond are discussed. We then show how precise assemblies of such dots
can be created to form artificial molecules. Such complex structures can be
used as systems with custom optical properties, circuit elements for
quantum-dot cellular automata, and quantum computing. Considerations on
macro-to-atom connections are discussed.Comment: 28 pages, 19 figure
Dynamical control of correlated states in a square quantum dot
In the limit of low particle density, electrons confined to a quantum dot
form strongly correlated states termed Wigner molecules, in which the Coulomb
interaction causes the electrons to become highly localized in space. By using
an effective model of Hubbard-type to describe these states, we investigate how
an oscillatory electric field can drive the dynamics of a two-electron Wigner
molecule held in a square quantum dot. We find that, for certain combinations
of frequency and strength of the applied field, the tunneling between various
charge configurations can be strongly quenched, and we relate this phenomenon
to the presence of anti-crossings in the Floquet quasi-energy spectrum. We
further obtain simple analytic expressions for the location of these
anti-crossings, which allows the effective parameters for a given quantum dot
to be directly measured in experiment, and suggests the exciting possibility of
using ac-fields to control the time evolution of entangled states in mesoscopic
devices.Comment: Replaced with version to be published in Phys. Rev.
Magnetization of noncircular quantum dots
We calculate the magnetization of quantum dots deviating from circular
symmetry for noninteracting electrons or electrons interacting according to the
Hartree approximation. For few electrons the magnetization is found to depend
on their number, and the shape of the dot. The magnetization is an ideal probe
into the many-electron state of a quantum dot.Comment: 11 RevTeX pages with 6 included Postscript figure
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