640 research outputs found
Aharonov-Bohm phase as quantum gate in two-electron charge qubits
We analyze the singlet-triplet splitting on a planar array of quantum dots
coupled capacitively to a set of external voltage gates. The system is modelled
using an extended Hubbard Hamiltonian keeping two excess electrons on the
array. The voltage dependence of the low-energy singlet and triplet states is
analyzed using the Feshbach formalism. The formation of a well decoupled
two-level system in the ground state is shown to rely on the fact of having two
particles in the system. Coherent operation of the array is studied with
respect to single quantum bit operations. One quantum gate is implemented via
voltage controls, while for the necessary second quantum gate, a uniform
external magnetic field is introduced. The Aharonov-Bohm phases on the closed
loop tunnel connections in the array are used to effectively suppress the
tunneling, despite a constant tunneling amplitude in the structure. This allows
one to completely stall the qubit in any arbitrary quantum superposition,
providing full control of this interesting quantum system.Comment: 6 pages, 5 figures (submitted to PRB
Sulphur in the protein molecule
METABOLISM EXPERIMENTS WITH METHIONINE:
(a) Feeding experiments with methionine and
a description of a characteristic
syndrone of cystine deficiency in the
albino rat.
(b) An apparatus and method for the
determination of the respiratory
quotient of small animals.
(c) The respiratory quotients of albino
rats on normal and cystine deficient
diets
Potential landscapes and induced charges near metallic islands in three dimensions
We calculate electrostatic potential landscapes for an external probe charge
in the presence of a set of metallic islands. Our numerical calculation in
three dimensions (3D)uses an efficient grid relaxation technique. The
well-known relaxation algorithm for solving the Poisson equation in two
dimensions is generalized to 3D. In addition,all charges on the system, free as
well as induced charges,are determined accurately and self-consistently to
satisfy the desired boundary conditions. This allows the straightforward
calculation of the potential on the outer boundary using the free space
electrostatic Green's function,as well as the calculation of the entire
capacitance matrix of the system. Physically interesting examples of nanoscale
systems are presented and analyzed.Comment: 6 pages, 6 figures, submitted to PR
Charge qubits and limitations of electrostatic quantum gates
We investigate the characteristics of purely electrostatic interactions with
external gates in constructing full single qubit manipulations. The quantum bit
is naturally encoded in the spatial wave function of the electron system.
Single-electron{transistor arrays based on quantum dots or insulating
interfaces typically allow for electrostatic controls where the inter-island
tunneling is considered constant, e.g. determined by the thickness of an
insulating layer. A representative array of 3x3 quantum dots with two mobile
electrons is analyzed using a Hubbard Hamiltonian and a capacitance matrix
formalism. Our study shows that it is easy to realize the first quantum gate
for single qubit operations, but that a second quantum gate only comes at the
cost of compromising the low-energy two-level system needed to encode the
qubit. We use perturbative arguments and the Feshbach formalism to show that
the compromising of the two-level system is a rather general feature for
electrostatically interacting qubits and is not just related to the specific
details of the system chosen. We show further that full implementation requires
tunable tunneling or external magnetic fields.Comment: 7 pages, 5 figures, submitted to PR
Spatial Distribution of the Incompressible Strips at Aharonov-Bohm Interferometer
In this work, the edge physics of an Aharonov-Bohm interferometer (ABI)
defined on a two dimensional electron gas, subject to strong perpendicular
magnetic field B, is investigated. We solve the three dimensional Poisson
equation using numerical techniques starting from the crystal growth parameters
and surface image of the sample. The potential profiles of etched and gate
defined geometries are compared and it is found that the etching yields a
steeper landscape. The spatial distribution of the incompressible strips is
investigated as a function of the gate voltage and applied magnetic field,
where the imposed current is confined to. AB interference is investigated due
to scattering processes between two incompressible "edge-states".Comment: 5 pages, 3 figure
Proposed Rabi-Kondo Correlated State in a Laser-Driven Semiconductor Quantum Dot
Spin exchange between a single-electron charged quantum dot and itinerant
electrons leads to an emergence of Kondo correlations. When the quantum dot is
driven resonantly by weak laser light, the resulting emission spectrum allows
for a direct probe of these correlations. In the opposite limit of vanishing
exchange interaction and strong laser drive, the quantum dot exhibits coherent
oscillations between the single-spin and optically excited states. Here, we
show that the interplay between strong exchange and non-perturbative laser
coupling leads to the formation of a new nonequilibrium quantum-correlated
state, characterized by the emergence of a laser-induced secondary spin
screening cloud, and examine the implications for the emission spectrum
Spatially resolved manipulation of single electrons in quantum dots using a scanned probe
The scanning metallic tip of a scanning force microscope was coupled
capacitively to electrons confined in a lithographically defined gate-tunable
quantum dot at a temperature of 300 mK. Single electrons were made to hop on or
off the dot by moving the tip or by changing the tip bias voltage owing to the
Coulomb-blockade effect. Spatial images of conductance resonances map the
interaction potential between the tip and individual electronic quantum dot
states. Under certain conditions this interaction is found to contain a
tip-voltage induced and a tip-voltage independent contribution.Comment: 4 pages, 4 figure
The trajectory of gait development in mice
OBJECTIVE: Gait irregularities are prevalent in neurodevelopmental disorders (NDDs). However, there is a paucity of information on gait phenotypes in NDD experimental models. This is in part due to the lack of understanding of the normal developmental trajectory of gait maturation in the mouse.
MATERIALS AND METHODS: Using the DigiGait system, we have developed a quantitative, standardized, and reproducible assay of developmental gait metrics in commonly used mouse strains that can be added to the battery of mouse model phenotyping. With this assay, we characterized the trajectory of gait in the developing C57BL/6J and FVB/AntJ mouse lines.
RESULTS: In both lines, a mature stride consisted of 40% swing and 60% stance in the forelimbs, which mirrors the mature human stride. In C57BL/6J mice, developmental trajectories were observed for stance width, paw overlap distance, braking and propulsion time, rate of stance loading, peak paw area, and metrics of intraindividual variability. In FVB/AntJ mice, developmental trajectories were observed for percent shared stance, paw overlap distance, rate of stance loading, and peak paw area, although in different directions than C57 mice. By accounting for the impact of body length on stride measurements, we demonstrate the importance of considering body length when interpreting gait metrics.
CONCLUSION: Overall, our results show that aspects of mouse gait development parallel a timeline of normal human gait development, such as the percent of stride that is stance phase and swing phase. This study may be used as a standard reference for developmental gait phenotyping of murine models, such as models of neurodevelopmental disease
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