507 research outputs found
Exciton lifetime in InAs/GaAs quantum dot molecules
The exciton lifetimes in arrays of InAs/GaAs vertically coupled quantum
dot pairs have been measured by time-resolved photoluminescence. A considerable
reduction of by up to a factor of 2 has been observed as compared
to a quantum dots reference, reflecting the inter-dot coherence. Increase of
the molecular coupling strength leads to a systematic decrease of with
decreasing barrier width, as for wide barriers a fraction of structures shows
reduced coupling while for narrow barriers all molecules appear to be well
coupled. The coherent excitons in the molecules gain the oscillator strength of
the excitons in the two separate quantum dots halving the exciton lifetime.
This superradiance effect contributes to the previously observed increase of
the homogeneous exciton linewidth, but is weaker than the reduction of .
This shows that as compared to the quantum dots reference pure dephasing
becomes increasingly important for the molecules
From laterally modulated two-dimensional electron gas towards artificial graphene
Cyclotron resonance has been measured in far-infrared transmission of
GaAs/AlGaAs heterostructures with an etched hexagonal lateral
superlattice. Non-linear dependence of the resonance position on magnetic field
was observed as well as its splitting into several modes. Our explanation,
based on a perturbative calculation, describes the observed phenomena as a weak
effect of the lateral potential on the two-dimensional electron gas. Using this
approach, we found a correlation between parameters of the lateral patterning
and the created effective potential and obtain thus insights on how the
electronic miniband structure has been tuned. The miniband dispersion was
calculated using a simplified model and allowed us to formulate four basic
criteria that have to be satisfied to reach graphene-like physics in such
systems
Classical percolation fingerprints in the high-temperature regime of the integer quantum Hall effect
We have performed magnetotransport experiments in the high-temperature regime
(up to 50 K) of the integer quantum Hall effect for two-dimensional electron
gases in semiconducting heterostructures. While the magnetic field dependence
of the classical Hall law presents no anomaly at high temperatures, we find a
breakdown of the Drude-Lorentz law for the longitudinal conductance beyond a
crossover magnetic field B_c ~ 1 T, which turns out to be correlated with the
onset of the integer quantum Hall effect at low temperatures. We show that the
high magnetic field regime at B > B_c can be understood in terms of classical
percolative transport in a smooth disordered potential. From the temperature
dependence of the peak longitudinal conductance, we extract scaling exponents
which are in good agreement with the theoretically expected values. We also
prove that inelastic scattering on phonons is responsible for dissipation in a
wide temperature range going from 1 to 50 K at high magnetic fields.Comment: 14 pages + 8 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
All-optical generation of states for "Encoding a qubit in an oscillator"
Both discrete and continuous systems can be used to encode quantum
information. Most quantum computation schemes propose encoding qubits in
two-level systems, such as a two-level atom or an electron spin. Others exploit
the use of an infinite-dimensional system, such as a harmonic oscillator. In
"Encoding a qubit in an oscillator" [Phys. Rev. A 64 012310 (2001)], Gottesman,
Kitaev, and Preskill (GKP) combined these approaches when they proposed a
fault-tolerant quantum computation scheme in which a qubit is encoded in the
continuous position and momentum degrees of freedom of an oscillator. One
advantage of this scheme is that it can be performed by use of relatively
simple linear optical devices, squeezing, and homodyne detection. However, we
lack a practical method to prepare the initial GKP states. Here we propose the
generation of an approximate GKP state by using superpositions of optical
coherent states (sometimes called "Schr\"odinger cat states"), squeezing,
linear optical devices, and homodyne detection.Comment: 4 pages, 3 figures. Submitted to Optics Letter
Pulsed squeezed light: simultaneous squeezing of multiple modes
We analyze the spectral properties of squeezed light produced by means of
pulsed, single-pass degenerate parametric down-conversion. The multimode output
of this process can be decomposed into characteristic modes undergoing
independent squeezing evolution akin to the Schmidt decomposition of the
biphoton spectrum. The main features of this decomposition can be understood
using a simple analytical model developed in the perturbative regime. In the
strong pumping regime, for which the perturbative approach is not valid, we
present a numerical analysis, specializing to the case of one-dimensional
propagation in a beta-barium borate waveguide. Characterization of the
squeezing modes provides us with an insight necessary for optimizing homodyne
detection of squeezing. For a weak parametric process, efficient squeezing is
found in a broad range of local oscillator modes, whereas the intense
generation regime places much more stringent conditions on the local
oscillator. We point out that without meeting these conditions, the detected
squeezing can actually diminish with the increasing pumping strength, and we
expose physical reasons behind this inefficiency
Noise reduction in 3D noncollinear parametric amplifier
We analytically find an approximate Bloch-Messiah reduction of a noncollinear
parametric amplifier pumped with a focused monochromatic beam. We consider type
I phase matching. The results are obtained using a perturbative expansion and
scaled to a high gain regime. They allow a straightforward maximization of the
signal gain and minimization of the parametric fluorescence noise. We find the
fundamental mode of the amplifier, which is an elliptic Gaussian defining the
optimal seed beam shape. We conclude that the output of the amplifier should be
stripped of higher order modes, which are approximately Hermite-Gaussian beams.
Alternatively, the pump waist can be adjusted such that the amount of noise
produced in the higher order modes is minimized.Comment: 18 pages, 9 figures, accepted to Applied Physics
Quantum interference and phonon-mediated back-action in lateral quantum dot circuits
Spin qubits have been successfully realized in electrostatically defined,
lateral few-electron quantum dot circuits. Qubit readout typically involves
spin to charge information conversion, followed by a charge measurement made
using a nearby biased quantum point contact. It is critical to understand the
back-action disturbances resulting from such a measurement approach. Previous
studies have indicated that quantum point contact detectors emit phonons which
are then absorbed by nearby qubits. We report here the observation of a
pronounced back-action effect in multiple dot circuits where the absorption of
detector-generated phonons is strongly modified by a quantum interference
effect, and show that the phenomenon is well described by a theory
incorporating both the quantum point contact and coherent phonon absorption.
Our combined experimental and theoretical results suggest strategies to
suppress back-action during the qubit readout procedure.Comment: 25 pages, 8 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 role of septal perforators and "myocardial bridging effect" in atherosclerotic plaque distribution in the coronary artery disease
The distribution of atherosclerotic plaque burden in the human coronary arteries is not uniform. Plaques are located mostly in the left anterior descending artery (LAD), then in the right coronary artery (RCA), circumflex branch (LCx) and the left main coronary artery (LM) in a decreasing order of frequency. In the LAD and LCx, plaques tend to cluster within the proximal segment, while in the RCA their distribution is more uniform. Several factors have been involved in this phenomenon, particularly flow patterns in the left and right coronary artery. Nevertheless, it does not explain the difference in lesion frequency between the LAD and the LCx as these are both parts of the left coronary artery. Branching points are considered to be the risk points of atherosclerosis. In the LCx, the number of side branches is lower than in the LAD or RCA and there are no septal perforators with intramuscular courses like in the proximal third of the LAD and the posterior descending artery (PDA). We hypothesized that septal branches generate disturbed flow in the LAD and PDA in a similar fashion to the myocardial bridge (myocardial bridging effect). This coronary architecture determines the non-uniform plaque distribution in coronary arteries and LAD predisposition to plaque formation
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