1,501 research outputs found
Quantum diffusion of dipole-oriented indirect excitons in coupled quantum wells
A model for diffusion of statistically-degenerate excitons in (coupled)
quantum wells is proposed and analysed. Within a microscopic approach, we
derive a quantum diffusion equation, calculate and estimate the self-diffusion
coefficient for excitons in quantum wells and derive a modified Einstein
relation adapted to statistically-degenerated quasi-two-dimensional bosons. It
is also shown that the dipole-dipole interaction of indirect excitons
effectively screens long-range-correlated disorder in quantum wells. Numerical
calculations are given for indirect excitons in GaAs/AlGaAs coupled quantum
wells.Comment: To appear in Europhysics Letter
Quantum and Classical Chaos in Kicked Coupled Jaynes-Cummings Cavities
We consider two Jaynes-Cummings cavities coupled periodically with a photon
hopping term. The semi-classical phase space is chaotic, with regions of
stability over some ranges of the parameters. The quantum case exhibits dynamic
localization and dynamic tunneling between classically forbidden regions. We
explore the correspondence between the classical and quantum phase space and
propose a scheme for implementing the system experimentally
Chaos assisted adiabatic passage
We study the exact dynamics underlying stimulated Raman adiabatic passage
(STIRAP) for a particle in a multi-level anharmonic system (the infinite
square-well) driven by two sequential laser pulses, each with constant carrier
frequency. In phase space regions where the laser pulses create chaos, the
particle can be transferred coherently into energy states different from those
predicted by traditional STIRAP. It appears that a transition to chaos can
provide a new tool to control the outcome of STIRAP
Scanning-gate-induced effects and spatial mapping of a cavity
Tailored electrostatic potentials are the foundation of scanning gate
microscopy. We present several aspects of the tip-induced potential on the
two-dimensional electron gas. First, we give methods on how to estimate the
size of the tip-induced potential. Then, a ballistic cavity is formed and
studied as a function of the bias-voltage of the metallic top gates and probed
with the tip-induced potential. It is shown how the potential of the cavity
changes by tuning the system to a regime where conductance quantization in the
constrictions formed by the tip and the top gates occurs. This conductance
quantization leads to a unprecedented rich fringe pattern over the entire
structure. Finally, the effect of electrostatic screening of the metallic top
gates is discussed.Comment: 10 pages, 6 figure
Quantum Phase Transitions and Bipartite Entanglement
We develop a general theory of the relation between quantum phase transitions
(QPTs) characterized by nonanalyticities in the energy and bipartite
entanglement. We derive a functional relation between the matrix elements of
two-particle reduced density matrices and the eigenvalues of general two-body
Hamiltonians of -level systems. The ground state energy eigenvalue and its
derivatives, whose non-analyticity characterizes a QPT, are directly tied to
bipartite entanglement measures. We show that first-order QPTs are signalled by
density matrix elements themselves and second-order QPTs by the first
derivative of density matrix elements. Our general conclusions are illustrated
via several quantum spin models.Comment: 5 pages, incl. 2 figures. v3: The version published in PRL, including
a few extra comments and clarifications for which there was no space in the
PR
Imaging magnetoelectric subbands in ballistic constrictions
We perform scanning gate experiments on ballistic constrictions in the
presence of small perpendicular magnetic fields. The constrictions form the
entrance and exit of a circular gate-defined ballistic stadium. Close to
constrictions we observe sets of regular fringes creating a checker board
pattern. Inside the stadium conductance fluctuations governed by chaotic
dynamics of electrons are visible. The checker board pattern allows us to
determine the number of transmitted modes in the constrictions forming between
the tip-induced potential and gate-defined geometry. Spatial investigation of
the fringe pattern in a perpendicular magnetic field shows a transition from
electrostatic to magnetic depopulation of magnetoelectric subbands. Classical
and quantum simulations agree well with different aspects of our observations.Comment: 18 pages, 7 figure
Impact of Quantum Phase Transitions on Excited Level Dynamics
The influence of quantum phase transitions on the evolution of excited levels
in the critical parameter region is discussed. The analysis is performed for 1D
and 2D systems with first- and second-order ground-state transitions. Examples
include the cusp and nuclear collective Hamiltonians.Comment: 6 pages, 4 figure
Locally induced quantum interference in scanning gate experiments
We present conductance measurements of a ballistic circular stadium
influenced by a scanning gate. When the tip depletes the electron gas below, we
observe very pronounced and regular fringes covering the entire stadium. The
fringes correspond to transmitted modes in constrictions formed between the
tip-induced potential and the boundaries of the stadium. Moving the tip and
counting the fringes gives us exquisite control over the transmission of these
constrictions. We use this control to form a quantum ring with a specific
number of modes in each arm showing the Aharonov-Bohm effect in low-field
magnetoconductance measurements.Comment: 10 pages, 4 figure
Electron backscattering in a cavity: ballistic and coherent effects
Numerous experimental and theoretical studies have focused on low-dimensional
systems locally perturbed by the biased tip of a scanning force microscope. In
all cases either open or closed weakly gate-tunable nanostructures have been
investigated, such as quantum point contacts, open or closed quantum dots, etc.
We study the behaviour of the conductance of a quantum point contact with a
gradually forming adjacent cavity in series under the influence of a scanning
gate. Here, an initially open quantum point contact system gradually turns into
a closed cavity system. We observe branches and interference fringes known from
quantum point contacts coexisting with irregular conductance fluctuations.
Unlike the branches, the fluctuations cover the entire area of the cavity. In
contrast to previous studies, we observe and investigate branches under the
influence of the confining stadium potential, which is gradually built up. We
find that the branches exist only in the area surrounded by cavity top gates.
As the stadium shrinks, regular fringes originate from tip-induced
constrictions leading to quantized conduction. In addition, we observe arc-like
areas reminiscent of classical electron trajectories in a chaotic cavity. We
also argue that electrons emanating from the quantum point contact spread out
like a fan leaving branch-like regions of enhanced backscattering.Comment: 7 pages, 4 figure
Nanosecond-timescale spin transfer using individual electrons in a quadruple-quantum-dot device
The ability to coherently transport electron-spin states between different
sites of gate-defined semiconductor quantum dots is an essential ingredient for
a quantum-dot-based quantum computer. Previous shuttles using electrostatic
gating were too slow to move an electron within the spin dephasing time across
an array. Here we report a nanosecond-timescale spin transfer of individual
electrons across a quadruple-quantum-dot device. Utilizing enhanced relaxation
rates at a so-called `hot spot', we can upper bound the shuttle time to at most
150 ns. While actual shuttle times are likely shorter, 150 ns is already fast
enough to preserve spin coherence in e.g. silicon based quantum dots. This work
therefore realizes an important prerequisite for coherent spin transfer in
quantum dot arrays.Comment: 7 pages including 2 pages of supplementary materia
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