776 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
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
Dynamic Model and Phase Transitions for Liquid Helium
This article presents a phenomenological dynamic phase transition theory --
modeling and analysis -- for superfluids. As we know, although the
time-dependent Ginzburg-Landau model has been successfully used in
superconductivity, and the classical Ginzburg-Landau free energy is still
poorly applicable to liquid helium in a quantitative sense. The study in this
article is based on 1) a new dynamic classification scheme of phase
transitions, 2) new time-dependent Ginzburg-Landau models for general
equilibrium transitions, and 3) the general dynamic transition theory. The
results in this article predict the existence of a unstable region H, where
both solid and liquid He II states appear randomly depending on fluctuations
and the existence of a switch point M on the lambda-curve, where the
transitions changes types
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
Coherent shuttle of electron-spin states
We demonstrate a coherent spin shuttle through a GaAs/AlGaAs
quadruple-quantum-dot array. Starting with two electrons in a spin-singlet
state in the first dot, we shuttle one electron over to either the second,
third or fourth dot. We observe that the separated spin-singlet evolves
periodically into the spin-triplet and back before it dephases due to
nuclear spin noise. We attribute the time evolution to differences in the local
Zeeman splitting between the respective dots. With the help of numerical
simulations, we analyse and discuss the visibility of the singlet-triplet
oscillations and connect it to the requirements for coherent spin shuttling in
terms of the inter-dot tunnel coupling strength and rise time of the pulses.
The distribution of entangled spin pairs through tunnel coupled structures may
be of great utility for connecting distant qubit registers on a chip.Comment: 21 pages, 10 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
Giant current fluctuations in an overheated single electron transistor
Interplay of cotunneling and single-electron tunneling in a thermally
isolated single-electron transistor (SET) leads to peculiar overheating
effects. In particular, there is an interesting crossover interval where the
competition between cotunneling and single-electron tunneling changes to the
dominance of the latter. In this interval, the current exhibits anomalous
sensitivity to the effective electron temperature of the transistor island and
its fluctuations. We present a detailed study of the current and temperature
fluctuations at this interesting point. The methods implemented allow for a
complete characterization of the distribution of the fluctuating quantities,
well beyond the Gaussian approximation. We reveal and explore the parameter
range where, for sufficiently small transistor islands, the current
fluctuations become gigantic. In this regime, the optimal value of the current,
its expectation value, and its standard deviation differ from each other by
parametrically large factors. This situation is unique for transport in
nanostructures and for electron transport in general. The origin of this
spectacular effect is the exponential sensitivity of the current to the
fluctuating effective temperature.Comment: 10 pages, 11 figure
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