826 research outputs found
Exact Master Equation and Non-Markovian Decoherence for Quantum Dot Quantum Computing
In this article, we report the recent progress on decoherence dynamics of
electrons in quantum dot quantum computing systems using the exact master
equation we derived recently based on the Feynman-Vernon influence functional
approach. The exact master equation is valid for general nanostructure systems
coupled to multi-reservoirs with arbitrary spectral densities, temperatures and
biases. We take the double quantum dot charge qubit system as a specific
example, and discuss in details the decoherence dynamics of the charge qubit
under coherence controls. The decoherence dynamics risen from the entanglement
between the system and the environment is mainly non-Markovian. We further
discuss the decoherence of the double-dot charge qubit induced by quantum point
contact (QPC) measurement where the master equation is re-derived using the
Keldysh non-equilibrium Green function technique due to the non-linear coupling
between the charge qubit and the QPC. The non-Markovian decoherence dynamics in
the measurement processes is extensively discussed as well.Comment: 15 pages, Invited article for the special issue "Quantum Decoherence
and Entanglement" in Quantum Inf. Proces
Free-induction decay and envelope modulations in a narrowed nuclear spin bath
We evaluate free-induction decay for the transverse components of a localized
electron spin coupled to a bath of nuclear spins via the Fermi contact
hyperfine interaction. Our perturbative treatment is valid for special
(narrowed) bath initial conditions and when the Zeeman energy of the electron
exceeds the total hyperfine coupling constant : . Using one unified
and systematic method, we recover previous results reported at short and long
times using different techniques. We find a new and unexpected modulation of
the free-induction-decay envelope, which is present even for a purely isotropic
hyperfine interaction without spin echoes and for a single nuclear species. We
give sub-leading corrections to the decoherence rate, and show that, in
general, the decoherence rate has a non-monotonic dependence on electron Zeeman
splitting, leading to a pronounced maximum. These results illustrate the
limitations of methods that make use of leading-order effective Hamiltonians
and re-exponentiation of short-time expansions for a strongly-interacting
system with non-Markovian (history-dependent) dynamics.Comment: 13 pages, 9 figure
Adiabatic information transport in the presence of decoherence
We study adiabatic population transfer between discrete positions. Being
closely related to STIRAP in optical systems, this transport is coherent and
robust against variations of experimental parameters. Thanks to these
properties the scheme is a promising candidate for transport of quantum
information in quantum computing. We study the effects of spatially registered
noise sources on the quantum transport and in particular model Markovian
decoherence via non-local coupling to nearby quantum point contacts which serve
as information readouts. We find that the rate of decoherence experienced by a
spatial superposition initially grows with spatial separation but surprisingly
then plateaus. In addition we include non-Markovian effects due to couplings to
nearby two level systems and we find that although the population transport
exhibits robustness in the presence of both types of noise sources, the
transport of a spatial superposition exhibits severe fragility.Comment: 11page
Microscopic theory of indistinguishable single-photon emission from a quantum dot coupled to a cavity: The role of non-Markovian phonon-induced decoherence
We study the fundamental limit on single-photon indistinguishability imposed
by decoherence due to phonon interactions in semiconductor quantum dot-cavity
QED systems. Employing an exact diagonalization approach we find large
differences compared to standard methods. An important finding is that
short-time non-Markovian effects limit the maximal attainable
indistinguishability. The results are explained using a polariton picture that
yields valuable insight into the phonon-induced dephasing dynamics.Comment: published version, comments are very welcom
Open system dynamics with non-Markovian quantum trajectories
A non-Markovian stochastic Schroedinger equation for a quantum system coupled
to an environment of harmonic oscillators is presented. Its solutions, when
averaged over the noise, reproduce the standard reduced density operator
without any approximation. We illustrate the power of this approach with
several examples, including exponentially decaying bath correlations and
extreme non-Markovian cases, where the `environment' consists of only a single
oscillator. The latter case shows the decay and revival of a `Schroedinger cat'
state. For strong coupling to a dissipative environment with memory, the
asymptotic state can be reached in a finite time. Our description of open
systems is compatible with different positions of the `Heisenberg cut' between
system and environment.Comment: 4 pages RevTeX, 3 figure
- …