2,327 research outputs found
Towards optimized suppression of dephasing in systems subject to pulse timing constraints
We investigate the effectiveness of different dynamical decoupling protocols
for storage of a single qubit in the presence of a purely dephasing bosonic
bath, with emphasis on comparing quantum coherence preservation under uniform
vs. non-uniform delay times between pulses. In the limit of instantaneous
bit-flip pulses, this is accomplished by establishing a new representation of
the controlled qubit evolution, where the resulting decoherence behaviour is
directly expressed in terms of the free evolution. Simple analytical
expressions are given to approximate the long- and short- term coherence
behaviour for both ohmic and supra-ohmic environments. We focus on systems with
physical constraints on achievable time delays, with emphasis on pure dephasing
of excitonic qubits in quantum dots. Our analysis shows that little advantage
of high-level decoupling schemes based on concatenated or optimal design is to
be expected if operational constraints prevent pulses to be applied
sufficiently fast. In such constrained scenarios, we demonstrate how simple
modifications of repeated periodic echo protocols can offer significantly
improved coherence preservation in realistic parameter regimes.Comment: 13 figures,1 tabl
Decoherence reduction via continuous dynamical decoupling: Analytical study of the role of the noise spectrum
We analyze the robust character against non-static noise of clock transitions
implemented via a method of continuous dynamical decoupling (CDD) in a
hyperfine Zeeman multiplet in ^{87}\textrm{Rb}. The emergence of features
specific to the quadratic corrections to the linear Zeeman effect is evaluated.
Our analytical approach, which combines methods of stochastic analysis with
time-dependent perturbation theory, allows tracing the decoherence process for
generic noise sources. Working first with a basic CDD scheme, it is shown that
the amplitude and frequency of the (driving) field of control can be
appropriately chosen to force the non-static random input to have a
(time-dependent) perturbative character. Moreover, in the dressed-state
picture, the effect of noise is described in terms of an operative random
variable whose properties, dependent on the driving field, can be analytically
characterized. In this framework, the relevance of the spectral density of the
fluctuations to the performance of the CDD technique is precisely assessed. In
particular, the range of noise correlation times where the method of
decoherence reduction is still efficient is identified. The results obtained in
the basic CDD framework are extrapolated to concatenated schemes. The
generality of our approach allows its applicability beyond the specific atomic
system considered
Reionization in Technicolor
We present the Technicolor Dawn simulations, a suite of cosmological
radiation-hydrodynamic simulations of the first 1.2 billion years. By modeling
a spatially-inhomogeneous UVB on-the-fly with 24 frequencies and resolving dark
matter halos down to within 12 Mpc volumes, our
simulations unify observations of the intergalactic and circumgalactic media,
galaxies, and reionization into a common framework. The only empirically-tuned
parameter, the fraction of ionizing photons that
escape the interstellar medium, is adjusted to match observations of the
Lyman- forest and the cosmic microwave background. With this single
calibration, our simulations reproduce the history of reionization; the stellar
mass-star formation rate relation of galaxies; the number density and
metallicity of damped Lyman- absorbers (DLAs) at ; the
abundance of weak metal absorbers; the ultraviolet background (UVB) amplitude;
and the Lyman- flux power spectrum at . The galaxy stellar mass
and UV luminosity functions are underproduced by , suggesting an
overly vigorous feedback model. The mean transmission in the Lyman-
forest is underproduced at , indicating tension between measurements of
the UVB amplitude and Lyman- transmission. The observed SiIV column
density distribution is reasonably well-reproduced ( low). By
contrast, CIV remains significantly underproduced despite being boosted by an
intense Ryd UVB. Solving this problem by increasing metal yields would
overproduce both weak absorbers and DLA metallicities. Instead, the observed
strength of high-ionization emission from high-redshift galaxies and absorption
from their environments suggest that the ionizing flux from conventional
stellar population models is too soft.Comment: 24 pages, 17 figures, accepted to MNRA
Investigation of Port Fuel Injector Spray Mass Distribution by Laser Induced Fluorescence
Modern internal combustion engines have stringent requirements for performance and reduced toxic emissions. The fuel delivery system, and particularly the fuel injectors, have a vital role in reducing unburned hydrocarbons (HC) and carbon monoxide (CO) in exhaust emission.
The main goal of this study is to map the spatial and temporal distribution of the spray from a low-pressure gasoline fuel injector. To attain this goal, three tasks were performed: (1) the experimental investigation of the spray oscillation as functions of operating pressure and injector timing, (2) the determination of the appropriate dye/fuel combinations for one particular experimental technique, and (3) the demonstration of the capabilities of a Computational Fluid Dynamics (CFD) code, Fluent, in the dispersed two-phase flow solutions.
An experimental technique, planar laser induced fluorescence (PLIF), was employed to investigate the spatial and temporal distribution of the spray mass from a set of four-hole, split-stream port fuel injectors. The spatial and temporal spray evolution in a horizontal cross-section was imaged instantaneously via detection of fluorescence intensities. The lateral displacement of the spray mass is clearly displayed in time sequence via the PLIF images, and the spray instability is shown to be sensitively dependent upon small geometric differences along the internal flow paths.
In the course of a study to develop a quantitative PLIF diagnostic for the mass distribution emanating from a liquid fuel injector, spectroscopic results were assembled for certain dye/fuel solutions. Experiments were performed with combinations of hydrocarbon solvents and organic dyes. Results are presented in the form of absorption and emission spectra, including extinction coefficients with error analysis, comparisons with data in the literature, and Stokes shift estimates.
A Computational Fluid Dynamics (CFD) code, Fluent, was employed to demonstrate its capabilities in the solution of dispersed two-phase flows. The dispersed two-phase flow consists of discrete elements surrounded by a continuous phase. The continuous phase equations were solved in an Eulerian reference frame. The Lagrangian approach was used to track packets of discrete phase elements. Inputs of the numerical dispersed two-phase flow model were obtained from the conditions of the PLIF experiments. Two cases were solved with the same input and boundary conditions. In the first case the spray consists of droplets with 100 μm diameter. A linear droplet diameter distribution between 40 and 100 μm was specified in the second case. Results indicate the existence of a core region with higher velocity values for both cases. The core region appears at the spray center close to the injection tip. The increase in the spray temperature towards the outlet boundary is larger for the constant droplet diameter case than the linear droplet diameter distribution case. Negligible evaporation is observed in the solution domain for both cases
Phenomenological Study of Decoherence in Solid-State Spin Qubits due to Nuclear Spin Diffusion
We present a study of the prospects for coherence preservation in solid-state
spin qubits using dynamical decoupling protocols. Recent experiments have
provided the first demonstrations of multipulse dynamical decoupling sequences
in this qubit system, but quantitative analyses of potential coherence
improvements have been hampered by a lack of concrete knowledge of the relevant
noise processes. We present simulations of qubit coherence under the
application of arbitrary dynamical decoupling pulse sequences based on an
experimentally validated semiclassical model. This phenomenological approach
bundles the details of underlying noise processes into a single experimentally
relevant noise power spectral density. Our results show that the dominant
features of experimental measurements in a two-electron singlet-triplet spin
qubit can be replicated using a noise power spectrum associated
with nuclear-spin-flips in the host material. Beginning with this validation we
address the effects of nuclear programming, high-frequency nuclear-spin
dynamics, and other high-frequency classical noise sources, with conjectures
supported by physical arguments and microscopic calculations where relevant.
Our results provide expected performance bounds and identify diagnostic metrics
that can be measured experimentally in order to better elucidate the underlying
nuclear spin dynamics.Comment: Updated References. Related articles at:
http://www.physics.usyd.edu.au/~mbiercuk/Publications.htm
Dynamical decoupling efficiency versus quantum non-Markovianity
We investigate the relationship between non-Markovianity and the
effectiveness of a dynamical decoupling protocol for qubits undergoing pure
dephasing. We consider an exact model in which dephasing arises due to a
bosonic environment with a spectral density of the Ohmic class. This is
parametrised by an Ohmicity parameter by changing which we can model both
Markovian and non-Markovian environments. Interestingly, we find that
engineering a non-Markovian environment is detrimental to the efficiency of the
dynamical decoupling scheme, leading to a worse coherence preservation. We show
that each dynamical decoupling pulse reverses the flow of quantum information
and, on this basis, we investigate the connection between dynamical decoupling
efficiency and the reservoir spectral density. Finally, in the spirit of
reservoir engineering, we investigate the optimum system-reservoir parameters
for achieving maximum stationary coherences.Comment: 6 pages, 4 figure
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