2,052 research outputs found
Optically induced spin gates in coupled quantum dots using the electron-hole exchange interaction
We propose a fast optically induced two-qubit \textsc{c-phase} gate between
two resident spins in a pair of coupled quantum dots. An excited bound state
which extends over the two dots provides an effective electron-electron
exchange interaction. The gate is made possible by the electron-hole exchange
interaction, which isolates a single transition in the system. When combined
with appropriate single qubit rotations, this gate generates an entangled state
of the two spins
Fast Two-Qubit Gates in Semiconductor Quantum Dots using a Photonic Microcavity
Implementations for quantum computing require fast single- and multi-qubit
quantum gate operations. In the case of optically controlled quantum dot qubits
theoretical designs for long-range two- or multi-qubit operations satisfying
all the requirements in quantum computing are not yet available. We have
developed a design for a fast, long-range two-qubit gate mediated by a photonic
microcavity mode using excited states of the quantum dot-cavity system that
addresses these needs. This design does not require identical qubits, it is
compatible with available optically induced single qubit operations, and it
advances opportunities for scalable architectures. We show that the gate
fidelity can exceed 90% in experimentally accessible systems
Carbon Ignition in Type Ia Supernovae: An Analytic Model
The observable properties of a Type Ia supernova are sensitive to how the
nuclear runaway ignites in a Chandrasekhar mass white dwarf - at a single point
at its center, off-center, or at multiple points and times. We present a simple
analytic model for the runaway based upon a combination of stellar
mixing-length theory and recent advances in understanding Rayleigh-Benard
convection. The convective flow just prior to runaway is likely to have a
strong dipolar component, though higher multipoles may contribute appreciably
at the very high Rayleigh number (10) appropriate to the white dwarf
core. A likely outcome is multi-point ignition with an exponentially increasing
number of ignition points during the few tenths of a second that it takes the
runaway to develop. The first sparks ignite approximately 150 - 200 km off
center, followed by ignition at smaller radii. Rotation may be important to
break the dipole asymmetry of the ignition and give a healthy explosion.Comment: 14 pages, 0 figures, submitted to ApJ, corrected typo in first
author's nam
The impact of beam deconvolution on noise properties in CMB measurements: Application to Planck LFI
We present an analysis of the effects of beam deconvolution on noise
properties in CMB measurements. The analysis is built around the artDeco beam
deconvolver code. We derive a low-resolution noise covariance matrix that
describes the residual noise in deconvolution products, both in harmonic and
pixel space. The matrix models the residual correlated noise that remains in
time-ordered data after destriping, and the effect of deconvolution on it. To
validate the results, we generate noise simulations that mimic the data from
the Planck LFI instrument. A test for the full 70 GHz covariance in
multipole range yields a mean reduced of 1.0037. We
compare two destriping options, full and independent destriping, when
deconvolving subsets of available data. Full destriping leaves substantially
less residual noise, but leaves data sets intercorrelated. We derive also a
white noise covariance matrix that provides an approximation of the full noise
at high multipoles, and study the properties on high-resolution noise in pixel
space through simulations.Comment: 22 pages, 25 figure
Detailed Spectral Modeling of a 3-D Pulsating Reverse Detonation Model: Too Much Nickel
We calculate detailed NLTE synthetic spectra of a Pulsating Reverse
Detonation (PRD) model, a novel explosion mechanism for Type Ia supernovae.
While the hydro models are calculated in 3-D, the spectra use an angle averaged
hydro model and thus some of the 3-D details are lost, but the overall average
should be a good representation of the average observed spectra. We study the
model at 3 epochs: maximum light, seven days prior to maximum light, and 5 days
after maximum light. At maximum the defining Si II feature is prominent, but
there is also a prominent C II feature, not usually observed in normal SNe Ia
near maximum. We compare to the early spectrum of SN 2006D which did show a
prominent C II feature, but the fit to the observations is not compelling.
Finally we compare to the post-maximum UV+optical spectrum of SN 1992A. With
the broad spectral coverage it is clear that the iron-peak elements on the
outside of the model push too much flux to the red and thus the particular PRD
realizations studied would be intrinsically far redder than observed SNe Ia. We
briefly discuss variations that could improve future PRD models.Comment: 15 pages, 4 figures, submitted to Ap
Surface detonation in type Ia supernova explosions?
We explore the evolution of thermonuclear supernova explosions when the
progenitor white dwarf star ignites asymmetrically off-center. Several
numerical simulations are carried out in two and three dimensions to test the
consequences of different initial flame configurations such as spherical
bubbles displaced from the center, more complex deformed configurations, and
teardrop-shaped ignitions. The burning bubbles float towards the surface while
releasing energy due to the nuclear reactions. If the energy release is too
small to gravitationally unbind the star, the ash sweeps around it, once the
burning bubble approaches the surface. Collisions in the fuel on the opposite
side increase its temperature and density and may -- in some cases -- initiate
a detonation wave which will then propagate inward burning the core of the star
and leading to a strong explosion. However, for initial setups in two
dimensions that seem realistic from pre-ignition evolution, as well as for all
three-dimensional simulations the collimation of the surface material is found
to be too weak to trigger a detonation.Comment: 5 pages, 3 figures, in: Proceedings of the SciDAC 2006 Meeting,
Denver June 25-26 2006, also available at
http://herald.iop.org/jpcs46/m51/gbr//link/40
On the Evolution of Thermonuclear Flames on Large Scales
The thermonuclear explosion of a massive white dwarf in a Type Ia supernova
explosion is characterized by vastly disparate spatial and temporal scales. The
extreme dynamic range inherent to the problem prevents the use of direct
numerical simulation and forces modelers to resort to subgrid models to
describe physical processes taking place on unresolved scales.
We consider the evolution of a model thermonuclear flame in a constant
gravitational field on a periodic domain. The gravitational acceleration is
aligned with the overall direction of the flame propagation, making the flame
surface subject to the Rayleigh-Taylor instability. The flame evolution is
followed through an extended initial transient phase well into the steady-state
regime. The properties of the evolution of flame surface are examined. We
confirm the form of the governing equation of the evolution suggested by
Khokhlov (1995). The mechanism of vorticity production and the interaction
between vortices and the flame surface are discussed. The results of our
investigation provide the bases for revising and extending previous
subgrid-scale model.Comment: 15 pages, 22 postscript figures. Accepted for publication by the
Astrophysical Journal. High resolution figures can be found at
http://flash.uchicago.edu/~zhang/research_paper.htm
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