7,606 research outputs found
Collective modes in the fluxonium qubit
Superconducting qubit designs vary in complexity from single- and
few-junction systems, such as the transmon and flux qubits, to the
many-junction fluxonium. Here we consider the question of wether the many
degrees of freedom in the fluxonium circuit can limit the qubit coherence time.
Such a limitation is in principle possible, due to the interactions between the
low-energy, highly anharmonic qubit mode and the higher-energy, weakly
anharmonic collective modes. We show that so long as the coupling of the
collective modes with the external electromagnetic environment is sufficiently
weaker than the qubit-environment coupling, the qubit dephasing induced by the
collective modes does not significantly contribute to decoherence. Therefore,
the increased complexity of the fluxonium qubit does not constitute by itself a
major obstacle for its use in quantum computation architectures.Comment: 22 pages, 15 figure
Asymmetric Totally-corrective Boosting for Real-time Object Detection
Real-time object detection is one of the core problems in computer vision.
The cascade boosting framework proposed by Viola and Jones has become the
standard for this problem. In this framework, the learning goal for each node
is asymmetric, which is required to achieve a high detection rate and a
moderate false positive rate. We develop new boosting algorithms to address
this asymmetric learning problem. We show that our methods explicitly optimize
asymmetric loss objectives in a totally corrective fashion. The methods are
totally corrective in the sense that the coefficients of all selected weak
classifiers are updated at each iteration. In contract, conventional boosting
like AdaBoost is stage-wise in that only the current weak classifier's
coefficient is updated. At the heart of the totally corrective boosting is the
column generation technique. Experiments on face detection show that our
methods outperform the state-of-the-art asymmetric boosting methods.Comment: 14 pages, published in Asian Conf. Computer Vision 201
How does gas cool in DM halos?
In order to study the process of cooling in dark-matter (DM) halos and assess
how well simple models can represent it, we run a set of radiative SPH
hydrodynamical simulations of isolated halos, with gas sitting initially in
hydrostatic equilibrium within Navarro-Frenk-White (NFW) potential wells. [...]
After having assessed the numerical stability of the simulations, we compare
the resulting evolution of the cooled mass with the predictions of the
classical cooling model of White & Frenk and of the cooling model proposed in
the MORGANA code of galaxy formation. We find that the classical model predicts
fractions of cooled mass which, after about two central cooling times, are
about one order of magnitude smaller than those found in simulations. Although
this difference decreases with time, after 8 central cooling times, when
simulations are stopped, the difference still amounts to a factor of 2-3. We
ascribe this difference to the lack of validity of the assumption that a mass
shell takes one cooling time, as computed on the initial conditions, to cool to
very low temperature. [...] The MORGANA model [...] better agrees with the
cooled mass fraction found in the simulations, especially at early times, when
the density profile of the cooling gas is shallow. With the addition of the
simple assumption that the increase of the radius of the cooling region is
counteracted by a shrinking at the sound speed, the MORGANA model is also able
to reproduce for all simulations the evolution of the cooled mass fraction to
within 20-50 per cent, thereby providing a substantial improvement with respect
to the classical model. Finally, we provide a very simple fitting function
which accurately reproduces the cooling flow for the first ~10 central cooling
times. [Abridged]Comment: 15 pages, accepted by MNRA
Random laser from engineered nanostructures obtained by surface tension driven lithography
The random laser emission from the functionalized thienyl-S,S-dioxide
quinquethiophene (T5OCx) in confined patterns with different shapes is
demonstrated. Functional patterning of the light emitter organic material in
well defined features is obtained by spontaneous molecular self-assembly guided
by surface tension driven (STD) lithography. Such controlled supramolecular
nano-aggregates act as scattering centers allowing the fabrication of
one-component organic lasers with no external resonator and with desired shape
and efficiency. Atomic force microscopy shows that different geometric pattern
with different supramolecular organization obtained by the lithographic process
tailors the coherent emission properties by controlling the distribution and
the size of the random scatterers
Thermopower in the Coulomb blockade regime for Laughlin quantum dots
Using the conformal field theory partition function of a Coulomb-blockaded
quantum dot, constructed by two quantum point contacts in a Laughlin quantum
Hall bar, we derive the finite-temperature thermodynamic expression for the
thermopower in the linear-response regime. The low-temperature results for the
thermopower are compared to those for the conductance and their capability to
reveal the structure of the single-electron spectrum in the quantum dot is
analyzed.Comment: 11 pages, 3 figures, Proceedings of the 10-th International Workshop
"Lie Theory and Its Applications in Physics", 17-23 June 2013, Varna,
Bulgari
Quantum Chaos, Delocalization, and Entanglement in Disordered Heisenberg Models
We investigate disordered one- and two-dimensional Heisenberg spin lattices
across a transition from integrability to quantum chaos from both a statistical
many-body and a quantum-information perspective. Special emphasis is devoted to
quantitatively exploring the interplay between eigenvector statistics,
delocalization, and entanglement in the presence of nontrivial symmetries. The
implications of basis dependence of state delocalization indicators (such as
the number of principal components) is addressed, and a measure of {\em
relative delocalization} is proposed in order to robustly characterize the
onset of chaos in the presence of disorder. Both standard multipartite and {\em
generalized entanglement} are investigated in a wide parameter regime by using
a family of spin- and fermion- purity measures, their dependence on
delocalization and on energy spectrum statistics being examined. A distinctive
{\em correlation between entanglement, delocalization, and integrability} is
uncovered, which may be generic to systems described by the two-body random
ensemble and may point to a new diagnostic tool for quantum chaos. Analytical
estimates for typical entanglement of random pure states restricted to a proper
subspace of the full Hilbert space are also established and compared with
random matrix theory predictions.Comment: 17 pages, 10 figures, revised versio
Dynamical Generation of Noiseless Quantum Subsystems
We present control schemes for open quantum systems that combine decoupling
and universal control methods with coding procedures. By exploiting a general
algebraic approach, we show how appropriate encodings of quantum states result
in obtaining universal control over dynamically-generated noise-protected
subsystems with limited control resources. In particular, we provide an
efficient scheme for performing universal encoded quantum computation in a wide
class of systems subjected to linear non-Markovian quantum noise and supporting
Heisenberg-type internal Hamiltonians.Comment: 4 pages, no figures; REVTeX styl
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