5,149 research outputs found
Neutral coding - A report based on an NRP work session
Neural coding by impulses and trains on single and multiple channels, and representation of information in nonimpulse carrier
Asymptotically Optimal Quantum Circuits for d-level Systems
As a qubit is a two-level quantum system whose state space is spanned by |0>,
|1>, so a qudit is a d-level quantum system whose state space is spanned by
|0>,...,|d-1>. Quantum computation has stimulated much recent interest in
algorithms factoring unitary evolutions of an n-qubit state space into
component two-particle unitary evolutions. In the absence of symmetry, Shende,
Markov and Bullock use Sard's theorem to prove that at least C 4^n two-qubit
unitary evolutions are required, while Vartiainen, Moettoenen, and Salomaa
(VMS) use the QR matrix factorization and Gray codes in an optimal order
construction involving two-particle evolutions. In this work, we note that
Sard's theorem demands C d^{2n} two-qudit unitary evolutions to construct a
generic (symmetry-less) n-qudit evolution. However, the VMS result applied to
virtual-qubits only recovers optimal order in the case that d is a power of
two. We further construct a QR decomposition for d-multi-level quantum logics,
proving a sharp asymptotic of Theta(d^{2n}) two-qudit gates and thus closing
the complexity question for all d-level systems (d finite.) Gray codes are not
required, and the optimal Theta(d^{2n}) asymptotic also applies to gate
libraries where two-qudit interactions are restricted by a choice of certain
architectures.Comment: 18 pages, 5 figures (very detailed.) MatLab files for factoring qudit
unitary into gates in MATLAB directory of source arxiv format. v2: minor
change
Dark Matter from Early Decays
Two leading dark matter candidates from supersymmetry and other theories of
physics beyond the standard model are WIMPs and weak scale gravitinos. If the
lightest stable particle is a gravitino, then a WIMP will decay into it with a
natural lifetime of order a month ~ M_{pl}^2/M_{weak}^3. We show that if the
bulk of dark matter today came from decays of neutral particles with lifetimes
of order a year or smaller, then it could lead to a reduction in the amount of
small scale substructure, less concentrated halos and constant density cores in
the smallest mass halos. Such beneficial effects may therefore be realized
naturally, as discussed by Cembranos, Feng, Rajaraman, and Takayama, in the
case of supersymmetry.Comment: Matches version accepted for publication in PRD. Added a paragraph to
Sec V. 9 pages, 3 figure
On the Kauffman bracket skein module of the quaternionic manifold
We use recoupling theory to study the Kauffman bracket skein module of the
quaternionic manifold over Z[A,A^{-1}] localized by inverting all the
cyclotomic polynomials. We prove that the skein module is spanned by five
elements. Using the quantum invariants of these skein elements and the Z_2
homology of the manifold, we determine that they are linearly independent.Comment: corrected summation signs in figures 14, 15, 17. Other minor change
Core Formation in Dwarf Halos with Self Interacting Dark Matter: No Fine-Tuning Necessary
We investigate the effect of self-interacting dark matter (SIDM) on the
density profiles of isolated dwarf dark
matter halos -- the scale of relevance for the too big to fail problem (TBTF)
-- using very high-resolution cosmological zoom simulations. Each halo has
millions of particles within its virial radius. We find that SIDM models with
cross sections per unit mass spanning the range \sigma/m =
alleviate TBTF and produce constant density cores of size
300-1000 pc, comparable to the half-light radii of ~
dwarfs. The largest, lowest density cores develop for cross sections
in the middle of this range, \sigma/m ~ . Our largest SIDM
cross section run (\sigma/m = ) develops a slightly denser core
owing to mild core-collapse behavior, but it remains less dense than the CDM
case and retains a constant density core profile. Our work suggests that SIDM
cross sections as large or larger than remain viable on
velocity scales of dwarf galaxies ( ~ ). The range
of SIDM cross sections that alleviate TBTF and the cusp/core problem spans at
least two orders of magnitude and therefore need not be particularly
fine-tuned.Comment: 9 pages, 7 figure
Efficient decomposition of quantum gates
Optimal implementation of quantum gates is crucial for designing a quantum
computer. We consider the matrix representation of an arbitrary multiqubit
gate. By ordering the basis vectors using the Gray code, we construct the
quantum circuit which is optimal in the sense of fully controlled single-qubit
gates and yet is equivalent with the multiqubit gate. In the second step of the
optimization, superfluous control bits are eliminated, which eventually results
in a smaller total number of the elementary gates. In our scheme the number of
controlled NOT gates is which coincides with the theoretical lower
bound.Comment: 4 pages, 2 figure
Venus climate stability and volcanic resurfacing rates
The climate of Venus is to a large degree controlled by the radiative properties of its massive atmosphere. In addition, outgassing due to volcanic activity, exospheric escape processes, and surface/atmosphere interactions may all be important in moderating the abundances of atmospheric CO2 and other volatiles. We have developed an evolutionary climate model for Venus using a systems approach that emphasizes feedbacks between elements in the climate system. Modules for atmospheric radiative transfer, surface/atmosphere interactions, tropospheric chemistry, and exospheric escape processes have so far been developed. Climate feedback loops result from interconnections between modules, in the form of the environmental parameters pressure, temperature, and atmospheric mixing ratios. The radiative transfer module has been implemented by using Rosseland mean opacities in a one dimensional grey radiative-convective model. The model has been solved for the static (time independent) case to determine climate equilibrium points. The dynamics of the model have also been explored by employing reaction/diffusion kinetics for possible surface atmosphere heterogeneous reactions over geologic timescales. It was found that under current conditions, the model predicts that the climate of Venus is at or near an unstable equilibrium point. The effects of constant rate volcanism and corresponding exsolution of volatiles on the stability of the climate model were also explored
3D stellar kinematics at the Galactic center: measuring the nuclear star cluster spatial density profile, black hole mass, and distance
We present 3D kinematic observations of stars within the central 0.5 pc of
the Milky Way nuclear star cluster using adaptive optics imaging and
spectroscopy from the Keck telescopes. Recent observations have shown that the
cluster has a shallower surface density profile than expected for a dynamically
relaxed cusp, leading to important implications for its formation and
evolution. However, the true three dimensional profile of the cluster is
unknown due to the difficulty in de-projecting the stellar number counts. Here,
we use spherical Jeans modeling of individual proper motions and radial
velocities to constrain for the first time, the de-projected spatial density
profile, cluster velocity anisotropy, black hole mass (), and
distance to the Galactic center () simultaneously. We find that the inner
stellar density profile of the late-type stars, to
have a power law slope , much more shallow than
the frequently assumed Bahcall Wolf slope of . The measured
slope will significantly affect dynamical predictions involving the cluster,
such as the dynamical friction time scale. The cluster core must be larger than
0.5 pc, which disfavors some scenarios for its origin. Our measurement of
and
kpc is consistent with that derived from stellar
orbits within 1 of Sgr A*. When combined with the orbit of
S0-2, the uncertainty on is reduced by 30% ( kpc).
We suggest that the MW NSC can be used in the future in combination with
stellar orbits to significantly improve constraints on .Comment: 7 pages, 3 figures, 2 tables, ApJL accepte
Looking for the Charged Higgs Boson
This review article starts with a brief introduction to the charged Higgs
boson (H^\pm) in the Minimal Supersymmetric Standard Model (MSSM). It then
discusses the prospects of a relatively light H^\pm boson search via top quark
decay at Tevatron/LHC, and finally a heavy H^\pm boson search at LHC. The
viable channels for H^\pm search are identified in both the cases, with
particular emphasis on the H^\pm --> tau + nu decay channel. The effects of NLO
QCD correction in the SM as well as the MSSM are discussed briefly.Comment: 17 pages with 8 eps figures, Invited review, Reference adde
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