2,239 research outputs found
On the Commutative Equivalence of Context-Free Languages
The problem of the commutative equivalence of context-free and regular languages is studied. In particular conditions ensuring that a context-free language of exponential growth is commutatively equivalent with a regular language are investigated
Quantum measurement of a mesoscopic spin ensemble
We describe a method for precise estimation of the polarization of a
mesoscopic spin ensemble by using its coupling to a single two-level system.
Our approach requires a minimal number of measurements on the two-level system
for a given measurement precision. We consider the application of this method
to the case of nuclear spin ensemble defined by a single electron-charged
quantum dot: we show that decreasing the electron spin dephasing due to nuclei
and increasing the fidelity of nuclear-spin-based quantum memory could be
within the reach of present day experiments.Comment: 8 pages, 2 figures; minor changes, published versio
Symbolic Dynamics Analysis of the Lorenz Equations
Recent progress of symbolic dynamics of one- and especially two-dimensional
maps has enabled us to construct symbolic dynamics for systems of ordinary
differential equations (ODEs). Numerical study under the guidance of symbolic
dynamics is capable to yield global results on chaotic and periodic regimes in
systems of dissipative ODEs which cannot be obtained neither by purely
analytical means nor by numerical work alone. By constructing symbolic dynamics
of 1D and 2D maps from the Poincare sections all unstable periodic orbits up to
a given length at a fixed parameter set may be located and all stable periodic
orbits up to a given length may be found in a wide parameter range. This
knowledge, in turn, tells much about the nature of the chaotic limits. Applied
to the Lorenz equations, this approach has led to a nomenclature, i.e.,
absolute periods and symbolic names, of stable and unstable periodic orbits for
an autonomous system. Symmetry breakings and restorations as well as
coexistence of different regimes are also analyzed by using symbolic dynamics.Comment: 35 pages, LaTeX, 13 Postscript figures, uses psfig.tex. The revision
concerns a bug at the end of hlzfig12.ps which prevented the printing of the
whole .ps file from page 2
Nitrate, sulphate and chloride contents in public drinking water supplies in Sicily, Italy
Water samples collected from public
drinking water supplies in Sicily were analysed
for electric conductivity and for their chloride,
sulphate and nitrate contents. The samples were
collected as uniformly as possible from throughout the Sicilian territory, with an average sampling density of about one sample for every 7,600
inhabitants. Chloride contents that ranged from
5.53 to 1,302 mg/l were correlated strongly with
electric conductivity, a parameter used as a proxy
for water salinity. The highest values are attributable to seawater contamination along the coasts
of the island. High chloride and sulphate values
attributable to evaporitic rock dissolution were
found in the central part of Sicily. The nitrate
concentrations ranged from 0.05 to 296 mg/l, with
31 samples (4.7% of the total) exceeding the
maximum admissible concentration of 50 mg/l.
Anomalous samples always came from areas of
intensive agricultural usage, indicating a clear anthropogenic origin. The same parameters were
also measured in bottled water sold in Sicily, and
they all were within the ranges for public drinking
water supplies. The calculated mean nitrate intake
from consuming public water supplies (16.1 mg/l)
did not differ significantly from that of bottled
water (15.2 mg/l). Although the quality of public
water supplies needs to be improved by eliminating those that do not comply with the current
drinking water limits, at present it does not justify
the high consumption of bottled water (at least for
nitrate contents)
Time-optimal rotation of a spin 1/2: application to the NV center spin in diamond
We study the applicability of the time optimal bang-bang control designed for
spin-1/2 [U. Boscain and P. Mason, J. Math. Phys. {\bf 47}, 062101 (2006)] to
the rotation of the electron spin of a nitrogen-vacancy (NV) center in diamond.
The spin of the NV center is a three-level system, with two levels forming a
relevant qubit subspace where the time-varying magnetic control field performs
rotation, and the third level being idle. We find that the bang-bang control
protocol decreases the rotation time by 20--25% in comparison with the
traditional oscillating sinusoidal driving. We also find that for most values
of the bias field the leakage to the idle level is very small, so that the NV
center is a suitable testbed for experimental study of the time-optimal
protocols. For some special values of the bias field, however, the unwanted
leakage is greatly increased. We demonstrate that this is caused by the
resonance with higher-order Fourier harmonics of the bang-bang driving field.Comment: 6 pages, 4 figure
Dynamical Quantum Error Correction of Unitary Operations with Bounded Controls
Dynamically corrected gates were recently introduced [Khodjasteh and Viola,
Phys. Rev. Lett. 102, 080501 (2009)] as a tool to achieve decoherence-protected
quantum gates based on open-loop Hamiltonian engineering. Here, we further
expand the framework of dynamical quantum error correction, with emphasis on
elucidating under what conditions decoherence suppression can be ensured while
performing a generic target quantum gate, using only available bounded-strength
control resources. Explicit constructions for physically relevant error models
are detailed, including arbitrary linear decoherence and pure dephasing on
qubits. The effectiveness of dynamically corrected gates in an illustrative
non-Markovian spin-bath setting is investigated numerically, confirming the
expected fidelity performance in a wide parameter range. Robutness against a
class of systematic control errors is automatically incorporated in the
perturbative error regime.Comment: 21 pages, 7 figures (errors fixed, figures added, text updated
CaloCube: a novel calorimeter for high-energy cosmic rays in space
In order to extend the direct observation of high-energy cosmic rays up to
the PeV region, highly performing calorimeters with large geometrical
acceptance and high energy resolution are required. Within the constraint of
the total mass of the apparatus, crucial for a space mission, the calorimeters
must be optimized with respect to their geometrical acceptance, granularity and
absorption depth. CaloCube is a homogeneous calorimeter with cubic geometry, to
maximise the acceptance being sensitive to particles from every direction in
space; granularity is obtained by relying on small cubic scintillating crystals
as active elements. Different scintillating materials have been studied. The
crystal sizes and spacing among them have been optimized with respect to the
energy resolution. A prototype, based on CsI(Tl) cubic crystals, has been
constructed and tested with particle beams. Some results of tests with
different beams at CERN are presented.Comment: Seven pages, seven pictures. Proceedings of INSTR17 Novosibirs
Quantum control theory for coupled 2-electron dynamics in quantum dots
We investigate optimal control strategies for state to state transitions in a
model of a quantum dot molecule containing two active strongly interacting
electrons. The Schrodinger equation is solved nonperturbatively in conjunction
with several quantum control strategies. This results in optimized electric
pulses in the THz regime which can populate combinations of states with very
short transition times. The speedup compared to intuitively constructed pulses
is an order of magnitude. We furthermore make use of optimized pulse control in
the simulation of an experimental preparation of the molecular quantum dot
system. It is shown that exclusive population of certain excited states leads
to a complete suppression of spin dephasing, as was indicated in Nepstad et al.
[Phys. Rev. B 77, 125315 (2008)].Comment: 24 pages, 9 figure
On equations over sets of integers
Systems of equations with sets of integers as unknowns are considered. It is
shown that the class of sets representable by unique solutions of equations
using the operations of union and addition S+T=\makeset{m+n}{m \in S, \: n \in
T} and with ultimately periodic constants is exactly the class of
hyper-arithmetical sets. Equations using addition only can represent every
hyper-arithmetical set under a simple encoding. All hyper-arithmetical sets can
also be represented by equations over sets of natural numbers equipped with
union, addition and subtraction S \dotminus T=\makeset{m-n}{m \in S, \: n \in
T, \: m \geqslant n}. Testing whether a given system has a solution is
-complete for each model. These results, in particular, settle the
expressive power of the most general types of language equations, as well as
equations over subsets of free groups.Comment: 12 apges, 0 figure
21 years of Timing PSR B1509-58
We present an updated timing solution for the young, energetic pulsar PSR
B1509-58 based on 21.3 years of radio timing data and 7.6 years of X-ray timing
data. No glitches have occurred in this time span, in contrast to other
well-studied young pulsars, which show frequent glitches. We report a
measurement of the third frequency derivative of (-1.28+/-0.21)x10^(-31)
s^(-4). This value is 1.65 standard deviations from, i.e. consistent with, that
predicted by the simple constant magnetic dipole model of pulsar spin-down. We
measured the braking index to be n=2.839+/-0.003 and show that it varies by
1.5% over 21.3 yr due to contamination from timing noise. Results of a
low-resolution power spectral analysis of the significant noise apparent in the
data yield a spectral index of alpha=-4.6+/-1.0 for the red noise component.Comment: 21 pages, 4 figures, 2 tables. Accepted by Ap
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