683 research outputs found
Multiple Quantum Well AlGaAs Nanowires
This letter reports on the growth, structure and luminescent properties of
individual multiple quantum well (MQW) AlGaAs nanowires (NWs). The composition
modulations (MQWs) are obtained by alternating the elemental flux of Al and Ga
during the molecular beam epitaxy growth of the AlGaAs wire on GaAs (111)B
substrates. Transmission electron microscopy and energy dispersive X-ray
spectroscopy performed on individual NWs are consistent with a configuration
composed of conical segments stacked along the NW axis. Micro-photoluminescence
measurements and confocal microscopy showed enhanced light emission from the
MQW NWs as compared to non-segmented NWs due to carrier confinement and
sidewall passivation
Exploring multivariate data structures with local principal curves.
A new approach to find the underlying structure of a multidimensional data cloud is proposed, which is based on a localized version of principal components analysis. More specifically, we calculate a
series of local centers of mass and move through the data in directions given by the first local principal axis.
One obtains a smooth ``local principal curve'' passing through the "middle" of a multivariate data cloud. The concept adopts to branched curves by considering the second local principal axis. Since the algorithm is based on a simple eigendecomposition, computation is fast and easy
Comparing the states of many quantum systems
We investigate how to determine whether the states of a set of quantum
systems are identical or not. This paper treats both error-free comparison, and
comparison where errors in the result are allowed. Error-free comparison means
that we aim to obtain definite answers, which are known to be correct, as often
as possible. In general, we will have to accept also inconclusive results,
giving no information. To obtain a definite answer that the states of the
systems are not identical is always possible, whereas, in the situation
considered here, a definite answer that they are identical will not be
possible. The optimal universal error-free comparison strategy is a projection
onto the totally symmetric and the different non-symmetric subspaces, invariant
under permutations and unitary transformations. We also show how to construct
optimal comparison strategies when allowing for some errors in the result,
minimising either the error probability, or the average cost of making an
error. We point out that it is possible to realise universal error-free
comparison strategies using only linear elements and particle detectors, albeit
with less than ideal efficiency. Also minimum-error and minimum-cost strategies
may sometimes be realised in this way. This is of great significance for
practical applications of quantum comparison.Comment: 13 pages, 2 figures. Corrected a misprint on p. 7 and added a few
references. Accepted for publication in J Mod Op
Descriptive Studies on Stylized Facts of the German Business Cycle
Thirteen Stylized Facts of the german economy are studied with different descriptive statistical methods. The results of this study are considered with respect to other results from Project B3 "Multivariate Bestimmung und Untersuchung von Konjunkturzyklen"
Variance Reduction with Monte Carlo Estimates of Error Rates in Multivariate Classification
In this paper, control variates are proposed to speed up Monte Carlo Simulations to estimate expected error rates in multivariate classification
A Fast and Compact Quantum Random Number Generator
We present the realization of a physical quantum random number generator
based on the process of splitting a beam of photons on a beam splitter, a
quantum mechanical source of true randomness. By utilizing either a beam
splitter or a polarizing beam splitter, single photon detectors and high speed
electronics the presented devices are capable of generating a binary random
signal with an autocorrelation time of 11.8 ns and a continuous stream of
random numbers at a rate of 1 Mbit/s. The randomness of the generated signals
and numbers is shown by running a series of tests upon data samples. The
devices described in this paper are built into compact housings and are simple
to operate.Comment: 23 pages, 6 Figs. To appear in Rev. Sci. Inst
Modally Resolved Fabry-Perot Experiment with Semiconductor Waveguides
Based on the interaction between different spatial modes, semiconductor
Bragg-reflection waveguides provide a highly functional platform for non-linear
optics. Therefore, the control and engineering of the properties of each
spatial mode is essential. Despite the multimodeness of our waveguide, the
well-established Fabry-Perot technique for recording fringes in the optical
transmission spectrum can successfully be employed for a detailed linear
optical characterization when combined with Fourier analysis. A prerequisite
for the modal sensitivity is a finely resolved transmission spectrum that is
recorded over a broad frequency band. Our results highlight how the features of
different spatial modes, such as their loss characteristics and dispersion
properties, can be separated from each other allowing their comparison. The
mode-resolved measurements are important for optimizing the performance of such
multimode waveguides by tailoring the properties of their spatial modes.Comment: 8 pages, 7 figure
Latent Factor Prediction Pursuit for Rank Deficient Regressors
In simulation studies Latent Factor Prediction Pursuit outperformed classical reduced rank regression methods. The algorithm described so far for Latent Factor Prediction Pursuit had two shortcomings. It was only implemented for situations where the explanatory variables were of full colum rank. Also instead of the projection matrix only the regression matrix was calculated. These problems are addressed by a new algorithm which finds the prediction optimal projection
Quantum entanglement distribution with 810 nm photons through telecom fibers
We demonstrate the distribution of polarization entangled photons of
wavelength 810 nm through standard telecom fibers. This technique allows
quantum communication protocols to be performed over established fiber
infrastructure, and makes use of the smaller and better performing setups
available around 800 nm, as compared to those which use telecom wavelengths
around 1550 nm. We examine the excitation and subsequent quenching of
higher-order spatial modes in telecom fibers up to 6 km in length, and perform
a distribution of high quality entanglement (visibility 95.6%). Finally, we
demonstrate quantum key distribution using entangled 810 nm photons over a 4.4
km long installed telecom fiber link.Comment: 5 pages, 5 figures, 1 tabl
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