Why you should not use the electric field to quantize in nonlinear optics

We show that using the electric field as a quantization variable in nonlinear optics leads to incorrect expressions for the squeezing parameters in spontaneous parametric down-conversion and conversion rates in frequency conversion. This observation is related to the fact that if the electric field is written as a linear combination of bosonic creation and annihilation operators one cannot satisfy Maxwell's equations in a nonlinear dielectric.Comment: This version corrects a minor typo from the published version in Optics Letters. Eq. 22 should have an \epsilon_0 that is lacking in the OL versio

Melting of Lennard-Jones rare gas clusters doped with a single impurity atom

Single impurity effect on the melting process of magic number Lennard-Jones, rare gas, clusters of up to 309 atoms is studied on the basis of Parallel Tempering Monte Carlo simulations in the canonical ensemble. A decrease on the melting temperature range is prevalent, although such effect is dependent on the size of the impurity atom relative to the cluster size. Additionally, the difference between the atomic sizes of the impurity and the main component of the cluster should be considered. We demonstrate that solid-solid transitions due to migrations of the impurity become apparent and are clearly differentiated from the melting up to cluster sizes of 147 atoms

High efficiency in mode selective frequency conversion

Frequency conversion (FC) is an enabling process in many quantum information protocols. Recently, it has been observed that upconversion efficiencies in single-photon, mode-selective FC are limited to around 80%.In this letter we argue that these limits can be understood as time-ordering corrections (TOCs) that modify the joint conversion amplitude of the process. Furthermore we show, using a simple scaling argument, that recently proposed cascaded FC protocols that overcome the aforementioned limitations act as "attenuators" of the TOCs. This observation allows us to argue that very similar cascaded architectures can be used to attenuate TOCs in photon generation via spontaneous parametric down-conversion. Finally, by using the Magnus expansion, we argue that the TOCs, which are usually considered detrimental for FC efficiency, can also be used to increase the efficiency of conversion in partially mode selective FC

Stability and noise spectra of relative Loran-C frequency comparisons

Relative comparisons of Loran-C frequency transmissions between the master station of Catanzaro (Simeri Crichi) and the X, Z slave stations of Estartit (Spain) and Lampedusa (Italy) are carrying out by the GG LORSTA monitor station of the Mediterranean Sea Loran-C chain. These comparisons are able to emphasize the relative and, under certain conditions, the absolute rate of the emitting standard frequencies of the slave stations and some relevant statistical properties of the Loran-C Method for frequency transmission and time synchronization. The stability of each Loran-C frequency standard transmission is subject to perturbations, more or less known, due to the propagation medium and other causes. Following the Allan (1966) method for data processing, the performance of the relative rate of frequency of the transmissions of the X, Z slave stations are described calculating the standard deviation of a set of N frequency measurements from its mean averaged during sampling times. This standard deviation is designated as the measure of the stability of the Loran-C frequency transmission

A Note on Methods for Vertical Accuracy Assessment of DEMs

This study investigates the vertical accuracy of a DEMs interpolated from 1:50,000 contours with universal kriging (UK) using DEMs interpolated from 1:10,000 contours as the independent source of higher accuracy following the geospatial accuracy standards proposed by the Federal Geographic Data Committee, FGDC, (1998). In accordance with the standards, the 1:10,000 contours do not qualify as well-defined points with known horizontal position and a high degree of accuracy that are easily visible and recoverable on the ground. Hence, DEMs are created from the 1:10,000 contours and used to check the vertical accuracy of the 1:50,000 DEMs with elevation errors, an RMSE and vertical accuracy measure calculated between the 1:10,000 and 1:50,000 DEMs. Other methods of vertical accuracy assessment for the 1:50,000 DEMs include a comparison of elevation ranges, visualization of error surfaces and latitudinal and longitudinal terrain profiles.Este estudio investiga la exactitud vertical de un modelo de elevación digital interpolado desde una escala 1:50,000, usando MED interpolado de 1:10,000 con líneas de contorno como una fuente independiente de alta precisión y siguiendo los indicadores propuestos por la Federal Geographic Data Committee, FGDC, (1998). De acuerdo con esos indicadores los contornos de una escala 1: 10 000 no califican como puntos bien definidos en una posición horizontal conocida y un alto nivel de precisión, los cuales pueden ser fácilmente visibles y recolectados en el campo. Por ello, los MEDs son creados a partir de contornos a escala 10 000 y usados para corroborar la precisión vertical a una escala 1 50 000. Además, los MEDs incluyen una comparación de los rangos de elevación, visualizan errores en la superficie y los perfiles latitudinales y longitudinales.Este estudo investiga a precisão vertical de um modelo de elevação digital interpolado a partir de uma escala de 1:50.000, utilizando MED pontilhadas de 1:10000 com linhas de contorno como uma fonte independente de alta precisão e seguindo os propostos pelos indicadores do Comitê Federal de Dados Geográficos, CFDC (1998). De acordo com estes indicadores, os contornos de uma escala de 1:10.000 não são qualificadas como pontos bem definidos em uma posição horizontal conhecido e um alto nível de precisão, o que pode ser facilmente visto e recolhido no campo. Portanto, MEDs são criados a partir de escala 10.000 contornos e, utilizados para corroborar a precisão vertical numa escala 1:50000. Além disso, as MEDs incluem uma comparação de gamas de elevação, erros de superfície de exibição e perfis latitude e longitude.UCR::Sedes Regionales::Sede de Occident

The influence of push-off timing in a robotic ankle-foot prosthesis on the energetics and mechanics of walking

Background: Robotic ankle-foot prostheses that provide net positive push-off work can reduce the metabolic rate of walking for individuals with amputation, but benefits might be sensitive to push-off timing. Simple walking models suggest that preemptive push-off reduces center-of-mass work, possibly reducing metabolic rate. Studies with bilateral exoskeletons have found that push-off beginning before leading leg contact minimizes metabolic rate, but timing was not varied independently from push-off work, and the effects of push-off timing on biomechanics were not measured. Most lower-limb amputations are unilateral, which could also affect optimal timing. The goal of this study was to vary the timing of positive prosthesis push-off work in isolation and measure the effects on energetics, mechanics and muscle activity. Methods: We tested 10 able-bodied participants walking on a treadmill at 1.25 m.s(-1). Participants wore a tethered ankle-foot prosthesis emulator on one leg using a rigid boot adapter. We programmed the prosthesis to apply torque bursts that began between 46% and 56% of stride in different conditions. We iteratively adjusted torque magnitude to maintain constant net positive push-off work. Results: When push-off began at or after leading leg contact, metabolic rate was about 10% lower than in a condition with Spring-like prosthesis behavior. When push-off began before leading leg contact, metabolic rate was not different from the Spring-like condition. Early push-off led to increased prosthesis-side vastus medialis and biceps femoris activity during push-off and increased variability in step length and prosthesis loading during push-off. Prosthesis push-off timing had no influence on intact-side leg center-of-mass collision work. Conclusions: Prosthesis push-off timing, isolated from push-off work, strongly affected metabolic rate, with optimal timing at or after intact-side heel contact. Increased thigh muscle activation and increased human variability appear to have caused the lack of reduction in metabolic rate when push-off was provided too early. Optimal timing with respect to opposite heel contact was not different from normal walking, but the trends in metabolic rate and center-of-mass mechanics were not consistent with simple model predictions. Optimal push-off timing should also be characterized for individuals with amputation, since meaningful benefits might be realized with improved timing

Alternative assessment of computer numerical control (CNC) machine tool students

Includes bibliographical references

Nucleon scattering on actinides using a dispersive optical model with extended couplings

Tamura coupling model has been extended to consider the coupling of additional low-lying rotational bands to the ground state band. Rotational bands are built on vibrational bandheads (even-even targets) or single particle bandheads (odd-$A$ targets) including both axial and non-axial deformations. These additional excitations are introduced as a perturbation to the underlying axially-symmetric rigid rotor structure of the ground state rotational band. Coupling matrix elements of the generalized optical model are derived for extended multi-band transitions in even-even and odd-$A$ nuclei. Isospin symmetric formulation of the optical model is employed. A coupled-channels optical model potential (OMP) containing a dispersive contribution is used to fit simultaneously all available optical experimental databases including neutron strength functions for nucleon scattering on $^{232}$Th, $^{233,235,238}$U and $^{239}$Pu nuclei and quasi-elastic ($p$,$n$) scattering data on $^{232}$Th and $^{238}$U. Lane consistent OMP is derived for all actinides if corresponding multi-band coupling schemes are defined. Calculations using the derived OMP potential reproduce measured total cross-section differences between several actinide pairs within experimental uncertainty for incident neutron energies from 50 keV up to 150MeV. Multi-band coupling is stronger in even-even targets due to the collective nature of the coupling; the impact of extended coupling on predicted compound-nucleus formation cross section reaches 5% below 3 MeV of incident neutron energy. Coupling of ground-state rotational band levels in odd-$A$ nuclei is sufficient for a good description of the compound-nucleus formation cross sections as long as the coupling is saturated (a minimum of 7 coupled levels are typically needed).Comment: 30 pages, 4 figures, 8 tables, 3 appendice

Predicting the optical observables for nucleon scattering on even-even actinides

Previously derived Lane consistent dispersive coupled-channel optical model for nucleon scattering on $^{232}$Th and $^{238}$U nuclei is extended to describe scattering on even-even actinides with $Z=$90--98. A soft-rotator-model (SRM) description of the low-lying nuclear structure is used, where SRM Hamiltonian parameters are adjusted to the observed collective levels of the target nucleus. SRM nuclear wave functions (mixed in $K$ quantum number) have been used to calculate coupling matrix elements of the generalized optical model. The "effective" deformations that define inter-band couplings are derived from SRM Hamiltonian parameters. Conservation of nuclear volume is enforced by introducing a dynamic monopolar term to the deformed potential leading to additional couplings between rotational bands. Fitted static deformation parameters are in very good agreement with those derived by Wang and collaborators using the Weizs\"acker-Skyrme global mass model (WS4), allowing to use the latter to predict cross section for nuclei without experimental data. A good description of scarce "optical" experimental database is achieved. SRM couplings and volume conservation allow a precise calculation of the compound-nucleus formation cross sections, which is significantly different from the one calculated with rigid-rotor potentials coupling the ground-state rotational band. Derived parameters can be used to describe both neutron and proton induced reactions.Comment: 6 pages, 4 figures, 5 table