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The design of inertial confinement fusion (ICF) ignition targets requires radiation-hydrodynamics simulations with accurate models of the fundamental material properties (i.e., equation of state, opacity, and conductivity). Validation of these models are required via experimentation. A feasibility study of using spatially-integrated, spectrally-resolved, X-ray Thomson scattering (XRTS) measurements to diagnose the temperature, density, and ionization of the compressed DT shell and hot spot of a laser direct-drive implosion at two-thirds convergence was conducted. Synthetic scattering spectra were generated using 1-D implosion simulations from the LILAC code that were post processed with the X-ray Scattering (XRS) model which is incorporated within SPECT3D. Analysis of two extreme adiabat capsule conditions showed that the plasma conditions for both compressed DT shells could be resolved., Comment: 11 pages, 8 figures

We present design and simulation of a Josephson parametric amplifier with bandwidth exceeding 1.6 GHz, and with high saturation power approaching -90 dBm at a gain of 22.8 dB. An improvement by a factor of roughly 50 in bandwidth over the state of the art is achieved by using well-established impedance matching techniques. An improvement by a factor of roughly 100 in saturation power over the state of the art is achieved by implementing the Josephson nonlinear element as an array of rf-SQUIDs with a total of 40 junctions. WRSpice simulations of the circuit are in excellent agreement with the calculated gain and saturation characteristics., Comment: 5 pages, 3 figures

The Voices of Youth Count initiative's systematic evidence review is the most comprehensive synthesis of evaluation evidence on programs and practices related to youth homelessness to date. This document is one in a series of seven topical evidence summaries derived from the longer evidence review brief. Herein, evaluations of individual counseling and treatment interventions for youth experiencing homelessness are summarized. These interventions typically involve non-housing, non-family-based interventions primarily focused on delivering therapeutic or health-related counseling or treatment to youth experiencing homelessness. The evidence here includes only impact evaluations designed to assess measurable changes in outcomes due to specific programs and practices. [For the longer evidence review brief, "Missed Opportunities: Evidence on Interventions for Addressing Youth Homelessness," see ED603003.]

This Research-to-Impact brief is the eighth in a series from Voices of Youth Count. Voices of Youth Count is a national research and policy initiative designed to fill critical gaps in the nation's knowledge about unaccompanied homelessness among youth and young adults ages 13 to 25. Voices of Youth Count research estimated that nearly 4.2 million youth and young adults in America experience some form of homelessness during a 12-month period. This brief summarizes research-based evidence on the effectiveness of interventions intended to prevent youth homelessness, reduce its duration and effects, and promote sustainable improvements in youth well-being. Accordingly, Voices of Youth screened nearly 4,000 studies, focusing on studies that evaluate the effectiveness of youth interventions. The screening process ultimately identified 62 studies that involved youth homelessness and evaluated the impacts of 51 programs. In this brief, critical evidence gaps are revealed that can prompt targeted investments in research and evaluation.

A scalable, multiplexed ion trap for quantum information processing is fabricated and tested. The trap design and fabrication process are optimized for scalability to small trap size and large numbers of interconnected traps, and for integration of control electronics and optics. Multiple traps with similar designs are tested with Cd+, Mg+, and Sr+ ions at room temperature and with Sr+ at 6 K, with respective ion lifetimes of 90 s, 300 +/- 30 s, 56 +/- 6 s, and 4.5 +/- 1.1 hours. The motional heating rate for Mg+ at room temperature and a trap frequency of 1.6 MHz is measured to be 7 +/- 3 quanta per millisecond. For Sr+ at 6 K and 540 kHz the heating rate is measured to be 220 +/- 30 quanta per second., Comment: 18 pages, 12 figures

We have measured motional heating rates of trapped atomic ions, a factor that can influence multi-ion quantum logic gate fidelities. Two simplified techniques were developed for this purpose: one relies on Raman sideband detection implemented with a single laser source, while the second is even simpler and is based on time-resolved fluorescence detection during Doppler recooling. We applied these methods to determine heating rates in a microfrabricated surface-electrode trap made of gold on fused quartz, which traps ions 40 microns above its surface. Heating rates obtained from the two techniques were found to be in reasonable agreement. In addition, the trap gives rise to a heating rate of 300 plus or minus 30 per second for a motional frequency of 5.25 MHz, substantially below the trend observed in other traps.

We investigate the temporal dynamics of Doppler cooling of an initially hot single trapped atom in the weak binding regime using a semiclassical approach. We develop an analytical model for the simplest case of a single vibrational mode for a harmonic trap, and show how this model allows us to estimate the initial energy of the trapped particle by observing the fluorescence rate during the cooling process. The experimental implementation of this temperature measurement provides a way to measure atom heating rates by observing the temperature rise in the absence of cooling. This method is technically relatively simple compared to conventional sideband detection methods, and the two methods are in reasonable agreement. We also discuss the effects of RF micromotion, relevant for a trapped atomic ion, and the effect of coupling between the vibrational modes on the cooling dynamics., Comment: 12 pages, 11 figures, Submitted to Phys. Rev. A

We cool the fundamental mode of a miniature cantilever by capacitively coupling it to a driven rf resonant circuit. Cooling results from the rf capacitive force, which is phase shifted relative to the cantilever motion. We demonstrate the technique by cooling a 7 kHz cantilever from room temperature to 45 K, obtaining reasonable agreement with a model for the cooling, damping, and frequency shift. Extending the method to higher frequencies in a cryogenic system could enable ground state cooling and may prove simpler than related optical experiments in a low temperature apparatus., Comment: 4 pages, 4 figures; minor changes to match published version

We consider a method to reduce the kinetic energy in a low-order mode of a miniature cantilever. If the cantilever contributes to the capacitance of a driven RF circuit, a force on the cantilever exists due to the electric field energy stored in the capacitance. If this force acts with an appropriate phase shift relative to the motion of the cantilever, it can oppose the velocity of the cantilever, leading to cooling. Such cooling may enable reaching the quantum regime of cantilever motion., Comment: 4 pages, 1 figure

We study the coupling of a single nitrogen-vacancy center in diamond to a nearby single nitrogen defect at room temperature. The magnetic dipolar coupling leads to a splitting in the electron spin resonance frequency of the nitrogen-vacancy center, allowing readout of the state of a single nitrogen electron spin. At magnetic fields where the spin splitting of the two centers is the same we observe a strong polarization of the nitrogen electron spin. The amount of polarization can be controlled by the optical excitation power. We combine the polarization and the readout in time-resolved pump-probe measurements to determine the spin relaxation time of a single nitrogen electron spin. Finally, we discuss indications for hyperfine-induced polarization of the nitrogen nuclear spin.

The prospect of building a quantum information processor underlies many recent advances ion trap fabrication techniques. Potentially, a quantum computer could be constructed from a large array of interconnected ion traps. We report on a micrometer-scale ion trap, fabricated from bulk silicon using micro-electromechanical systems (MEMS) techniques. The trap geometry is relatively simple in that the electrodes lie in a single plane beneath the ions. In such a trap we confine laser-cooled 24Mg+ ions approximately 40 microns above the surface. The fabrication technique and planar electrode geometry together make this approach amenable to scaling up to large trap arrays. In addition we observe that little laser cooling light is scattered by the electrodes., Comment: 4 pages, 6 figures

We demonstrate confinement of individual atomic ions in a radio-frequency Paul trap with a novel geometry where the electrodes are located in a single plane and the ions confined above this plane. This device is realized with a relatively simple fabrication procedure and has important implications for quantum state manipulation and quantum information processing using large numbers of ions. We confine laser-cooled Mg-24 ions approximately 40 micrometer above planar gold electrodes. We measure the ions' motional frequencies and compare them to simulations. From measurements of the escape time of ions from the trap, we also determine a heating rate of approximately five motional quanta per millisecond for a trap frequency of 5.3 MHz., Comment: 4 pages, 4 figures

The nitrogen-vacancy (N-V) center in diamond is a promising atomic-scale system for solid-state quantum information processing. Its spin-dependent photoluminescence has enabled sensitive measurements on single N-V centers, such as: electron spin resonance, Rabi oscillations, single-shot spin readout and two-qubit operations with a nearby 13C nuclear spin. Furthermore, room temperature spin coherence times as long as 58 microseconds have been reported for N-V center ensembles. Here, we have developed an angle-resolved magneto-photoluminescence microscopy apparatus to investigate the anisotropic electron spin interactions of single N-V centers at room temperature. We observe negative peaks in the photoluminescence as a function of both magnetic field magnitude and angle that are explained by coherent spin precession and anisotropic relaxation at spin level anti-crossings. In addition, precise field alignment unmasks the resonant coupling to neighboring dark nitrogen spins that are not otherwise detected by photoluminescence. The latter results demonstrate a means of investigating small numbers of dark spins via a single bright spin under ambient conditions., Comment: 13 pages, 4 figures

We describe the growth of modulation-doped ZnSe/(Zn,Cd)Se quantum wells on (110) GaAs substrates. Unlike the well-known protocol for the epitaxy of ZnSe-based quantum structures on (001) GaAs, we find that the fabrication of quantum well structures on (110) GaAs requires significantly different growth conditions and sample architecture. We use magnetotransport measurements to confirm the formation of a two-dimensional electron gas in these samples, and then measure transverse electron spin relaxation times using time-resolved Faraday rotation. In contrast to expectations based upon known spin relaxation mechanisms, we find surprisingly little difference between the spin lifetimes in these (110)-oriented samples in comparison with (100)-oriented control samples., Comment: To appear in Journal of Superconductivity (Proceedings of 3rd Conference on Physics and Applications of Spin-dependent Phenomena in Semiconductors)

The Voices of Youth Count initiative's systematic evidence review is the most comprehensive synthesis of evaluation evidence on programs and practices related to youth homelessness to date. This document is one in a series of seven topical evidence summaries derived from the longer evidence review brief. Herein, evaluations of non-housing case management and support interventions for addressing youth homelessness are summarized. These interventions involve casework, mentoring, and/or youth development programming as key features, without including shelter or direct housing assistance. The evidence here includes only impact evaluations designed to assess measurable changes in outcomes due to specific programs and practices. [For the longer evidence review brief, "Missed Opportunities: Evidence on Interventions for Addressing Youth Homelessness," see ED603003.]

The Voices of Youth Count initiative's systematic evidence review is the most comprehensive synthesis of evaluation evidence on programs and practices related to youth homelessness to date. This document is one in a series of seven topical evidence summaries derived from the longer evidence review brief. Herein, evaluations of family interventions for youth experiencing homelessness, not including housing-related services, are summarized. These interventions involve both young people and their family members and usually include a counseling component. The evidence here includes only impact evaluations designed to assess measurable changes in outcomes due to specific programs and practices. [For the longer evidence review brief, "Missed Opportunities: Evidence on Interventions for Addressing Youth Homelessness," see ED603003.]

Shelter and housing interventions provide residential facilities or rental assistance along with other services. Despite the substantial policy focus on shelter and housing interventions to address youth homelessness, few studies of shelter and housing interventions for youth met in inclusion criteria. The Voices of Youth Count initiative's systematic evidence review is the most comprehensive synthesis of evaluation evidence on programs and practices related to youth homelessness to date. This document is one in a series of seven topical evidence summaries derived from the longer evidence review brief. Evaluations of shelter and housing interventions for youth experiencing homelessness are summarized. The evidence includes only impact evaluations designed to assess measurable changes in outcomes due to specific programs and practices. Other kinds of evaluation, including assessments of program implementation, processes, or participant experiences, will be summarized and reported elsewhere. [For the longer evidence review brief, "Missed Opportunities: Evidence on Interventions for Addressing Youth Homelessness," see ED603003.]

We propose a method based on optically detected magnetic resonance (ODMR) to measure the decoherence time $T_{2}$ of a single electron spin in a semiconductor quantum dot. The electron spin resonance (ESR) of a single excess electron on a quantum dot is probed by circularly polarized laser excitation. Due to Pauli blocking, optical excitation is only possible for one of the electron-spin states. The photoluminescence is modulated due to the ESR which enables the measurement of electron spin decoherence. We study different possible schemes for such an ODMR setup., Comment: 5 pages, 4 figures, v2: minor changes and added references

We employ optical pump-probe spectroscopy to investigate the voltage dependence of spontaneous electron and nuclear spin polarizations in hybrid MnAs/n-GaAs and Fe/n-GaAs Schottky diodes. Through the hyperfine interaction, nuclear spin polarization that is imprinted by the ferromagnet acts on conduction electron spins as an effective magnetic field. We demonstrate tuning of this nuclear field from <0.05 to 2.4 kG by varying a small bias voltage across the MnAs device. In addition, a connection is observed between the diode turn-on and the onset of imprinted nuclear polarization, while traditional dynamic nuclear polarization exhibits relatively little voltage dependence., Comment: Submitted to Physical Review B Rapid Communications. 15 pages, 3 figures

We find that photoexcited electrons in an n-GaAs epilayer rapidly (< 50 ps) spin-polarize due to the proximity of an epitaxial ferromagnetic metal. Comparison between MnAs/GaAs and Fe/GaAs structures reveals that this coherent spin polarization is aligned antiparallel and parallel to their magnetizations, respectively. In addition, the GaAs nuclear spins are dynamically polarized with a sign determined by the spontaneous electron spin orientation. In Fe/GaAs, competition between nuclear hyperfine and applied magnetic fields results in complete quenching of electron spin precession., Comment: 13 pages, 4 figures, in press, Phys. Rev. B Rapid Communications (2002)

The long-held notion that the highest-energy cosmic rays are of distant extragalactic origin is challenged by observations that events above $\sim 10^{20}$ eV do not exhibit the expected high-energy cutoff from photopion production off the cosmic microwave background. We suggest that these unexpected ultra-high-energy events are due to iron nuclei accelerated from young strongly magnetized neutron stars through relativistic MHD winds. We find that neutron stars whose initial spin periods are shorter than $\sim 4 (B_S/10^{13}{\rm G})^{1/2}$ ms, where $B_S$ is the surface magnetic field, can accelerate iron cosmic rays to greater than $\sim 10^{20}$ eV. These ions can pass through the remnant of the supernova explosion that produced the neutron star without suffering significant spallation reactions. For plausible models of the Galactic magnetic field, the trajectories of the iron ions curve sufficiently to be consistent with the observed arrival directions of the highest energy events., Comment: 11 pages, 1 figure, replaced with revised version, some references added

With the aim of constraining the structural properties of neutron stars and the equation of state of dense matter, we study sudden spin-ups, glitches, occurring in the Vela pulsar and in six other pulsars. We present evidence that glitches represent a self-regulating instability for which the star prepares over a waiting time. The angular momentum requirements of glitches in Vela indicate that at least 1.4% of the star's moment of inertia drives these events. If glitches originate in the liquid of the inner crust, Vela's `radiation radius' must exceed ~12 km for a mass of 1.4 solar masses. Observational tests of whether other neutron stars obey this constraint will be possible in the near future., Comment: 5 pages, including figures. To appear in Physical Review Letters

The absence of the expected GZK cutoff strongly challenges the notion that the highest-energy cosmic rays are of distant extragalactic origin. We discuss the possibility that these ultra-high-energy events originate in our Galaxy and propose that they may be due to iron nuclei accelerated from young, strongly magnetic neutron stars. Newly formed pulsars accelerate ions from their surface through relativistic MHD winds. We find that pulsars whose initial spin periods are shorter than $\sim 4 (B_S/10^{13}{\rm G})$ ms, where $B_S$ is the surface magnetic field, can accelerate iron ions to greater than $10^{20} eV$. These ions can pass through the remnant of the supernova explosion that produced the pulsar without suffering significant spallation reactions. Depending on the structure of the galactic magnetic field, the trajectories of the iron ions from galactic sources can be consistent with the observed arrival directions of the highest energy events., Comment: 4 pages, 1 figure, to appear in the Proceedings of the 26th ICRC, OG3303

Variation in the angle $\alpha$ between a pulsar's rotational and magnetic axes would change the torque and spin-down rate. We show that sudden increases in $\alpha$, coincident with glitches, could be responsible for the persistent increases in spin-down rate that follow glitches in the Crab pulsar. Moreover, changes in $\alpha$ at a rate similar to that inferred for the Crab pulsar account naturally for the very low braking index of the Vela pulsar. If $\alpha$ increases with time, all pulsar ages obtained from the conventional braking model are underestimates. Decoupling of the neutron star liquid interior from the external torque cannot account for Vela's low braking index. Variations in the Crab's pulse profile due to changes in $\alpha$ might be measurable., Comment: 14 pages and one figure, Latex, uses aasms4.sty. Accepted to ApJ Letters

We examine the thermal and dynamical response of a neutron star to a sudden perturbation of the inner crust temperature. During the star's evolution, starquakes and other processes may deposit $\gap 10^{42}$ ergs, causing significant internal heating and increased frictional coupling between the crust and the more rapidly rotating neutron superfluid the star is expected to contain. Through numerical simulation we study the propagation of the thermal wave created by the energy deposition, the induced motion of the interior superfluid, and the resulting spin evolution of the crust. We find that energy depositions of $\sim 10^{40}$ ergs produce gradual spin-ups above the timing noise level, while larger energy depositions produce sudden spin jumps resembling pulsar glitches. For a star with a temperature in the observed range of the Vela pulsar, an energy deposition of $\sim 10^{42}$ ergs produces a large spin-up taking place over minutes, similar to the Vela ``Christmas'' glitch. Comparable energy deposition in a younger and hotter ``Crab-like'' star produces a smaller spin-up taking place over $\sim 1$ day, similar to that seen during the partially time-resolved Crab glitch of 1989., Comment: 21 pages plus 17 figures, uuencode compressed Postscript. Accepted for publication in the Astrophysical Journal

Differential rotation between the neutron star crust and a more rapidly rotating interior superfluid leads to frictional heating that affects the star's long-term thermal evolution and resulting surface emission. Here we present the results of thermal evolution simulations based on two models of the vortex pinning forces that bracket a range of plausible pinning strengths. We include the effects of superfluidity, magnetic fields, and temperature gradients. As representative standard and accelerated neutrino emission processes taking place in the core, we consider the modified Urca process in normal baryonic matter, and the much faster quark Urca process. Comparison of our results with neutron star surface temperature data, including the recent temperature measurement of the Geminga pulsar, shows that stars with soft equations of state and modest frictional heating are in closest agreement with the data; stars with stronger frictional heating have temperatures inconsistent with the upper limit of PSR 1929+10. Stiffer stars undergoing standard cooling generally have temperatures lying above the Vela detection, a situation worsened by the inclusion of frictional heating. Stars undergoing accelerated cooling without frictional heating have temperatures that fall far below most temperature measurements; the Vela and Geminga detections being the most compelling examples. Only in stiff stars, which have thick crusts, can the inclusion of strong frictional heating raise the temperature at late stages in the evolution to a level consistent with the data. However, such a large amount of heating leads to a temperature at $\sim 1000$ yr in excess of the Crab upper limit. Suppression of accelerated neutrino emission processes, perhaps by superfluid pairing in the core, may yield acceptable cooling models., Comment: AASTeX, 24 pages, 7 figures (Substantial Changes)