Surface-electrode ion trap with integrated light source

An atomic ion is trapped at the tip of a single-mode optical fiber in a cryogenic (8 K) surface-electrode ion trap. The fiber serves as an integrated source of laser light, which drives the quadrupole qubit transition of $^{88}$Sr$^+$. Through \emph{in situ} translation of the nodal point of the trapping field, the Gaussian beam profile of the fiber output is imaged, and the fiber-ion displacement, in units of the mode waist at the ion, is optimized to within $0.13\pm0.10$ of the mode center despite an initial offset of $3.30\pm0.10$. Fiber-induced charging at $125 \mu$W is observed to be ${\sim}10$ V/m at an ion height of $670 \mu$m, with charging and discharging time constants of $1.6\pm0.3$ s and $4.7\pm0.6$ s respectively. This work is of importance to large-scale, ion-based quantum information processing, where optics integration in surface-electrode designs may be a crucial enabling technology.Comment: 4 pages, 4 figure

Failure Thresholds for RCC Crest Caps

Overtopping protection using roller-compacted concrete (RCC) overlays is a common improvement made to address hydrologic deficiencies at existing small- to medium-height embankment dams. Overlays allow floods to pass over the dam while preventing erosion of the original embankment. The United States has many examples of such projects on both private and public dams, including many associated with federal agencies of the Departments of the Interior and Agriculture. For one project modified with an overlay in 1993, deterioration of the RCC and poor bonding between lifts has caused concern for stability of the 0.3-m (1-ft) thick RCC lifts that cap the dam. Analytical studies from 2013 to the present have assessed potential failure modes during overtopping flow and have produced widely varying estimates of the probability of failure. To address this uncertainty, physical scale modeling in the Bureau of Reclamation Hydraulics Laboratory was used to evaluate crest cap failure modes for the conservative condition of unbonded RCC lifts. The testing defines the threshold conditions for failure, which will enable effective evaluation of risk for similar facilities in the future

Posture Affects How Robots and Infants Map Words to Objects

For infants, the first problem in learning a word is to map the word to its referent; a second problem is to remember that mapping when the word and/or referent are again encountered. Recent infant studies suggest that spatial location plays a key role in how infants solve both problems. Here we provide a new theoretical model and new empirical evidence on how the body – and its momentary posture – may be central to these processes. The present study uses a name-object mapping task in which names are either encountered in the absence of their target (experiments 1–3, 6 & 7), or when their target is present but in a location previously associated with a foil (experiments 4, 5, 8 & 9). A humanoid robot model (experiments 1–5) is used to instantiate and test the hypothesis that body-centric spatial location, and thus the bodies’ momentary posture, is used to centrally bind the multimodal features of heard names and visual objects. The robot model is shown to replicate existing infant data and then to generate novel predictions, which are tested in new infant studies (experiments 6–9). Despite spatial location being task-irrelevant in this second set of experiments, infants use body-centric spatial contingency over temporal contingency to map the name to object. Both infants and the robot remember the name-object mapping even in new spatial locations. However, the robot model shows how this memory can emerge –not from separating bodily information from the word-object mapping as proposed in previous models of the role of space in word-object mapping – but through the body’s momentary disposition in space

Ultrafast nonlinear optical response of Dirac fermions in graphene

The speed of solid-state electronic devices, determined by the temporal dynamics of charge carriers, could potentially reach unprecedented petahertz frequencies through direct manipulation by optical fields, consisting in a million-fold increase from state-of-the-art technology. In graphene, charge carrier manipulation is facilitated by exceptionally strong coupling to optical fields, from which stems an important back-action of photoexcited carriers. Here we investigate the instantaneous response of graphene to ultrafast optical fields, elucidating the role of hot carriers on sub-100 fs timescales. The measured nonlinear response and its dependence on interaction time and field polarization reveal the back-action of hot carriers over timescales commensurate with the optical field. An intuitive picture is given for the carrier trajectories in response to the optical-field polarization state. We note that the peculiar interplay between optical fields and charge carriers in graphene may also apply to surface states in topological insulators with similar Dirac cone dispersion relations.Peer ReviewedPostprint (published version

Water Ice and Dust in the Innermost Coma of Comet 103P/Hartley 2

On November 4th, 2010, the Deep Impact eXtended Investigation (DIXI) successfully encountered comet 103P/Hartley 2, when it was at a heliocentric distance of 1.06 AU. Spatially resolved near-IR spectra of comet Hartley 2 were acquired in the 1.05-4.83 micron wavelength range using the HRI-IR spectrometer. We present spectral maps of the inner ~10 kilometers of the coma collected 7 minutes and 23 minutes after closest approach. The extracted reflectance spectra include well-defined absorption bands near 1.5, 2.0, and 3.0 micron consistent in position, bandwidth, and shape with the presence of water ice grains. Using Hapke's radiative transfer model, we characterize the type of mixing (areal vs. intimate), relative abundance, grain size, and spatial distribution of water ice and refractories. Our modeling suggests that the dust, which dominates the innermost coma of Hartley 2 and is at a temperature of 300K, is thermally and physically decoupled from the fine-grained water ice particles, which are on the order of 1 micron in size. The strong correlation between the water ice, dust, and CO2 spatial distribution supports the concept that CO2 gas drags the water ice and dust grains from the nucleus. Once in the coma, the water ice begins subliming while the dust is in a constant outflow. The derived water ice scale-length is compatible with the lifetimes expected for 1-micron pure water ice grains at 1 AU, if velocities are near 0.5 m/s. Such velocities, about three order of magnitudes lower than the expansion velocities expected for isolated 1-micron water ice particles [Hanner, 1981; Whipple, 1951], suggest that the observed water ice grains are likely aggregates.Comment: 51 pages, 12 figures, accepted for publication in Icaru

Characterizing the transition from diffuse atomic to dense molecular clouds in the Magellanic clouds with [CII], [CI], and CO

We present and analyze deep Herschel/HIFI observations of the [CII] 158um, [CI] 609um, and [CI] 370um lines towards 54 lines-of-sight (LOS) in the Large and Small Magellanic clouds. These observations are used to determine the physical conditions of the line--emitting gas, which we use to study the transition from atomic to molecular gas and from C^+ to C^0 to CO in their low metallicity environments. We trace gas with molecular fractions in the range 0.1<f(H2)<1, between those in the diffuse H2 gas detected by UV absorption (f(H2)<0.2) and well shielded regions in which hydrogen is essentially completely molecular. The C^0 and CO column densities are only measurable in regions with molecular fractions f(H2)>0.45 in both the LMC and SMC. Ionized carbon is the dominant gas-phase form of this element that is associated with molecular gas, with C^0 and CO representing a small fraction, implying that most (89% in the LMC and 77% in the SMC) of the molecular gas in our sample is CO-dark H2. The mean X_CO conversion factors in our LMC and SMC sample are larger than the value typically found in the Milky Way. When applying a correction based on the filling factor of the CO emission, we find that the values of X_CO in the LMC and SMC are closer to that in the Milky Way. The observed [CII] intensity in our sample represents about 1% of the total far-infrared intensity from the LOSs observed in both Magellanic Clouds.Comment: 32 pages, 21 figures, Accepted to Ap