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A4BLiP Anti-Racist Description Resources User Survey Results
No full textThe related items include the link to the original PDF copy on GitHub and the link to a set of guidelines that this survey reflects
A geometric approach to inelastic collapse
No full textWe show in this note how to interpret logarithmic spiral tilings as one-dimensional particle systems undergoing inelastic collapse. By deforming the spirals appropriately, we can simulate collisions among particles with distinct or varying coefficients of restitution. Our geometric constructions provide a strikingly simple illustration of a widely studied phenomenon in the physics of dissipative gases: the collapse of inelastic particles
Aδ: Autodiff for Discontinuous Programs - Applied to Shaders
No full textOver the last decade, automatic differentiation (AD) has profoundly impacted graphics and vision applications --- both broadly via deep learning and specifically for inverse rendering. Traditional AD methods ignore gradients at discontinuities, instead treating functions as continuous. Rendering algorithms intrinsically rely on discontinuities, crucial at object silhouettes and in general for any branching operation. Researchers have proposed fully-automatic differentiation approaches for handling discontinuities by restricting to affine functions, or semi-automatic processes restricted either to invertible functions or to specialized applications like vector graphics. This paper describes a compiler-based approach to extend reverse mode AD so as to accept arbitrary programs involving discontinuities. Our novel gradient rules generalize differentiation to work correctly, assuming there is a single discontinuity in a local neighborhood, by approximating the prefiltered gradient over a box kernel oriented along a 1D sampling axis. We describe when such approximation rules are first-order correct, and show that this correctness criterion applies to a relatively broad class of functions. Moreover, we show that the method is effective in practice for arbitrary programs, including features for which we cannot prove correctness. We evaluate this approach on procedural shader programs, where the task is to optimize unknown parameters in order to match a target image, and our method outperforms baselines in terms of both convergence and efficiency. Our compiler outputs gradient programs in TensorFlow, PyTorch (for quick prototypes) and Halide with an optional auto-scheduler (for efficiency). The compiler also outputs GLSL that renders the target image, allowing users to interactively modify and animate the shader, which would otherwise be cumbersome in other representations such as triangle meshes or vector art
Vertical Stacked LEGO-PoL CPU Voltage Regulator
No full textThis article presents a 48–1 V merged-two-stage hybrid-switched-capacitor converter with a linear extendable group operated point-of-load (LEGO-PoL) architecture for ultrahigh-current microprocessors, featuring 3-D stacked packaging and coupled inductors for miniaturized size, fast speed, and vertical power delivery. The architecture is highly modular and scalable. The switched-capacitor circuits are connected in series on the input side to split the high input voltage into multiple stacked voltage domains. The multiphase buck circuits are connected in parallel to distribute the high output current into multiple parallel current paths. It leverages the advantages of switched-capacitor circuits and multiphase buck circuits to achieve soft charging, current sharing, and voltage balancing. The inductors of the multiphase buck converters are used as current sources to soft-charge and soft-switch the switched-capacitor circuits, and the switched-capacitor circuits are utilized to ensure current sharing among the multiphase buck circuits. A 780-A vertical stacked CPU voltage regulator with a peak efficiency of 91.1% and a full load efficiency of 79.2% at an output voltage of 1 V with liquid cooling is built and tested. The switched capacitor circuits operate at 286 kHz and the buck circuits operate at 1 MHz. It regulates output voltage between 0.8 and 1.5 V through the entire 780-A current range. This is the first demonstration of a 48–1 V CPU voltage regulator to achieve over 1-A/mm2 current density and the first to achieve 1000-W/in3 power density
Probing Spin Dynamics on Diamond Surfaces Using a Single Quantum Sensor
No full textUnderstanding the dynamics of a quantum bit’s environment is essential for the realization of practical systems for quantum information processing and metrology. We use single nitrogen-vacancy (NV) centers in diamond to study the dynamics of a disordered spin ensemble at the diamond surface. Specifically, we tune the density of “dark” surface spins to interrogate their contribution to the decoherence of shallow NV center spin qubits. When the average surface spin spacing exceeds the NV center depth, we find that the surface spin contribution to the NV center free induction decay can be described by a stretched exponential with variable power n. We show that these observations are consistent with a model in which the spatial positions of the surface spins are fixed for each measurement, but some of them reconfigure between measurements. In particular, we observe a depth-dependent critical time associated with a dynamical transition from Gaussian (n = 2) decay to n = 2/3, and show that this transition arises from the competition between the small decay contributions of many distant spins and strong coupling to a few proximal spins at the surface. These observations demonstrate the potential of a local sensor for understanding complex systems and elucidate pathways for improving and controlling spin qubits at the surface
Impact of organic solutes on capillary phenomena in water-CO2-quartz systems
No full textThe migration of supercritical CO2 (scCO2) injected into underground reservoirs as part of carbon capture and storage is influenced by organic contamination affecting mineral wettability. Molecular
dynamics (MD) simulations of relevant systems that incorporate representative organic solutes allow
detailed investigation of changes in fundamental interfacial and capillary properties. Experiments: We
use MD simulations to explore the effects of four organic solutes (quinoline, decanoic acid, coronene, sorgoleone) on the wettability of quartz by water in the presence of scCO2. We examine the impacts of polar,
alkyl, and aromatic moieties as well as fluid flow velocity at elevated temperatures and pressures.
Findings: Organic molecules accumulate at the water-CO2 interface, where they distribute according to
their size and functional groups. Certain organics penetrate the adsorbed water film at the quartz-CO2
interface, revealing two modes of hydrogen bonding between polar organic functional group, water,
and quartz surface –OH groups. Interfacial energies and contact angles are minimally impacted by
organic adsorption at the water-CO2 interface, possibly due to simultaneous CO2 desorption. Nonequilibrium MD simulations reveal that flow-induced redistribution of organic compounds modulates
the radii of curvature of the advancing and receding water-CO2 interfaces. Our results indicate that
organic adsorption on water surfaces is likely ubiquitous during multi-phase flow in soils and sedimentary rocks, with implications for the mobilization and transport of organic compounds
Molecular Laser Cooling in a Dynamically Tunable Repulsive Optical Trap
No full textRecent work with laser-cooled molecules in attractive optical traps has shown that the differential ac
Stark shifts arising from the trap light itself can become problematic, limiting collisional shielding
efficiencies, rotational coherence times, and laser-cooling temperatures. In this Letter, we explore trapping
and laser cooling of CaF molecules in a ring-shaped repulsive optical trap. The observed dependences of
loss rates on temperature and barrier height show characteristic behavior of repulsive traps and indicate
strongly suppressed average ac Stark shifts. Within the trap, we find that Λ-enhanced gray molasses cooling is effective, producing similar minimum temperatures as those obtained in free space. By combining in-trap laser cooling with dynamical reshaping of the trap, we also present a method that allows highly efficientand rapid transfer from molecular magneto-optical traps into conventional attractive optical traps, which has been an outstanding challenge for experiments to date. Notably, our method could allow nearly lossless transfer over millisecond timescales
Two-Dimensional Programmable Tweezer Arrays of Fermions
No full textWe prepare high-filling two-component arrays of tens of fermionic 6Li atoms in optical tweezers, with
the atoms in the ground motional state of each tweezer. Using a stroboscopic technique, we configure the
arrays in various two-dimensional geometries with negligible Floquet heating. A full spin- and density-
resolved readout of individual sites allows us to postselect near-zero entropy initial states for fermionic
quantum simulation. We prepare a correlated state in a two-by-two tunnel-coupled Hubbard plaquette,
demonstrating all the building blocks for realizing a programmable fermionic quantum simulator
The Variable CTCF Site from Drosophila melanogaster Ubx Gene is Redundant and Has no Insulator Activity
No full textCTCF is the most thoroughly studied chromatin architectural protein and it is found in both Drosophila and mammals. CTCF preferentially binds to promoters and insulators and is thought to facilitate formation of chromatin loops. In a subset of sites, CTCF binding depends on the epigenetic status of the surrounding chromatin. One such variable CTCF site (vCTCF) was found in the intron of the Ubx gene, in close
proximity to the BRE and abx enhancers. CTCF binds to the variable site in tissues where Ubx gene is active, suggesting that the vCTCF site plays a role in facilitating contacts between the Ubx promoter and its
enhancers. Using CRISPR/Cas9 and attP/attB site-specific integration methods, we investigated the functional role of vCTCF and showed that it is not required for normal Drosophila development. Furthermore, a 2161-bp fragment containing vCTCF does not function as an effective insulator when substituted for the Fab-7 boundary in the Bithorax complex. Our results suggest that vCTCF function is redundant in the regulation of Ubx
Monodromy defects from hyperbolic space
No full textAbstract We study monodromy defects in O(N) symmetric scalar field theories in d dimensions. After a Weyl transformation, a monodromy defect may be described by placing the theory on S1 × Hd−1, where Hd−1 is the hyperbolic space, and imposing on the fundamental fields a twisted periodicity condition along S1. In this description, the codimension two defect lies at the boundary of Hd−1. We first study the general monodromy defect in the free field theory, and then develop the large N expansion of the defect in the interacting theory, focusing for simplicity on the case of N complex fields with a one-parameter monodromy condition. We also use the ϵ-expansion in d = 4 − ϵ, providing a check on the large N approach. When the defect has spherical geometry, its expectation value is a meaningful quantity, and it may be obtained by computing the free energy of the twisted theory on S1 × Hd−1. It was conjectured that the logarithm of the defect expectation value, suitably multiplied by a dimension dependent sine factor, should decrease under a defect RG flow. We check this conjecture in our examples, both in the free and interacting case, by considering a defect RG flow that corresponds to imposing alternate boundary conditions on one of the low-lying Kaluza-Klein modes on Hd−1. We also show that, adapting standard techniques from the AdS/CFT literature, the S1 × Hd−1 setup is well suited to the calculation of the defect CFT data, and we discuss various examples, including one-point functions of bulk operators, scaling dimensions of defect operators, and four-point functions of operator insertions on the defect.</jats:p