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MIT Open Access Articles

Mostrando recursos 41 - 60 de 12,636

41.
Science with the Murchison Widefield Array - Cappallo, Roger J.; Corey, Brian E.; Coster, Anthea J.; Erickson, Philip J.; Goeke, Robert F.; Hewitt, Jacqueline N.; Kincaid, Barton B.; Kratzenberg, Eric; Lonsdale, Colin John; Matthews, Lynn D.; McWhirter, Stephen R.; Morgan, Edward H.; Remillard, Ronald Alan; Rogers, Alan E. E.; Salah, Joseph E.; Whitney, Alan R.; Williams, Christopher Leigh
Significant new opportunities for astrophysics and cosmology have been identified at low radio frequencies. The Murchison Widefield Array is the first telescope in the southern hemisphere designed specifically to explore the low-frequency astronomical sky between 80 and 300 MHz with arcminute angular resolution and high survey efficiency. The telescope will enable new advances along four key science themes, including searching for redshifted 21-cm emission from the EoR in the early Universe; Galactic and extragalactic all-sky southern hemisphere surveys; time-domain astrophysics; and solar, heliospheric, and ionospheric science and space weather. The Murchison Widefield Array is located in Western Australia at the...

42.
The Murchison Widefield Array: solar science with the low frequency SKA Precursor - Tingay, S. J.; Cappallo, Roger J.; Corey, Brian E.; Goeke, Robert F.; Hewitt, Jacqueline N.; Kincaid, B. B.; Kratzenberg, E.; Lonsdale, Colin John; McWhirter, Stephen R.; Morgan, Edward H.; Remillard, Ronald Alan; Rogers, Alan E. E.; Salah, J. E.; Whitney, Alan R.; Williams, Christopher Leigh
The Murchison Widefield Array is a low frequency (80 – 300 MHz) SKA Precursor, comprising 128 aperture array elements (known as tiles) distributed over an area of 3 km diameter. The MWA is located at the extraordinarily radio quiet Murchison Radioastronomy Observatory in the mid-west of Western Australia, the selected home for the Phase 1 and Phase 2 SKA low frequency arrays. The MWA science goals include: 1) detection of fluctuations in the brightness temperature of the diffuse redshifted 21 cm line of neutral hydrogen from the epoch of reionisation; 2) studies of Galactic and extragalactic processes based on deep,...

43.
The EoR sensitivity of the Murchison Widefield Array - Cappallo, Roger J.; Corey, Brian E.; Goeke, Robert F.; Hewitt, Jacqueline N.; Kincaid, B. B.; Kratzenberg, E.; Lonsdale, Colin John; McWhirter, Stephen R.; Morgan, Edward H.; Oberoi, D.; Remillard, Ronald Alan; Rogers, Alan E. E.; Salah, J. E.; Whitney, Alan R.; Williams, Christopher Leigh
Using the final 128 antenna locations of the Murchison Widefield Array (MWA), we calculate its sensitivity to the epoch of reionization (EoR) power spectrum of redshifted 21 cm emission for a fiducial model and provide the tools to calculate the sensitivity for any model. Our calculation takes into account synthesis rotation, chromatic and asymmetrical baseline effects, and excludes modes that will be contaminated by foreground subtraction. For the fiducial model, the MWA will be capable of a 14σ detection of the EoR signal with one full season of observation on two fields (900 and 700 h).

44.
LOW-FREQUENCY OBSERVATIONS OF THE MOON WITH THE MURCHISON WIDEFIELD ARRAY - Cappallo, Roger J.; Corey, Brian E.; Goeke, Robert F.; Hewitt, Jacqueline N.; Kincaid, B. B.; Kratzenberg, E.; Lonsdale, Colin John; McWhirter, Stephen R.; Morgan, Edward H.; Remillard, Ronald Alan; Rogers, Alan E. E.; Salah, J. E.; Whitney, Alan R.; Williams, Christopher Leigh
A new generation of low-frequency radio telescopes is seeking to observe the redshifted 21 cm signal from the epoch of reionization (EoR), requiring innovative methods of calibration and imaging to overcome the difficulties of wide-field low-frequency radio interferometry. Precise calibration will be required to separate the expected small EoR signal from the strong foreground emission at the frequencies of interest between 80 and 300 MHz. The Moon may be useful as a calibration source for detection of the EoR signature, as it should have a smooth and predictable thermal spectrum across the frequency band of interest. Initial observations of the...

45.
Adversarial hypothesis testing and a quantum stein's lemma for restricted measurements - Harrow, Aram W.; Lee, James R.; Peres, Yuval; Brandao, Fernando G.S.L.
Recall the classical hypothesis testing setting with two convex sets of probability distributions P and Q. One receives either n i.i.d. samples from a distribution p ∈ P or from a distribution q ∈ Q and wants to decide from which set the points were sampled. It is known that the optimal exponential rate at which errors decrease can be achieved by a simple maximum-likelihood ratio test which does not depend on p or q, but only on the sets P and Q.
We consider an adaptive generalization of this model where the choice of p ∈ P and q ∈...

46.
Uselessness for an Oracle Model with Internal Randomness - Harrow, Aram W.; Rosenbaum, David J.
We consider a generalization of the standard oracle model in which the oracle acts on the target with a permutation selected according to internal random coins. We describe several problems that are impossible to solve classically but can be solved by a quantum algorithm using a single query; we show that such infinity-vs-one separations between classical and quantum query complexities can be constructed from much weaker separations. We also give conditions to determine when oracle problems--either in the standard model, or in any of the generalizations we consider--cannot be solved with success probability better than random guessing would achieve. In...

47.
Generalized Two-Temperature Model for Coupled Phonon-Magnon Diffusion - Zhou, Jiawei; Chen, Gang; Liao, Bolin
We generalize the two-temperature model [Sanders and Walton, Phys. Rev. B 15, 1489 (1977)] for coupled phonon-magnon diffusion to include the effect of the concurrent magnetization flow, with a particular emphasis on the thermal consequence of the magnon flow driven by a nonuniform magnetic field. Working within the framework of the Boltzmann transport equation, we derive the constitutive equations for coupled phonon-magnon transport driven by gradients of both temperature and external magnetic fields, and the corresponding conservation laws. Our equations reduce to the original Sanders-Walton two-temperature model under a uniform external field, but predict a new magnon cooling effect driven...

48.
Bio-Artificial Synergies for Grasp Posture Control of Supernumerary Robotic Fingers - Wu, Faye Y.; Asada, Harry
A new type of wrist-mounted robot, the Supernumerary Robotic (SR) Fingers, is proposed to work closely with the human hand and aid the human in performing a variety of prehensile tasks. For people with diminished functionality of their hands, these robotic fingers could provide the opportunity to live with more independence and work more productively. A natural and implicit coordination between the SR Fingers and the human fingers is required so the robot can be transformed to act as part of the human body. This paper presents a novel control algorithm, termed “Bio-Artificial Synergies”, which enables the SR and human...

49.
Measurement of Υ production in pp collisions at √s = 2.76 TeV - Counts, Ian Thomas Hunt; Ilten, Philip James; Williams, Michael
The production of Υ(1S), Υ(2S) and Υ(3S) mesons decaying into the dimuon final state is studied with the LHCb detector using a data sample corresponding to an integrated luminosity of 3.3 pb[superscript −1] collected in proton–proton collisions at a centre-of-mass energy of √s = 2.76 TeV. The differential production cross-sections times dimuon branching fractions are measured as functions of the Υ transverse momentum and rapidity, over the ranges p[subscript T] < 15 GeV/c and 2.0 < y < 4.5. The total cross-sections in this kinematic region, assuming unpolarised production, are measured to be σ(pp → Υ(1S)X) × B(Υ(1S) → μ[superscript...

50.
Product-state approximations to quantum ground states - Brandao, Fernando G.S.L.; Harrow, Aram W.
The local Hamiltonian problem consists of estimating the ground-state energy (given by the minimum eigenvalue) of a local quantum Hamiltonian. It can be considered as a quantum generalization of constraint satisfaction problems (CSPs) and has a key role in quantum complexity theory, being the first and most natural QMA-complete problem known. An interesting regime for the local Hamiltonian problem is that of extensive error, where one is interested in estimating the mean ground-state energy to constant accuracy. The problem is NP-hard by the PCP theorem, but whether it is QMA-hard is an important open question in quantum complexity theory. A...

51.
Quantum de finetti theorems under local measurements with applications - Brandao, Fernando G.S.L.; Harrow, Aram W.
Quantum de Finetti theorems are a useful tool in the study of correlations in quantum multipartite states. In this paper we prove two new quantum de Finetti theorems, both showing that under tests formed by local measurements in each of the subsystems one can get a much improved error dependence on the dimension of the subsystems. We also obtain similar results for non-signaling probability distributions. We give the following applications of the results to quantum complexity theory, polynomial optimization, and quantum information theory: We prove the optimality of the Chen-Drucker protocol for 3-SAT, under the assumption there is no subexponential-time...

52.
Efficient distributed quantum computing - Beals, Robert; Brierley, Stephen; Gray, Oliver; Harrow, Aram W.; Kutin, Samuel; Linden, Noah; Shepherd, Dan; Stather, Mark
We provide algorithms for efficiently moving and addressing quantum memory in parallel. These imply that the standard circuit model can be simulated with a low overhead by a more realistic model of a distributed quantum computer. As a result, the circuit model can be used by algorithm designers without worrying whether the underlying architecture supports the connectivity of the circuit. In addition, we apply our results to existing memory-intensive quantum algorithms. We present a parallel quantum search algorithm and improve the time–space trade-off for the element distinctness and collision finding problems.

53.
A SLOWLY EVOLVING HOST MOVES FIRST IN SYMBIOTIC INTERACTIONS - Gore, Jeff; Damore, James
Symbiotic relationships, both parasitic and mutualistic, are ubiquitous in nature. Understanding how these symbioses evolve, from bacteria and their phages to humans and our gut microflora, is crucial in understanding how life operates. Often, symbioses consist of a slowly evolving host species with each host only interacting with its own subpopulation of symbionts. The Red Queen hypothesis describes coevolutionary relationships as constant arms races with each species rushing to evolve an advantage over the other, suggesting that faster evolution is favored. Here, we use a simple game theoretic model of host–symbiont coevolution that includes population structure to show that if...

54.
Observation of Floquet-Bloch States on the Surface of a Topological Insulator - Wang, Y. H.; Steinberg, Hadar; Jarillo-Herrero, Pablo; Gedik, Nuh
The unique electronic properties of the surface electrons in a topological insulator are protected by time-reversal symmetry. Circularly polarized light naturally breaks time-reversal symmetry, which may lead to an exotic surface quantum Hall state. Using time- and angle-resolved photoemission spectroscopy, we show that an intense ultrashort midinfrared pulse with energy below the bulk band gap hybridizes with the surface Dirac fermions of a topological insulator to form Floquet-Bloch bands. These photon-dressed surface bands exhibit polarization-dependent band gaps at avoided crossings. Circularly polarized photons induce an additional gap at the Dirac point, which is a signature of broken time-reversal symmetry on...

55.
Circular dichroism in angle-resolved photoemission spectroscopy of topological insulators - Wang, Yihua; Gedik, Nuh
Topological insulators are a new phase of matter that exhibits exotic surface electronic properties. Determining the spin texture of this class of material is of paramount importance for both fundamental understanding of its topological order and future spin-based applications. In this article, we review the recent experimental and theoretical studies on the differential coupling of left-circularly versus right-circularly polarized light to the topological surface states in angle-resolved photoemission spectroscopy. These studies have shown that the polarization of light and the experimental geometry play a veryimportant role in both photocurrent intensity and spin polarization of photoelectrons emitted from the topological surface...

56.
Fluctuating charge-density waves in a cuprate superconductor - Torchinsky, Darius H.; Mahmood, Fahad; Bollinger, Anthony T.; Božović, Ivan; Gedik, Nuh
Cuprate materials hosting high-temperature superconductivity (HTS) also exhibit various forms of charge and spin ordering1, 2, 3, 4, 5, 6 whose significance is not fully understood7. So far, static charge-density waves8 (CDWs) have been detected by diffraction probes only at particular doping levels9, 10, 11 or in an applied external field12 . However, dynamic CDWs may also be present more broadly and their detection, characterization and relationship with HTS remain open problems. Here we present a method based on ultrafast spectroscopy to detect the presence and measure the lifetimes of CDW fluctuations in cuprates. In an underdoped La[subscript 1.9]Sr[subscript 0.1]CuO4...

57.
METAL-POOR STARS OBSERVED WITH THE MAGELLAN TELESCOPE. I. CONSTRAINTS ON PROGENITOR MASS AND METALLICITY OF AGB STARS UNDERGOING s-PROCESS NUCLEOSYNTHESIS - Placco, Vinicius M.; Beers, Timothy C.; Karakas, Amanda I.; Kennedy, Catherine R.; Rossi, Silvia; Christlieb, Norbert; Stancliffe, Richard J.; Frebel, Anna L.
We present a comprehensive abundance analysis of two newly discovered carbon-enhanced metal-poor (CEMP) stars. HE 2138?3336 is a s-process-rich star with [Fe/H] = -2.79, and has the highest [Pb/Fe] abundance ratio measured thus far, if non-local thermodynamic equilibrium corrections are included ([Pb/Fe] = +3.84). HE 2258?6358, with [Fe/H] = -2.67, exhibits enrichments in both s- and r-process elements. These stars were selected from a sample of candidate metal-poor stars from the Hamburg/ESO objective-prism survey, and followed up with medium-resolution (R ~ 2000) spectroscopy with GEMINI/GMOS. We report here on derived abundances (or limits) for a total of 34 elements in...

58.
CHEMICAL SIGNATURES OF THE FIRST GALAXIES: CRITERIA FOR ONE-SHOT ENRICHMENT - Bromm, Volker; Frebel, Anna L.
We utilize metal-poor stars in the local, ultra-faint dwarf galaxies (UFDs; L [subscript tot] ≤ 10[superscript 5] L [subscript ☉]) to empirically constrain the formation process of the first galaxies. Since UFDs have much simpler star formation histories than the halo of the Milky Way, their stellar populations should preserve the fossil record of the first supernova (SN) explosions in their long-lived, low-mass stars. Guided by recent hydrodynamical simulations of first galaxy formation, we develop a set of stellar abundance signatures that characterize the nucleosynthetic history of such an early system if it was observed in the present-day universe. Specifically,...

59.
SEARCHES FOR HIGH-FREQUENCY VARIATIONS IN THE [superscript 8]B SOLAR NEUTRINO FLUX AT THE SUDBURY NEUTRINO OBSERVATORY - Formaggio, Joseph A.; Miller, M. L.; Monreal, B.; Monroe, J.; Ott, R. A.; Sonley, T. J.
We have performed three searches for high-frequency signals in the solar neutrino flux measured by the Sudbury Neutrino Observatory, motivated by the possibility that solar g-mode oscillations could affect the production or propagation of solar [superscript 8]B neutrinos. The first search looked for any significant peak in the frequency range 1-144 day[superscript –1], with a sensitivity to sinusoidal signals with amplitudes of 12% or greater. The second search focused on regions in which g-mode signals have been claimed by experiments aboard the Solar and Heliospheric Observatory satellite, and was sensitive to signals with amplitudes of 10% or greater. The third...

60.
FERMI-LAT OBSERVATIONS OF SUPERNOVA REMNANTS INTERACTING WITH MOLECULAR CLOUDS: W41, MSH 17-39, AND G337.7-0.1 - Castro, Daniel; Slane, Patrick; Carlton, Ashley; Figueroa-Feliciano, Enectali
We report the detection of γ-ray emission coincident with three supernova remnants (SNRs) using data from the Large Area Telescope on board the Fermi Gamma-ray Space Telescope. W41, G337.7-0.1, and MSH 17-39 are SNRs known to be interacting with molecular clouds, as evidenced by observations of hydroxyl (OH) maser emission at 1720 MHz in their directions and other observational information. SNR shocks are expected to be sites of cosmic-ray acceleration, and clouds of dense material can provide effective targets for production of γ-rays from π[superscript 0]-decay. The observations reveal unresolved sources in the direction of G337.7-0.1 and MSH 17-39 and...