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Time-reversal symmetry and universal conductance fluctuations in a driven two-level system

Published in:
Phys. Rev. Lett., Vol. 110, No. 1, 2 January 2013, 016603.

Summary

In the presence of time-reversal symmetry, quantum interference gives strong corrections to the electric conductivity of disordered systems. The self-interference of an electron wave function traveling time-reversed paths leads to effects such as weak localization and universal conductance fluctuations. Here, we investigate the effects of broken time-reversal symmetry in a driven artificial two-level system. Using a superconducting flux qubit, we implement scattering events as multiple Landau-Zener transitions by driving the qubit periodically back and forth through an avoided crossing. Interference between different qubit trajectories gives rise to a speckle pattern in the qubit transition rate, similar to the interference patterns created when coherent light is scattered off a disordered potential. Since the scattering events are imposed by the driving protocol, we can control the time-reversal symmetry of the system by making the drive waveform symmetric or asymmetric in time. We find that the fluctuations of the transition rate exhibit a sharp peak when the drive is time symmetric, similar to universal conductance fluctuations in electronic transport through mesoscopic systems.
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Summary

In the presence of time-reversal symmetry, quantum interference gives strong corrections to the electric conductivity of disordered systems. The self-interference of an electron wave function traveling time-reversed paths leads to effects such as weak localization and universal conductance fluctuations. Here, we investigate the effects of broken time-reversal symmetry in a...

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Graph embedding for speaker recognition

Published in:
Chapter in Graph Embedding for Pattern Analysis, 2013, pp. 229-60.

Summary

This chapter presents applications of graph embedding to the problem of text-independent speaker recognition. Speaker recognition is a general term encompassing multiple applications. At the core is the problem of speaker comparison-given two speech recordings (utterances), produce a score which measures speaker similarity. Using speaker comparison, other applications can be implemented-speaker clustering (grouping similar speakers in a corpus), speaker verification (verifying a claim of identity), speaker identification (identifying a speaker out of a list of potential candidates), and speaker retrieval (finding matches to a query set).
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Summary

This chapter presents applications of graph embedding to the problem of text-independent speaker recognition. Speaker recognition is a general term encompassing multiple applications. At the core is the problem of speaker comparison-given two speech recordings (utterances), produce a score which measures speaker similarity. Using speaker comparison, other applications can be...

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Dwell scheduling algorithms for phased array antenna

Published in:
IEEE Trans. Aerosp. Electron. Syst., Vol. 49, No. 1, January 2013, pp. 42-54.

Summary

In a multifunctional radar performing searching and tracking operations, the maximum number of targets that can be managed is an important measure of performance. One way a radar can maximize tracking performance is to optimize its dwell scheduling. The problem of designing efficient dwell scheduling algorithms for various tracking and searching scenarios with respect to various objective functions has been considered many times in the past and many solutions have been proposed. We consider the dwell scheduling problem for two different scenarios where the only objective is to maximize the number of dwells scheduled during a scheduling period. We formulate the problem as a distributed and a nondistributed bin packing problem and present optimal solutions using an integer programming formulation. Obtaining an optimal solution gives the limit of radar performance. We also present a more computationally friendly but less optimal solution using a greedy approach.
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Summary

In a multifunctional radar performing searching and tracking operations, the maximum number of targets that can be managed is an important measure of performance. One way a radar can maximize tracking performance is to optimize its dwell scheduling. The problem of designing efficient dwell scheduling algorithms for various tracking and...

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The MIT-LL/AFRL IWSLT-2011 MT System

Summary

This paper describes the MIT-LL/AFRL statistical MT system and the improvements that were developed during the IWSLT 2011 evaluation campaign. As part of these efforts, we experimented with a number of extensions to the standard phrase-based model that improve performance on the Arabic to English and English to French TED-talk translation tasks. We also applied our existing ASR system to the TED-talk lecture ASR task. We discuss the architecture of the MIT-LL/AFRL MT system, improvements over our 2010 system, and experiments we ran during the IWSLT-2011 evaluation. Specifically, we focus on 1) speech recognition for lecture-like data, 2) cross-domain translation using MAP adaptation, and 3) improved Arabic morphology for MT preprocessing.
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Summary

This paper describes the MIT-LL/AFRL statistical MT system and the improvements that were developed during the IWSLT 2011 evaluation campaign. As part of these efforts, we experimented with a number of extensions to the standard phrase-based model that improve performance on the Arabic to English and English to French TED-talk...

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Atomic layer deposition of Sc2O3 for passivating AlGaN/GaN high electron mobility transistor devices

Published in:
Appl. Phys. Lett., Vol. 101, No. 23, 3 December 2012, 232109.
Topic:

Summary

Polycrystalline, partially epitaxial Sc2O3 films were grown on AlGaN/GaN substrates by atomic layer deposition (ALD). With this ALD Sc2O3 film as the insulator layer, the Sc2O3/AlGaN/GaN metal-insulator-semiconductor high electron mobility transistors showed excellent electrical performance with a high Ion/Ioff ratio of over 108 and a low subthreshold slope of 75 mV/dec. The UV/NH4OH surface treatment on AlGaN/GaN prior to ALD was found to be critical for achieving these excellent figures. In addition, the Sc2O3 dielectric is found to be negatively charged, which facilitates the enhancement-mode operation. While bare Sc2O3 suffers from moisture degradation, depositing a moisture blocking layer of ALD Al2O3 can effectively eliminate this effect.
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Summary

Polycrystalline, partially epitaxial Sc2O3 films were grown on AlGaN/GaN substrates by atomic layer deposition (ALD). With this ALD Sc2O3 film as the insulator layer, the Sc2O3/AlGaN/GaN metal-insulator-semiconductor high electron mobility transistors showed excellent electrical performance with a high Ion/Ioff ratio of over 108 and a low subthreshold slope of 75...

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High efficiency coherent beam combining of semiconductor optical amplifiers

Published in:
Opt. Lett., Vol. 37, No. 23, 1 December 2012, pp. 5006-5008.

Summary

We demonstrate 40 W coherently combined output power in a single diffraction-limited beam from a one-dimensional 47-element array of angled-facet slab-coupled optical waveguide amplifiers at 1064 nm. The output from each emitter was collimated and overlapped onto a diffractive optical element combiner using a common transform lens. Phase locking was achieved via active feedback on each amplifier's drive current to maximize the power in the combined beam. The combining efficiency at all current levels was nearly constant at 87%.
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Summary

We demonstrate 40 W coherently combined output power in a single diffraction-limited beam from a one-dimensional 47-element array of angled-facet slab-coupled optical waveguide amplifiers at 1064 nm. The output from each emitter was collimated and overlapped onto a diffractive optical element combiner using a common transform lens. Phase locking was...

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Characterizing the optical variability of bright blazars: variability-based selection of fermi active galactic nuclei

Summary

We investigate the use of optical photometric variability to select and identify blazars in large-scale time-domain surveys, in part to aid in the identification of blazar counterparts to the ~30% of y -ray sources in the Fermi 2FGL catalog still lacking reliable associations. Using data from the optical LINEAR asteroid survey, we characterize the optical variability of blazars by fitting a damped random walk model to individual light curves with two main model parameters, the characteristic timescales of variability T , and driving amplitudes on short timescales ^sigma. Imposing cuts on minimum T and ^sigma allows for blazar selection with high efficiency E and completeness C. To test the efficacy of this approach, we apply this method to optically variable LINEAR objects that fall within the several arcminute error ellipses of y -ray sources in the Fermi 2FGL catalog. Despite the extreme stellar contamination at the shallow depth of the LINEAR survey, we are able to recover previously associated optical counterparts to Fermi active galactic nuclei with E >/ 88% and C = 88% in Fermi 95% confidence error ellipses having semimajor axis r < 8'. We find that the suggested radio counterpart to Fermi source 2FGL J1649.6+5238 has optical variability consistent with other y -ray blazars and is likely to be the y -ray source. Our results suggest that the variability of the non-thermal jet emission in blazars is stochastic in nature, with unique variability properties due to the effects of relativistic beaming. After correcting for beaming, we estimate that the characteristic timescale of blazar variability is ~3 years in the rest frame of the jet, in contrast with the ~320 day disk flux timescale observed in quasars. The variability-based selection method presented will be useful for blazar identification in time-domain optical surveys and is also a probe of jet physics.
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Summary

We investigate the use of optical photometric variability to select and identify blazars in large-scale time-domain surveys, in part to aid in the identification of blazar counterparts to the ~30% of y -ray sources in the Fermi 2FGL catalog still lacking reliable associations. Using data from the optical LINEAR asteroid...

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Tower Flight Data Manager benefits assessment: initial investment decision interim report

Summary

This document provides an overview of MIT Lincoln Laboratory's activities in support of the interim stage of the Initial Investment Decision benefits assessment for the Tower Flight Data Manager. It outlines the rationale for the focus areas, and the background, methodology, and scope in the focus areas of departure metering, sequence optimization, airport configuration optimization, and safety assessment. Estimates of the potential benefits enabled by TFDM deployment are presented for each of these areas for a subset of airports and conditions considered within the scope of the analyses. These benefits are monetized where possible. Recommendations for follow-on work, for example, to support future benefits assessment efforts for TFDM, are also discussed.
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Summary

This document provides an overview of MIT Lincoln Laboratory's activities in support of the interim stage of the Initial Investment Decision benefits assessment for the Tower Flight Data Manager. It outlines the rationale for the focus areas, and the background, methodology, and scope in the focus areas of departure metering...

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Switchable electrowetting of droplets on dual-scale structured surfaces

Published in:
J. Vac. Sci. Technol. B, Microelectron. Process. Phenon., Vol. 30, No. 6, November 2012, 06F801.

Summary

The authors report on the development of surfaces containing artificially fabricated structures of dual nanometer and micrometer surfaces that allow an aqueous droplet to be reversibly switched by electrowetting from a Cassie state with low adhesion to a Wenzel state with high adhesion. A variety of geometries were fabricated to study parameters that affect switchable wetting-dewetting. Nanometer parallel corrugations, posts, and holes were fabricated and combined with micrometer features consisting of parallel corrugations, streets, and checkerboard patterns of varying widths and pitches. It was observed that many combinations of the dual-textured surfaces produced superhydrophobic wetting states and aqueous droplets on these surfaces could be electrically controlled to switch from a Cassie state to a Wenzel state. Reversible switching between these wetting states occurred on specific combinations of surface geometries, namely surfaces that had parallel corrugations.
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Summary

The authors report on the development of surfaces containing artificially fabricated structures of dual nanometer and micrometer surfaces that allow an aqueous droplet to be reversibly switched by electrowetting from a Cassie state with low adhesion to a Wenzel state with high adhesion. A variety of geometries were fabricated to...

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Multifunction Phased Array Radar wind shear experiment

Published in:
26th Conf. on Sever Local Storms, 5-8 November 2012.

Summary

Terminal Doppler Weather Radars (TDWRs) provide near-ground wind shear detection that is critical for aircraft safety at 46 airports across the United States. These systems are part of the larger network of 510 weather and aircraft surveillance radars owned and operated by government agencies in the continental United States. As the TDWR and other radar systems approach their engineering design life cycles, the Federal Aviation Administration (FAA), National Weather Service (NWS), and Department of Defense (DoD) are considering potential replacement systems (OFCM 2006; Weber et al. 2007). One option under consideration that would maintain the current airspace coverage is a replacement network of 334 Multifunction Phased Array Radars (MPARs) (Weber et al. 2007). The MPAR network described by Weber et al. (2007) would include two classes of systems: A high-resolution, full-scale version with an 8-m diameter antenna, and a lower-resolution terminal version with a 4-m diameter antenna, termed Terminal MPAR, or TMPAR. As the proposed TMPAR design has lower azimuthal beam resolution and less sensitivity than TDWRs, it is crucial to determine the impacts of that design on the detection of low-altitude wind shear. The design of the SPY-1A PAR, a research radar at the National Weather Radar Test Bed in Norman, Oklahoma (Zrnić et al. 2007), makes it a good proxy for examining the potential wind shear detection performance of the TMPAR. Therefore, in spring 2012, the National Oceanic and Atmospheric Administration (NOAA) National Severe Storms Laboratory organized and executed the MPAR Wind Shear Experiment (WSE) in collaboration with the FAA, NOAA's NWS Radar Operations Center, the University of Oklahoma Advanced Radar Research Center (OU ARRC), and the Massachusetts Institute of Technology Lincoln Laboratory (MIT LL). The primary objective of the MPAR WSE was to collect low-altitude observations with the SPY-1A PAR (hereafter, PAR) for comparison with observations from the nearby Oklahoma City (OKC) TDWR. Of particular interest is comparison of MIT LL wind shear detection algorithm performance using data from these two radars; this analysis is reported in Cho et al. (2013). Data were also collected from other radars in central Oklahoma to facilitate basic research on microbursts and other wind-producing storms. This paper provides an overview of the MPAR WSE and observed wind shear events.
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Summary

Terminal Doppler Weather Radars (TDWRs) provide near-ground wind shear detection that is critical for aircraft safety at 46 airports across the United States. These systems are part of the larger network of 510 weather and aircraft surveillance radars owned and operated by government agencies in the continental United States. As...

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