156 research outputs found
Radiation-pressure-mediated control of an optomechanical cavity
No full textWe describe and demonstrate a method to control a detuned movable-mirror Fabry-Pérot cavity using radiation pressure in the presence of a strong optical spring. At frequencies below the optical spring resonance, self-locking of the cavity is achieved intrinsically by the optomechanical (OM) interaction between the cavity field and the movable end mirror. The OM interaction results in a high rigidity and reduced susceptibility of the mirror to external forces. However, due to a finite delay time in the cavity, this enhanced rigidity is accompanied by an antidamping force, which destabilizes the cavity. The cavity is stabilized by applying external feedback in a frequency band around the optical spring resonance. The error signal is sensed in the amplitude quadrature of the transmitted beam with a photodetector. An amplitude modulator in the input path to the cavity modulates the light intensity to provide the stabilizing radiation pressure force.National Science Foundation (U.S.) (Grants PHY1707840)National Science Foundation (U.S.) (Grant PHY-1404245)National Science Foundation (U.S.) (Grant PHY-1404245
Understanding the Impacts of Altering Safe Harbor Plans within Small Business Organizations
No full textIn the past three decades, many companies have adopted less expensive, relatively easier to establish, modify, amend, and terminate safe harbor 401 (k) retirement plans. By adopting the safe harbor, 401(k) plans, smaller private employers have enacted retirement incentives, shared the investment and managing responsibilities, and assisted their employees with building their retirement savings (Ali & Frank, 2019; Clark et al., 2019a). Furthermore, many small companies have reflected the impacts of the economic fluctuations and the retirement legislation changes, amended their safe harbor 401(k) plans, and lowered their retirement investments (Card & Ransom, 2011). The high number of plan amendments and the economic uncertainties have limited the employees’ abilities to accumulate retirement savings (Benartzi & Thaler, 2013; Libson, 2017). As a result, the increasing number of safe harbor amendments and the lack of financial education among employers and employees have amplified the retirement crisis in the United States (Acevedo, 2016; Clark et al., 2019b; Smith, 2016). Enhancing an understanding of the employees’ perceptions of engaging in retirement planning was essential for this research. The study focused on exploring the adverse impacts of amendments in the safe harbor 401(k) retirement plans as perceived and described by the retirement plan participants. It revealed the importance of pursuing proactive participation in retirement through employment. Finally, the research explored the impacts of the employees’ understanding and awareness of their opportunities to maximize their retirement benefits
THE MOLECULAR COILGUN AND SINGLE PHOTON COOLING: TOWARDS ULTRA-COLD MOLECULES
No full textAuthor Institution: Center for Nonlinear Dynamics and Department of Physics, The University of Texas at Austin, Austin, Texas 78712-1081, USA; Sackler School of Chemistry, Tel-Aviv University, Tel-Aviv, Israel; Center for Nonlinear Dynamics and Department of Physics, The University of Texas at Austin, Austin, Texas 78712-1081, USAWe report the stopping of a molecular beam of oxygen using a series of pulsed electromagnetic coils. This molecular coilgun can be used stop and trap any paramagnetic molecules at temperatures of the supersonic expansion. Further cooling of molecules near the single photon recoil limit can be accomplished with the method of single photon cooling as was recently demonstrated in our group with atoms. The application of these methods to cold chemistry will be discussed
Room-temperature optomechanical squeezing
No full text© 2020, The Author(s), under exclusive licence to Springer Nature Limited. Squeezed light—light with quantum noise lower than shot noise in some quadratures and higher in others—can be used to improve the sensitivity of precision measurements. In particular, squeezed light sources based on nonlinear optical crystals are being used to improve the sensitivity of gravitational wave detectors. In optomechanical squeezers, the radiation-pressure-driven interaction of a coherent light field with a mechanical oscillator induces correlations between the amplitude and phase quadratures of the light, which induce the squeezing. However, thermally driven fluctuations of the mechanical oscillator’s position make it difficult to observe the quantum correlations at room temperature and at low frequencies. Here, we present a measurement of optomechanically squeezed light, performed at room temperature in a broad band near the audio-frequency regions relevant to gravitational wave detectors. We observe sub-Poissonian quantum noise in a frequency band of 30–70 kHz with a maximum reduction of 0.7 ± 0.1 dB below shot noise at 45 kHz. We present two independent methods of measuring this squeezing, one of which does not rely on the calibration of shot noise
Thomas Gainsborough and The Imagery of Passage
No full textABSTRACT
This study aims to look closely at the road in Gainsborough’s landscape paintings, and to establish what they may tell us about passage beyond their accepted recessional or structural role. The clarity of detail in these early landscapes enables us to speculate the likely forms of passage that are being enacted within the context of Gainsborough’s native county of Suffolk; thus isolating them from his later stylistic developments. Understanding the circumstances pertaining to Suffolk’s roads and their uses during the first half of the eighteenth century - how they looked and the volume of traffic they sustained - will inform this investigation.
Coming to terms with the actual historical, rural environment, and applying these findings to the fictional plasticity of the painted road, and the landscape through which it passes, will bring us closer to understanding how Gainsborough’s landscapes may have contributed to a more local process; a preoccupation that was concerned with movement through, or around a particular location. We will seek to establish how particular spatial areas, created through an illusionistic and fictional depth of field, together with the manner of representation, inform passage in a broader sense
Finding apparent horizons in dynamic 3D numerical spacetimes
No full textWe have developed a general method for finding apparent horizons in 3D numerical relativity. Instead of
solving for the partial differential equation describing the location of the apparent horizons, we expand the
closed 2D surfaces in terms of symmetric trace-free tensors and solve for the expansion coefficients using a
minimization procedure. Our method is applied to a number of different spacetimes, including numerically
constructed spacetimes containing highly distorted axisymmetric black holes in spherical coordinates, and 3D
rotating, and colliding black holes in Cartesian coordinates
Photostop
No full textThe use of photodissociation to produce cold, slow molecules from a molecular beam of cold, fast molecules is dubbed “photostop”. The essence of the scheme is thus: a pulsed molecular beam of a precursor molecule AB seeded in a noble carrier gas is crossed by a laser beam. The laser light dissociates AB, producing fragments A and B. These recoil from the dissociation site. The speed of the molecular beam and the wavelength of the laser beam are tuned so that the recoil velocity of those A molecules recoiling opposite to the molecular beam direction cancels out their initial velocity. This leaves a certain amount of A close to stationary in the laboratory frame.
The photostop technique was first demonstrated with the production of cold, slow NO radicals by the photodissociation of nitrogen dioxide, both at Durham and in South Korea. The work described in this thesis began as a development of the NO photostop experiment, with an attempt made to magnetically trap NO. Latterly, the photostop project became part of the Millikelvin Molecules in a Quantum Array (MMQA) collaboration, the aim of which is the trapping of ten million cold, polar molecules. To this end, the photostop of SH radicals by the photodissociation of hydrogen disulphide was demonstrated
Transport and catabolism of murein tripeptide in Escherichia coli K-12
No full textIn bacteria, extracellular peptides are not only a source of nutrients but also play important roles in cell-cell communication. The primary mode of uptake of these peptides in bacteria is via ATP Binding Cassette (ABC) transporters. The substrate-binding protein (SBP) of these transporters captures extracellular peptides and delivers them to membrane-bound components for transport. Bacterial peptide-binding SBPs that function in ABC transporters have evolved predominantly to function as generalists, recognising peptides of particular lengths but with low or no sequence specificity. However, within the peptide-specific SBPs are examples that appear to have evolved subsequently to recognise specific peptides.
Here we have provided the first biophysical evidence, using native electrospray mass spectrometry, for the binding of a specific peptide to an Escherichia coli SBP. This peptide is a cell wall component named murein tripeptide (Mtp, L-Ala--D-Glu-meso-Dap) and is transported by an ABC transporter containing the SBP MppA. We demonstrate that MppA recognises Mtp specifically and with high affinity (KD ~ 250 nM), as determined by protein fluorescence spectroscopy and isothermal titration calorimetry. The crystal structure of MppA in complex with Mtp has been solved in order to understand the structural basis of specific binding of Mtp to MppA. Comparison of the structure of MppA-Mtp with structures of general tripeptides bound to oligopeptide binding protein OppA, reveals close similarity in the protein chains which fold to form an enclosed interdomain pocket in which the respective peptides reside. The peptide ligands superimpose remarkably closely given the profound differences in their structure. Strikingly, the effect of the D-stereochemistry, which projects the side chain of residue 2 in Mtp in the direction of the main chain in a conventional tripeptide, is compensated by the side chain γ-linkage of the carboxylate of the D-Glu to the amino group of diaminopimelic acid. The resulting amide linkage mimicks the peptide bond between residues 2 and 3 of a conventional tripeptide. Specificity for Mtp is conferred by a series of ionic and polar residues of MppA which make charge-charge and dipole-charge interactions with the highly ionised peptide.
Analysis of multiple bacterial genomes revealed that mppA is often situated near a gene called mpaA which encodes a putative amidase enzyme which is thought to hydrolyze murein tripeptide. This genetic organization is conserved among -proteobacteria and suggests that mppA and mpaA function together and are a part of peptide catabolic pathway. The MpaA enzyme from E. coli was purified to homogeneity as a His-tagged form, and its kinetic properties and parameters were determined. MpaA was found to hydrolyze Mtp efficiently but it did not cleave murein tetrapeptide. Replacement of meso-Dap by L-Lys in the tripeptide resulted in much lower efficiency. Mass spectrometric analysis of purified MpaA suggested that it co-purifies with a zinc ion and exists as a dimer in solution. The first crystal structure of an MpaA protein from Vibrio harveyi was solved to 2.17Å. The structural fold of V. harveyi MpaA was similar to eukaryotic carboxypeptidases. Analysis of the structure showed an obvious dimer interface between two monomers of MpaA and presence of a zinc ion in the active centre of each monomer consistent with the mass spectrometric data. The structure provides the basis for future modelling and mutational studies for extensive functional characterization of MpaA
First all-sky search for continuous gravitational waves from unknown sources in binary systems
No full textWe present the first results of an all-sky search for continuous gravitational waves from unknown spinning neutron stars in binary systems using LIGO and Virgo data. Using a specially developed analysis program, the TwoSpect algorithm, the search was carried out on data from the sixth LIGO science run and the second and third Virgo science runs. The search covers a range of frequencies from 20 Hz to 520 Hz, a range of orbital periods from 2 to ∼2,254 h and a frequency- and period-dependent range of frequency modulation depths from 0.277 to 100 mHz. This corresponds to a range of projected semimajor axes of the orbit from ∼0.6 × 10[superscript −3] ls to ∼6,500 ls assuming the orbit of the binary is circular. While no plausible candidate gravitational wave events survive the pipeline, upper limits are set on the analyzed data. The most sensitive 95% confidence upper limit obtained on gravitational wave strain is 2.3 × 10[superscript −24] at 217 Hz, assuming the source waves are circularly polarized. Although this search has been optimized for circular binary orbits, the upper limits obtained remain valid for orbital eccentricities as large as 0.9. In addition, upper limits are placed on continuous gravitational wave emission from the low-mass x-ray binary Scorpius X-1 between 20 Hz and 57.25 Hz.National Science Foundation (U.S.)United States. National Aeronautics and Space AdministrationCarnegie TrustDavid & Lucile Packard FoundationResearch CorporationAlfred P. Sloan Foundatio
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