Cells of mutant strains of the bacterium B. subtilis fail to separate upon dividing, and form long fibers. A given fiber is initially straight, but once it grows to around 100 microns, it buckles and writhes. Soon aft...
Cells of mutant strains of the bacterium B. subtilis fail to separate upon dividing, and form long fibers. A given fiber is initially straight, but once it grows to around 100 microns, it buckles and writhes. Soon after this initial instability the fiber rapidly braids up into a plectoneme. Modeling a fiber as a twisted elastic filament in a viscous fluid, we exploit the “natural” frame of space curves to formulate the dynamics of the filament’s shape and twist density. The resulting coupled nonlinear equations are used to display a remarkable nonlinear phenomenon: geometric untwisting of open filaments, in which twist strain relaxes through a transient writhing instability without axial rotation. Experimentally observed motions of the fibers of B. subtilis may be examples of this twisting process.
Density-matrix calculations provide the steady-state conditions for probe amplification or lasing between atomic levels with an uninverted population, but additional insight into the underlying physics is given by a p...
Density-matrix calculations provide the steady-state conditions for probe amplification or lasing between atomic levels with an uninverted population, but additional insight into the underlying physics is given by a probability amplitude approach. In this paper we derive the gain coefficient from the Feynman diagrams for a probe-laser incident on a resonantly pumped, V-type system using time-dependent perturbation theory in a dressed basis. The connection is made to density-matrix calculations for this model, which have been used recently to describe experiments in Rb [A. S. Zibrov et al., Laser Phys. 5, 553 (1995); Phys. Rev. Lett. 75, 1499 (1995)]. In the density-matrix calculation the overall gain is possible because the pump-induced coherence of a strongly driven transition leads to probe amplification, despite the lack of inversion on the probe transition. In our amplitude approach we associate a specific physical process with each of the scattering channels for the probe and show how amplification without inversion can be achieved. The amplitude calculation reveals a distinction between stepwise and two-quantum processes. Interference is shown to result from the two-quantum processes, constructive for the amplification channels and destructive for the absorption. Terms appearing in the gain coefficient are traced to different sources in the amplitude and density-matrix approaches. The physical origin of each term is discussed and compared for both approaches. Terms that arise from coherences in the density-matrix approach are shown to correspond to noninterfering stepwise contributions in the amplitude approach. In deriving these results, we find that the Feynman rules that we construct for forming the probability amplitude for an arbitrary scattering process of the electromagnetic field from the coupled atom-strong pump system are consistent with Rayleigh-Schrödinger perturbation theory in the quantum dressed basis. In addition, the spontaneously emitted phot
作者：
Schulte, DPSkolnick, AHe has supported the development and operation of several naval systems
including advanced component selection for Trident II fire control and navigation systems. He served as branch manager of the Surface Ship ASW Combat System Branch which acted as the acquisition engineering agent for the AN/SQQ-89 Surface Ship Anti-Submarine Warfare Weapon System. He was then selected to manage the Module Engineering Department which provided engineering support to numerous naval systems including the AN/BSY-1 Submarine Combat System and the Trident II fire control and navigation system. He then served as the deputy program manager for NAVSEA Progressive Maintenance (2M/ATE). He holds a B.S. degree in Electrical Engineering from Purdue University and currently is pursuing a Maste's degree in Public Environmental Affairs at Indiana University—Purdue University
Indianapolis. He served at Applied Physics Laboratory/The Johns Hopkins University in missile development
then aboard USS Boston (CAG-1) and played leading roles in several weapon system developments (Regulus Terrier Tartar Talos) inertial navigation (Polaris) deep submergence (DSRV) and advanced ship designs (SES). He later was director Combat System Integration Naval Sea Systems Command and head Combat Projects Naval Ship Engineering Center. He led the Navy's High Energy Lasers and Directed Energy Weapons development efforts. He was vice president advanced technology at Operations Research Inc. and vice president maritime engineering at Defense Group Inc. before starting SSC in 1991. Dr. Skolnick holds a B.S. degree in Mathematics and Economics
Queens College an M.A. degree in Mathematics and Philosophy Columbia University an M.S. degree in Electrical/Aeronautical Engineering U.S. Naval Postgraduate School and a Ph.D. in Electrical Engineering and Applied Mathematics from Polytechnic University in New York. He is the author of many published papers on engineering design issues source selection procedures and large-scale complex technology problems
The Fleet continues to require high performance systems that can operate with dependability in the seas' unforgiving environments and under hostile action. Those demands are not new. What has changed is the urgent...
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The Fleet continues to require high performance systems that can operate with dependability in the seas' unforgiving environments and under hostile action. Those demands are not new. What has changed is the urgent priority formerly assigned to national defense issues. The arguments for continued superpower military strength are now roiled in politics along with unsettled budgets and uncertain force level projections. Current expectations revolve about indefinite fiscal and operational issues (difficult funding constraints and broadband threats). In the actual event of ''doing more with less,'' a practical response is to apply the creative power available from sound engineering judgement and the crucible of experience to the immediate needs of the Fleet. The attempt to shorten the path between advanced development effort and Fleet use has been tried occasionally in the past, often, without exemplary results. The Sustainable Hardware and Affordable Readiness Practices (SHARP) program, is a generic R&D effort under OpNav sponsorship that has been working steadily on sensible solutions to product engineering problems. Armed today with fast-time, large-scale computation abilities and modern tools for technical problem solving coupled with specialized engineering knowledge, it has been refreshed and is underway satisfying existing Fleet needs. The relationship between fully responsive engineering services and current operational needs is always demanding. The connection between advanced engineering development (6.3 category funds) and immediate Fleet usage brings added complexity and challenge, both technical and organizational. Illustrative examples of affordable engineering solutions to ''retain, revise, replace or retire'' questions are presented within the context of both Fleet realities and budgetary limitations. The discussion covers legacy system support, civil/military considerations and Fleet maintenance issues. It describes the substantial and critical payoffs i
The influence of a laterally inhomogeneous electrostatic potential of an adsorbate-covered surface on the resonant charge transfer in ion-surface scattering is investigated by means of the time-dependent Anderson-Newn...
The influence of a laterally inhomogeneous electrostatic potential of an adsorbate-covered surface on the resonant charge transfer in ion-surface scattering is investigated by means of the time-dependent Anderson-Newns Hamiltonian. The adsorbate-induced random modulation of the projectile orbital energy level is treated as a stochastic process, and its role in the charge transfer is evaluated analytically, in the Gaussian-process approximation, for low projectile speeds and low surface coverage by alkali atoms.
Modular scintillation cameras are gamma cameras with relatively small crystal faces, a small number of photomultiplier tubes (PMTs), and independent processing electronics. Our prototypical module has a 10 cm square c...
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作者：
M. SteslickaM. RadnyS. G. DavisonQuantum Theory Group
Department of Applied Mathematics and Department of Physics University of Waterloo Waterloo Ontario Canada N2L?1 and Guelph-Waterloo Program for Graduate Work in Physics University of Guelph Guelph Ontario Canada N1G?1
The effective-range approximation is adopted to study surface states of a linear array of scattering centers. It is shown that, depending on the values of the scattering length and effective range for the surface atom...
The effective-range approximation is adopted to study surface states of a linear array of scattering centers. It is shown that, depending on the values of the scattering length and effective range for the surface atom, two, one, or zero surface states can exist in the forbidden energy gap. The possibility of a smooth transition from true (stationary) surface states to virtual (decaying) states is discussed. A comparison is also made with the crystal-potential approach.
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