16,767 research outputs found
Ramsey interferometry with ultracold atoms
We examine the passage of ultracold two-level atoms through two separated
laser fields for the nonresonant case. We show that implications of the atomic
quantized motion change dramatically the behavior of the interference fringes
compared to the semiclassical description of this optical Ramsey
interferometer. Using two-channel recurrence relations we are able to express
the double-laser scattering amplitudes by means of the single-laser ones and to
give explicit analytical results. When considering slower and slower atoms, the
transmission probability of the system changes considerably from an
interference behavior to a regime where scattering resonances prevail. This may
be understood in terms of different families of trajectories that dominate the
overall transmission probability in the weak field or in the strong field
limit.Comment: 5 figures, 4 page
Arcfinder: An algorithm for the automatic detection of gravitational arcs
We present an efficient algorithm designed for and capable of detecting
elongated, thin features such as lines and curves in astronomical images, and
its application to the automatic detection of gravitational arcs. The algorithm
is sufficiently robust to detect such features even if their surface brightness
is near the pixel noise in the image, yet the amount of spurious detections is
low. The algorithm subdivides the image into a grid of overlapping cells which
are iteratively shifted towards a local centre of brightness in their immediate
neighbourhood. It then computes the ellipticity for each cell, and combines
cells with correlated ellipticities into objects. These are combined to graphs
in a next step, which are then further processed to determine properties of the
detected objects. We demonstrate the operation and the efficiency of the
algorithm applying it to HST images of galaxy clusters known to contain
gravitational arcs. The algorithm completes the analysis of an image with
3000x3000 pixels in about 4 seconds on an ordinary desktop PC. We discuss
further applications, the method's remaining problems and possible approaches
to their solution.Comment: 12 pages, 12 figure
Variable gain for a wind turbine pitch control
The gain variation is made in the software logic of the pitch angle controller. The gain level is changed depending upon the level of power error. The control uses low gain for low pitch activity the majority of the time. If the power exceeds ten percent offset above rated, the gain is increased to a higher gain to more effectively limit power. A variable gain control functioned well in tests on the Mod-0 wind turbine
Ground Instrumentation for Mariner IV OCCULTATION Experiment
Deep Space Instrumentation Facility /DSIF/ GROUND receiver stations for Mariner IV space probe occulation experimen
Preparation, Structure, and Reactivity of Nonstabilized Organoiron Compounds. Implications for Iron-Catalyzed Cross Coupling Reactions
A series of unprecedented organoiron complexes of the formal oxidation states −2, 0, +1, +2, and +3 is presented, which are largely devoid of stabilizing ligands and, in part, also electronically unsaturated (14-, 16-, 17- and 18-electron counts). Specifically, it is shown that nucleophiles unable to undergo β-hydride elimination, such as MeLi, PhLi, or PhMgBr, rapidly reduce Fe(3+) to Fe(2+) and then exhaustively alkylate the metal center. The resulting homoleptic organoferrate complexes [(Me4Fe)(MeLi)][Li(OEt2)]2 (3) and [Ph4Fe][Li(Et2O)2][Li(1,4-dioxane)] (5) could be characterized by X-ray crystal structure analysis. However, these exceptionally sensitive compounds turned out to be only moderately nucleophilic, transferring their organic ligands to activated electrophiles only, while being unable to alkylate (hetero)aryl halides unless they are very electron deficient. In striking contrast, Grignard reagents bearing alkyl residues amenable to β-hydride elimination reduce FeXn (n = 2, 3) to clusters of the formal composition [Fe(MgX)2]n. The behavior of these intermetallic species can be emulated by structurally well-defined lithium ferrate complexes of the type [Fe(C2H4)4][Li(tmeda)]2 (8), [Fe(cod)2][Li(dme)]2 (9), [CpFe(C2H4)2][Li(tmeda)] (7), [CpFe(cod)][Li(dme)] (11), or [Cp*Fe(C2H4)2][Li(tmeda)] (14). Such electron-rich complexes, which are distinguished by short intermetallic Fe−Li bonds, were shown to react with aryl chlorides and allyl halides; the structures and reactivity patterns of the resulting organoiron compounds provide first insights into the elementary steps of low valent iron-catalyzed cross coupling reactions of aryl, alkyl, allyl, benzyl, and propargyl halides with organomagnesium reagents. However, the acquired data suggest that such C−C bond formations can occur, a priori, along different catalytic cycles shuttling between metal centers of the formal oxidation states Fe(+1)/Fe(+3), Fe(0)/Fe(+2), and Fe(−2)/Fe(0). Since these different manifolds are likely interconnected, an unambiguous decision as to which redox cycle dominates in solution remains difficult, even though iron complexes of the lowest accessible formal oxidation states promote the reactions most effectively
Operator normalized quantum arrival times in the presence of interactions
We model ideal arrival-time measurements for free quantum particles and for
particles subject to an external interaction by means of a narrow and weak
absorbing potential. This approach is related to the operational approach of
measuring the first photon emitted from a two-level atom illuminated by a
laser. By operator-normalizing the resulting time-of-arrival distribution, a
distribution is obtained which for freely moving particles not only recovers
the axiomatically derived distribution of Kijowski for states with purely
positive momenta but is also applicable to general momentum components. For
particles interacting with a square barrier the mean arrival time and
corresponding ``tunneling time'' obtained at the transmission side of the
barrier becomes independent of the barrier width (Hartman effect) for
arbitrarily wide barriers, i.e., without the transition to the ultra-opaque,
classical-like regime dominated by wave packet components above the barrier.Comment: 10 pages, 5 figures, RevTe
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