19,229 research outputs found
Elementary solution to the time-independent quantum navigation problem
A quantum navigation problem concerns the identification of a time-optimal Hamiltonian that realizes a required quantum process or task, under the influence of a prevailing ‘background’ Hamiltonian that cannot be manipulated. When the task is to transform one quantum state into another, finding the solution in closed form to the problem is nontrivial even in the case of timeindependent Hamiltonians. An elementary solution, based on trigonometric analysis, is found here when the Hilbert space dimension is two. Difficulties arising from generalizations to higher-dimensional systems are discussed
Overcoming the false-minima problem in direct methods: Structure determination of the packaging enzyme P4 from bacteriophage φ13
The problems encountered during the phasing and structure determination of the packaging enzyme P4 from bacteriophage φ13 using the anomalous signal from selenium in a single-wavelength anomalous dispersion experiment (SAD) are described. The oligomeric state of P4 in the virus is a hexamer (with sixfold rotational symmetry) and it crystallizes in space group C2, with four hexamers in the crystallographic asymmetric unit. Current state-of-the-art ab initio phasing software yielded solutions consisting of 96 atoms arranged as sixfold symmetric clusters of Se atoms. However, although these solutions showed high correlation coefficients indicative that the substructure had been solved, the resulting phases produced uninterpretable electron-density maps. Only after further analysis were correct solutions found (also of 96 atoms), leading to the eventual identification of the positions of 120 Se atoms. Here, it is demonstrated how the difficulties in finding a correct phase solution arise from an intricate false-minima problem. © 2005 International Union of Crystallography - all rights reserved
Information requirements for supersonic transport operation Final report
Effects of meteorological parameters and instrument errors on vertical flight performance of supersonic transport
Magnetization transport and quantized spin conductance
We analyze transport of magnetization in insulating systems described by a
spin Hamiltonian. The magnetization current through a quasi one-dimensional
magnetic wire of finite length suspended between two bulk magnets is determined
by the spin conductance which remains finite in the ballistic limit due to
contact resistance. For ferromagnetic systems, magnetization transport can be
viewed as transmission of magnons and the spin conductance depends on the
temperature T. For antiferromagnetic isotropic spin-1/2 chains, the spin
conductance is quantized in units of order at T=0.
Magnetization currents produce an electric field and hence can be measured
directly. For magnetization transport in electric fields phenomena analogous to
the Hall effect emerge.Comment: 4 pages, 3 figures, minor change
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