Inversion for Anisotropic Velocity Parameter

The problem under study concerns the robust computation of a certain parameter of anisotropy from observed travel-times of a seismic shear wave propagating through a geological medium. We have obtained an exact mathematical description of a geoseismic signal propagating through an anisotropic medium using a constant coefficient wave equation as the basic model. This model captures exactly the elliptical velocity profile required in the formulation of the geophysical model from which we obtained exact formulas describing the travel-time through a two layer geological structure, and an exact inversion formula for computing the anisotropic velocity parameter (gamma). A robust numerical method based on a minimization technique was presented as an accurate method of computing both travel-time and the inverted gamma. The exact formulas and robust numerical methods are significant improvements over the approximations and root finding methods discussed in the background material, and we note our formulation is no more difficult than these background methods. We derived asymptotic formulas valid for the near vertical case, which describe accurately the high sensitivity of gamma to the input parameters in this case. Our numerical work also confirms this sensitivity, even using exact formulas and robust numerical methods. We conclude that the computation of the anisotropic velocity parameter (gamma) for the given physical measurements from a series of surface signals and single borehole receiver is intrinsically unstable. By changing to the alpha,beta velocity parameter space, we obtain an inversion method that is much less sensitive to input errors. For certain geophysical problems, the alpha,beta parameters may suffice for an accurate description of the material. When the anisotropic velocity parameter (gamma) is needed directly, a different measurement technique is required. This route will require further investigation, and we have proposed a number of promising possibilities involving a differential time measure

Role of gravitational stress in land plant evolution - The gravitational factor in lignification Semiannual status report, period ending 31 Oct. 1968

Gravitational and mechanical stress effects on lignification in land plant

Experimental quantum key distribution over highly noisy channels

Error filtration is a method for encoding the quantum state of a single particle into a higher dimensional Hilbert space in such a way that it becomes less sensitive to phase noise. We experimentally demonstrate this method by distributing a secret key over an optical fiber whose noise level otherwise precludes secure quantum key distribution. By filtering out the phase noise, a bit error rate of 15.3% +/- 0.1%, which is beyond the security limit, can be reduced to 10.6% +/- 0.1%, thereby guaranteeing the cryptographic security.Comment: 4 pages, 2 figure

Provably Secure Experimental Quantum Bit-String Generation

Coin tossing is a cryptographic task in which two parties who do not trust each other aim to generate a common random bit. Using classical communication this is impossible, but non trivial coin tossing is possible using quantum communication. Here we consider the case when the parties do not want to toss a single coin, but many. This is called bit string generation. We report the experimental generation of strings of coins which are provably more random than achievable using classical communication. The experiment is based on the ``plug and play'' scheme developed for quantum cryptography, and therefore well suited for long distance quantum communication.Comment: 4 pages, 3 figures. Submitted to Phys. Rev. Lett. A complete security analysis for the experiment is given in quant-ph/040812

Cloning the entanglement of a pair of quantum bits

It is shown that any quantum operation that perfectly clones the entanglement of all maximally-entangled qubit pairs cannot preserve separability. This ``entanglement no-cloning'' principle naturally suggests that some approximate cloning of entanglement is nevertheless allowed by quantum mechanics. We investigate a separability-preserving optimal cloning machine that duplicates all maximally-entangled states of two qubits, resulting in 0.285 bits of entanglement per clone, while a local cloning machine only yields 0.060 bits of entanglement per clone.Comment: 4 pages Revtex, 2 encapsulated Postscript figures, one added autho

Bremsstrahlung analysis through the microwave cutoff and afterglow performances

Bremsstralung spectra with a very good energy resolution have been obtained for various time slabs of a few ms throughout the microwave cutoff. In a recent work (1) we had noticed+ and explained why the enhancement of the extracted high charge currents by the afterglow effect is more pronounced when the X-ray emission in the heating stage is more intense. In the present communication, we give some additional information deduced from our spectra. We indicate estimates of the temperature parameter and of the density of the hot electron population at various times. For this purpose the method presented in ref.(3) was adapted to argon. We also determine the maximum energy reached by the electrons in the steady state; the spare results seem to follow the scaling law indicated in Geller's book (4)

Layer-Resolved Ultrafast XUV Measurement of Hole Transport in a Ni-TiO2-Si Photoanode

Metal-oxide-semiconductor junctions are central to most electronic and optoelectronic devices. Here, the element-specificity of broadband extreme ultraviolet (XUV) ultrafast pulses is used to measure the charge transport and recombination kinetics in each layer of a Ni-TiO2-Si junction. After photoexcitation of silicon, holes are inferred to transport from Si to Ni ballistically in ~100 fs, resulting in spectral shifts in the Ni M2,3 XUV edge that are characteristic of holes and the absence of holes initially in TiO2. Meanwhile, the electrons are observed to remain on Si. After picoseconds, the transient hole population on Ni is observed to back-diffuse through the TiO2, shifting the Ti spectrum to higher oxidation state, followed by electron-hole recombination at the Si-TiO2 interface and in the Si bulk. Electrical properties, such as the hole diffusion constant in TiO2 and the initial hole mobility in Si, are fit from these transient spectra and match well with values reported previously

Extremal quantum cloning machines

We investigate the problem of cloning a set of states that is invariant under the action of an irreducible group representation. We then characterize the cloners that are "extremal" in the convex set of group covariant cloning machines, among which one can restrict the search for optimal cloners. For a set of states that is invariant under the discrete Weyl-Heisenberg group, we show that all extremal cloners can be unitarily realized using the so-called "double-Bell states", whence providing a general proof of the popular ansatz used in the literature for finding optimal cloners in a variety of settings. Our result can also be generalized to continuous-variable optimal cloning in infinite dimensions, where the covariance group is the customary Weyl-Heisenberg group of displacements.Comment: revised version accepted for publicatio

Thermal phase diagrams of columnar liquid crystals

In order to understand the possible sequence of transitions from the disordered columnar phase to the helical phase in hexa(hexylthio)triphenylene (HHTT), we study a three-dimensional planar model with octupolar interactions inscribed on a triangular lattice of columns. We obtain thermal phase diagrams using a mean-field approximation and Monte Carlo simulations. These two approaches give similar results, namely, in the quasi one-dimensional regime, as the temperature is lowered, the columns order with a linear polarization, whereas helical phases develop at lower temperatures. The helicity patterns of the helical phases are determined by the exact nature of the frustration in the system, itself related to the octupolar nature of the molecules.Comment: 12 pages, 9 figures, ReVTe