Department of Physics

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Augmenting the unitary transformation which generates a quantum walk by a generalized phase gate G is a symmetry for both noisy and noiseless quantum walk on a line, in the sense that it leaves the position probability distribution invariant. However, this symmetry breaks down in the case of a quantum walk on an n cycle, and hence can be regarded as a probe of the walk topology. Noise, modeled here as phase flip and generalized amplitude damping channels, tends to restore the symmetry because it classicalizes the walk. However, symmetry restoration happens even in the regime where the walker is not entirely classical, because noise also has the effect of desensitizing the operation G to the walk topology. We discuss methods for physical implementation, and talk about the wider implications to condensed matter systems. © 2008 The American Physical Society.

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Physical Review A - Atomic, Molecular, and Optical Physics

Symmetry-noise interplay in a quantum walk on an n -cycle

High energy density (ε{lunate}) and temperature (T) links general relativity and hydrodynamics leading to a lower bound for the ratio of shear viscosity (η) and entropy density (s). We get the interesting result that the bound is saturated in the simple model for quark matter that we use for strange stars at the surface for T ∼ 80 MeV. At this T we have the possibility of cosmic separation of phases. At the surface of the star where the pressure is zero-the density ε{lunate} has a fixed value for all stars of various masses with correspondingly varying central energy density ε{lunate}c. Inside the star where this density is higher, the ratio of η / s is larger and are like the known results found for perturbative QCD. This serves as a check of our calculation. The deconfined quarks at the surface of the strange star at T = 80 MeV seem to constitute the most perfect interacting fluid permitted by nature. © 2008 Elsevier B.V. All rights reserved.

Physics Letters, Section B: Nuclear, Elementary Particle and High-Energy Physics

Bound for entropy and viscosity ratio of strange quark matter

Second harmonic amplitude (V2f) of Giant Magneto-Impedance (GMI) voltage of (Fe6Co94)72.5Si12.5B15 amorphous wires has been studied systematically for as-cast and Joule-heated states. Joule heating was carried out at current densities (J) of 20-40 A/mm2 for a period of 1-300 min. The amplitude of the second harmonic voltage (V2f) initially increased with J, attained maximum and then decreased. The maximum amplitude of V2f was observed for the sample annealed at J=25 A/mm2 for 5 min. The second harmonic peak shifted towards the higher field side with the increase of annealing current density and annealing period (tp). Few-fold increase in the amplitude of the second harmonic voltage (V2f) was observed due to the presence of a small dc bias current (Ib) of amplitude 1 mA. The asymmetry in V2f was also observed in a Joule-heated sample. The observed asymmetry has been correlated with the bias field generated between the surface crystallized layer and the amorphous core. A small bias current of amplitude of 1 mA was applied along the direction of the applied field to minimize the asymmetry. © 2007 Elsevier B.V. All rights reserved.

Journal of Magnetism and Magnetic Materials

Effect of annealing on the second harmonic amplitude of Giant Magneto-Impedance (GMI) voltage of a Co-Fe-Si-B amorphous wire

We reported a single step process for the synthesis of Ni having wire, sponge and flower type microstructures that are obtained via a microwave-assisted decomposition of Ni-TEA (triethanolamine) complex precursor with and without amino acid. The products formed at different stages of the growth process were characterized using scanning electron microscope (SEM), transmission electron microscope (TEM) and magnetometer. Morphological modification enhanced the coercivity from 40 Oe for the Ni-wire to 120 Oe for the Ni-flower with exchange bias effect, which mechanistically arises due to the presence of NiO phase. © 2008 American Institute of Physics.

AIP Conference Proceedings

Single step synthesis of ni wire, sponge and flower, and their magnetic properties

Different resistance spot welded joints of interstitial free steel have been tested by the magnetic hysteresis technique. The spot welded joints are also characterized through a peel-off test, as well as through optical and transmission electron microscopy. It is observed tlrnt the magnetic coercivity of the weld nuggets increased with increasing dislocation dmsity and precipitation. Coercivity, however, decreased with increasing nugget size and grain coarsening. The results of the study indicate that the magnetic hysteresis method could be successfully employed as a rapid and effective technique for quality testing of spot welded joints of interstitial free steels.

Materials Evaluation

Magnetic hysteresis of resistance spot welded interstitial free steels

Squeezing of a thermal bath introduces features absent in an open quantum system interacting with an uncorrelated (zero squeezing) thermal bath. The resulting dynamics, governed by a Lindblad-type evolution, extends the concept of a generalized amplitude damping channel, which corresponds to a dissipative interaction with a purely thermal bath. Here we present the Kraus representation of this map, which we call the squeezed generalized amplitude damping channel. As an application of this channel to quantum information, we study the classical capacity of this channel. © 2008 The American Physical Society.

Squeezed generalized amplitude damping channel

Organic photodetectors based on various organic materials (small molecule and polymer) have been studied quite extensively. Number of properties those can be helpful for preparing an optimized device structure has also been studied. Some of them which we have studied are; variation of number of donor/acceptor interface and by studying the decay characteristics of the carriers, by providing a gradient in the concentration in the molecular level. © 2007 IEEE.

Proceedings of the 14th International Workshop on the Physics of Semiconductor Devices, IWPSD

Photodetectors based on organic semiconductors: Optimization of device architectures

The energetics and structural, electronic and optical absorption properties of thallium nanowires, Tln with n = 1-18, have been investigated by employing a first-principles density functional theory in the local density approximation. The spin-orbit (SO) interaction has also been considered. We study four types of stable structures: planar, caged, pyramidal and helical. In general, the binding energy increases with the coordination number except in a few cases where the nearest-neighbours lie at comparatively larger separations. The maximum stability is seen for the helical configurations containing pentagons, hexagons, heptagons and octagons. Nanowires containing a core linear chain of atoms on the tube axis are more stable than the corresponding nanotubes having no such chains. All the wires or tubes are found to be metallic with or without consideration of the SO interaction. The electronic structures of the pentagonal-, hexagonal- and octagonal-configuration wires provide a large number of channels, which may give rise to large quantum ballistic conduction. One finds large differences between the optical absorption calculated with and without the SO interaction. Consideration of the SO interaction enhances the number of absorption peaks by approximately a factor of two. A strong and multi-peaked optical absorption, extending up to 4.0 eV including the visible region, appears for wires containing pentagons and octagons. These wires may thus be useful as a source of white radiation.

Philosophical Magazine

Structural, electronic and optical properties of ultrathin thallium nanowires - An ab initio study

We quantitatively analyze the dynamics of the quantum phase distribution associated with the reduced density matrix of a system, as the system evolves under the influence of its environment with an energy-preserving quantum nondemolition (QND) type of coupling. We take the system to be either an oscillator (harmonic or anharmonic) or a two-level atom (or equivalently, a spin-1/2 system), and model the environment as a bath of harmonic oscillators, initially in a general squeezed thermal state. The impact of the different environmental parameters is explicitly brought out as the system starts out in various initial states. The results are applicable to a variety of physical systems now studied experimentally with QND measurements. © 2007 The American Physical Society.

Phase-diffusion pattern in quantum-nondemolition systems

Environment-induced decoherence presents a great challenge to realizing a quantum computer. We point out the somewhat surprising fact that decoherence can be useful, indeed necessary, for practical quantum computation, in particular, for the effective erasure of quantum memory in order to initialize the state of the quantum computer. The essential point behind the deleter is that the environment, by means of a dissipative interaction, furnishes a contractive map towards a pure state. We present a specific example of an amplitude damping channel provided by a two-level system's interaction with its environment in the weak Born-Markov approximation. This is contrasted with a purely dephasing, non-dissipative channel provided by a two-level system's interaction with its environment by means of a quantum nondemolition interaction. We point out that currently used state preparation techniques, for example using optical pumping, essentially perform as quantum deleters. © 2007 Elsevier B.V. All rights reserved.