31,654 research outputs found
Geometric phase and gauge theory structure in quantum computing
We discuss the presence of a geometrical phase in the evolution of a qubit
state and its gauge structure. The time evolution operator is found to be the
free energy operator, rather than the Hamiltonian operator.Comment: 5 pages, presented at Fifth International Workshop DICE2010:
Space-Time-Matter - current issues in quantum mechanics and beyond,
Castiglioncello (Tuscany), September 13-17, 201
Improvements on analytic modelling of stellar spots
In this work we present the solution of the stellar spot problem using the
Kelvin-Stokes theorem. Our result is applicable for any given location and
dimension of the spots on the stellar surface. We present explicitely the
result up to the second degree in the limb darkening law. This technique can be
used to calculate very efficiently mutual photometric effects produced by
eclipsing bodies occulting stellar spots and to construct complex spot shapes.Comment: Resubmitted to MNRAS after accounting for minor comments of second
review, 9 pages, 5 figures, software available at
http://eduscisoft.com/KSINT
Some Aspects of the Exact Foldy-Wouthuysen Transformation for a Dirac Fermion
The Foldy-Wouthuysen transformation (FWT) is used to separate distinct
components of relativistic spinor field, e.g. electron and positron. Usually,
the FWT is perturbative, but in some cases there is an involution operator and
the transformation can be done exactly. We consider some aspects of an exact
FWT and show that, even if the theory does not admit an involution operator,
one can use the technique of exact FWT to obtain the conventional perturbative
result. Several particular cases can be elaborated as examples
Anharmonic stabilization of the high-pressure simple cubic phase of calcium
The phonon spectrum of the high-pressure simple cubic phase of calcium, in
the harmonic approx- imation, shows imaginary branches that make it
mechanically unstable. In this letter, the phonon spectrum is recalculated
using density-functional theory (DFT) ab initio methods fully including
anharmonic effects up to fourth order at 50 GPa. Considering that perturbation
theory cannot be employed with imaginary harmonic frequencies, a variational
procedure based on the Gibbs- Bogoliubov inequality is used to estimate the
renormalized phonon frequencies. The results show that strong quantum
anharmonic effects make the imaginary phonons become positive even at zero
temperature so that the simple cubic phase becomes mechanically stable, as
experiments suggest. Moreover, our calculations find a superconducting Tc in
agreement with experiments and predict an anomalous behavior of the specific
heat.Comment: 5 pages, 3 figure
A sweep algorithm for massively parallel simulation of circuit-switched networks
A new massively parallel algorithm is presented for simulating large asymmetric circuit-switched networks, controlled by a randomized-routing policy that includes trunk-reservation. A single instruction multiple data (SIMD) implementation is described, and corresponding experiments on a 16384 processor MasPar parallel computer are reported. A multiple instruction multiple data (MIMD) implementation is also described, and corresponding experiments on an Intel IPSC/860 parallel computer, using 16 processors, are reported. By exploiting parallelism, our algorithm increases the possible execution rate of such complex simulations by as much as an order of magnitude
A dynamical systems approach to the discrimination of the modes of operation of cryptographic systems
Evidence of signatures associated with cryptographic modes of operation is
established. Motivated by some analogies between cryptographic and dynamical
systems, in particular with chaos theory, we propose an algorithm based on
Lyapunov exponents of discrete dynamical systems to estimate the divergence
among ciphertexts as the encryption algorithm is applied iteratively. The
results allow to distinguish among six modes of operation, namely ECB, CBC,
OFB, CFB, CTR and PCBC using DES, IDEA, TEA and XTEA block ciphers of 64 bits,
as well as AES, RC6, Twofish, Seed, Serpent and Camellia block ciphers of 128
bits. Furthermore, the proposed methodology enables a classification of modes
of operation of cryptographic systems according to their strength.Comment: 14 pages, 10 figure
Electron paramagnetic resonance studies of slowly tumbling vanadyl spin probes in nematic liquid crystals
An analysis of EPR line shapes by the method of Polnaszek, Bruno, and Freed is made for slowly tumbling vanadyl spin probes in viscous nematic liquid crystals. The use of typical vanadyl complexes as spin probes for nematic liquid crystals is shown to simplify the theoretical analysis and the subsequent interpretation. Rotational correlation times tau and orientational ordering parameters S sub Z where slow tumbling effects are expected to be observed in vanadyl EPR spectra are indicated in a plot. Analysis of the inertial effects on the probe reorientation, which are induced by slowly fluctuating torque components of the local solvent structure, yield quantitative values for tau and S sub Z. The weakly ordered probe VOAA is in the slow tumbling region and displays these inertial effects throughout the nematic range of BEPC and Phase V. VOAA exhibits different reorientation behavior near the isotropic-nematic transition temperature than that displayed far below this transition temperature
Synaptic tagging and capture : differential role of distinct calcium/calmodulin kinases in protein synthesis-dependent long-term potentiation
Weakly tetanized synapses in area CA1 of the hippocampus that ordinarily display long-term potentiation lasting ~3 h (called early-LTP) will maintain a longer-lasting change in efficacy (late-LTP) if the weak tetanization occurs shortly before or after strong tetanization of an independent, but convergent, set of synapses in CA1. The synaptic tagging and capture hypothesis explains this heterosynaptic influence on persistence in terms of a distinction between local mechanisms of synaptic tagging and cell-wide mechanisms responsible for the synthesis, distribution, and capture of plasticity-related proteins (PRPs). We now present evidence that distinct CaM kinase (CaMK) pathways serve a dissociable role in these mechanisms. Using a hippocampal brain-slice preparation that permits stable long-term recordings in vitro for >10 h and using hippocampal cultures to validate the differential drug effects on distinct CaMK pathways, we show that tag setting is blocked by the CaMK inhibitor KN-93 (2-[N-(2-hydroxyethyl)]-N-(4-methoxybenzenesulfonyl)amino-N-(4-chlorocinnamyl)-N-methylbenzylamine) that, at low concentration, is more selective for CaMKII. In contrast, the CaMK kinase inhibitor STO-609 [7H-benzimidazo(2,1-a)benz(de)isoquinoline-7-one-3-carboxylic acid] specifically limits the synthesis and/or availability of PRPs. Analytically powerful three-pathway protocols using sequential strong and weak tetanization in varying orders and test stimulation over long periods of time after LTP induction enable a pharmacological dissociation of these distinct roles of the CaMK pathways in late-LTP and so provide a novel framework for the molecular mechanisms by which synaptic potentiation, and possibly memories, become stabilized
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