Qubit teleportation and transfer across antiferromagnetic spin chains.

We explore the capability of spin-1/2 chains to act as quantum channels for both teleportation and transfer of qubits. Exploiting the emergence of long-distance entanglement in low-dimensional systems [Phys. Rev. Lett. 96, 247206 (2006)10.1103/Phys.Rev.Lett.96, 247206(2006)], here we show how to obtain high communication fidelities between distant parties. An investigation of protocols of teleportation and state transfer is presented, in the realistic situation where temperature is included. Basing our setup on antiferromagnetic rotationally invariant systems, both protocols are represented by pure depolarizing channels. We propose a scheme where channel fidelity close to 1 can be achieved on very long chains at moderately small temperature.

Incommmensurability and unconventional superconductor to insulator transition in the hubbard model with bond-charge interaction.

We determine the quantum phase diagram of the one-dimensional Hubbard model with bond-charge interaction X in addition to the usual Coulomb repulsion U>0 at half-filling. For large enough X

Long-distance entanglement in spin systems.

Most quantum system with short-ranged interactions show a fast decay of entanglement with the distance. In this Letter, we focus on the peculiarity of some systems to distribute entanglement between distant parties. Even in realistic models, like the spin-1 Heisenberg chain, sizable entanglement is present between arbitrarily distant particles. We show that long-distance entanglement appears for values of the microscopic parameters which do not coincide with known quantum critical points, hence signaling a transition detected only by genuine quantum correlations.

Effects of fluctuations on electrical properties of gap-junction connected cells in the turtle retina.

Electrical properties of gap-junction connected cells (input voltage and length constant) are shown to depend strongly on fluctuations in membrane and contact conductances. This opens new possibilities and incorporates a further difficulty to the analysis of electrophysiological data, since four, instead of two, parameters (the average values and the magnitude of fluctuations of the two conductances) have to be used in fitting the experimental data. The discussion is illustrated by investigating the effects of dopamine on signal spreading in horizontal cells of turtle retina, assuming a linear cell arrangement. It is shown that while a standard fitting with the average values of the two conductances leads to the conclusion that both are equally affected by dopamine, including fluctuations allows fitting the data by varying just the average contact conductance plus the magnitude of fluctuations.

Detecting determinism in high-dimensional chaotic systems.

A method based upon the statistical evaluation of the differentiability of the measure along the trajectory is used to identify determinism in high-dimensional systems. The results show that the method is suitable for discriminating stochastic from deterministic systems even if the dimension of the latter is as high as 13. The method is shown to succeed in identifying determinism in electroencephalogram signals simulated by means of a high-dimensional system.

Instability versus nonlinearity in certain nonautonomous oscillators: A critical dynamical transition driven by the initial energy.

The equation of motion q+Omega(2)(t)q+alpha/q/(gamma-2)q=0 (gamma>2) for the real coordinate q(t) is studied, as an example of the interplay between nonlinearity and instability. Two contrasting mechanisms determine the behavior of q(t), when the time-varying frequency Omega(t) does produce exponential instability in the linear equation q (lin)+Omega(2)(t)q(lin)=0. At low energy, the exponential instability is the dominant effect, while at high energy the bounding effect of the autonomous nonlinear term prevails. Starting from low initial energies, the result of this competition is a time-varying energy characterized by quasiperiodic peaks, with an average recurrence time T(peak). A closed critical curve S(omega) exists in the initial phase space, whose crossing corresponds to a divergence of the recurrence time T(peak). The divergence of T(peak) has a universal character, expressed by a critical exponent a=1. The critical curve S(omega) is the initial locus of the solutions that vanish asymptotically. A close relationship exists between this dynamical transition and the transition from mobile to self-trapped polarons in one spatial dimension. The application to a number of physical problems is addressed, with special attention to the Fermi-Pasta-Ulam problem and to transitions to chaos.

Nonautonomous and nonlinear effects in generalized classical oscillators: A boundedness theorem

The phenomenon of exponential instability, arising in certain nonautonomous linear oscillators, motivates the question about the boundedness of amplitude and energy of the oscillators describing many physical situations. We present a rigorous result ensuring the boundedness for a class of generalized classical oscillators, characterized by symmetric potentials with only one equilibrium point. The key elements turn out to be the oscillating nature of the solutions and the presence of an autonomous part in the potential, diverging more than quadratically with the coordinate.