We have used a multiscale statistical analysis to interpret the mobility of water molecules diffusing within nematic aqueous dispersions of charged anisotropic nanocomposites (synthetic Laponite clays). The nematic ordering of dense aqueous suspensions (29-52% w/w) prepared by uniaxial compression is detected by analyzing the splitting of the nuclear magnetic resonance line of the quadrupolar counterions ( 7 Li and 23 Na) neutralizing the negative charges of the clay. The tensor describing the water self-diffusion is measured by 1 H pulsed gradient spin-echo (PGSE) NMR spectroscopy. It exhibits a large anisotropy of water mobility in these nematic dispersions. The macroscopic mobility of the water molecules is obtained from numerical simulations of Brownian dynamics (BD), by integrating the water trajectories over a time scale of 1 µs. The local mobility of the water molecules in the vicinity of the surface of the Laponite particles is deduced from preliminary molecular dynamics (MD) simulations of the trajectories of the water molecules confined between two clay fragments by integrating their trajectories over a time scale of a few picoseconds. The equilibrium density and initial configuration of these confined water molecules are deduced from grand canonical Monte Carlo (GCMC) simulations, by using a new clay/water force field determined from semi-empirical periodic (MINDO) quantum calculations coupled to perturbation theory for dispersion forces. This multiscale statistical analysis of the water mobility bridges the gap between the time scale (nanoseconds) accessible by MD simulations and the time scale (microseconds) accessible by BD, leading to macroscopic behavior comparable with experimental data. (I) IntroductionClays are charged anisotropic colloids used in many industrial applications (waste management and storage, heterogeneous catalysis, drilling, ionic exchange, etc.) exploiting their various physico-chemical properties (large affinity for water molecules and polar solvents, high specific surface and surface charge density, gelling, thixotropy, surface acidity, etc.). By contrast with natural clays which contains numerous impurities, Laponite is a synthetic clay frequently used for scientific investigations 1-13 because of its high chemical purity. Thus we have used this synthetic clay in order to validate our experimental procedures before investigating the behavior of the natural clay materials which are used for the industrial applications. Laponite clay results from the sandwiching of one layer of magnesium oxides, with an octahedral geometry, between two layers of silicium oxides, with an tetrahedral geometry. In dilute aqueous regime, Laponite clays behave as isolated rigid disks (diameter, 200-300 Å; thickness, 10 Å; and density, 2.7). 8,14,15 Because of the substitution of some Mg 2+ cations from their octahedral network by Li + cations, each Laponite disk bears about a thousand of negative electric charges neutralized by hydrated sodium counterions.The purpose of this study is to determine t...

Future quantum technologies rely heavily on good protection of quantum entanglement against environment-induced decoherence. A recent study showed that an extension of Uhrig's dynamical decoupling (UDD) sequence can (in theory) lock an arbitrary but known two-qubit entangled state to the N th order using a sequence of N control pulses [Mukhtar et al., Phys. Rev. A 81, 012331 (2010)]. By nesting three layers of explicitly constructed UDD sequences, here we first consider the protection of unknown two-qubit states as superposition of two known basis states, without making assumptions of the system-environment coupling. It is found that the obtained decoherence suppression can be highly sensitive to the ordering of the three UDD layers and can be remarkably effective with the correct ordering. The detailed theoretical results are useful for general understanding of the nature of controlled quantum dynamics under nested UDD. As an extension of our three-layer UDD, it is finally pointed out that a completely unknown two-qubit state can be protected by nesting four layers of UDD sequences. This work indicates that when UDD is applicable (e.g., when environment has a sharp frequency cut-off and when control pulses can be taken as instantaneous pulses), dynamical decoupling using nested UDD sequences is a powerful approach for entanglement protection.

A detailed line shape analysis of 23 Na nuclear magnetic resonance spectra within dense suspensions (12% vol/vol) of Laponite clay exhibits a macroscopic ordering of these charged anisotropic colloids within a nematic phase. The angular variation of the order parameter limits to 20% of the maximum amount of disorder in these dense suspensions. By contrast, dilute Laponite suspensions (1-4% vol/vol) remain isotropic while the variation of 23 Na relaxation rates over a broad range of frequencies indicates a local ordering of the clay platelets within microdomains of the same spatial extent than the particle diameter (300 Å).

We report on the measurement of the spectral functions of noninteracting ultracold atoms in a three-dimensional disordered potential resulting from an optical speckle field. Varying the disorder strength by 2 orders of magnitude, we observe the crossover from the "quantum" perturbative regime of low disorder to the "classical" regime at higher disorder strength, and find an excellent agreement with numerical simulations. The method relies on the use of state-dependent disorder and the controlled transfer of atoms to create well-defined energy states. This opens new avenues for experimental investigations of three-dimensional Anderson localization.