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  • Spin polarization of quantum Hall states for filling factors 1 ⩽ ν ⩽ ...
    Spin polarization measurements were performed in three 2D electron gases in GaAs with densities , 7.2 and 6.5 cm−2, in the quantum Hall regime. Full spin polarization at ν = 1 surrounded by rapid depolarization due to Skyrmion formation was observed in all devices, consistent with past measurements. Depolarization of the , 8/5 states and repolarization of the state was also measured, in remarkable agreement with a non-interacting, disorder-free Composite Fermion model. Optical power and temperature dependent measurements of the ν = 1 state suggest a regime of non-linear optics.

  • Variational formulation of active nematic fluids: theory and simulation
    The structure and dynamics of important biological quasi-two-dimensional systems, ranging from cytoskeletal gels to tissues, are controlled by nematic order, flow, defects and activity. Continuum hydrodynamic descriptions combined with numerical simulations have been used to understand such complex systems. The development of thermodynamically consistent theories and numerical methods to model active nemato-hydrodynamics is eased by mathematical formalisms enabling systematic derivations and structured-preserving algorithms. Alternative to classical nonequilibrium thermodynamics and bracket formalisms, here we develop a theoretical and computational framework for active nematics based on Onsager’s variational formalism to irreversible thermodynamics, according to which the dynamics result from the minimization of a Rayleighian functional capturing the competition between free-energy release, dissipation and activity. We show that two standard incompressible models of active nemato-hydrodynamics can be framed in the variational formalism, and develop a new compressible model for density-dependent active nemato-hydrodynamics relevant to model actomyosin gels. We show that the variational principle enables a direct and transparent derivation not only of the governing equations, but also of the finite element numerical scheme. We exercise this model in two representative examples of active nemato-hydrodynamics relevant to the actin cytoskeleton during wound healing and to the dynamics of confined colonies of elongated cells.

  • Characterization of the thorium-229 defect structure in CaF2 crystals
    Recent advancements in laser excitation of the low-energy thorium-229 (229Th) nuclear isomeric state in calcium fluoride (CaF2) single crystals render this system a promising candidate for a solid-state nuclear clock. Nonetheless, the precise experimental determination of the microscopic ion configuration surrounding the doped 229Th and its electronic charge state remains a critical challenge. Such characterization is essential for precisely controlling the clock transition and evaluating the performance of this solid-state nuclear clock system. In this study, we use x-ray absorption fine structure spectroscopy of 229Th:CaF2 to investigate the charge state and coordination environment of doped 229Th. The results indicate that 229Th displays a 4+ oxidation state at the substitutional site of a Ca2+ ion, with charge compensated provided by two F− ions positioned at interstitial sites adjacent to 229Th.

  • Species interconversion of deformable particles yields transient phase separation
    We consider a dense assembly of repulsive particles whose fluctuating sizes are subject to an energetic landscape that defines three species: two distinct states of particles with a finite size, and point particles as an intermediate state between the two previous species. We show that the nonequilibrium synchronization of sizes systematically leads to a homogeneous configuration associated with the survival of a single species. Remarkably, the relaxation towards such a configuration features a transient phase separation. By delineating and analyzing the dominant kinetic factors at play during relaxation, we recapitulate the phase diagram of species survival in terms of the parameters of the size landscape. Finally, we obtain a hydrodynamic mapping to equilibrium by coarse-graining the microscopic dynamics, which leads to predicting the nature of the transitions between various regimes where distinct species survive.

  • Machine learning for optimal parameter prediction in free space continuous-variable quantum key distribution
    For a practical continuous-variable quantum key distribution (CV-QKD) system, the optimization of modulation variance is crucial for promoting protocol performance. The optimization relies on algorithms like local search in general. But the efficiency of local search methods is limited in low latency and limited computing power scenarios due to their high computational consumption. Hence, this optimization approach is infeasible for satellite-based CV-QKD due to satellites’ low power feature. In this paper, a neural network model that directly predicting the optimal modulation variance in nearly real time is proposed for free space gaussian modulated CV-QKD protocols. This work enhances the feasibility of implementing CV-QKD in low-Earth-orbit satellite scenarios, where typical link durations of few seconds demand rapid parameter optimization. Moreover, a simulation platform for free space CV-QKD protocol, which employs the precise orbital model to extract the elevation angle and the transmission length, is designed and developed to generate training sets that make the neural network model more practical. Our work can be used as a foundation to support parameter prediction for future quantum communication satellite missions.