In the last 30 days

• Open/close Ramsey Resonance in a Zacharias Fountain

  A. Clairon et al 1991 Europhys. Lett. 16 165 Tag this article

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  We report a realization of Zacharias's 1953 proposal for observing a Ramsey resonance in an atomic fountain. Launched upward from a moving optical molasses where they have been cooled to ~ 5 μK, cesium atoms pass once through a microwave cavity, continue to the summit of their trajectory, then fall again through the same cavity, completing the separated-fields interaction. The atoms spend 0.25 s in free flight above the cavity. Linewidth (2 Hz) and S/N imply a stability of 3·10 ^-12 τ ^-1/2, at least as good as in existing Cs clocks, with eventual expected improvements of 10 ².

• Open/close Observing different quantum trajectories in cavity QED

  M. F. Santos and A. R. R. Carvalho 2011 EPL 94 64003 Tag this article

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  The experimental observation of quantum jumps is an example of single open quantum systems that, when monitored, evolve in terms of stochastic trajectories conditioned on measurements results. Here we present a proposal that allows the experimental observation of a much larger class of quantum trajectories in cavity QED systems. In the microwave regime, three-level atoms probe the cavity field while in the optical case the field probes atomic states. In the first situation the cavity field evolves stochastically between highly non-classical states while the second case allows for the monitoring of engineered thermal baths that are crucial for recent proposals for probing entanglement decay and also for entanglement protection.

• Open/close The asymmetric Kerr metric as a source of CP violation

  M. J. Hadley 2011 EPL 95 21003 Tag this article

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  All experimental evidence for violation of discrete spacetime symmetries: Parity and Time reversal and the related Charge conjugation/Parity combination (P, T and CP, respectively) has been obtained on Earth in a gravitational potential that is P and T anisotropic. It is suggested that the origin of the observed CP violation is the scalar field equal to the frame dragging term d d t in the Kerr metric of a spinning massive body. The Galaxy would be the largest such source. Indirect evidence of such an effect would be anisotropic decay products when plotted in a reference frame defined by the fixed stars. As a consequence, CP violation would be very much greater near compact astrophysical objects with large angular momentum.

• Open/close Evidence of deep-blue photon emission at high efficiency by common plastic

  H. Nakamura et al 2011 EPL 95 22001 Tag this article

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  Various scintillation devices are used in many countries and wide scientific fields. Key elements that determine the performance of a scintillation device are the number of photons emitted per incident radiation event and the emission of easy-to-measure blue photons. It is generally known that only materials with very complex compositions perform well as scintillators. However, we demonstrated that the scintillation performance of a newly developed plastic such as 100 percent pure polyethylene naphthalate exceeds that of conventional organic scintillators. By measuring the light output spectra and emission spectra of several samples, we revealed that the plastic emits a high number of photons per incident radiation event (~10500 photons/MeV), and, surprisingly, deep-blue photons (425 nm). Even though the plastic has a simple composition, it could replace the expensive organic scintillators that have been used in many applications.

• Open/close CPT symmetry and antimatter gravity in general relativity

  M. Villata 2011 EPL 94 20001 Tag this article

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  The gravitational behavior of antimatter is still unknown. While we may be confident that antimatter is self-attractive, the interaction between matter and antimatter might be either attractive or repulsive. We investigate this issue on theoretical grounds. Starting from the CPT invariance of physical laws, we transform matter into antimatter in the equations of both electrodynamics and gravitation. In the former case, the result is the well-known change of sign of the electric charge. In the latter, we find that the gravitational interaction between matter and antimatter is a mutual repulsion, i.e. antigravity appears as a prediction of general relativity when CPT is applied. This result supports cosmological models attempting to explain the Universe accelerated expansion in terms of a matter-antimatter repulsive interaction.

• Open/close Lattice BGK Models for Navier-Stokes Equation

  Y. H. Qian et al 1992 Europhys. Lett. 17 479 Tag this article

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  We propose the lattice BGK models, as an alternative to lattice gases or the lattice Boltzmann equation, to obtain an efficient numerical scheme for the simulation of fluid dynamics. With a properly chosen equilibrium distribution, the Navier-Stokes equation is obtained from the kinetic BGK equation at the second-order of approximation. Compared to lattice gases, the present model is noise-free, has Galileian invariance and a velocity-independent pressure. It involves a relaxation parameter that influences the stability of the new scheme. Numerical simulations are shown to confirm the speed of sound and the shear viscosity.

• Open/close Impact of network structure on a model of diffusion and competitive interaction

  V. Nicosia et al 2011 EPL 94 68009 Tag this article

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  We consider a model in which agents of different species move over a complex network, are subject to reproduction and compete for resources. The complementary roles of competition and diffusion produce a variety of fixed points, whose stability depends on the structure of the underlying complex network. The survival and death of species is influenced by the network degree distribution, clustering, degree-degree correlations and community structures. We found that the invasion of all the nodes by just one species is possible only in Erdös-Renyi and regular graphs, while networks with scale-free degree distribution, as those observed in real social, biological and technological systems, guarantee the coexistence of different species and therefore help enhancing species diversity.

• Open/close Scale-free networks by preferential depletion

  C. M. Schneider et al 2011 EPL 95 16005 Tag this article

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  We show that not only preferential attachment but also preferential depletion leads to scale-free networks. In our model there is neither growth of new connections nor growth of new nodes. We start from a well connected network and implement a probabilistic purely depletion procedure. The resulting degree distribution exponent is typically less than two (5/3) as opposed to the case of the growth models studied before where the exponents are larger. We investigate the most important properties characterizing these networks, as the cluster size distribution, the average shortest path and the clustering coefficient.

• Open/close Terahertz Faraday rotation induced by an anomalous Hall effect in the itinerant ferromagnet SrRuO₃

  R. Shimano et al 2011 EPL 95 17002 Tag this article

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  We report on the observation of Faraday rotation in the terahertz frequency range in a thin film of itinerant ferromagnet, SrRuO ₃. As optical counterpart of the anomalous Hall effect (AHE), the complex optical Hall conductivity spectrum σ _xy (ω) was extracted from the Faraday rotation and ellipticity spectrum. The temperature and frequency (ω) dependences of σ _xy (ω) in the terahertz regime are consistently connected with the dc AHE. A clear resonance structure is identified in the σ _xy (ω) spectrum right below the Fermi level as the plausible origin of the large dc AHE.

• Open/close The fragility of protein-protein interaction networks

  C. M. Schneider et al 2011 EPL 95 16001 Tag this article

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  The capacity to resist perturbations from the environment is crucial to the survival of all organisms. We quantitatively analyze the susceptibility of protein interaction networks of numerous organisms to random and targeted failures. We find for all organisms studied that random rewiring improves protein network robustness, so that actual networks are more fragile than rewired surrogates. This unexpected fragility contrasts with the behavior of networks such as the Internet, whose robustness decreases with random rewiring. We trace this surprising effect to the modular structure of protein networks.