307,881 research outputs found

    Out of equilibrium quantum field dynamics in external fields

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    The quantum dynamics of the symmetry broken \lambda (\Phi^2)^2 scalar field theory in the presence of an homogeneous external field is investigated in the large N limit. We consider an initial thermal state of temperature T for a constant external field J. A subsequent sign flip of the external field, J to -J, gives rise to an out of equilibrium nonperturbative quantum field dynamics. We review here the dynamics for the symmetry broken lambda(\Phi^2)^2 scalar N component field theory in the large N limit, with particular stress in the comparison between the results when the initial temperature is zero and when it is finite. The presence of a finite temperature modifies the dynamical effective potential for the expectation value, and also makes that the transition between the two regimes of the early dynamics occurs for lower values of the external field. The two regimes are characterized by the presence or absence of a temporal trapping close to the metastable equilibrium position of the potential. In the cases when the trapping occurs it is shorter for larger initial temperatures.Comment: LaTeX, 3 pages, 2 figures. Presented at the IVth International Conference on Quarks and Nuclear Physics (QNP06). Selected to appear in Eur. Phys. J.

    Remarks on Quantum Modular Exponentiation and Some Experimental Demonstrations of Shor's Algorithm

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    An efficient quantum modular exponentiation method is indispensible for Shor's factoring algorithm. But we find that all descriptions presented by Shor, Nielsen and Chuang, Markov and Saeedi, et al., are flawed. We also remark that some experimental demonstrations of Shor's algorithm are misleading, because they violate the necessary condition that the selected number q=2sq=2^s, where ss is the number of qubits used in the first register, must satisfy n2≤q<2n2n^2 \leq q < 2n^2, where nn is the large number to be factored.Comment: 12 pages,5 figures. The original version has 6 pages. It did not point out the reason that some researchers took for granted that quantum modlar exponentiation is in polynomial time. In the new version, we indicate the reason and analyze some experimental demonstrations of Shor's algorithm. Besides, the author Zhenfu Cao is added to the version for his contribution. arXiv admin note: text overlap with arXiv:1409.735

    Phase Transition in Evolutionary Games

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    The evolution of cooperative behaviour is studied in the deterministic version of the Prisoners' Dilemma on a two-dimensional lattice. The payoff parameter is set at the critical region 1.8<b<2.01.8 < b < 2.0 , where clusters of cooperators are formed in all spatial sizes. Using the factorial moments developed in particle and nuclear physics for the study of phase transition, the distribution of cooperators is studied as a function of the bin size covering varying numbers of lattice cells. From the scaling behaviour of the moments a scaling exponent is determined and is found to lie in the range where phase transitions are known to take place in physical systems. It is therefore inferred that when the payoff parameter is increased through the critical region the biological system of cooperators undergoes a phase transition to defectors. The universality of the critical behaviour is thus extended to include also this particular model of evolution dynamics.Comment: 12 pages + 3 figures, latex, submitted to Natur
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