Evolution of an Atom Impeded by Measurement: The Quantum Zeno Effect

A quantum system being observed evolves more slowly. This `'quantum Zeno effect'' is reviewed with respect to a previous attempt of demonstration, and to subsequent criticism of the significance of the findings. A recent experiment on an {\it individual} cold trapped ion has been capable of revealing the micro-state of this quantum system, such that the effect of measurement is indeed discriminated from dephasing of the quantum state by either the meter or the environment.Comment: 6 pages, 5 figures. Presented at the 3rd Workshop on Mysteries, Puzzles and Paradoxes in Quantum Mechanics, Gargagno, Italy, September 17-23, 200

Density-dependent diffusion in the periodic Lorentz gas

We study the deterministic diffusion coefficient of the two-dimensional periodic Lorentz gas as a function of the density of scatterers. Results obtained from computer simulations are compared to the analytical approximation of Machta and Zwanzig [Phys.Rev.Lett. 50, 1959 (1983)] showing that their argument is only correct in the limit of high densities. We discuss how the Machta-Zwanzig argument, which is based on treating diffusion as a Markovian hopping process on a lattice, can be corrected systematically by including microscopic correlations. We furthermore show that, on a fine scale, the diffusion coefficient is a non-trivial function of the density. We finally argue that, on a coarse scale and for lower densities, the diffusion coefficient exhibits a Boltzmann-like behavior, whereas for very high densities it crosses over to a regime which can be understood qualitatively by the Machta-Zwanzig approximation.Comment: 9 pages (revtex) with 9 figures (postscript

The Danish Cultural Heritage: Economics and Politics

This paper deals with the Danish cultural heritage understood as the tangible heritage, mainly buildings. The paper briefly describes the political economy of conserving the cultural heritage and the means of preserving it. The main focus of the paper is, however, on the present Danish conservation policy. The paper presents estimates of the costs of preserving the cultural heritage on a national scale. It is concluded that while the level of conservation probably conforms to the wishes of the Danish people marginal decisions are probably misguided. In particular, a detailed examination of the practices of the Danish Conservation Board is presented. The Board is not guided by the recommended cost-benefit perspective but rather in practice devoted to preserving architectonical values.cultural heritage, cultural policy, preservation

The Danish Museum System

This paper deals with the Danish Museum System and presents a detailed examination of the Danish Museums, though restricted to museums devoted to cultural history rather than arts. The emphasis of the paper is on the system rather than the individual museum, in marked contrast to most contributions in the literature where the individual museum is analyzed. Due to the information policy of the Danish government it is possible to get closer look of the actual workings of the museums than is standard in the literature. The paper concludes that the Danish museums are autonomous to a degree that makes it unlikely that an optimal system of museums is achieved. The paper concludes by discussing some proposals for rearrangement of the museum system.museums, cultural policy, preservation

On free lunches in random walk markets with short-sale constraints and small transaction costs, and weak convergence to Gaussian continuous-time processes

This paper considers a sequence of discrete-time random walk markets with a safe and a single risky investment opportunity, and gives conditions for the existence of arbitrages or free lunches with vanishing risk, of the form of waiting to buy and selling the next period, with no shorting, and furthermore for weak convergence of the random walk to a Gaussian continuous-time stochastic process. The conditions are given in terms of the kernel representation with respect to ordinary Brownian motion and the discretisation chosen. Arbitrage and free lunch with vanishing risk examples are established where the continuous-time analogue is arbitrage-free under small transaction costs - including for the semimartingale modifications of fractional Brownian motion suggested in the seminal Rogers (1997) article proving arbitrage in fBm models.Comment: To appear in the Brazilian Journal of Probability and Statistics, http://www.imstat.org/bjps

Bisimulations and Logical Characterizations on Continuous-time Markov Decision Processes

In this paper we study strong and weak bisimulation equivalences for continuous-time Markov decision processes (CTMDPs) and the logical characterizations of these relations with respect to the continuous-time stochastic logic (CSL). For strong bisimulation, it is well known that it is strictly finer than CSL equivalence. In this paper we propose strong and weak bisimulations for CTMDPs and show that for a subclass of CTMDPs, strong and weak bisimulations are both sound and complete with respect to the equivalences induced by CSL and the sub-logic of CSL without next operator respectively. We then consider a standard extension of CSL, and show that it and its sub-logic without X can be fully characterized by strong and weak bisimulations respectively over arbitrary CTMDPs.Comment: The conference version of this paper was published at VMCAI 201

Prospects for a Nuclear Optical Frequency Standard based on Thorium-229

The 7.6-eV-isomer of Thorium-229 offers the opportunity to perform high resolution laser spectroscopy of a nuclear transition. We give a brief review of the investigations of this isomer. The nuclear resonance connecting ground state and isomer may be used as the reference of an optical clock of very high accuracy using trapped and laser-cooled thorium ions, or in a compact solid-state optical frequency standard of high stability.Comment: 5 pages, 1 figure; Proceedings of the 7th Symposium on Frequency Standards and Metrology, 5-11 October 2008; reference added for section