Measurement induced entanglement and quantum computation with atoms in optical cavities

We propose a method to prepare entangled states and implement quantum computation with atoms in optical cavities. The internal state of the atoms are entangled by a measurement of the phase of light transmitted through the cavity. By repeated measurements an entangled state is created with certainty, and this entanglement can be used to implement gates on qubits which are stored in different internal degrees of freedom of the atoms. This method, based on measurement induced dynamics, has a higher fidelity than schemes making use of controlled unitary dynamics.Comment: 4 pages including 2 figures. v2+3: minor change

The Economics of Delaying Policy Change: An Application to the 1992 CAP Reform

Positive political economy is usually concerned with economic explanations of observed policy choices, while the timing of a policy reform has not gained similar attention. This is somewhat surprising since policy makers most often are free to decide both the design and timing of a policy reform. Drawing on insights from recent developments in the finance literature on investment under uncertainty, here we apply the idea of option value to the analysis of government policy making. Common political-economic explanations of the 1992 CAP reform are that policymakers felt domestic political pressure to make the CAP more efficient, and also international political pressure and to bring the CAP in line with treaty obligations. Although these arguments are sound, they fail to explain why policy-makers did not enact the reform earlier, especially during times of decreasing world market prices prior to 1992. We address this question using the theory of option value, which is the value of being able to wait in decisionmaking. Commonly governments are free to decide when to reform policy. Waiting to reform policy can improve government decisions. For while waiting decision-makers may observe market parameter changes as they occur. (For example, they may obtain better information about changes in world prices.) This reduces their uncertainty about the effects of their decisions. Giving up the option to wait incurs a cost which has to be taken into account in policy decisions. We illustrate the option value concept using a political-economy model of the 1992 CAP reform. We show empirically that if decision-makers had not had the option to wait to reform policy, it would have been more efficient to implement the 1992 CAP reform in the mid 1980s.Agricultural and Food Policy,

Towards a Theory of Policy Making

The paper presents a theory of policy timing that relies on uncertainty and transaction costs to explain the optimal timing and length of policy reforms. Delaying reforms resolves some uncertainty by gaining valuable information and saves transaction costs. Implementing reforms without waiting increases welfare by adjusting domestic policies to changed market parameters. Optimal policy timing is found by balancing the trade-off between delaying reforms and implementing reforms without waiting. Our theory offers an explanation of why countries differ with respect to the length of their policy reforms, and why applied studies often judge agricultural policies to be inefficient.Policy analysis, Uncertainty, Dynamic model, Transaction costs, Agricultural and Food Policy,

The C*-algebra of an affine map on the 3-torus

We study the C*-algebra of an affine map on a compact abelian group and give necessary and sufficient conditions for strong transitivity when the group is a torus. The structure of the C*-algebra is completely determined for all strongly transitive affine maps on a torus of dimension one, two or three

Boundary Integral Method for Stationary States of Two-Dimensional Quantum Systems

The boundary integral method for calculating the stationary states of a quantum particle in nano-devices and quantum billiards is presented in detail at an elementary level. According to the method, wave functions inside the domain of the device or billiard are expressed in terms of line integrals of the wave function and its normal derivative along the domain's boundary; the respective energy eigenvalues are obtained as the roots of Fredholm determinants. Numerical implementations of the method are described and applied to determine the energy level statistics of billiards with circular and stadium shapes and demonstrate the quantum mechanical characteristics of chaotic motion. The treatment of other examples as well as the advantages and limitations of the boundary integral method are discussed.Comment: RevTeX3.0, 24 pages, 9 EPS figures (included); To be published in Int. J. of Mod. Phys.

Reply to the Comment of den Hartog and van Wees on "Conductance Fluctuations in Mesoscopic Normal-Metal/Superconductor Samples"

In their comment cond-mat/9710285 [Phys. Rev. Lett. 80, 5024 (1998)] den Hartog and van Wees (HW) raise objections against our analysis of the experimental data presented in cond-mat/9708162 [Phys. Rev. Lett. 79, 1547 (1997)]. According to HW, we did not account for the quantum phase incoherence introduced by the Niobium compounds of the investigated Nb/Au hybrid samples. Here we show that and why this criticism is not justified. Some difficulties associated with a precise determination of the coherence lengths are discussed. It is discussed why these uncertainties do not have a qualitative impact on the results reported in our paper.Comment: Reply to the comment cond-mat/9710285 by den Hartog and van Wees; 1 page REVTE

Detecting entanglement in spatial interference

We discuss an experimentally amenable class of two-particle states of motion giving rise to nonlocal spatial interference under position measurements. Using the concept of modular variables, we derive a separability criterion which is violated by these non-Gaussian states. While we focus on the free motion of material particles, the presented results are valid for any pair of canonically conjugate continuous variable observables and should apply to a variety of bipartite interference phenomena.Comment: 4 pages; corresponds to published versio