Vortex patterns in a fast rotating Bose-Einstein condensate

For a fast rotating condensate in a harmonic trap, we investigate the structure of the vortex lattice using wave functions minimizing the Gross Pitaveskii energy in the Lowest Landau Level. We find that the minimizer of the energy in the rotating frame has a distorted vortex lattice for which we plot the typical distribution. We compute analytically the energy of an infinite regular lattice and of a class of distorted lattices. We find the optimal distortion and relate it to the decay of the wave function. Finally, we generalize our method to other trapping potentials

Stable single-photon interference in a 1 km fiber-optical Mach-Zehnder interferometer with continuous phase adjustment

We experimentally demonstrate stable and user-adjustable single-photon interference in a 1 km long fiber- optical Mach-Zehnder interferometer, using an active phase control system with the feedback provided by a classical laser. We are able to continuously tune the single-photon phase difference between the interferometer arms using a phase modulator, which is synchronized with the gate window of the single-photon detectors. The phase control system employs a piezoelectric fiber stretcher to stabilize the phase drift in the interferometer. A single-photon net visibility of 0.97 is obtained, yielding future possibilities for experimental realizations of quantum repeaters in optical fibers, and violation of Bell's inequalities using genuine energy-time entanglementComment: 3 pages, 3 figure

Social Security, Retirement, and the Single-Mindedness of the Electorate

We propose a positive theory that is consistent with two important features of social security programs around the world: (1) they redistribute income from young to old and (2) they induce retirement. We construct a voting model that includes a political campaign' or debate' prior to the election. The model incorporates single-mindedness' of the groups that do not work: while the workers divide their political capital between their age concerns' and occupational concerns', the retired concentrate all their political capital to support their age group. In our model, the elderly end up getting transfers from the government (paid by the young) and distortionary labor income taxes induce the retirement of the elderly. In addition, our model predicts that occupational groups that work more will tend to have more political power. The opposite is true for non-occupational groups (such as the elderly). We provide some evidence that supports these additional predictions.

Social security in theory and practice (I): Facts and political theories

166 countries have some kind of public old age pension. What economic forces create and sustain old age Social Security as a public program? We document some of the internationally and historically common features of Social Security programs including explicit and implicit taxes on labor supply, pay-as-you-go features, intergenerational redistribution, benefits which are increasing functions of lifetime earnings and not means-tested. We partition theories of Social Security into three groups: "political", "efficiency" and "narrative" theories. We explore three political theories in this paper: the majority rational voting model (with its two versions: "the elderly as the leaders of a winning coalition with the poor" and the "once and for all election" model), the "time-intensive model of political competition" and the "taxpayer protection model". Each of the explanations is compared with the international and historical facts. A companion paper explores the "efficiency" and "narrative" theories, and derives implications of all the theories for replacing the typical pay-as-you-go system with a forced savings plan.Social Security, retirement, gerontocracy, retirement incentives, political theories of Social Security

Social Security in Theory and Practice (II): Efficiency Theories, Narrative Theories, and Implications for Reform

166 countries have some kind of public old age pension. What economic forces create and sustain old age Social Security as a public program? Mulligan and Sala-i-Martin (1999) document several of the internationally and historically common features of social security programs, and explore political' theories of Social Security. This paper discusses the efficiency theories,' which view creation of the SS program as a full or partial solution to some market failure. Efficiency explanations of social security include the SS as welfare for the elderly', the retirement increases productivity to optimally manage human capital externalities', optimal retirement insurance', the prodigal father problem', the misguided Keynesian', the optimal longevity insurance', the government economizing transaction costs' and the return on human capital investment'. We also analyze four narrative' theories of social security: the chain letter theory', the lump of labor theory', the monopoly capitalism theory', and the Sub-but-Nearly-Optimal policy response to private pensions theory'. The political and efficiency explanations are compared with the international and historical facts and used to derive implications for replacing the typical pay-as-you-go system with a forced savings plan. Most of the explanations suggest that forced savings does not increase welfare, and may decrease it.