Overcoming loss of contrast in atom interferometry due to gravity gradients

Long-time atom interferometry is instrumental to various high-precision measurements of fundamental physical properties, including tests of the equivalence principle. Due to rotations and gravity gradients, the classical trajectories characterizing the motion of the wave packets for the two branches of the interferometer do not close in phase space, an effect which increases significantly with the interferometer time. The relative displacement between the interfering wave packets in such open interferometers leads to a fringe pattern in the density profile at each exit port and a loss of contrast in the oscillations of the integrated particle number as a function of the phase shift. Paying particular attention to gravity gradients, we present a simple mitigation strategy involving small changes in the timing of the laser pulses which is very easy to implement. A useful representation-free description of the state evolution in an atom interferometer is introduced and employed to analyze the loss of contrast and mitigation strategy in the general case. (As a by-product, a remarkably compact derivation of the phase-shift in a general light-pulse atom interferometer is provided.) Furthermore, exact results are obtained for (pure and mixed) Gaussian states which allow a simple interpretation in terms of the alignment of Wigner functions in phase-space. Analytical results are also obtained for expanding Bose-Einstein condensates within the time-dependent Thomas-Fermi approximation. Finally, a combined strategy for rotations and nonaligned gravity gradients is considered as well.Comment: 14+7 pages including appendices, 9 figures; v2 minor changes, matches published versio

World population in 2050: assessing the projections: discussion

I would like to follow up on the last sentence of the excellent presentation by Joel Cohen. It is indeed true that the lag between new methodological developments and their actual implementation by statistical agencies is regrettably long, but I think there is some hope that the speed of applying innovations has been accelerating over this century. This is the case in many other areas, and it would be surprising if it were not the case in the field of population forecasting. I would assess with a probability of well above 90 percent that by the end of this century, institutional procedures for projecting population will include probabilistic elements. ; In the following, I would like to mention three additional aspects [new methods, extreme events, and projections of population by level of education] complementing the important remarks made by Cohen.Demography ; Economic conditions

Conformal Mapping and Bound States in Bent Waveguides

Is it possible to trap a quantum particle in an open geometry? In this work we deal with the boundary value problem of the stationary Schroedinger (or Helmholtz) equation within a waveguide with straight segments and a rectangular bending. The problem can be reduced to a one dimensional matrix Schroedinger equation using two descriptions: oblique modes and conformal coordinates. We use a corner-corrected WKB formalism to find the energies of the one dimensional problem. It is shown that the presence of bound states is an effect due to the boundary alone, with no classical counterpart for this geometry. The conformal description proves to be simpler, as the coupling of transversal modes is not essential in this case.Comment: 16 pages, 10 figures. To appear in the Proceedings of the Symposium "Symmetries in Nature, in memoriam Marcos Moshinsky

Generation of tunable subpicosecond light pulses in the midinfrared between 4.5 and 11.5 Mum

Stable subpicosecond infrared pulses in the spectral region of 4.5-11.5 ,um are generated by difference-frequency mixing in AgGaS2. The system uses femtosecond pulses from a Ti:sapphire regenerative amplifier and from a tunable traveling-wave dye laser. The infrared pulses have a duration of 400 fs, an energy of more than 10 nJ, and a repetition rate of 1 kHz