Test of the He-McKellar-Wilkens topological phase by atom interferometry. Part I: theoretical discussion

We have recently tested the topological phase predicted by He and McKellar and by Wilkens: this phase appears when an electric dipole propagates in a transverse magnetic field. In the present paper, we first recall the physical origin of this phase and its relations to the Aharononov-Bohm and Aharonov-Casher phases. We then explain possible detection schemes and we briefly describe the lithium atom interferometer we have used for this purpose. Finally, we analyze in great detail the phase shifts induced by electric and magnetic fields acting on such an interferometer, taking into account experimental defects. The experiment and its results are described in a companion paper

Optical pumping of a lithium atomic beam for atom interferometry

We apply optical pumping to prepare the lithium beam of our atom interferometer in a single hyperfine-Zeeman sublevel: we use two components of the D1-line for pumping the 7Li atoms in a dark state F,mF=+2 (or -2) sublevel. The optical pumping efficiency has been characterized by two techniques: state-selective laser atom deflection or magnetic dephasing of the atom interferometer signals. The first technique has not achieved a high sensitivity, because of a limited signal to noise ratio, but magnetic dephasing signals have shown that about 95% of the population has been transferred in the aimed sublevel, with similar results for three mean velocities of the atomic beam covering the range 744-1520m/s

Test of the He-McKellar-Wilkens topological phase by atom interferometry. Part II: the experiment and its results

In this paper, we describe an experimental test of the He-McKellar-Wilkens (HMW) topological phase with our lithium atom interferometer. The expected value of the HMW phase shift in our experiment is small and its measurement was difficult because of stray phase shifts due to small experimental defects. We start by describing our setup and we characterize the effects of the electric and magnetic fields needed to observe the HMW effect. Then, we develop a model of our interferometer signals including all the defects we have identified. After various tests of this model, we use it to suppress the largest part of the stray phase shifts. We thus obtain a series of measurements of the HMW phase: the results are 31% larger than expected and this discrepancy is probably due to some limitations of our model

Vibration induced phase noise in Mach-Zehnder atom interferometers

The high inertial sensitivity of atom interferometers has been used to build accelerometers and gyrometers but this sensitivity makes these interferometers very sensitive to the laboratory seismic noise. This seismic noise induces a phase noise which is large enough to reduce the fringe visibility in many cases. We develop here a model calculation of this phase noise in the case of Mach-Zehnder atom interferometers and we apply this model to our thermal lithium interferometer. We are thus able to explain the observed dependence of the fringe visibility with the diffraction order. The dynamical model developed in the present paper should be very useful to further reduce this phase noise in atom interferometers and this reduction should open the way to improved interferometers

Measurement of the Aharonov-Casher geometric phase with a separated-arm atom interferometer

In this letter, we report a measurement of the Aharonov-Casher (AC) geometric phase with our lithium atom interferometer. The AC phase appears when a particle carrying a magnetic dipole propagates in a transverse electric field. The first measurement of the AC phase was done with a neutron interferometer in 1989 by A. Cimmino \textit{et al.} (Phys. Rev. Lett. \textbf{63}, 380, 1989) and all the following experiments were done with Ramsey or Ramsey-Bord\'e interferometers with molecules or atoms. In our experiment, we use lithium atoms pumped in a single hyperfine-Zeeman sublevel and we measure the AC-phase by applying opposite electric fields on the two interferometer arms. Our measurements are in good agreement with the expected theoretical values and they prove that this phase is independent of the atom velocity.Comment: 6 page

Dispersion compensation in atom interferometry by a Sagnac phase

We reanalyzed our atom interferometer measurement of the electric polarizability of lithium now accounting for the Sagnac effect due to Earth rotation. The resulting correction to the polarizability is very small but the visibility as a function of the applied phase shift is now better explained. The fact that the Sagnac and polarizability phase shifts are both proportional to $v^{-1}$, where $v$ is the atom velocity, suggests that a phase shift of the Sagnac type could be used as a counterphase to compensate the electric polarizability phase shift. This exact compensation opens the way to higher accuracy measurements of atomic polarizabilities and we discuss how this can be practically done and the final limitations of the proposed technique

Essays in empirical asset pricing with machine learning

This thesis consists of four papers on topics in empirical asset pricing with a particular focus on applications of machine learning. The first paper investigates the interplay of predictable trading behaviour and asset prices. We show that predictable order ow is associated with improved liquidity and market efficiency. In addition, we find evidence for a priced factor constructed from order ow predictability, contributing to the literature that connects market microstructure features and asset prices. The second paper evaluates the efficacy of machine learning based forecasts of bond excess returns and contributes to a better understanding of the formation of bond risk premia. We show that machine learning techniques outperform the principal components benchmarks used in extant literature and deliver substantial economic gains to investors. The third paper investigates the risk-reward trade-off in index options through the lens of a factor modelling approach. We show that a factor model with five factors and time-varying loadings instrumented with option characteristics, explains the vast majority of variation in delta-hedged option returns. The recovered factors point to jump, volatility and term structure spread risks. Finally, the fourth paper studies the systematic drivers of asset holdings in a novel factor modelling approach. I document the existence of a factor structure in holdings changes that points to distinct, well-understood economic channels as drivers of asset holdings. Using investor-specific factor loadings I find evidence for pro-cyclical trading of banks and mutual funds as well as counter-cyclical trading of investment advisors and pension funds. Furthermore, I document that changes to institutional investor holdings driven by systematic factors are negatively associated with future returns, suggesting a price pressure channel as a driver for return reversals

Signatures of electronic polarons in La1−x_{1-x}Sr1+x_{1+x}MnO4_4 observed by electron energy-loss spectroscopy

The dielectric properties of La$_{1-x}$Sr$_{1+x}$MnO$_4$ single crystals with x = 0, 0.125, 0.25, and 0.5 were studied by means of electron energy-loss spectroscopy as a function of temperature and momentum transfer. A clear signature of the doped holes is observed around 1.65 eV energy loss, where spectral weight emerges with increasing x. For all $x \neq 0$, this doping-induced excitation can propagate within the ab-plane, as revealed by a clear upward dispersion of the corresponding loss peak with increasing momentum transfer. The hole-induced excitation also shifts to higher energies with the onset of magnetic correlations for x = 0.5, implying a strong coupling of charge and spin dynamics. We conclude that (i) the loss feature at 1.65 eV is a signature of electronic polarons, which are created around doped holes, and that (ii) this low-energy excitation involves the charge transfer between manganese and oxygen. The finite dispersion of these excitations further indicates significant polaron-polaron interactions.Comment: 7 pages, 4 figure

Index of refraction of gases for matter waves: effect of the motion of the gas particles on the calculation of the index.

International audienceTwo different formulae relating the index of refraction $n$ of gases for atom waves to the scattering amplitude have been published. We show here that these two formulae are not consistent with the definition of the total scattering cross-section while the formula developed by one of us (C.C.) in her thesis is in agreement with this standard knowledge. We discuss this result, in particular in the neutron case for which such an index was first introduced. We finally evaluate the index of refraction as a function of well known quantities and we discuss the order of magnitude of the ratio of $(n-1)/n_t$, where $n_t$ is the gas density