Effective velocity distribution in an atom gravimeter: Effect of the convolution with the response of the detection

Abstract

We present here a detailed study of the influence of the transverse motion of the atoms in a free-fall gravimeter. By implementing Raman selection in the horizontal directions at the beginning of the atoms free fall, we characterize the effective velocity distribution, ie the velocity distribution of the detected atom, as a function of the laser cooling and trapping parameters. In particular, we show that the response of the detection induces a pronounced asymetry of this effective velocity distribution that depends not only on the imbalance between molasses beams but also on the initial position of the displaced atomic sample. This convolution with the detection has a strong influence on the averaging of the bias due to Coriolis acceleration. The present study allows a fairly good understanding of results previously published in Louchet-Chauvet et al., NJP 13, 065025 (2011), where the mean phase shift due to Coriolis acceleration was found to have a sign different from expected.

Active Control of Laser Wavefronts in Atom Interferometers

“…This Monte-Carlo simulation reproduced the experiment described in Fig. 1, taking into account the parameters of the atomic source, the inhomogeneity of the detection response (as in [37]) and the wavefront aberrations. To characterize the DM, the bias of different aberrations on g were measured by varying the mirror shape for T =58 ms. For these measurements, the short term sensitivity was in the range 100-200 µGal at 1 s. A summary of the comparison between the numerical simulations and the experimental results is shown in Table II.…”

“…We observed relatively large and well resolved variations, displayed on Fig. 4(b), of the order of ±30 µGal over a day, which we attribute to slow fluctuations of the atomic source initial position (of the order of ±200 µm) [37]. These position fluctuations bias the gravity measurement in the presence of asymmetric wavefront distortions such as coma aberrations.…”

Improving the accuracy of atom interferometers with ultracold sources

“…There is thus a finite sensitivity to other polynomials if the position of the cloud is not centred onto this aperture, or if there is a residual mean velocity. In addition, coupling inhomogeneities arising from the Gaussian Raman beam profile, and more important in our case, offset positions with respect to the centre of the detection [29], will play a role.…”

“…Description of the model To interpret these data, we have developed a full Monte Carlo model of the experiment, which averages the contributions to the interferometer signal of atoms randomly drawn in the initial position and velocity distributions. It takes into account the selection and interferometer processes, by including the finite size and finite coupling of the Raman lasers, and the detection process, whose finite field of view cuts the contribution of the hottest atoms to the measured atomic populations [29]. This model is used to calculate the effect of wavefront aberrations onto the gravity measurement as a function of the experimental parameters.…”

The effect of wavefront aberrations in atom interferometry

“…This assumption is based on measurements of the velocity distribution of the atomic cloud in the z direction using velocity selective Raman pulses. Although others found Lorentzian velocity distributions [22], we fitted the data to a Gaussian, as it yields the cloud temperature as a parameter. The differences in the simulation results between both distributions are negligible.…”