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arXiv:1807.05860 · New Journal of Physics

A trapped ultracold atom force sensor with a μm-scale spatial resolution

Xavier Alauze, Alexis Bonnin, Cyrille Solaro, Franck Pereira dos Santos

Published Jul 31, 2018• 33 citations
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Abstract

We report on the use of an ultracold ensemble of 87 Rb atoms trapped in a vertical lattice as a source for a quantum force sensor based on a Ramsey–Raman type interferometer. We reach spatial resolution in the low micrometer range in the vertical direction thanks to evaporative cooling down to ultracold temperatures in a crossed optical dipole trap. In this configuration, the coherence time of the atomic ensemble is degraded by inhomogeneous dephasing arising from atomic interactions. By weakening the confinement in the transverse direction only, we dilute the cloud and drastically reduce the strength of these interactions, without affecting the vertical resolution. This allows us to maintain an excellent relative sensitivity on the Bloch frequency, which is related to the local gravitational force, of 5 × 10 −6 at 1 s which integrates down to 8 × 10 −8 after one hour averaging time.

DOI
10.1088/1367-2630/aad716

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We report on the use of an ultracold ensemble of 87 Rb atoms trapped in a vertical lattice as a source for a quantum force sensor based on a Ramsey–Raman type interferometer. We reach spatial resolution in the low micrometer range in the vertical direction thanks to evaporative cooling down to ultracold temperatures in a crossed optical dipole trap. In this configuration, the coherence time of the atomic ensemble is degraded by inhomogeneous dephasing arising from atomic interactions. By weakening the confinement in the transverse direction only, we dilute the cloud and drastically reduce the strength of these interactions, without affecting the vertical resolution. This allows us to maintain an excellent relative sensitivity on the Bloch frequency, which is related to the local gravitational force, of 5 × 10 −6 at 1 s which integrates down to 8 × 10 −8 after one hour averaging time.
A Ramsey-Raman interferometer is used to realize a compact force sensor using cold 87Rb^{87}\mathrm{Rb}87Rb atoms trapped in a vertical optical lattice. The sensitivity and accuracy of the force measurements are discussed and the limits in short-term sensitivity evaluated. We reach a relative sensitivity on the Bloch frequency and thus on the gravity acceleration of $3.9\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}6}$ at 1 s. We perform an experimental study of the influence of the transverse confinement onto the decay of the interferometer contrast and compare the measurements with a simple semiclassical model. It is shown that vertical gradients of the trapping potential can contribute significantly to the loss of contrast.
We report the realization of a quantum device for force sensing at the micrometric scale. We trap an ultracold 88Sr^{88}\mathrm{Sr}88Sr atomic cloud with a one-dimensional (1D) optical lattice; then we place the atomic sample close to a test surface using the same optical lattice as an elevator. We demonstrate precise positioning of the sample at the micrometer scale. By observing the Bloch oscillations of atoms into the 1D optical standing wave, we are able to measure the total force on the atoms along the lattice axis, with a spatial resolution of few micrometers. We also demonstrate a technique for transverse displacement of the atoms, allowing us to perform measurements near either transparent or reflective test surfaces. In order to reduce the minimum distance from the surface, we compress the longitudinal size of the atomic sample by means of an optical tweezer. This system is suited for studies of atom-surface interaction at short distance, such as measurement of the Casimir force and the search for possible non-Newtonian gravity effects.
Summary form only given. Alkali-earth atoms have an electronic level structure particularly suited for applications in matter-wave interferometry and high-precision laser spectroscopy. Recently atomic strontium was the subject of active research in several fields such as all-optical cooling towards quantum degeneracy and detection of ultra-narrow optical transitions. Because of its small elastic cross-section, we show that ultra-cold88Sr in presence of a lattice potential is also particularly well suited for the realization of inertial sensors to accurately measure forces with micro-metric spatial resolution. By loading the sample into a vertical lattice potential we observe persistent Bloch oscillations with a damping time longer than 10 seconds, and from the measured Bloch frequency we determine the local gravity with a sensitivity of 5 x 10-6g. Our result has direct implications in force measurements at small distances from surfaces such as tests of possible deviations form the Newtonian gravity potential at sub-millimetre distances. We will present the status of a force measurement at distances below 20 mum from dielectric and metallic surfaces, which based on ultra-cold atoms undergoing Bloch oscillations. We will also report on the progress towards the realization of an optical frequency standard based on the highly forbidden1S0-3Po 88Sr intercombination line at 698 nm as well as the realization of a laser suited for precision spectroscopy on this transition.
Using cold 87{}^{87}87Rb atoms trapped in a one-dimensional (1D)-optical lattice, atomic interferometers involving coherent superpositions between different Wannier-Stark atomic states are realized. Two different kinds of trapped interferometer schemes are presented: a Ramsey-type interferometer sensitive both to clock frequency and external forces, and a symmetric accordion-type interferometer, sensitive to external forces only. We evaluate the limits in terms of sensitivity and accuracy of those schemes and discuss their application as force sensors. As a first step, we apply these interferometers to the measurement of the Bloch frequency and the demonstration of a compact gravimeter.
We report on a trapped atom interferometer based on Bragg diffraction and Bloch oscillations with alkaline-earth-metal atoms. We use a Ramsey--Bord\'e Bragg interferometer with 88Sr^{88}\mathrm{Sr}88Sr atoms combined with Bloch oscillations to extend the interferometer time. Thanks to a long coherence time for Bloch oscillations of 88Sr^{88}\mathrm{Sr}88Sr atoms, we observed interference up to 1 s evolution time in the lattice. A detailed study of decoherence sources during the Bloch phase is also presented. While still limited in sensitivity by lattice lifetime and beam inhomogeneity this result opens the way to high contrast trapped interferometers with extended interrogation time.
Most cold atoms experiments in microgravity platforms or in Space are achieved using atom chips, leading to limitations in terms of optical access and inhomogeneous magnetic fields. Optical dipole traps do not have these drawbacks but have difficulties producing atomic samples with a large number of atoms at ultra low temperature in the absence of gravity. Here, we report on an efficient evaporative cooling in two-crossed laser beams during parabolic flights. Time-averaged potentials combine the advantages of large capture volume and trap compression, increasing the initial phase space density and collision rate to favor the evaporative process. With this technique we demonstrate the production of an ultra cold gas of 2.5×1042.5\times 10^42.5×104 rubidium atoms at a temperature below 100 nK in less than 4 seconds. Our experiment paves the way for the development of quantum sensors applied to fundamental physics and geodesy as well as the study of ultracold atomic physics in Space.