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LaserPhys Youth School-Conference

Preparation of the initial wave packet for the quantum interferometer-gravimeter

Kirill Adamov, Dmitry P. Kapusta, Anatoly E. Bonert, Andrei Goncharov

Published Jan 1, 2025• 0 citations
Publisher

Abstract

This work was aimed at improving metrological characteristics of the developed atomic interferometer-based gravimeter. We investigated a preparation of ultracold 87Rb atoms in a magnetically insensitive quantum state. Numerical modeling of optical pumping of rubidium atoms to the ∣F=1, m=0⟩ state was performed using the ∣F=1⟩↔∣F′=0⟩ transition by solving rate equations that describe the population distribution among magnetic sublevels of the ground state. Simulation results demonstrate the high efficiency of the selected pumping scheme (~100%) without significant heating of the atomic cloud. Implementation of this method enables preparation of 87Rb atoms in the magnetically insensitive ∣F=1, m=0⟩ state with efficiency >95% and a temperature increase of only 1 μK. The use of optical pumping results in more than a fourfold improvement in the gravimeter sensitivity, achieving 5.2 mGal/√Hz. A compensation of the gravimeter vibration noise was performed using a post-correction method together with an accelerometer data. Implementation of this technique improved the output metrological characteristics of the gravimeter by a factor of two.

DOI
10.15862/83mnnpk25.07

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•Jan 1, 2019•2 citations
This work was aimed at improving metrological characteristics of the developed atomic interferometer-based gravimeter. We investigated a preparation of ultracold 87Rb atoms in a magnetically insensitive quantum state. Numerical modeling of optical pumping of rubidium atoms to the ∣F=1, m=0⟩ state was performed using the ∣F=1⟩↔∣F′=0⟩ transition by solving rate equations that describe the population distribution among magnetic sublevels of the ground state. Simulation results demonstrate the high efficiency of the selected pumping scheme (~100%) without significant heating of the atomic cloud. Implementation of this method enables preparation of 87Rb atoms in the magnetically insensitive ∣F=1, m=0⟩ state with efficiency >95% and a temperature increase of only 1 μK. The use of optical pumping results in more than a fourfold improvement in the gravimeter sensitivity, achieving 5.2 mGal/√Hz. A compensation of the gravimeter vibration noise was performed using a post-correction method together with an accelerometer data. Implementation of this technique improved the output metrological characteristics of the gravimeter by a factor of two.
In this work, we created a source of ultracold atoms for a quantum interferometer-gravimeter. A cloud of 107−10810^{7}-10^{8}107−108 atoms with the temperature of around 6 uK was obtained. An effective selection of rubidium atoms in the nonmagnetic sub-state was carried out.
We present an ultrahigh-sensitivity gravimeter based on an 87{}^{87}87Rb atom interferometer using stimulated Raman transitions. Compared with our previous work, a two-dimensional magneto-optical trap is added in the new gravimeter to increase the atom number and improve the detection signal-to-noise ratio, and a better optical phase-locked loop system is used to reduce the phase noise of Raman beams. Benefiting from these efforts and the excellent performance of the active vibration isolator, a short-term sensitivity of about 4.2 $\ensuremath{\mu}\text{Gal}/\sqrt{\text{Hz}}$ ($1\phantom{\rule{4pt}{0ex}}\ensuremath{\mu}\mathrm{Gal}=1\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}8}$ m/s2\text{m}/{\text{s}}^{2}m/s2) is reached, which improves the sensitivity by a factor of 2 compared with the former best reported value. By a modulation experiment, we further indicate that the residual vibration noise contribution is about 1.2 $\ensuremath{\mu}\text{Gal}/\sqrt{\text{Hz}}$, which implies a possible improvement over the present absolute gravity measurement level by about one order of magnitude. Moreover, we demonstrate a calibration experiment to directly evaluate the sub-$\ensuremath{\mu}$Gal resolution of our gravimeter.
The results of experimental studies on creating a source of ultracold atoms 87 Rb for an absolute quantum gravimeter based on atomic interference are presented. The studies resulted in sub-Doppler cooling of atoms 87 Rb in a magneto-optical trap and in obtaining a cloud ~10 7 -10 8 of atoms with a temperature of 6 мкК. Using microwave radiation and optical pumping, the preparation of ultracold atoms in the initial state |F = 1, mF = 0〉is carried out and experiments are performed to observe Ramsey resonances at the transition |F = 1, mF = 0〉 → |F = 2, mF = 0〉during the interaction of the atomic cloud with unidirectional Raman radiation pulses.
The presented thesis describes the setting up of an inertial sensitive atom interferometer for navigation using Rubidium 87 atoms, which is able to perform horizontal acceleration measurement. An ultra-high vacuum interferometer chamber is designed and built. The cold atoms are prepared in a compact 2D MOT setup and loaded into the main MOT. These atoms are then cooled down to few K and prepared in a magnetically insensitive ground state using a sequence of optical and microwave pulses, after which atoms are selected in a narrow band of the velocity. A three-pulse Mach-Zehnder ( pi/2 -pi-pi/2) interferometer using stimulated Raman transitions is used to measure the horizontal acceleration. To improve the sensitivity to acceleration, the fringe contrast of the interferometer requires to be optimised. By investigating the intensity profile of the Raman beams using atoms, a procedure is developed to operate the interferometer in an improved condition, which is able to enhance the fringe contrast by a factor of 3. This procedure improves the shot-to-shot sensitivity of the interferometer below 5 um/s^2. In addition, the atom interferometer is compared with a classical state-of-the-art accelerometer.
Atomic vapour magnetometers sense the local magnetic field strength by measuring the resulting precession rate of a well-defined quantum state. An essential prerequisite for this approach is a requirement to drive the media into this quantum state, which is frequently achieved via optical pumping. In real-world alkali-metal atoms, with their multiplicity of ground states, the optical pumping process is necessarily lossy, with a large fraction of the atoms being lost to quantum states that do not contribute to the useful magnetically sensitive signal. This consequently reduces the sensitivity of all optically-pumped atomic sensors. Here we theoretically and experimentally study the population changes of the quantum ground states of 87 Rb during optical pumping. We use this understanding to develop a repumping scheme that allows us to increase the number of atoms that are contributing to the useful magnetic sensing output. Unlike prior schemes, our approach delivers this improved sensitivity with significantly suppressed fictitious magnetic fields associated with the repumping, which would otherwise reduce the accuracy of the sensor. When operated at Earth’s field strength (∼50 µ T), the repumped sensor shows a magnetic sensitivity of 200 fT/ Hz , that is nearly three times higher than the non-repumped version.
The quantum gravimeter QG-1 is designed to perform local gravity measurements in field with residual uncertainties surpassing state-of-the-art absolute gravimeters. It will use delta-kick collimated Bose-Einstein condensates (BEC) of 87 Rb atoms as the source for matter-wave interferometry and aims for an uncertainty level below 3 nm/s 2 with a repetition rate of 0.5 Hz. In the frame of this thesis, the atom-chip source has been set up and characterized. In order to meet the requirements for transportability, the apparatus needs to be compact and robust. Additionally, a high BEC flux is desired for the targeted statistical uncertainty. To meet these requirements a double magneto-optical trap (MOT) configuration based on a 3-layer atom chip has been implemented. Furthermore, a compact high power fiber-based laser system has been developed to provide the light for atomic manipulation. To power and control this laser system and generate the magnetic fields necessary to trap atoms, a miniaturized electronic system with low power consumption has been utilized and characterized. The steps towards trapping the atoms inside a magnetic trap on which the evaporation will be implemented have been realized and optimized. A two-dimensional MOT has been employed and an optimized atomic beam flux of 2.2 × 10 9 atoms/s with a mean longitudinal velocity of 17.2 m/s and a narrow longitudinal velocity distribution of 2.2 m/s has been used to load a 3D-chip MOT. The resulting 3D-MOT has been characterized and a loading rate of 8.1×10 8 atoms/s has been attained. After loading for 300 ms, 2.4×10 8 atoms are trapped in the 3D-MOT. Applying a subsequent compressed MOT stage, the cloud density has been increased by a factor of 10. This denser atomic ensemble with 2.9 × 10 10 atoms/cm 3 has been further cooled in a polarization gradient cooling (PGC) stage to 5.6 μK in order to increase the mode match with the subsequent magnetic trap. In the initial large volume magnetic trap generated by the wire structures on the mesoscopic and the base layers of the atom-chip assembly, 3.6 × 10 7 atoms have been captured. This trap features a temperature of 44.3 μK, a trapping frequency of 9.1 Hz in its weak confining axis and a phase-space density of 4.5 × 10 −7 . The process of transferring the atoms into the final magnetic trap with higher trapping frequency for an efficient BEC evaporation, which is formed by the wire structures on the base and science chip has been tested. With this, all the necessary functionalities of a high-flux BEC source setup have been demonstrated.