Romain Karcher’s research while affiliated with French National Centre for Scientific Research and other places

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Publications (12)


Calibration of a superconducting gravimeter with an absolute atom gravimeter
  • Article

May 2021

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95 Reads

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14 Citations

Journal of Geodesy

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Pierre Gillot

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We present a 27-day-long common view measurement of an absolute cold atom gravimeter (CAG) and a relative iGrav superconducting gravimeter, which we use to calibrate the iGrav scale factor. This allowed us to push the CAG long-term stability down to the level of 0.5 nm s2^{-2}. We investigate the impact of the duration of the measurement on the uncertainty in the determination of the correlation factor and show that it is limited to about 3‰ by the coloured noise of our cold atom gravimeter. A 3-day-long measurement session with an additional FG5X absolute gravimeter allows us to directly compare the calibration results obtained with two different absolute meters. Based on our analysis, we expect that with an improvement of its long-term stability, the CAG will allow to calibrate the iGrav scale factor to better than the per mille level (1σ\sigma level of confidence) after only 1 day of concurrent measurements for maximum tidal amplitudes.


Calibration of a superconducting gravimeter with an absolute atom gravimeter
  • Preprint
  • File available

July 2020

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258 Reads

We present in this article a 27 days long common view measurement of an absolute cold atom gravimeter (CAG) and a relative iGrav superconducting gravimeter, which we use to calibrate the iGrav scale factor. We investigate the impact of the duration of the measurement on the uncertainty in the determination of the correlation factor and show that it is limited to about 3\textperthousand~ by the coloured noise of our cold atom gravimeter. A 3 days long measurement session with an additional FG5X absolute gravimeter allows us to directly compare the calibration results obtained with two different absolute meters. Based on our analysis, we expect that with an improvement of its long term stability, the CAG will allow to calibrate the iGrav scale factor to the per mille level after only one day of concurrent measurements.

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Impact of direct-digital-synthesizer finite resolution on atom gravimeters

April 2020

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46 Reads

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12 Citations

Physical Review A

We report on the study of the impact of the finite resolution of the chirp rate applied on the frequency difference between the Raman lasers beamsplitters onto the phase of a free fall atom gravimeter. This chirp induces a phase shift that compensates the one due to gravity acceleration, allowing for its precise determination in terms of frequencies. In practice, it is most often generated by a direct digital synthesizer (DDS). Besides the effect of eventual rounding errors, we evaluate here the bias on the g measurement due to the finite time and frequency resolution of the chirp generated by the DDS, and show that it can compromise the measurement accuracy. However, this effect can be mitigated by an adequate choice of the DDS chirp parameters resulting from a trade-off between interferometer phase resolution and induced bias.


FIG. 1: Frequency chirp applied to the Raman lasers frequency difference. The perfect linear case is represented in black and the actual chirp, composed of frequency steps of δν = 125 Hz and temporal steps δt = 10 µs, in red. The insert shows the parameters of the DDS (δt, δν), ∆t being the adjustable delay between the starts of the interferometer and the chirp. Bottom: residual or frequency difference ∆f between the ideal and actual chirps, represented as a sawtooth function.
FIG. 2: Calculated bias g δt as a function of the temporal step of the chirp δt for T = 80 ms and τ = 16 µs. Focus on the [0 − 30] µs interval.
FIG. 3: Calculated bias g ∆t as a function of the delay ∆t between the chirp sequence and the interferometer sequence for a) δt = 10 µs in black and 20 µs in red. b) δt = 50 µs in black and 100 µs in red.
FIG. 5: Measurements of the truncation error ∆g T E (in blue) as a function of the temporal step of the DDS δt. The results of the calculation are displayed as a continuous red line. The asymptotic line, which corresponds to a null bias and to a difference of 4.15 µGal with respect to the reference configuration of ∆t = 0 µs, τ = 16 µs, is represented in black. The residues between theory and measurements are displayed at the bottom as blue open circles.
Impact of direct digital synthesizer finite resolution on atom gravimeters

January 2020

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126 Reads

We report on the study of the impact of the finite resolution of the chirp rate applied on the frequency difference between the Raman lasers beamsplitters onto the phase of a free fall atom gravimeter. This chirp induces a phase shift that compensates the one due to gravity acceleration, allowing for its precise determination in terms of frequencies. In practice, it is most often generated by a direct digital synthesizer (DDS). Besides the effect of eventual truncation errors, we evaluate here the bias on the g measurement due to the finite time and frequency resolution of the chirp generated by the DDS, and show that it can compromise the measurement accuracy. However, this effect can be mitigated by an adequate choice of the DDS chirp parameters resulting from a trade-off between interferometer phase resolution and induced bias.


Implémentation d'une source d'atomes ultra-froids pour l'amélioration de l'exactitude d'un gravimètre atomique

October 2019

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8 Reads

Cette thèse a pour objectif l’amélioration de l’exactitude du gravimètre atomique du LNE-SYRTE. Ce gravimètre utilise des techniques d’interférométrie atomique pour déterminer l’accélération de la pesanteur g subie par des atomes de Rubidium 87 en chute libre. Il constitue la référence métrologique nationale pour la mesure de g et ses biais doivent donc être évalués avec la meilleure incertitude.Au début de cette thèse, le budget d’incertitude total était de 4.3µGal,dominé par l’incertitude de 4.0µGal associée au biais lié aux aberrations du front d’onde des lasers Raman utilisés pour réaliser les séparatrices lumineuses de l’interféromètre. Pour améliorer l’évaluation de cet effet, nous avons implémenté une source d’atomes ultra froids et mesuré g dans une large gamme de températures. Le développement d’un modèle de l’expérience accompagné par une simulation de l’impact des fronts d’ondes sur la mesure nous a permis d’évaluer pour la première fois ce biais, avec une incertitude record de 1.3µGal, soit trois fois meilleure que précédemment.Le gravimètre participe au projet de balance de Kibble du LNE qui visait à mesurer la constante de Planck par la pesée du kg et dans lequel une mesure exacte de g est nécessaire.Nous avons ainsi contribué à la révolution qu’a connue le système international d’unités, avec la décision de réviser le SI en fixant la valeur numérique de la constante de Planck et en modifiant la définition du kg, entrée en vigueur le 20 Mai 2019.



Scheme of the experimental setup, illustrating the effect of wavefront aberrations. Due to their ballistic expansion across the Raman beam, the atoms sample different parasitic phase shifts at the three π/2 − π − π/2 Raman pulses due to wavefront distortions (displayed in blue as a distorted surface). This leads to a bias in the gravity measurement, resulting from the average of the effect over all atomic trajectories, filtered by finite size effects, such as related to the waist and clear aperture of the Raman beam and to the finite field of view at the detection.
Gravity measurements as a function of the atom temperature. The measurements, displayed as black circles, are performed in a differential way, with respect to a reference temperature of 1.8 μK (displayed as a red circle). The red line is a fit to the data with a subset of five Zernike polynomials and the filled area the corresponding 68% confidence area.
Calculation of the impact of the size of the Raman beam waist (RB) and of the detection field of view (DFoV) on the gravity shift induced by a defocus as a function of the atomic temperature. The peak-to-peak amplitude of the defocus is 20 nm. The total interferometer time is $2T=160$ ms. The results displayed here correspond to four different cases, depending on whether the sizes of the Raman beam waist and detection field of view are taken as finite or infinite.
Gravity shifts calculated for the parameters of our experiment, for the five first Zernike polynomials with rotation symmetry ${Z}_{n}^{0}$ (n = 2, 4, 6, 8, 10) for peak-to-peak amplitudes of 20 nm.
Improving the accuracy of atom interferometers with ultracold sources

November 2018

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202 Reads

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128 Citations

We report on the implementation of ultracold atoms as a source in a state of the art atom gravimeter. We perform gravity measurements with 10 nm/s 2 statistical uncertainties in a so-far unexplored temperature range for such a high accuracy sensor, down to 50 nK. This allows for an improved characterization of the most limiting systematic effect, related to wavefront aberrations of light beam splitters. A thorough model of the impact of this effect onto the measurement is developed and a method is proposed to correct for this bias based on the extrapolation of the measurements down to zero temperature. Finally, an uncertainty of 13 nm/s 2 is obtained in the evaluation of this systematic effect, which can be improved further by performing measurements at even lower temperatures. Our results clearly demonstrate the benefit brought by ultracold atoms to the metrological study of free falling atom interferometers. By tackling their main limitation, our method allows reaching record-breaking accuracies for inertial sensors based on atom interferometry.



Improving the accuracy of atom interferometers with ultracold sources

April 2018

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1 Read

We report on the implementation of ultracold atoms as a source in a state of the art atom gravimeter. We perform gravity measurements with 10 nm/s 2 statistical uncertainties in a so-far unexplored temperature range for such a high accuracy sensor, down to 50 nK. This allows for an improved characterization of the most limiting systematic effect, related to wavefront aberrations of light beam splitters. A thorough model of the impact of this effect onto the measurement is developed and a method is proposed to correct for this bias based on the extrapolation of the measurements down to zero temperature. Finally, an uncertainty of 13 nm/s 2 is obtained in the evaluation of this systematic effect, which can be improved further by performing measurements at even lower temperatures. Our results clearly demonstrate the benefit brought by ultracold atoms to the metrological study of free falling atom interferometers. By tackling their main limitation, our method allows reaching record-breaking accuracies for inertial sensors based on atom interferometry.



Citations (4)


... While atomic interferometry techniques are predominantly employed for gravimetry [24][25][26][27][28][29][30][31], several other high-precision gravimeters have also been investigated. These include optomechanical systems [32][33][34][35][36][37][38][39][40], ultracold Bose-Einstein condensate systems [41][42][43][44][45], micro-electro-mechanical systems [46][47][48][49][50][51], diamagnetic-levitated micro-oscillator [52], gravity-induced electric currents scheme [53], and superconducting systems [54]. Inspired by matter-wave interferometers [55,56], a gravimeter can be devised using a parametric spinmechanical system [57], which can now be implemented with levitated optomechanics [20,55,58] or potentially in a more compact solid-state platform thanks to spin engineering [59,60]. ...

Reference:

Heisenberg-limited spin-mechanical gravimetry
Calibration of a superconducting gravimeter with an absolute atom gravimeter
  • Citing Article
  • May 2021

Journal of Geodesy

... Enhancing its hardware performance [33] and researching related testing methods [34] can help analyze and reduce the uncertainties it introduces. However, due to limitations in the time and frequency resolution of the DDS [35], the change in the Raman laser's frequency is not continuous, leading to step-wise frequency changes. This deviation from ideal frequency changes results in periodic frequency errors and phase shifts [35] in the atom interferometer, inducing bias in gravity measurements. ...

Impact of direct-digital-synthesizer finite resolution on atom gravimeters
  • Citing Article
  • April 2020

Physical Review A

... The use of a GPU to enhance the parallel computation efficiency is discussed in section 4.3. The subsequent outcomes of the state selection and interference are detailed below and compared with the experimental results [49,[51][52][53]. ...

Improving the accuracy of atom interferometers with ultracold sources

... More than a dozen Kibble balance experiments are ongoing at National Metrology Institutes (NMIs) [6], [7], [8], [9], [10], [11], [12], [13], [14], as well as the International Bureau of Weights and Measures (BIPM) [15] and other metrology laboratories [16]. The most accurate Kibble balance can calibrate a kilogram-level mass with a relative uncertainty of approximately 1 × 10 −8 . ...

A determination of the Planck constant using the LNE Kibble balance in air