Quantum Gravimeter

Absolute Quantum Gravimeter

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Jean Lautier-Gaud

Director of Business Development & Sales, Quantum Sensors

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Thanks to the advances of Quantum Technologies, iXblue is able to provide a turn-key transportable quantum sensor measuring gravity at a level of 10-8 m/s2. The Absolute Quantum Gravimeter (AQG) measures the acceleration of a free-falling test mass in vacuum: the ballistic free-fall of an ensemble of laser-cooled atoms is accurately monitored, and the acceleration of gravity is then inferred. This technique is one of the ballistic free-fall methods proclaimed by the BIPM (Bureau International des Poids et Mesures) as an official primary method for the measurement of gravity.

A free-fall absolute gravimeter using Quantum Technologies and laser-cooled atoms

It is today the only commercial industry-grade gravity meter to enable continuous absolute measurements from a few seconds to several years. Being at the same time easily transportable it can be deployed anywhere on the planet both indoor and outdoor. This makes the AQG highly suitable for a wide range of applications in geophysics reservoir monitoring, geodesy, metrology and sub-surface imaging for civil engineering.

For high-precision gravity measurements

The Absolute Quantum Gravimeter (AQG) is the first commercially available gravimeter based on Quantum Technologies and exploits the principle of atom interferometry with laser-cooled atoms. This unique solution is the result of more than 15 years of research conducted by our academic partners (LP2N and LNE-SYRTE)

It offers very attractive features for high-precision gravity measurements:

  • absolute gravity measurement at a level of 10-8 m/s2 (1 μGal) in terms of sensitivity, stability and repeatability
  • continuous data acquisition from a few seconds to several years
  • transportable device allowing to perform surveys, time-lapse measurement of a network of reference stations or stationary measurements with the same instrument

The AQG comes in 2 versions: the AQG-A for indoor use and the AQG-B for field measurements.

A turn-key transportable and easy-to-operate quantum sensor

A year-long field campaign on Mount Etna to prove the Absolute Quantum Gravimeter value for volcanology

Installation time: 20 minutes (no optical alignment, no mechanical assembly, no pumping required prior to measuring)

Maintenance: Low maintenance effort (no moving parts, no gasket nor belt to replace)

Robustness to ground vibration: Sub-μGal resolution even in urban environments (without any spring-based mechanical isolation device)

Autonomy: Transport and storage for several weeks without any power supply will not affect measurement capability nor require additional pumping (sealed dropping chamber)

Software: Dedicated and user-friendly data acquisition and system controller software
Automated starting procedure
Automated self-calibration procedures
Remote monitoring and real-time data retrieval

Data processing: Gravity data is real-time processed with corrections from local tides, atmospheric pressure, tilt variations, ocean loading and polar motion

Auxiliary sensors: The sensor head of the AQG houses a classical 3D accelerometer, a pressure gauge, tiltmeters, temperature sensors.

Providing high-quality gravity data

The AQG is a flexible gravity sensor able to perform: surveys reaching a resolution of a few µGal with short integration time (Fig. 1), reference station and time-lapse measurements at 1 µGal (Fig. 2) and drift-free long-term continuous gravity monitoring with ultra-high sensitivity (Fig. 3).

10-8 m.s-2 = 1 µGal

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Data from V. Ménoret, et al., « Gravity measurements below 10−9 g with a transportable absolute quantum gravimeter », Nature Scientific Report, 8:12300 (2018)

These values of g are corrected from local tides, atmospheric pressure, tilt deviations and ocean loading. In Fig. 2, error bars correspond to the value of the Allan Deviation of the entire data-set calculated at one hour integration time, which corresponds to the statistical uncertainty of this measurement.

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This uninterrupted time series has lasted one month and shows local tides. When data are averaged over long durations, the AQG provides drift-free sub-μGal stability.

Simple and compact quantum gravimeter

The design of the AQG relies on a patented opto-mechanical architecture using a pyramidal retro-reflector. This configuration allows to perform the measurement sequence with a single laser beam instead of up to eight usually, leading to a drastic simplification of the instrument.

High-reliability fibered optical components

The laser system developed by iXblue relies on the use of lasers operating at 1560 nm. This approach therefore gives access to a wide variety of high performance fibered optical components, originally developed for high-bit-rate optical communication systems. Thanks to the technological efforts conducted over the last 20 years by the telecom industry, these components present unique features:

  • fibered components: no optical alignment required
  • remarkable optical and electrical performances
  • compliance with Telcordia qualification procedures (extended temperature range)
  • high reliability (lifetime > 50 000 h).

High immunity to ground vibrations

The AQG efficiently rejects ground vibration noise with an active compensation technique that does not require any mechanical isolation device or superspring.

Specifications of the field AQG

PERFORMANCES SPECIFICATIONS

Sensitivity (at quiet site) 50 µGal/√t
5 µGal in 1.5 min
2 µGal in 10 min
1 µGal in 40 min
Long-term stability < 2 µGal
Repeatability < 5 µGal
Announced Accuracy ≤ 10 µGal
(under metrological evaluation)
Cycling frequency > 1 Hz

PHYSICS PACKAGE

MASS & DIMENSIONS
Number of boxes 4
Maximum mass of each box 40 kg
Total mass < 140 kg
Dimensions of sensor head Diam: 40 cm
Height : 100 cm (tripod incl.)
Dimensions of control units 2 modules, each:
h = 41 cm x w = 59 cm x l = 105 cm
OPERATION
Warm up time (typical) 2h
Nb of connexions 10
Time of installation < 20 minutes
INTERFACES
Length of the cable between sensor head and laser system 15 m

SYSTEM INTERFACE & CONTROL

Control system On-board computer
User Interface Dedicated software
OS of the operating computer Linux
Remote Monitoring ? Yes
Connector
Protocol
Location
Ethernet
IP
Computer
Data format .csv

ENVIRONMENTAL & PHYSICAL SPECIFICATIONS

Operating Temperature [0 ; 40] °C
Storage temperature [5 ; 30] °C
Humidity 0 – 100 %

POWER REQUIREMENTS (AC and DC)

Operating power consumption < 500 W
Video of the installation of the AQG

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Datasheet

(2 MB)
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Publications

  • Gravity measurements below 10−9 g with a transportable absolute quantum gravimeter

    V. Ménoret, Pierre Vermeulen, Nicolas Le Moigne, Sylvain Bonvalot, Philippe Bouyer, Arnaud Landragin, Bruno Desruelle, « Gravity measurements below 10−9 g with a transportable absolute quantum gravimeter

    Nature Scientific Report, 8:12300 (2018)

    Gravimetry is a well-established technique for the determination of sub-surface mass distribution needed in several fields of geoscience, and various types of gravimeters have been developed over the last 50 years. Among them, quantum gravimeters based on atom interferometry have shown toplevel performance in terms of sensitivity, long-term stability and accuracy.

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  • The added value of time-variable microgravimetry to the understanding of how volcanoes work

    D. Carbone, M. P. Poland, M. Diament, F. Greco

    Earth-Science Rev. 169, 146 – 179 (2017)

    During the past few decades, time-variable volcano gravimetry has shown great potential for imaging subsurface processes at active volcanoes (including some processes that might otherwise remain “hidden”), especially when combined with other methods (e.g., ground deformation, seismicity, and gas emissions). By supplying information on changes in the distribution of bulk mass over time, gravimetry can provide information regarding processes such as magma accumulation in void space, gas segregation at shallow depths, and mechanisms driving volcanic uplift and subsidence.

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  • Geophysics From Terrestrial Time-Variable Gravity Measurements

    M. Van Camp, O. de Viron, A. Watlet, B. Meurers, O. Francis, C. Caudron

    Rev. Geophys. (2017)

    In a context of global change and increasing anthropic pressure on the environment, monitoring the Earth system and its evolution has become one of the key missions of geosciences. Geodesy is the geoscience that measures the geometric shape of the Earth, its orientation in space, and gravity field. Time-variable gravity, because of its high accuracy, can be used to build an enhanced picture and understanding of the changing Earth.

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  • Cold-atom absolute gravimetry

    F. Pereira dos Santos, S. Bonvalot

    Encyclopedia of Geodesy, pp 1-6 (2016)

    The principle of measurement of the gravitational acceleration by dropping atoms rose up in the 1990s. The first applications quickly revealed the promising alternative of coldatom gravimetry to classical free-fall techniques currently used to perform accurate and absolute measurements of the Earth gravity field.

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  • Stability comparison of two absolute gravimeters at their best capabilities: optical versus atomic interferometers

    P. Gillot, O. Francis, A. Landragin, F. Pereira Dos Santos and S. Merlet

    Metrologia 51, L15-L17 (2014)

    We report the direct comparison between the stabilities of two mobile absolute gravimeters of different technology: the LNE-SYRTE Cold Atom Gravimeter (CAG) and FG5X#216 of the Université du Luxembourg. These instruments rely on two different principles of operation: atomic and optical interferometry.

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