An Atomic Gravity Gradiometer for Earth Gravity Mapping and Monitoring Measurements

IIP-10-0009

PI: Nan Yu, JPL

October 29, 2014

Task Co-Is: Jim Kohel, Robert Thompson, Xiaoping Wu, JPL Current team members: Sheng-wey Chiow, Jason Williams, and Dah-Ning Yuan, JPL Many others have been on the task and made significant contributions.

Gravity – Part of Whole Earth Science Measurements

Geodesy Earth and Planetary Interiors

–  Lithospheric thickness, composition –  Lateral mantle density heterogeneity –  Deep interior studies –  Translational oscillation between core/mantle

Earth and Planetary Climate Effects –  Oceanic circulation –  Tectonic and glacial movements –  Tidal variations –  Surface and ground water storage –  Polar ice sheets –  Earthquake monitoring

GOCE

N. Yu, J. M. Kohel, L. Romans, and L. Maleki, “Quantum Gravity Gradiometer Sensor for Earth Science Applications,” NASA Earth Science and Technology Conference 2002. Paper B3P5. Pasadena, California (June 2002).

Why Atomic Sensors?

σ−

σ+

σ+σ−

σ−

σ+

B field

coils

Laser-cooled Cs atom cloud at µK

Atomic beam

fringes

π/2 pulse π pulse

π/2 pulse

Atoms are stable clocks (laser-based atom optics) atom-wave interferometer

Freefall test mass Displacement Detection Atomic system stability

NIST atomic clock

+ +

•  Use totally freefall atomic particles as ideal test masses identical atomic particles are collected, cooled, and set in free fall in vacuum with no external

perturbation other than gravity/inertial forces; laser-cooling and trapping are used to produce the atomic test masses at µK and even pK; no cryogenics and little/no mechanical moving parts.

•  Matter-wave interference for displacement measurements displacement measurements trough interaction of lasers and atoms, pm/Hz1/2 when in space;

laser control and manipulation of atoms with opto-atomic optics. •  Intrinsic high stability of atomic system

use the very same atoms and measurement schemes as those for the most precise atomic clocks, allowing high measurement stabilities.

•  Enable orders of magnitude sensitivity gain when in space microgravity environment in space offers long interrogation times with atoms, resulting orders of

magnitude higher sensitivity compared terrestrial operations.

Inertial Phase Shifts in Atom Interferometers

Atomic fountain on ground

!ΔΦ = 2k a T2

With over 106 atoms, the shot-noise limited SNR ~ 1000. Per shot sensitivity = 2 × 10–10⁄ T2 m/s2.

Light-pulse atom interferometer accelerometer

π pulse

π/2" pulses

•  Independent of atom initial velocity.!•  The laser wavenumber k is the only

reference parameter.!•  Sensitivity increases with T2.!

Raman Beam

Counter-propagating Raman Beam

z

t

π/2

π

π/2

0 T 2T

Master laser II

Master laser I

Slave laser 2

Slave laser 1

Cs lock

Raman lasers

Amplifier 2D MOT Laser 1

2D MOT Laser 2

3D MOT Laser 5

3D MOT Laser 6

3D MOT Laser 1

3D MOT Laser 2

3D MOT Laser 3

3D MOT Laser 4

3D MOT

2D MOT

Gravity Gradiometer = Atom Interferometer (x2)

Upper Detector

3D MOT

2D MOT

Raman

Lower Detector

Stabilized Retro-Mirror

Single Chamber

Laser and Optical Subsystem

Raman

•  A ground transportable gravity gradiometer with the system design comparable with microgravity operation in space

LabVIEW Control

Data Acquisition/ Processing

Gradiometer CPU 2D MOT Coil Drivers

Timing Control

Control System

Frequency Control

Bias Field Coil Drivers

3D MOT Coil Drivers

IIP Gravity Gradiometer: Instrument Overview

2D MOT, 3D MOT, Repumps, Clearing beams

Fountain launch and detection.

The configuration for a space instrument will look like this without long fountain tubes.

Single Vacuum Chamber

Subsystem: Atomic Physics Package

Rack-mounted enclosures housing laser and optical system modules.

Subsystem: Laser and Optics

Repumper Laser module, complete

High Frequency AOM

module, x2 complete

Booster Laser Module, complete 

Slave module stack 

Laser module drawers

The optics and electronics rack contains all laser systems and electronics. Electronics has a combination of COTS, semi-custom parts, and in-house fabrications.

Subsystem: Electronics and Control

Added optical amplifier for a larger sized optical beam.

Instrument Analysis and Optimization

The plots show the expected and measured contrast of Raman fringes that impact the overall instrument performance.

Example: Atom interferometer contrast loss limitation and optimization. The fringe contrast is limited by the atom cloud residual thermal expansion and size of the laser beam.

Amplitude after Raman upgrade

0.2 0.3 0.4 0.5 0.6 0.70.2

0.3

0.4

0.5

0.6

0.7

Low

er N

EF

(a.u

.)

Upper NEF (a.u.)

Closed Loop Real-time Measurement

!  Applicable for space-based missions

!  Addressed SNR-dominant sensitivity discrepancy

!  First-order insensitive to fluctuations in the atom interferometer contrast and offsets.

0.2 0.3 0.4 0.5 0.6 0.70.2

0.3

0.4

0.5

0.6

0.7

Low

er N

EF

(a.u

.)

Upper NEF (a.u.) NEF: Normalized Excitation Fraction of atoms

Instrument Analysis and Optimization

1.0

0.8

0.6

0.4

0.2

0.0

Phas

e100806040200

Run #

Upper Lower

Run # S

igna

l (N

EF)

Transportable gradiometer Instrument Sensor Rack

Instrument Sensitivity Evaluation

•  40E @ 1s (E: Eotvos, 10-9/s) •  At the state-of-the-art reported in research lab experiments

0.2 0.3 0.4 0.5 0.6 0.7 0.80.2

0.3

0.4

0.5

0.6

0.7

0.8

Upper NEF (a.u.)

Low

er N

EF

(a.u

.)

1 5 10 50 100 500100012

51020

50100

Time HsL

GradientHEL

(Ellipse Fit)

Designed performance

(Closed loop)

Sensitivity Validation Measurements

•  Five lead bricks (total 33kg) were placed near the apparatus •  The instrument is sensitive to minute structural distortion due to additional mass

Vibration isolation platform

Mass supporting beam, independent of the instrument and platform

•  Disturbance to the instrument was minimized by supporting the mass from outside of the vibration isolation box

Stack of 5 lead bricks

Sensitivity Demonstration

•  Observed clear modulated closed loop signal of 36.4 (1.0) E •  Agrees with the estimate of test mass gradient signal of 34.4 (4.0) E (Error due to 1

cm positioning precision.)

3300

3280

3260

3240

3220

3200

3180

Grad

ient (

Eotv

os)

300020001000Time (s)

No extra mass With lead mass

Modulation of 5 lead bricks (~30 kg)Differential gravity gradient: 36.4 +/- 1.0 Eotvos

4

5

6

789

0.001

2

3

4

Alla

n De

viat

ion

(1/S

NR)

12 3 4 5 6 7 8 9

102 3 4 5 6 7 8 9

1002 3 4

Averaging Time (s)

Apex Upper Return Lower Return

Apex measurement: Atoms launched from the lower chamber, detected in the upper chamber when stationary at apex. No degradation in SNR with stationary clouds.

Detecting stationary clouds

400 420 440 460 480 5000.20.40.60.81.01.2

Time HmsL

SignalHVL

Releasing vs launching: Atoms released from the upper chamber rather than launched, detected in the lower chamber. No degradation in atom number by releasing.

Launched Released

Generating stationary clouds

Microgravity Operation Evaluations

Current Status

•  The JPL instrument is designed as a ground transportable gradiometer with an operation configuration compatible to space operation mode under microgravity condition.

•  The instrument is capable of operating continuously with a sensitivity within a factor of four of the designed performance. Implementation to achieve the remaining factor is underway.

•  The instrument sensitivity is current at the state of the art of atom-interferometer gravity gradiometers demonstrated in research labs.

•  We are actively investigating and developing a technology infusion path to space missions for Earth Science gravity measurements.