Researchers Suppress Noise in Atom Gravimeters, Improving Resolution to 0.092 Radians

Scientists at the Wuhan Institute of Physics and Mathematics, in collaboration with Hefei National Laboratory, Wuhan Institute of Quantum Technology, Chinese Academy of Sciences, and University of Chinese Academy of Sciences, have developed a new method to suppress noise in dynamic atom gravimeters, substantially enhancing their precision. Wen-Zhang Wang and colleagues identified amplitude modulation noise (AMN) as a key factor limiting performance when conducting absolute gravity measurements on moving platforms. The research, focused on the behaviour of cold atomic clouds, demonstrates a reduction in gravity measurement noise from 2.69 mGal to 1.68 mGal, a significant improvement with implications for a range of sensing technologies. This advancement is vital for improving the precision of dynamic atom gravimeters and has potential applications in other atom interferometer-based sensors, including gradiometers and gyroscopes.

Cold atom velocity fluctuations identified and mitigated in mobile gravity measurements

Gravity measurement noise in moving devices reduced from 2.69 mGal to 1.68 mGal, a threshold previously unattainable due to limitations in suppressing amplitude modulation noise within moving atom gravimeters. Atom gravimeters operate on the principle of atom interferometry, where the wave-like nature of atoms exploits to measure acceleration due to gravity with high precision. However, when these instruments deploy on moving platforms, such as vehicles or aircraft, vibrations, accelerations, and other disturbances subject them to noise into the measurements. Understanding the origin of this noise, fluctuations in the velocity and trajectory of cold atoms distorting gravity readings akin to static on a radio signal, was key to the breakthrough. These cold atoms, typically rubidium or caesium, cool to extremely low temperatures, approaching absolute zero, to enhance their wave-like properties and minimise Doppler broadening. Any variation in their velocity during the interferometer sequence directly impacts the measured phase shift, and thus the accuracy of the gravity measurement. Modelling and fitting this noise to the instrument’s kinematic parameters, position, velocity, and acceleration, effectively decoupled it from the desired gravity signal, simultaneously improving fringe phase resolution from 0.244 rad to 0.092 rad. This improved resolution allows for more precise determination of the gravity gradient, enhancing the sensitivity of the instrument.

A correlation between this noise and the instrument’s kinematic parameters revealed through detailed analysis of field data, enabling suppression from 0.11 to 0.038. The researchers employed sophisticated signal processing techniques to isolate the AMN component from the overall noise spectrum. This involved analysing the frequency content of the noise and identifying components that were correlated with the motion of the platform. By constructing a model of the AMN based on the kinematic parameters, they were able to subtract it from the measured gravity signal. Clearer interference fringes observed during dynamic measurements, confirming this noise reduction; these shifted from blurred patterns at sea to smoother signals resembling those obtained in static conditions. The interference fringes are formed by the superposition of the atomic wave packets after they have been split, reflected, and recombined. Sharper, more distinct fringes indicate a higher signal-to-noise ratio and improved measurement accuracy. Histograms of the signals demonstrated a transition from broadened, unimodal shapes to sharper, bimodal peaks, indicating improved data quality. This change in the histogram shape reflects a reduction in the uncertainty of the gravity measurements, with the bimodal distribution indicating a more precise determination of the gravity value.

Despite successful suppression of amplitude modulation noise in the current apparatus, a key gap in understanding remained regarding performance in dynamic environments. The gravimeter achieved a standard deviation of acceleration below 0.001m/s² when moored, but long-term stability and performance require further investigation for continuous gravity mapping. Continuous gravity mapping, for example, could be used to monitor changes in subsurface density, track groundwater levels, or detect underground cavities. Cold atomic cloud trajectory and velocity variation induce amplitude modulation noise, a factor that previously limited performance. The AMN arises because the atoms are not perfectly stationary during the interferometer sequence. Any deviation from a perfectly defined trajectory introduces a phase shift that is indistinguishable from the gravity-induced phase shift. This effect is particularly pronounced in dynamic environments where the platform is subjected to accelerations and vibrations.

Laboratory demonstrations rarely translate perfectly to field work, acknowledging that improvements haven’t yet been validated under rapidly changing conditions. The complexities of real-world environments, such as unpredictable vibrations, temperature fluctuations, and magnetic field variations, can introduce additional noise sources that were not accounted for in the laboratory experiments. Successfully identifying and mitigating a significant noise source represents an important step forward for mobile gravimeters. Reducing measurement noise demonstrably enhances precision and broadens the scope of potential applications, including subsurface mapping and resource detection, opening avenues for more detailed geophysical surveys. For instance, improved gravity mapping can aid in the identification of mineral deposits, oil and gas reserves, and geological structures. Previously, fluctuations in the velocity and path of cold atoms caused amplitude modulation noise, a primary source of error limiting device sensitivity. Precision measurement now extends to moving platforms thanks to a new understanding of noise affecting atom gravimeters, and by modelling this noise and linking it to the instrument’s motion, the noise successfully suppressed, paving the way for more accurate mobile sensors. Future research will focus on refining the noise model, developing more robust signal processing algorithms, and testing the technique on different gravimeter designs and in more challenging dynamic environments.

The researchers successfully identified and suppressed a key source of noise, amplitude modulation noise, in dynamic atom gravimeters. This noise, caused by variations in the cold atomic cloud’s trajectory and velocity, previously limited the precision of gravity measurements on moving platforms. By modelling this noise and fitting it to kinematic parameters, they reduced dynamic gravity measurement noise from 2.69 to 1.68 mGal and improved fringe phase resolution from 0.244 to 0.092 rad. The authors intend to refine the noise model and test the technique on different gravimeter designs in more challenging environments.