Quantum sensor identifies underground tunnel

Sensitive gravity measurements are essential in fundamental explorations of the fine-structure constant and the gravitational constant. (See the article by Markus Arndt, Physics Today, May 2014, page 30 .) The sensors used for those measurements work by optically putting a cloud of atoms into a superposition of different momentum and energy states and letting them freely fall. The atomic wavepackets then move in two spatial paths before being recombined. The resulting interference pattern provides a sensitive measure of the local gravitational field.

Attempts have been made to construct gravity sensors that use minute changes in the surface-level gravitational field to probe subsurface density inhomogeneities for real-world applications, such as monitoring volcanoes and detecting underground cavities that might interfere with building projects. But such sensors’ practical use is limited, paradoxically, by their sensitivity: Long measurements are needed to cancel out microseismic vibrations. Mapping the local gravitational field at high resolution would simply take too much time.

Michael Holynski of the University of Birmingham in the UK and his colleagues have now demonstrated a solution to reducing the measurement duration. They prepared rubidium-87 atoms at microkelvin temperatures in two antiparallel magneto-optical traps. Instead of one cloud of atoms, the researchers used a single beam to simultaneously interfere two clouds, with one created a meter above the other. Comparing the two interference patterns reveals the vertical gravity gradient and almost entirely removes vibrational noise.

The researchers tested the detector, shown in the photo above, on a Birmingham street that had a 2-m-wide utility tunnel underneath. They took a measurement every half meter over an 8.5 m stretch. The results picked up not only tiny gravitational differences because of underground structures but also the gravitational pull of nearby buildings and local terrain. The researchers modeled the total expected signal (the dashed line in the graph).

The atom interferometer detected the expected gravitational variation, particularly the well-defined dip in the gravity gradient when just over the tunnel at position 0 in the plot. After some statistical analysis, the results also offered quantitative information about the placement of subterranean structures—in this case, that the center of the tunnel was measured to be at a depth of 1.89 (−0.59/+2.3) m.

Although the prototype’s sensitivity is only about 1/30 that of the best laboratory-based atom interferometers, its uncertainty is smaller by a factor of 1.5 to 4 than the uncertainties of commercial classical gravimeters. And if mounted on a rail or vehicle, it should be able to detect similar meter-scale subterranean structures with a 10-point line scan in as little as 15 minutes—comparable to the time needed for a single measurement with a classical gravimeter.

The potential applications are plentiful, such as searching for archaeological sites and mapping aquifers. The researchers already have suggestions for how to boost the sensitivity, which would decrease the measurement times even further and pick out smaller and deeper features than is possible currently. (B. Stray et al., Nature 602, 590, 2022 .)