Probing the quantum Earth

01 May 2023
Mark Wolverton

At first thought, quantum physics and geophysics would seem to have nothing at all in common. The quantum world is all about the very small, the realm of the subatomic, the mysterious and paradoxical phenomena of spins, entanglement, and cats alive and dead at the same time. Geophysics, on the other hand, is about as tangible and large-scale as it gets: the planet we’re standing on and the immensity of its many moods, including earthquakes, volcanoes, and tsunamis. How can the two disciplines possibly be linked? Yet there’s a strong connection between the extremes of the quantum microworld and the planetary macroworld, one that researchers are using to study the inaccessible depths of the Earth.

One of the links between the quantum world and the macroworld involves gravity.  Although it’s not noticeable in our everyday lives, the force of gravity is not exactly the same everywhere on Earth. It varies from one place to another depending on local conditions such as the types and density of rock and other forms of mass. Geologists use these tiny variations to find water, minerals, and other materials, and to study volcanoes and fault lines and various other geological phenomena. Devices for measuring gravity, known as gravimeters, have been around for almost a century, but the traditional variety, generally based on a mass suspended on a spring, have some serious limitations.

“They’re prone to drift as the spring stretches over time and have differences between instruments due to manufacturing tolerances and so on, which means that different instruments have slight differences in their response,” explains Daniel Boddice, a civil engineer at the UK’s University of Birmingham. “Springs are also very sensitive to vibration in the ground, so-called microseismic noise, which can be from various sources such as wind, traffic and people movement, ocean waves, etc.”

Boddice is part of a research group at Birmingham that is taking a different approach to gravimetry, using a technique called cold-atom interferometry. This exploits the quantum physics principle of wave-particle duality, in which atoms and subatomic particles can behave both as particles and waves of light or other electromagnetic radiation. Atoms of a particular type are cooled by lasers to within a fraction of absolute zero and then placed into the strange quantum state known as superposition, in which they briefly exist in two states at the same time—much like the famous example of Schrödinger’s unfortunate cat, locked in a box and simultaneously alive and dead. The fleeting quantum states of the atoms are measured, and the slight differences in measurements mathematically combined to create an interference pattern from which information—in this case, gravimetric data—can be obtained. The idea of using supercooled atoms to measure gravitational fields by interferometry can be traced back about 30 years to the groundbreaking work of Nobel Prize-winning physicist and former US Energy Secretary Steven Chu.

Previous work in cold-atom interferometry was extremely promising, but essentially confined to the laboratory because of the bulky and sensitive equipment required. Although the technique avoids the problems of the traditional spring-based instruments, the extreme fragility and transience of the quantum phenomena involved in cold-atom interferometry has made practical application a huge challenge. 

Boddice and his colleagues are changing all that by perfecting an instrument that’s portable, robust, and small enough to take out into the field. Their quantum technology (QT) gravity gradiometer, described in a 2022 Nature paper, is a cylindrical instrument a little less than two meters high, mounted on a wheeled cart, with its controlling electronics also on wheels and tied to it by cable. Arranged in an hourglass configuration inside the instrument are two counter- oriented single-beam magneto-optical traps (MOTs), one aimed upward and the other downward. Clouds of supercooled rubidium atoms are dropped through the device, vertically separated by a meter, and measured simultaneously as they fall. Using two clouds of atoms as test masses not only adds valuable redundancy, but allows more sensitive measurements of the gravity gradient, the subtle differences between the two.

“There are several advantages of a QT gravity gradiometer based on cold-atom interferometry,” Boddice explains.  The two atom clouds used in the instrument have what’s called a uniform test mass—that is, they weigh exactly the same today or in a year.  “That makes the measurements extremely accurate and highly repeatable, even between different instruments,” says Boddice. The lack of mechanical parts also means less wear on the system and the functional drift during a survey is much lower. Using a single laser on atom clouds at two different heights “locks” the two clouds of atoms, suppressing microseismic vibrational noise, simplifying the measurements, and allowing more survey points to be collected in a shorter amount of time. “Taking a gradient is also less sensitive to the instrument being tilted, which relaxes the need to level the instrument as accurately in the field, which is especially challenging in certain ground conditions,” Boddice adds. And unlike other instruments, which can only take readings when stationary, the cold-atom technique can potentially take measurements even on a moving platform.

The Nature paper describes a successful field test of the Birmingham team’s instrument in which they detected a 2 x 2 m utility tunnel buried half-a-meter deep beneath the ground between two buildings. Their equipment accurately located it solely via the subtle gravitational effects the tunnel created, despite the possibly complicating interference, both gravitational and vibrational, from the nearby buildings, other structures, and passing vehicles. The test both validated the team’s previous computer models of how the QT gradiometer might operate, and it opened the door toward the development of a truly practical and wholly portable field instrument for a wide range of applications.

“We’re working towards other applications such as aquifer monitoring and planning application-specific field trials,” says Boddice, “especially for moving platforms such as railways, as well as how to integrate the instruments with other sensors.” For example, it might be possible to take a hybrid approach and combine highly accurate quantum sensors and cheaper, more ubiquitous MEMS devices.

Although the Birmingham team’s instrument will still need further refinement and development before it’s convenient and mobile enough for a surveyor’s toolkit, Boddice is enthusiastic about the promise of quantum technology gravimetry. “I would love to see quantum gravity sensors being used more widely and think they have the capability to revolutionize our understanding of underground space, enabling us to make better planning decisions. Gravity surveys have potentially the best resolution with depth of any geophysical technique,” he says.

While the technology has obvious applications for construction and civil engineering companies looking for hidden tunnels, pipelines, abandoned mineshafts, and other subterranean hazards, that’s only one possibility. QT gravity sensors could also find underground mineral and water deposits, help archaeologists locate buried artifacts without digging, and even be used by the military for underwater navigation. 

“I’d like to see QT gravity gradiometers used more proactively than reactively to manage the underground and do preventative maintenance on infrastructure,” Boddice notes. “I hope that by making gravity a faster and more reliable method for underground mapping, there will be a more widespread adoption and more opportunities to use the sensors to solve challenges for societal good.”

A quantum phase transition called spin crossover can be used to visualize deep-Earth processes like subducting tectonic plates. Photo credit: Nature Communications

While some investigators are using quantum physics to build instruments to directly measure and map the larger world, others are finding ways to use quantum phenomena to study the unseen depths of the planet. Renata Wentzcovitch, a condensed matter physicist at Columbia University, is using a particular quantum phenomenon found in certain minerals to help geophysicists understand and describe what’s happening deep in Earth, a thousand miles or more underneath our feet.

In 2003, a group of researchers found that iron in ferropericlase, a magnesium-iron oxide mineral that comprises the second largest component of the Earth’s lower mantle, displays a type of quantum phase transition called a spin crossover at certain extreme pressures and temperatures—the same conditions that exist in that region. Wentzcovitch specializes in ab initio materials simulations and began investigating the spin crossover in  ferropericlase,  in particular, how it worked and how it might be detected in the depths of Earth.

Without direct access to the realm thousands of miles inside our planet, geologists have to use indirect means to visualize those regions, mostly based on constructing tomographic and other images using seismological data, much as a CT exam provides pictures of the interior of the human body. Wentzcovitch’s calculations showed that the spin crossover in ferropericlase would affect the characteristics of lower mantle rocks in detectable ways, such as its compressibility. “Wherever the material exists, the compressibility of that region is affected,” Wentzcovitch explains. “It’s a unique fingerprint.”

Wentzcovitch’s work provided a firm theoretical basis for that fingerprint, showing that the spin crossover would affect the Fe-O bond length in ferropericlase and thus its particular properties, especially elasticity and compressibility. Those changes would consequently affect how P- and S-waves, the main types of seismological waves studied by geophysicists, propagate through the mineral in the inner Earth in ways that are directly related to earthquake and volcanic behavior, convection in the mantle, and tectonic plate motion. “In collaboration with my group, I developed a theory for the elastic properties, and we were able to predict the changes in compressive and shear wave speeds of ferropericlase at lower mantle conditions,” she says.

But theoretical predictions can be fiendishly difficult to isolate and detect, especially when dealing with something as delicate as quantum phenomena. Wentzcovitch started her investigations of the ferropericlase spin crossover in 2005, calculating every possible permutation of the phenomenon and how it might be detected. But it wasn’t until 2021 that she and her geophysicist collaborators could announce the actual detection of the ferropericlase spin crossover in the lower mantle. “The prediction of the velocity changes in lower mantle rocks is very, very subtle,” Wentzcovitch notes. She worked with seismologists and geodynamicists to search for these patterns of velocity changes in the lower mantle and identify them specifically in seismological data.

The identification of an exotic quantum phenomenon deep within our own planet is important not only in the understanding of Earth but also has implications reaching back to the origins of the solar system. An important open question in geophysics is related to the amount of ferropericlase in the lower mantle, says Wentzcovitch. If the composition of the lower mantle is the same as the upper mantle, “then the Earth’s  composition is different from the solar composition and that of primitive meteorites that formed the Earth.” New mineralogical/thermal models of the mantle based on tomographic images can show the amount and extent of ferropericlase in the lower mantle. “The answer to this question will shed light on the planetary formation process that produced Earth from the solar nebula,” Wentzcovitch says.   

From the origins of Earth, to its present-day and future moods and behaviors; from the infinitesimal realm of the quantum, to the scale of our entire planet, the work of these and other scientists is tying together the extremes of small and large, deep within Earth. Although quantum effects are rarely immediately obvious at the scale of our human perceptions, they still govern our world. One way or another, as Wentzcovitch points out, in the final analysis, everything is quantum.

Mark Wolverton is a freelance science writer and author based in Philadelphia.


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