In a new study, physicists at the University of Colorado Boulder have used a cloud of atoms chilled down to incredibly cold temperatures to simultaneously measure acceleration in three dimensions—a feat that many scientists didn't think was possible.
The device, a new type of atom "interferometer," could one day help people navigate submarines, spacecraft, cars and other vehicles more precisely.
"Traditional atom interferometers can only measure acceleration in a single dimension, but we live within a three-dimensional world," said Kendall Mehling, a co-author of the new study and a graduate student in the Department of Physics at CU Boulder. "To know where I'm going, and to know where I've been, I need to track my acceleration in all three dimensions."
The researchers published their paper, titled " Vector atom accelerometry in an optical lattice ," this month in the journal Science Advances. The team included Mehling; Catie LeDesma, a postdoctoral researcher in physics; and Murray Holland, professor of physics and fellow of JILA, a joint research institute between CU Boulder and the National Institute of Standards and Technology (NIST).
In 2023, NASA awarded the CU Boulder researchers a $5.5 million grant through the agency's Quantum Pathways Institute to continue developing the sensor technology.
The new device is a marvel of engineering: Holland and his colleagues employ six lasers as thin as a human hair to pin a cloud of tens of thousands of rubidium atoms in place. Then, with help from artificial intelligence, they manipulate those lasers in complex patterns—allowing the team to measure the behavior of the atoms as they react to small accelerations, like pressing the gas pedal down in your car.
Today, most vehicles track acceleration using GPS and traditional, or "classical," electronic devices known as accelerometers. The team's quantum device has a long way to go before it can compete with these tools. But the researchers see a lot of promise for navigation technology based on atoms.
"If you leave a classical sensor out in different environments for years, it will age and decay," Mehling said. "The springs in your clock will change and warp. Atoms don't age."
Fingerprints of motion
Interferometers, in some form or another, have been around for centuries—and they've been used to do everything from transporting information over optical fibers to searching for gravitational waves, or ripples in the fabric of the universe.
The general idea involves splitting things apart and bringing them back together, not unlike unzipping, then zipping back up a jacket.
In laser interferometry, for example, scientists first shine a laser light, then split it into two, identical beams that travel over two separate paths. Eventually, they bring the beams back together. If the lasers have experienced diverging effects along their journeys, such as gravity acting in different ways, they may not mesh perfectly when they recombine. Put differently, the zipper might get stuck. Researchers can make measurements based on how the two beams, once identical, now interfere with each other—hence the name.
In the current study, the team achieved the same feat, but with atoms instead of light.
Here's how it works: The device currently fits on a bench about the size of an air hockey table. First, the researchers cool a collection of rubidium atoms down to temperatures just a few billionths of a degree above absolute zero.
In that frigid realm, the atoms form a mysterious quantum state of matter known as a Bose-Einstein Condensate (BEC). Carl Wieman, then a physicist at CU Boulder, and Eric Cornell of JILA won a Nobel Prize in 2001 for creating the first BEC.
Next, the team uses laser light to jiggle the atoms, splitting them apart. In this case, that doesn't mean that groups of atoms are separating. Instead, each individual atom exists in a ghostly quantum state called a superposition, in which it can be simultaneously in two places at the same time.
When the atoms split and separate, those ghosts travel away from each other following two different paths. (In the current experiment, the researchers didn't actually move the device itself but used lasers to push on the atoms, causing acceleration).
"Our Bose-Einstein Condensate is a matter-wave pond made of atoms, and we throw stones made of little packets of light into the pond, sending ripples both left and right," Holland said. "Once the ripples have spread out, we reflect them and bring them back together where they interfere."
When the atoms snap back together, they form a unique pattern, just like the two beams of laser light zipping together but more complex. The result resembles a thumb print on a glass.
"We can decode that fingerprint and extract the acceleration that the atoms experienced," Holland said.
Planning with computers
The group spent almost three years building the device to achieve this feat.
"For what it is, the current experimental device is incredibly compact. Even though we have 18 laser beams passing through the vacuum system that contains our atom cloud, the entire experiment is small enough that we could deploy in the field one day," LeDesma said.
One of the secrets to that success comes down to an artificial intelligence technique called machine learning. Holland explained that splitting and recombining the rubidium atoms requires adjusting the lasers through a complex, multi-step process. To streamline the process, the group trained a computer program that can plan out those moves in advance.
So far, the device can only measure accelerations several thousand times smaller than the force of Earth's gravity. Currently available technologies can do a lot better.
But the group is continuing to improve its engineering and hopes to increase the performance of its quantum device many times over in the coming years. Still, the technology is a testament to just how useful atoms can be.
"We're not exactly sure of all the possible ramifications of this research, because it opens up a door," Holland said.
