Blog

The experiment captures atoms in free fall to look for gravitational anomalies caused by the universe’s missing energy.

Researchers at the University of California at Berkeley have improved the precision of gravity experiments using atom interferometer combined with an optical grating, which greatly extends the time that atoms can be held in free fall. Although no deviations from Newtonian gravity have yet been detected, these advances could potentially reveal new quantum aspects of gravity and test theories about exotic particles such as chameleons or symmetrons.

Twenty-six years ago, physicists discovered dark energy, a mysterious force that is pushing the universe apart at an ever-increasing rate. Since then, scientists have been searching for a new and exotic particle causing the expansion.

Crossing the boundaries of this search, University of California, Berkeley physicists have now built the most precise experiment yet to look for tiny deviations from the accepted theory of gravity that could be evidence of such a particle, which theorists have dubbed a chameleon or symmetron.

The experiment, which combines an atomic interferometer for precise gravity measurements with an optical array to hold the atoms in place, allowed the researchers to immobilize freely falling atoms in seconds instead of milliseconds to look for gravitational effects, surpassing the current most precise measurement by a factor of five .

The purple glow of an infrared laser illuminates the optical bench used in the experiment. The laser is used to precisely control the quantum states of cesium atoms in a vacuum chamber. Credit: Holger Müller Laboratory

Exploring the quantum nature of gravity

Although the researchers did not find a deviation from what was predicted by the theory laid out by Isaac Newton 400 years ago, the expected improvements in the precision of the experiment could eventually show evidence that supports or refutes theories of a hypothetical fifth force mediated by chameleons or symmetrons .

The lattice atom interferometer’s ability to hold atoms for up to 70 seconds — and potentially 10 times longer — also opens up the possibility of studying gravity at the quantum level, said Holger Müller, a professor of physics at UC Berkeley. While physicists have well-tested theories describing the quantum nature of three of the four forces of nature—electromagnetism and the strong and weak forces—the quantum nature of gravity has never been demonstrated.

“Most theorists probably agree that gravity is quantum. But no one has ever seen an experimental signature of this,” Muller said. “It’s very difficult to even know if gravity is quantum, but if we could hold our atoms 20 or 30 times longer than anyone else, because our sensitivity increases by the second or fourth power of the holding time, we could have 400 to 800,000 times greater chance of finding experimental evidence that gravity is indeed quantum mechanical.”

The optical grating captures groups of atoms (blue disks) in a regular array so that they can be studied for more than a minute in a grating atom interferometer. The individual atoms (blue dots) are placed in quantum spatial superposition, that is, in two layers of the lattice at once, indicated by the extended yellow bars. Credit: Sarah Davis

Applications and future directions in quantum sensors

Besides precise gravity measurements, other applications of the lattice atom interferometer include quantum sensing.

“Atomic interferometry is particularly sensitive to gravitational or inertial effects. You can create gyroscopes and accelerometers,” said UC Berkeley postdoctoral researcher Christian Panda, who is the first author of a paper on the gravity measurements that will be published this week in the journal Nature and co-authored with Muller. “But it provides a new direction in atom interferometry, where quantum sensing of gravity, acceleration and rotation can be done with atoms held in optical lattices in a compact package that is robust to environmental imperfections or noise.”

Because the optical lattice holds the atoms rigidly in place, the lattice atom interferometer can even work in the sea, where sensitive measurements of gravity are used to map the geology of the ocean floor.

Insights into dark energy and the chameleon particle

Dark energy was discovered in 1998 by two teams of scientists: a group of physicists based at Lawrence Berkeley National Laboratory, led by Sol Perlmutter, now a professor of physics at Berkeley, and a group of astronomers that included postdoctoral fellow Adam Rees of the University of California, Berkeley. The two shared the 2011 Nobel Prize in Physics for the discovery.

The realization that the universe is expanding faster than it should comes from tracking distant supernovae and using them to measure cosmic distances. Despite much speculation by theorists about what actually divides space, dark energy remains an enigma—a big enigma, since about 70% of all matter and energy in the universe is in the form of dark energy.

In this photo, clusters of about 10,000 cesium atoms can be seen floating in a vacuum chamber, levitated by intersecting laser beams that create a stable optical lattice. A cylindrical tungsten weight and its support can be seen at the top. Credit: Christian Panda, UC Berkeley

One theory is that dark energy is simply the vacuum energy of space. Another is that it is an energy field called quintessence that varies in time and space.

Another proposal is that dark energy is a fifth force, much weaker than gravity and mediated by a particle that exerts a repulsive force that varies with the density of the surrounding matter. In the void of space, it would exert a long-range repulsive force capable of spreading space apart. In a laboratory on Earth, with matter around to shield it, the particle would have an extremely short range.

This particle is called a chameleon, as if hiding in plain sight.

Advances in Atomic Interferometry Techniques

In 2015, Muller adapted an atom interferometer to look for evidence of chameleons using cesium atoms fired into a vacuum chamber that mimics the emptiness of space. During the 10 to 20 milliseconds it took for the atoms to rise and fall above a heavy aluminum sphere, he and his team found no deviation from what would be expected from the normal gravitational attraction of the sphere and Earth.

The key to using free-falling atoms to test gravity is the ability to excite each atom into a quantum superposition of two states, each with a slightly different momentum, which carries them different distances from a heavy tungsten weight hanging overhead. The higher momentum, higher altitude state experiences more gravitational pull on the tungsten, changing its phase. When the atom’s wave function collapses, the phase difference between the two parts of the matter wave reveals the difference in gravitational attraction between them.

“Atomic interferometry is the art and science of exploiting the quantum properties of a particle, that is, the fact that it is both a particle and a wave. We split the wave so that the particle takes two paths at the same time and then interfere with them at the end,” Muller said. “The waves can either be in phase and add up, or the waves can be out of phase and cancel each other out. The trick is that whether they are in phase or out of phase depends very sensitively on some quantity you might want to measure, such as acceleration, gravity, rotation, or fundamental constants.

Expanding the frontiers of experimental physics

In 2019, Muller and his colleagues added an optical lattice to hold the atoms close to the tungsten weight for a much longer time—an astonishing 20 seconds—to increase the effect of gravity on the phase. The optical array uses two crossed laser beams that create a lattice array of stable collection sites for atoms levitating in the vacuum. But is 20 seconds the limit, he wondered?

During the height of COVID 19 pandemic, Panda worked tirelessly to increase dwell time by systematically fixing a list of 40 possible obstacles until he determined that the swaying tilt of the laser beam caused by vibrations was a major limitation. By stabilizing the beam in a resonance chamber and adjusting the temperature to be slightly colder – in this case less than a millionth of the kelvin above absolute zeroor a billion times colder than room temperature—he was able to extend the retention time to 70 seconds.

He and Mueller published these results in the June 11, 2024, issue Natural physics.

Gravitational entanglement

In the recently reported gravity experiment, Panda and Müller traded a shorter time, 2 seconds, for a greater separation of wave packets down to a few microns, or a few thousandths of a millimeter. There are about 10,000 cesium atoms in the vacuum chamber for each experiment – too sparsely distributed to interact with each other – scattered by the optical array in clouds of about 10 atoms each.

“Gravity is trying to push them down with a force a billion times stronger than pulling them to the tungsten mass, but you have the restoring force from the optical array holding them up, sort of like a shelf,” Panda said. “Then we take each atom and split it into two wave packets, so now it’s in a superposition of two heights. And then we take each of those two wave packets and load them into a separate rack, a separate shelf, so it looks like a cabinet. When we turn off the grating, the wave packets recombine and all the quantum information that was acquired during the hold can be read out.

Panda plans to build his own lattice atom interferometer at the University of Arizona, where he has just been hired as an assistant professor of physics. He hopes to use it, among other things, to more accurately measure the gravitational constant, which relates the force of gravity to mass.

Meanwhile, Muller and his team are building from scratch a new lattice atom interferometer with better vibration control and lower temperature. The new device could produce results that are 100 times better than the current experiment, sensitive enough to detect the quantum properties of gravity. The planned experiment to detect gravitational entanglement, if successful, would be similar to the first demonstration of quantum photon entanglement, performed at UC Berkeley in 1972 by the late Stuart Friedman and former postdoctoral fellow John Clauser. Clauser shared the 2022 Nobel Prize in Physics for this work.

Reference: “Measurement of Gravitational Attraction with a Lattice Atom Interferometer” by Christian D. Panda, Matthew J. Tao, Miguel Ceja, Justin Khoury, Guglielmo M. Tino, and Holger Müller, 26 Jun 2024, Nature.
DOI: 10.1038/s41586-024-07561-3

Other co-authors of the gravity paper are graduate student Matthew Tao and former student Miguel Ceja of UC Berkeley, Justin Khoury of University of Pennsylvania in Philadelphia and Guglielmo Tino of the University of Florence in Italy. The work was supported by the National Science Foundation (1708160, 2208029), the Office of Naval Research (N00014-20-1-2656), and the Jet Propulsion Laboratory (1659506, 1669913).