Physicists at UC Berkeley immobilized small groups of cesium atoms (pink blobs) in a vertical vacuum chamber, then split each atom into a quantum state in which half of the atom was closer to the weight of tungsten (the shiny cylinder) than the other half. (split spheres below tungsten). By measuring the phase difference between the two halves of the atomic wave function, they were able to calculate the difference in gravitational attraction between the two parts of the atom, which matched what would be expected from Newtonian gravity. Credit: Cristian Panda/UC Berkeley
The experiment captures free-falling atoms to look for gravitational anomalies caused by the Universe’s missing energy.
Researchers at UC Berkeley have increased the accuracy of gravity experiments using a atom interferometer combined with an optical grating, significantly extending the time that atoms can be kept in free fall. Although no deviations from Newtonian gravity have yet been found, 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 pushing the universe apart at an ever-increasing rate. Since then, scientists have searched for a new and exotic particle that causes the expansion.
Pushing the boundaries of this research, University of California, Berkeley physicists have now built the most precise experiment yet to look for small deviations from the accepted theory of gravity that could be evidence for 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 lattice to hold atoms in place, allowed researchers to immobilize freely falling atoms for seconds instead of milliseconds to look for gravitational effects, improve the current most accurate 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 lab
Exploring the quantum nature of gravity
Although the researchers found no deviation from what is predicted by the theory written by Isaac Newton 400 years ago, the expected improvements in the precision of the experiment may eventually yield evidence supporting or refuting theories of a hypothetical fifth force mediated by chameleons. or symmetric. .
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 probing gravity at the quantum level, said Holger Müller, 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,” Müller said. “It’s very hard to know if gravity is quantum, but if we can hold our atoms 20 or 30 times longer than anyone else, because the sensitivity ours increases by the second or fourth power of the retention time, we might have 400 to 800,000 times more chance of finding experimental evidence that gravity is indeed quantum mechanical.
An optical grating traps groups of atoms (blue discs) in a regular array so that they can be studied for more than a minute inside a grating atom interferometer. Individual atoms (blue dots) are placed in a quantum spatial overlap, that is, in two layers of the lattice at once, indicated by the elongated yellow bands. Credit: Sarah Davis
Applications and future directions in quantum sensing
In addition to precise gravity measurements, other applications of the lattice atom interferometer include quantum sensing.
“Atom interferometry is particularly sensitive to gravity or inertial effects. You can build gyroscopes and accelerometers,” said UC Berkeley postdoctoral fellow Cristian Panda, who is first author of a paper about gravity measurements to be published this week in the journal. Nature and is co-authored by Müller. “But it provides a new direction in atomic interferometry, where quantum sensing of gravity, acceleration and rotation can be done with atoms held in optical lattices in a compact package that is resilient to environmental imperfections or noise.”
Because the optical lattice holds the atoms in place rigidly, the lattice atom interferometer can also operate at sea, where sensitive gravity measurements 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 Saul Perlmutter, now a professor of physics at UC Berkeley, and a group of astronomers that included UC Berkeley postdoctoral fellow , Adam Riess. The two shared the Nobel Prize in Physics in 2011 for the discovery.
The realization that the universe was expanding faster than expected came from tracking distant supernovae and using them to measure cosmic distances. Despite much speculation by theorists about what’s actually driving space apart, dark energy remains an enigma—a big one, since about 70% of all matter and energy in the universe is in the form of dark energy.
In this picture, clusters of about 10,000 cesium atoms can be seen floating in a vacuum chamber, lifted by intersecting laser beams that create a stable optical lattice. A cylindrical tungsten weight and its support are visible at the top. Credit: Cristian Panda, UC Berkeley
One theory is that dark energy is simply the energy of the vacuum of space. Another is that it is an energy field called quintessence, which changes over 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 repulsive force over long distances, capable of repelling space. In a laboratory on Earth, with surrounding matter to shield it, the particle would have an extremely small range.
This particle has been called a chameleon, as if hiding in plain sight.
Advances in atomic interferometry techniques
In 2015, Müller adapted an atom interferometer to look for evidence of chameleons using cesium atoms released 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 on a heavy aluminum sphere, he and his team detected no deviation from what would be expected from the normal gravitational pull 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 that carries them different distances from a heavy tungsten weight that it hangs above. The higher moment, higher altitude state experiences more gravitational pull on 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 using the quantum properties of a particle, namely 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 interferes with them at the end,” Müller said. “The waves can either be in phase and add up, or the waves can be out of phase and cancel each other. 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, Müller and his colleagues added an optical lattice to keep the atoms close to the tungsten weight for a much longer time—an astonishing 20 seconds—to enhance the effect of gravity on the phase. The optical lattice uses two intersecting laser beams that create a stable array of lattice-like sites for atoms to collect, levitating in a vacuum. But was 20 seconds the limit, he asked?
During the height of COVID 19 pandemic, Panda worked tirelessly to extend hold time, systematically adjusting a list of 40 possible obstacles until concluding that tilting the laser beam’s motion, caused by vibrations, was a major limitation. By stabilizing the beam inside a resonant chamber and changing the temperature to be slightly cooler – in this case less than a millionth of a Kelvin above absolute zeroor a billion times colder than room temperature – he was able to extend the retention time to 70 seconds.
He and Müller published those results in the June 11, 2024, issue of Nature Physics.
Gravitational entanglement
In the newly reported gravity experiment, Panda and Müller traded a shorter time, 2 seconds, for a larger separation of wave packets of a few microns, or several thousandths of a millimeter. There are about 10,000 cesium atoms in the vacuum chamber for each experiment—scattered too thinly to interact with each other—scattered by the optical lattice in clouds of about 10 atoms each.
“Gravity is trying to push them down with a force a billion times stronger than their attraction to the mass of tungsten, but you have the restoring force from the optical lattice holding them up, kind of like a shelf,” Panda said. “Then we take each atom and split it into two wave packets, so now it’s at a superposition of two heights. And then we take each of those two wave packets and load them into a separate grid location, a separate rack, so it looks like a closet. When we turn off the lattice, the wave packets recombine and all the quantum information that was picked up during the hold can be read out.”
Panda plans to build his lattice atom interferometer at the University of Arizona, where he was just named an assistant professor of physics. He hopes to use it, among other things, to more accurately measure the gravitational constant that relates the force of gravity to mass.
Meanwhile, Müller and his team are building a new atomic lattice interferometer from scratch with better vibration control and a 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 Freedman and former postdoctoral fellow John Clauser. Clauser shared the 2022 Nobel Prize in Physics for that work.
Reference: “Measuring gravitational pull with an atom grating interferometer” by Cristian 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 undergraduate student Miguel Ceja of UC Berkeley, Justin Khoury of University of Pennsylvania in Philadelphia and Guglielmo Tino from the University of Florence in Italy. The work is supported by the National Science Foundation (1708160, 2208029), the Office of Naval Research (N00014-20-1-2656), and the Jet Propulsion Laboratory (1659506, 1669913).