Physicists at the University of California, Berkeley fixed a small chunk of cesium atoms (the pink blob) in a vertical vacuum chamber and split each atom into a quantum state. In this state, half of the atom was close to a tungsten weight (the shiny cylinder) and the other half was close to a segmented sphere below the tungsten. By measuring the phase difference between the two halves of the atomic wave function, they could calculate the difference in gravitational attraction between the two parts of the atom, which matches what you would expect from Newtonian gravity. Credit: Cristian Panda/University of California, Berkeley
The experiment captures atoms in free fall and looks for gravitational anomalies caused by missing energy in the universe.
Researchers at the University of California, Berkeley atom Combining interferometers with optical lattices can greatly extend the time that atoms can fall freely. Although we have not yet found any deviations from Newtonian gravity, these advances could reveal new quantum aspects of gravity and test theories about exotic particles such as chameleons and symmetrons.
26 years ago, physicists discovered dark energy, the mysterious force that’s causing the universe to accelerate at an ever-increasing rate. Since then, scientists have been searching for new, exotic particles that could cause the expansion.
Pushing the boundaries of this exploration, University of California, Berkeley Physicists have now built the most precise experiment ever to look for tiny deviations from conventional theories of gravity that could provide evidence of the existence of such a particle, which theorists have named the chameleon, or symmetron.
The experiment combined an atom interferometer for precise gravity measurements with an optical lattice to hold the atoms in place, allowing the researchers to freeze the free-falling atoms for seconds instead of milliseconds to study the effects of gravity, improving results by a factor of five over current most precise measurements.
![Dark Energy Experiment Laser Optical Bench](https://scitechdaily.com/images/Dark-Energy-Experiment-Laser-Optical-Bench-777x444.jpg)
The optical bench used in the experiment is illuminated by the violet light of an infrared laser. The laser is used to precisely control the quantum state of the cesium atoms in the vacuum chamber. Credit: Holger Müller Laboratory
Exploring the quantum nature of gravity
While the researchers found no deviations from what was predicted by the theory uncovered by Isaac Newton 400 years ago, future improvements in experimental precision may ultimately provide evidence to support or refute the theory of a hypothetical fifth force mediated by chameleons or symmetrons.
Because lattice atom interferometers can hold atoms for up to 70 seconds, and potentially 10 times longer, they also open up the possibility of probing gravity at the quantum level, says Holger Mueller, a professor of physics at the University of California, Berkeley. Physicists have well-tested theories that explain the quantum nature of three of the four forces of nature — electromagnetic force and the strong and weak forces — but the quantum nature of gravity has never been demonstrated before.
“Most theorists would probably agree that gravity is quantum, but nobody has seen any experimental evidence of it,” Muller says. “It’s very hard to even know if gravity is quantum, but if we could hold the atoms for 20 or 30 times longer than anyone else, then our sensitivity would increase with the square or fourth power of the holding time, so we might be 400 to 800,000 times more likely to find experimental evidence that gravity is indeed quantum mechanical.”
![Atoms in quantum superposition states in an optical lattice](https://scitechdaily.com/images/Atoms-Quantum-Superposition-Inside-Optical-Lattice-777x971.jpg)
An optical lattice traps a group of atoms (blue disks) in a regular array, allowing them to be studied for over a minute in a lattice atom interferometer. Individual atoms (blue dots) are placed in quantum spatial superposition, that is, simultaneously in two layers of the lattice, as indicated by the thin yellow bands. Credit: Sarah Davis
Applications and future directions of quantum sensing
Besides precision measurements of gravity, applications of lattice atom interferometers also include quantum sensing.
“Atom interferometry is particularly sensitive to gravitational and inertial effects. It can be used to make gyroscopes and accelerometers,” said Christian Panda, a postdoctoral researcher at the University of California, Berkeley, and lead author of a paper on the gravity measurements appearing in the journal Nature this week. Nature “But it gives a new direction to atom interferometry, allowing quantum sensing of gravity, acceleration and rotation to be performed with atoms held in an optical lattice in a compact package that is robust to environmental imperfections and noise,” said the paper, co-authored by Muller.
Because the optical lattice holds the atoms firmly in place, lattice atom interferometers can also operate at sea, where sensitive gravity measurements have been employed to create geological maps 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 the University of California, Berkeley) and a group of astronomers that included UC Berkeley postdoctoral researcher Adam Riess. 2011 Nobel Prize in Physics For discovery.
The realization that the universe is expanding more rapidly than it should have came about by tracking distant supernovae and using them to measure distances in the universe. While theorists have speculated about what is actually pushing space apart, dark energy remains a mystery. It’s a big mystery because about 70% of all the matter and energy in the universe is in the form of dark energy.
![Cesium atoms floating in an optical lattice](https://scitechdaily.com/images/Cesium-Atoms-Levitating-Optical-Lattice-777x930.jpg)
This photo shows a cluster of about 10,000 cesium atoms suspended in a vacuum chamber, levitated by crossed laser beams to form a stable optical lattice. A cylindrical tungsten weight and its support are visible on top. Credit: Christian Panda, University of California, Berkeley
One theory holds that dark energy is simply the vacuum energy of space. Another holds that dark energy is an energy field called quintessence that varies with time and space.
Another proposal is that dark energy is a fifth force, much weaker than gravity, mediated by particles that exert a repulsive force that varies with the density of the surrounding matter. In the empty void of space, dark energy can exert a repulsive force over long distances, expanding space. In a laboratory on Earth, the reach of dark energy is extremely limited due to the surrounding material shielding.
This particle has been dubbed a chameleon, as if it were hiding in plain sight.
Advances in atom interferometry
In 2015, Mueller modified an atom interferometer to look for signs of chameleons by firing cesium atoms into a vacuum chamber that simulated outer space. During the 10 to 20 milliseconds it took the atoms to rise and fall on the heavy aluminum sphere, Mueller and his team detected no deviations from what would be expected from the sphere and Earth’s normal gravity.
The key to testing gravity with free-falling atoms is the ability to excite each atom into a quantum superposition of two states. Each atom has a slightly different momentum, carrying it a different distance from a heavy tungsten weight hanging overhead. The state with higher momentum and higher altitude has a greater gravitational attraction to 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.
“Atom interferometry is the technology and science that exploits the quantum properties of particles – their properties as both particles and waves. We split the waves so that the particles take two paths at the same time, and then we make them interfere at the end,” says Muller. “The waves are either in phase and add, or out of phase and cancel each other out. The key is that whether they are in phase or out of phase depends very sensitively on the quantities we want to measure, such as acceleration, gravity, rotation, or fundamental constants.”
Pushing the boundaries of experimental physics
In 2019, Muller and his colleagues decided to enhance the effect of gravity on the phase by adding an optical lattice, keeping the atoms close to a tungsten weight for much longer (a whopping 20 seconds). An optical lattice uses two crossed laser beams to create a lattice of stable places for the atoms to gather while suspended in a vacuum. But was 20 seconds the limit?, he wondered.
At its peak COVID-19 During the pandemic, Panda worked tirelessly to extend the hold time, systematically revising a list of 40 possible obstacles and identifying vibration-induced wobble in the laser beam as the major limitation. By stabilizing the beam in a resonant chamber and adjusting the temperature a little lower (in this case, less than a millionth of a kelvin), they were able to reduce the hold time. absolute temperatureBy achieving a temperature one billion times lower than room temperature, the holding time could be extended to 70 seconds.
He and Muller The results were announced June 11, 2024 issue Natural Physics.
Gravitational entanglement
In the newly reported gravity experiment, Panda and Muller managed to separate the wave packets by a few microns, or a few thousandths of a millimeter, instead of for periods as short as two seconds. Inside the vacuum chamber of each experiment are about 10,000 caesium atoms, which are too sparsely distributed to interact with each other, and are dispersed by an optical lattice into clouds of about 10 atoms each.
“Gravity tries to push them down with a force a billion times greater than its gravitational force on a block of tungsten, but there’s a restoring force from the optical lattice that’s holding them like a shelf,” Panda said. “Then we split each atom into two wave packets – now they’re at two superimposed heights. We then load each of these two wave packets onto a different lattice site, a different shelf – that’s why it looks like a cupboard. When we turn the lattice off, the wave packets recombine, and we can read off all the quantum information that was captured during the holding.”
At the University of Arizona, where he was recently appointed assistant professor of physics, Panda will build his own lattice-atom interferometer, which he hopes to use to more precisely measure the gravitational constant, which links gravity to mass.
Meanwhile, Muller and his team are building from the ground up a new lattice-atom interferometer with better vibration control and cooler temperatures. This new instrument could produce results 100 times better than current experiments. Sensitive enough to detect the quantum properties of gravityIf successful, the planned experiment to detect gravitational entanglement would be similar to the first demonstration of quantum entanglement of photons, performed in 1972 by the late Stuart Friedman and a former postdoctoral researcher at the University of California, Berkeley. John CrowtherCrowther shared the 2022 Nobel Prize in Physics for this research.
Reference: “Measuring Gravitational Attraction with a Lattice Atom Interferometer,” Cristian D. Panda, Matthew J. Tao, Miguel Ceja, Justin Khoury, Guglielmo M. Tino, Holger Müller, 26 June 2024, Nature.
Publication date: 10.1038/s41586-024-07561-3
Other co-authors on the gravity paper are graduate student Matthew Tao and former undergraduate student Miguel Ceja, both of the University of California, Berkeley; University of Pennsylvania The research was conducted by Richard E. Schmidt of Philadelphia in collaboration with Guglielmo Tino of the University of Florence in Italy. This research 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).