GRASIAN: Shaping and characterization of the cold hydrogen and deuterium beams for the forthcoming first demonstration of gravitational quantum states of atoms

Gravity is the force we feel every moment, the invisible tether that keeps our feet on the ground and the oceans in their basins. For centuries, we have understood gravity as a smooth, continuous pull, a rule that applies equally to a falling apple and a drifting cloud. But in the quantum world, where the rules of physics change for the smallest particles, gravity behaves differently. It does not pull smoothly; instead, it forces particles into specific, discrete levels of height, much like a ladder where you can stand on a rung but never in the space between them. These are called gravitational quantum states. While scientists have already seen this strange behavior in neutrons, a new effort is now trying to catch it in atoms, a feat that could reveal whether gravity works the same way for all matter and even for antimatter.

The GRASIAN collaboration, a team of physicists from Austria, Switzerland, Finland, and France, has been building a specialized machine to observe these quantum states in hydrogen atoms. Their goal is to create a beam of hydrogen that moves so slowly and smoothly that the atoms can settle into these invisible rungs of gravity. To do this, they must cool the atoms to near absolute zero and guide them through a series of narrow openings, filtering out any that are moving too fast or too erratically. The challenge is immense because the atoms are incredibly light and difficult to control, and the signal they produce is easily drowned out by background noise from stray gas molecules in the vacuum chamber.

In their latest work, the team reports significant progress in taming this chaotic environment. They began by shaping the hydrogen beam with a series of precision holes and slits, narrowing the stream of atoms to a width of just two millimeters. This collimation process acts like a sieve, allowing only the most orderly atoms to pass through. By carefully adjusting the height of these slits, the researchers can select atoms with very specific vertical speeds, filtering for those moving at just a few centimeters per second. This slow, steady motion is essential, as faster atoms would simply fly over the quantum rungs without ever settling into them.

However, even with a perfectly shaped beam, the experiment faced a major hurdle: background noise. The detectors were picking up signals from stray hydrogen atoms that had leaked into the chamber from the walls, making it difficult to distinguish the real signal from the noise. To solve this, the team first tried swapping hydrogen for its heavier cousin, deuterium. Because deuterium is heavier, it moves more slowly at the same temperature, and it is much rarer in the vacuum system, which naturally reduced the background noise. This switch improved the clarity of the signal, allowing the team to detect deuterium atoms moving at speeds below 95 meters per second.

The most dramatic improvement came when they redesigned the detection chamber itself. They installed a double-layered shield cooled by liquid helium, effectively turning the inside of the detector into a cryopump that freezes out stray gas molecules. This simple but powerful change eliminated the beam-related background almost entirely. With the noise suppressed, the team successfully detected hydrogen atoms moving at speeds as low as 72 meters per second, a threshold that brings them within striking distance of the speeds required to observe gravitational quantum states.

The researchers then used computer simulations to test whether their current setup is ready for the final experiment. They modeled how the atoms would behave as they passed through a gravitational spectrometer, a device designed to measure the height of these quantum states. The simulations showed that with the speeds they have now achieved, they should be able to see the distinct steps that mark the presence of gravitational quantum states. The team is now confident that their apparatus is ready to replace the current detection chamber with the final spectrometer. If successful, this experiment will mark the first time gravitational quantum states have been observed in atoms, opening a new window into the fundamental nature of gravity and potentially allowing scientists to test how gravity acts on antimatter in the future.