Scientists at Goethe University, Frankfurt, have directly measured the zero-point motion of atoms within molecules prepared in their lowest quantum energy state—an achievement the team said was previously considered impossible.
Although zero-point motion was predicted by quantum physics due to another theoretical phenomenon called “zero-point energy,” classical physics still cannot explain how this motion can occur in systems lacking thermal energy.
“This zero-point motion is a purely quantum mechanical phenomenon that cannot be explained classically.” Professor Till Jahnke from the Institute for Nuclear Physics at Goethe University Frankfurt and the Max Planck Institute for Nuclear Physics in Heidelberg stated, in a press release announcing the team’s findings.
In classical physics, molecules with no thermal energy—such as those at absolute zero—should theoretically remain perfectly motionless. However, modern studies have shown that the atoms that make up molecules remain in motion even in this state. This zero-point motion has been attributed to zero-point energy, the minimum energy permitted by quantum mechanics even at the ground state.
Although zero-point motion and the theoretical energy driving this motion can be characterized by quantum physics, directly measuring this motion has been considered impossible. Along with the cost and complexity of cooling samples to absolute zero, the primary impediment to measuring zero-point motion is a concept in quantum physics known as the Heisenberg Uncertainty Principle. According to Heisenberg, it is impossible to measure both the location and the speed of quantum particles simultaneously.
“It’s like observing a dance without being able to see simultaneously exactly where someone is dancing and how fast they’re moving – you always must choose to focus on one,” the study authors explain.
The researchers also note that measuring the properties of multiple atoms within a molecule can be challenging, particularly for molecules containing two or three atoms. In iodopyridine, the molecule that the team chose to study, there are eleven atoms vibrating in 27 different modes, making the task even more complex.
To achieve the impossible, the team gained access to the world’s largest X-ray laser, the European XFEL in Hamburg, Germany. Using a technique called Coulomb Explosion Imaging, they directed the high-energy laser to bombard an iodopyridine sample with ultrashort, high-intensity X-ray pulses. These laser pulses triggered the molecules to undergo a controlled explosion, which allowed the team to capture high-resolution images of their underlying structure.
The team explained that a powerful X-ray pulse “knocks many electrons out of the molecule,” causing the atoms to become positively charged. This newly positive charge state causes the atoms to repel each other and fly apart “in a fraction of a trillionth of a second.” During that brief interval, a special apparatus measures the atom’s time of impact and its position, allowing the team to reconstruct the molecule’s original structure.
As hoped, the experiments captured the zero-point motion of the atoms. The team noted that by capturing this “dance of the atoms,” they revealed each atom’s precise choreography and something else unexpected: the atoms seemed to dance in a coordinated manner.
“The exciting thing about our work is that we were able to see that the atoms don’t just vibrate individually, but that they vibrate in a coupled manner, following fixed patterns,” Jahnke explained. “We directly measured this behavior for the first time in individual medium-sized molecules that were also in their lowest energy state.”
While the team’s imaging of zero-point motion was unprecedented, Jahnke said it has a long history. Specifically, the actual data used in the study was gathered six years earlier during an unrelated experiment.
“We originally collected the data in 2019 during a measurement campaign led by Rebecca Boll at the European XFEL, which had an entirely different goal,” he explained. “It wasn’t until two years later that we realized we were actually seeing signs of zero-point motion.”
Dr. Gregor Kastirke, who built a customized version of the COLTRIMS reaction microscope tailored specifically for the European XFEL, which was used to image the exploding atoms, said seeing the tool in action in such a significant way was a memorable experience.
“Witnessing such groundbreaking results makes me feel a little proud,” Kastirke said. “After all, they only come about through years of preparation and close teamwork.”
In the study’s conclusion, Jahnke notes the significant collaboration with colleagues from the Center for Free-Electron Laser Science in Hamburg. Benoît Richard and Ludger Inhester, who invented all-new analysis methods, “that elevated our data interpretation to an entirely new level.”
“Looking back, many puzzle pieces had to come together perfectly,” Jahnke added.
The team leader also noted that his team is continuously improving their method and is “already planning” the next series of zero-point motion experiments.
“Our goal is to go beyond the dance of atoms and observe in addition the dance of electrons—a choreography that is significantly faster and also influenced by atomic motion,” he explained. “With our apparatus, we can gradually create real short films of molecular processes—something that was once unimaginable.”
The study, “Imaging collective quantum fluctuations of the structure of a complex molecule,” was published in Science.
Christopher Plain is a Science Fiction and Fantasy novelist and Head Science Writer at The Debrief. Follow and connect with him on X, learn about his books at plainfiction.com, or email him directly at christopher@thedebrief.org.