Physicists have demonstrated that even tiny chunks of metal can behave according to the strange rules of quantum mechanics, existing in states that spread across multiple locations at once. In a new study published in Nature, researchers from the University of Vienna and the University of Duisburg-Essen showed that metallic nanoparticles made of thousands of sodium atoms still display quantum behavior despite being far larger and heavier than particles typically used in such experiments.
The achievement represents one of the strongest tests yet of quantum mechanics on scales approaching the macroscopic world.
Quantum Behavior Beyond Tiny Particles
Quantum physics describes a world where matter can behave both like a particle and a wave. Scientists have repeatedly confirmed this unusual behavior in electrons, atoms, and small molecules using interference and double-slit experiments. But in daily life, ordinary objects such as rocks, dust, or marbles appear to follow the predictable laws of classical physics, staying in one place and moving along defined paths.
The Vienna research team, led by Markus Arndt and Stefan Gerlich, has now extended these quantum effects to much larger metallic nanoparticles for the first time. The sodium clusters used in the experiment measured roughly 8 nanometers across, similar in scale to modern transistor components. Each cluster also had a mass exceeding 170,000 atomic mass units, making them heavier than most proteins.
Even at that scale, the particles still produced measurable quantum interference.
“Intuitively, one would expect such a large lump of metal to behave like a classical particle,” says lead author and doctoral student Sebastian Pedalino. “The fact that it still interferes shows that quantum mechanics is valid even on this scale and does not require alternative models.”
Creating a “Schrödinger’s Metal Lump”
To perform the experiment, the researchers created ultracold sodium clusters containing between 5,000 and 10,000 atoms. The particles then traveled through three diffraction gratings generated by ultraviolet laser beams.
The first laser beam established the position of each cluster with an accuracy of about 10 nm and placed the particles into a quantum superposition, meaning they could follow multiple paths through the apparatus simultaneously. As these possible paths overlapped later in the experiment, they produced a detectable striped interference pattern that matched the predictions of quantum theory.
The results indicate that the particles did not occupy one fixed position during their flight. Instead, their quantum state spread over a region dozens of times larger than the particles themselves.
Physicists describe these conditions as Schrödinger cat states, referencing Austrian physicist Erwin Schrödinger’s famous thought experiment involving a cat that is simultaneously dead and alive until observed. In this case, the researchers describe the metal clusters as effectively being “here and not here” at the same time.
Record-Breaking Test of Quantum Mechanics
The theoretical foundation for this type of near-field interferometry has been developed over the past two decades by Klaus Hornberger (University of Duisburg Essen), who also co-authored the new study. Hornberger and Stefan Nimmrichter (then University of Vienna) previously introduced the concept of macroscopicity, a way to compare how strongly different experiments test the limits of quantum mechanics.
Macroscopicity allows scientists to evaluate experiments involving systems such as nano-oscillators, atomic interferometers, and nanoacoustic resonators by measuring how effectively they rule out even tiny deviations from standard quantum theory.
In the new experiment, the team achieved a macroscopicity value of μ = 15.5. According to the researchers, this is roughly an order of magnitude beyond previous experiments worldwide.
To match the same level of testing precision using electrons, scientists would need to preserve electron quantum superpositions for nearly 100 million years. The metallic nanoparticles in Vienna achieved this benchmark in only about one hundredth of a second.
Future Applications and Larger Quantum Experiments
Beyond testing the foundations of physics, the work may help researchers understand why quantum effects dominate the microscopic world while everyday objects appear normal and classical.
The team plans to investigate even larger particles and additional materials in future studies, potentially pushing these tests several orders of magnitude further. Improved experimental infrastructure and upgraded equipment are expected to make even more sensitive measurements possible.
The Vienna interferometer also functions as an extremely precise force sensor capable of detecting forces as small as 10-26 N. Researchers say future versions could become even more sensitive, opening possibilities for highly accurate measurements of electrical, magnetic, and optical properties in isolated nanoparticles. These capabilities could eventually support new advances in nanotechnology and precision sensing.
Researchers at the University of Vienna led by Markus Arndt and Stefan Gerlich carried out the study in collaboration with Klaus Hornberger from the University of Duisburg-Essen. The findings were published in Nature.
The experiment was substantially funded by:
- Der Gordon & Betty Moore Foundation grant GMBF10771
- Fonds zur Förderung Wissenschaftlicher Forschung, FWF, MUSCLE #32542-N
