Physicists have demonstrated that even tiny chunks of metal can exist spread out in multiple places at once, behaving according to the strange rules of quantum mechanics. In a new study published in natureResearchers from the University of Vienna and the University of Duisburg-Essen have shown that metal nanoparticles made of thousands of sodium atoms still exhibit quantum behavior despite being much larger and heavier than the particles typically used in such experiments.
This result represents one of the most powerful tests yet of quantum mechanics at a scale approaching the macroscopic world.
Quantum behavior beyond microparticles
Quantum physics describes a world in which matter can behave like both particles and waves. Scientists have repeatedly confirmed this unusual behavior in electrons, atoms, and small molecules using interference and double-slit experiments. However, in everyday life, ordinary objects such as rocks, dust, and marble appear to follow the predictable laws of classical physics, staying in one place and moving along defined paths.
A research team in Vienna led by Markus Arndt and Stefan Gerlich has extended these quantum effects to larger metal nanoparticles for the first time. The sodium clusters used in the experiment were approximately 8 nanometers in diameter, similar in size to modern transistor components. Each cluster also has a mass of more than 170,000 atomic mass units, making it heavier than most proteins.
Even at that scale, the particles still produced measurable quantum interference.
“Intuitively, you would expect such a large chunk of metal to behave like a classical particle,” says first author and doctoral student Sebastian Pedalino. “The fact that they still interfere shows that quantum mechanics is valid even at this scale and does not require alternative models.”
Creation of “Schrödinger’s Metal Block”
To conduct the experiment, the researchers created ultracold sodium clusters containing 5,000 to 10,000 atoms. The particles then pass through three diffraction gratings generated by an ultraviolet laser beam.
The first laser beam established the position of each cluster with an accuracy of about 10 nm, placing the particles in a quantum superposition. This means that particles can take multiple paths through the device at the same time. When these possible paths overlapped later in the experiment, they produced a detectable fringe interference pattern consistent with the predictions of quantum theory.
The results show that the particles did not occupy one fixed position during flight. Instead, the particle’s quantum state extends over an area tens of times larger than the particle itself.
Physicists describe these states as Schrödinger cat states, citing Austrian physicist Erwin Schrödinger’s famous thought experiment about a cat that is simultaneously dead and alive until observed. In this case, the researchers explain that the metal cluster is effectively “both here and not here.”
Record-breaking tests of quantum mechanics
The theoretical basis for this type of near-field interferometry has been developed over the past 20 years by Klaus Hornberger (University of Duisburg-Essen), who is also a co-author of the new study. Hornberger and Stefan Nimrichter (then at the 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 nanooscillators, atom interferometers, and nanoacoustic resonators by measuring how effectively they eliminate even small deviations from standard quantum theory.
In the new experiment, the team achieved a macroscopic value of μ = 15.5. The researchers say this is about an order of magnitude larger than any previous experiment conducted anywhere in the world.
To achieve the same level of test accuracy using electrons, scientists would need to preserve quantum superpositions of electrons for about 100 million years. Vienna’s metal nanoparticles achieved this criterion in just about one-hundredth of a second.
Future applications and large-scale quantum experiments
In addition to testing the fundamentals of physics, this research may help researchers understand why quantum effects dominate the microscopic world, even though everyday objects appear normal and classical.
The researchers plan to investigate larger particles and additional materials in future studies, which could push these tests several orders of magnitude further. It is expected that improved experimental infrastructure and equipment upgrades will enable even more sensitive measurements.
The Wien interferometer also serves as a highly accurate force sensor that can detect forces as small as 10 to 26 N. The researchers say future versions could be even more sensitive, allowing them to measure the electrical, magnetic and optical properties of isolated nanoparticles with high precision. These capabilities may ultimately support new advances in nanotechnology and precision sensing.
Researchers from the University of Vienna, led by Markus Arndt and Stefan Gerlich, conducted the study in collaboration with Klaus Hornberger from the University of Duisburg-Essen. The survey results are nature.
This experiment was substantially funded by:
- Dare Gordon and Betty Moore Foundation Grant GMBF10771
- Grant-in-Aid for Scientific Research FWF MUSCLE #32542-N
