Can a small lump of metal be in a quantum state that extends over distant locations? A research team at the University of Vienna answers this question with a resounding yes. In the journal Nature, physicists from the University of Vienna and the University of Duisburg-Essen show that even massive nanoparticles consisting of thousands of sodium atoms follow the rules of quantum mechanics. The experiment is currently one of the best tests of quantum mechanics on a macroscopic scale.
Matter as a wave
In quantum mechanics, not only light but also matter can behave both as a particle and as a wave. This has been proven many times for electrons, atoms, and small molecules through double-slit diffraction or interference experiments. However, we do not see this in everyday life: marbles, stones, and dust particles have a well-defined location and a predictable trajectory; they follow the rules of classical physics.
At the University of Vienna, the team led by Markus Arndt and Stefan Gerlich has now demonstrated for the first time that the wave nature of matter is also preserved in massive metallic nanoparticles. The scale of the particles is impressive: the clusters have a diameter of around 8 nanometers, which is comparable to the size of modern transistor structures. With a mass of more than 170,000 atomic mass units, they are also more massive than most proteins. Nevertheless, quantum interference of these nanoparticles can be detected.
"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."
"Schrödinger's metal lump"
The scientists generate cold sodium clusters consisting of 5,000 to 10,000 atoms. These are sent through three diffraction gratings generated by ultraviolet laser beams. In the first laser beam, the location of each cluster is initially predetermined with a period of one ten-thousandth of a millimeter to an accuracy of around 10 nm, thus bringing it into a superposition of paths that the particle can take through the apparatus. When these possibilities superimpose at the end of the machine, a measurable striped pattern of metal is created, in good agreement with quantum theory.
This shows that the location of the particles is not fixed during unobserved flight. This delocalization is dozens of times larger than the size of each individual particle. Physicists refer to such states as Schrödinger cat states because they mimic a thought experiment by Austrian Nobel Prize winner Erwin Schrödinger. He considered whether it was possible to put a cat in a state in which it is both dead and alive. The analogy in the experiment: "every piece of metal is here and not here."
New scale achieved in the University of Vienna laboratory
A comprehensive theory on near-field interferometry has been formulated over the past two decades by Klaus Hornberger (University of Duisburg Essen) who is also a co-author of this study. Hornberger and Stefan Nimmrichter (then University of Vienna) introduced macroscopicity as a measure to make a wide variety of quantum experiments comparable, including nano-oscillators, atomic interferometers, and nanoacoustic resonators. Macroscopicity measures how strictly a quantum experiment can rule out even the smallest deviations from quantum theory.
In the new experiment, a value of μ = 15.5 has now been achieved. This is around one order of magnitude higher than in all other experiments worldwide to date. To achieve an equally rigorous test with electrons, their quantum superposition would have to be maintained for around 100 million years. The massive nanoparticles in the University of Vienna laboratory only needed around one hundredth of a second to do this.
Outlook and applications
The experiment is primarily designed to help us understand why quantum physics seems so strange, yet our everyday lives seem so 'normal'. In the future, even larger objects and other classes of materials will be investigated, which are expected to yield even better tests of quantum physics. In an improved infrastructure and using new equipment, the aim is to improve their own record by several orders of magnitude in the coming years. The Vienna interferometer is also a highly sensitive force sensor that can currently measure forces in the range of 10-26 N and will be even more sensitive in the future. This opens up new perspectives for precision measurements, such as electrical, magnetic, or optical properties of isolated nanoparticles—an exciting addition to established methods in nanotechnology.
