Vienna Team Puts 8-Nanometer Sodium Clusters in a Schrödinger Cat State, Smashing the Quantum Macroscopicity Record by an Order of Magnitude
Sodium nanoparticles of 5,000 to 10,000 atoms diffract through ultraviolet laser gratings, reaching macroscopicity μ = 15.5 in a Nature paper from the Arndt group.
Overview
Physicists at the University of Vienna and the University of Duisburg-Essen have sent metallic nanoparticles containing thousands of sodium atoms through a matter-wave interferometer and recovered an interference pattern, demonstrating that solid lumps of metal heavier than 170,000 atomic mass units can still occupy a quantum superposition. The result, published in Nature under the title Probing quantum mechanics with nanoparticle matter-wave interferometry, reports a macroscopicity value of μ = 15.5 — about an order of magnitude beyond what any previous interference experiment had reached, according to ScienceDaily.
What we know
The lead author is Sebastian Pedalino, a doctoral student in the Quantum Nanophysics group, with senior authors Markus Arndt and Stefan Gerlich of the University of Vienna and Klaus Hornberger of the University of Duisburg-Essen, as listed by PubMed. The paper appears in Nature volume 649, issue 8098, pages 866–870, with DOI 10.1038/s41586-025-09917-9.
The team prepared cold sodium clusters of roughly 5,000 to 10,000 atoms and sent them through three diffraction gratings generated by ultraviolet laser beams — an apparatus the group calls MUSCLE, for Multi-Scale Cluster Interference Experiment, the University of Vienna explains. Each cluster is approximately 8 nanometers across, with a mass exceeding 170,000 atomic mass units. The arrangement is a Talbot–Lau configuration and the particles travelled at around 160 metres per second with de Broglie wavelengths between 10 and 22 femtometres, as detailed by Physics World.
The central figure of merit is macroscopicity, a parameter introduced more than a decade ago to compare how strongly different interference experiments rule out hypothetical modifications to quantum theory at the boundary between the microscopic and the everyday. The Vienna result reaches μ = 15.5, a value the University of Vienna release describes as approximately one order of magnitude higher than any previous experiment. The PubMed abstract summarises the achievement as “quantum interference of sodium nanoparticles…in a Schrödinger cat state with a macroscopicity of μ = 15.5, surpassing previous experiments by an order of magnitude.”
Pedalino put the surprise in plain terms: “Intuitively, one would expect such a large lump of metal to behave like a classical particle. The fact that it still interferes shows that quantum mechanics is valid even on this scale and does not require alternative models,” he told ScienceDaily. The same outlet notes that the apparatus held each particle in superposition for about one hundredth of a second; matching that level of testing precision with single electrons, ScienceDaily writes, would require preserving electron superpositions for nearly 100 million years.
The experiment is also a sensitive force sensor. The University of Vienna release reports a force sensitivity of 10⁻²⁶ newtons, far below the gravitational pull a single atom exerts on its neighbour.
Why it matters
Matter-wave interferometry has been climbing the mass ladder for decades, from electrons and neutrons to atoms, molecules, and increasingly heavy macromolecules. Pushing the technique into the realm of metallic nanoparticles is qualitatively different: a sodium cluster of 8 nanometres is comparable in scale to features in modern semiconductor electronics and is heavier than many viruses. Demonstrating that such an object still behaves as a delocalised quantum wave — and not as a tiny ball with a well-defined position — is a direct test of whether standard quantum mechanics holds, or whether some yet-to-be-detected mechanism collapses the wavefunction once objects get large enough.
The macroscopicity score is the way the field keeps that score. A μ of 15.5 means the experiment rules out a substantially broader class of speculative collapse models than any previous interferometer had managed. Arndt framed the open question philosophically: “The interpretation of this phenomenon, the duality between this delocalization and the apparently local nature in the act of measurement, is still an open conundrum,” Physics World quoted him as saying.
What we don’t know
The paper does not claim to settle whether the wavefunction of arbitrarily heavy objects can be coherently delocalised; it only extends the empirical envelope. How far the MUSCLE platform can be pushed beyond the current cluster regime depends on the team’s ability to cool, control and detect ever heavier particles without losing coherence. The work was supported by the Gordon & Betty Moore Foundation (grant GMBF10771) and the Austrian Science Fund FWF (MUSCLE #32542-N), per the Vienna release; how quickly the platform scales beyond the current 170,000-amu envelope is the next experimental question.