Schrödinger’s cat has gotten a little fat. Physicists have created the largest ‘superposition’ ever – a quantum state in which an object exists simultaneously in a blur of possible locations.
A team based at the University of Vienna placed individual groups of about 7,000 atoms of sodium metal about 8 nanometers wide in a superposition of separate locations, each spaced 133 nanometers apart. Instead of shooting through the experimental set up like a billiard ball, each chunky cluster behaves like a wave, propagating in a superposition of spatially distinct paths and then interfering to form a pattern that researchers can detect.
“This is a fantastic result,” says physicist Sandra Eibenberger-Arias of the Fritz Haber Institute in Berlin.
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She points out that quantum theory places no limits on how large a superposition can be, but everyday objects obviously do not behave in a quantum way. This experiment – which places an object as huge as a protein or a tiny virus particle in a superposition – answers the question “Is there a transition between quantum and classical?” is helping to answer the big, almost philosophical question, she says. The authors “show that, at least for clusters of this size, quantum mechanics is still valid”.
The experiment, described in Nature Giulia Rubino, a quantum physicist at the University of Bristol, Britain, says January 21 also has practical significance. Quantum computers will eventually need to maintain potentially millions of objects in one large quantum state to perform useful calculations. “If nature collapses the system beyond a certain point, and that scale is smaller than the scale needed to build a quantum computer, then that’s problematic,” she says.
superposition size limit
Physicists have long debated how the classical, everyday world emerges from an underlying quantum state. “Quantum theory never says it stops working above a certain mass or size,” says Sebastian Padalino, a physicist at the University of Vienna and co-author of the study.
In 1935, Austrian physicist Erwin Schrödinger showed the absurdity of common interpretations of quantum mechanics with his famous cat-based thought experiment. The cat is put in a box with a vial of poison, which will be released when the radioactive atom decays. If the box remains isolated from its environment, the atom exists in a superposition of both decay and non-decay, and unless observed, the cat is an undefined state of both dead and alive.
In the real world, objects eventually become too complex or interact too much to maintain a superposition, an idea known as deformity. But there are also extensions of quantum mechanics, known as collapse theories, which suggest that beyond a certain point, a system will essentially reduce to a classical state, even in isolation. These theories were chosen by 4% of researchers as their preferred explanation of quantum mechanics in 2025 Nature survey. “The only way to answer this question is to scale up quantum experiments,” says Rubino.
To do this, Padalino and his team prepared a beam of clusters at 77 degrees Kelvin (-196 ºC) in an ultra-high vacuum. The researchers put the beam through an interferometer consisting of three gratings created from the laser beam. The first transmitted the groups through narrow gaps, causing them to spread out and travel in sync as waves; They then passed through a second set of slits that caused the waves to interfere in a specific pattern, which could be detected using the final grating.
laborious process
Such quantum effects are difficult to observe on a large scale, because stray gas molecules, light or electric fields can disrupt the delicate quantum state, and even the slightest misalignment of the grating or minute force can blur the fine interference patterns. Pedrolino says it took the team two years to see the signal. Before that, he says, he had spent “thousands of hours” in a basement laboratory looking at “flat lines and noise.”
The team’s superposition is ten times larger than the previous record. This is according to a measure called ‘macroscopicity’, which links mass with how long a quantum state lasts and how far apart the states are. However, that doesn’t mean it’s the largest mass ever put into superposition, Rubino says. In 2023, another team placed 16-microgram vibrating crystals in a superposition – but that was only over a distance of two billionths of a nanometer.
Moving forward won’t be easy, says co-author Stefan Gerlich of the University of Vienna. More massive particles have shorter wavelengths, making it difficult to distinguish quantum predictions from classical predictions. However, Gerlich says that 15 years ago, he thought today’s experiment was “not possible.”
The team is also working on putting organic matter through the same experimental set-up. Some viruses are similar in shape to clusters, but they are more fragile and can break into pieces during flight, making experiments difficult – though not impossible. “I think it’s not too far out of reach now,” says Padalino.
Although a virus is not considered alive, experiments with biological matter “will take the whole quantum interference into a new regime,” he added.
This article is reproduced with permission and was first published On 21 January 2026.
