Schrödinger’s cat just got a little bit fatter. Physicists have created the largest ever ‘superposition’ — a quantum state in which an object exists in a haze of possible locations at once.
A team based at the University of Vienna put individual clusters of around 7,000 atoms of sodium metal some 8 nanometres wide into a superposition of different locations, each spaced 133 nanometres apart. Rather than shoot through the experimental set up like a billiard ball, each chunky cluster behaved like a wave, spreading out into a superposition of spatially distinct paths and then interfering to form a pattern researchers could detect.
“It’s a fantastic result,” says Sandra Eibenberger-Arias, a physicist at the Fritz Haber Institute in Berlin.
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Quantum theory doesn’t put a limit on how big a superposition can be, but everyday objects clearly do not behave in a quantum way, she explains. This experiment — which puts an object as massive as a protein or small virus particle into a superposition — is helping to answer the “big, almost philosophical question of ‘is there a transition between the quantum and classical?’,” she says. The authors “show that, at least for clusters of this size, quantum mechanics is still valid”.
The experiment, described in Nature on 21 January, is of practical importance, too, says Giulia Rubino, a quantum physicist at the University of Bristol, UK. Quantum computers will ultimately need to maintain perhaps millions of objects in a large quantum state to perform useful calculations. If nature were to make systems collapse past a certain point, and that scale was smaller than what is needed to make 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 one. Quantum theory “never states it stops working above a certain mass or size,” says Sebastian Pedalino, a physicist at the University of Vienna and a co-author of the study.
In 1935, the Austrian physicist Erwin Schrödinger showed the absurdity of common interpretations of quantum mechanics with his famous cat-based thought experiment. The cat is put into a box with vial of poison, which will be released if a radioactive atom decays. If the box remains isolated from its environment, the atom exists in a superposition of both decayed and not-decayed, and until 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 decoherence. But there are also extensions to quantum mechanics, known as collapse theories, that suggest that beyond a certain point, a system will inevitably reduce to a classical state, even in isolation. These theories were picked by 4% of researchers as their favourite interpretation of quantum mechanics in a 2025 Nature survey. “The only way to answer this question is by scaling up” quantum experiments, says Rubino.
To do this, Pedalino and his team generated 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 constructed with laser beams. The first channelled the clusters through narrow gaps, from which they spread out and travelled in sync as waves; they then passed through a second set of slits that made the waves interfere in a distinctive pattern, which could be detected using the final grating.
Painstaking process
Viewing such quantum effects at scale is difficult, because stray gas molecules, light or electric fields can disrupt the delicate quantum state, and the slightest misalignment of the gratings or minute force can blur the fine interference pattern. It took two years for the team to be able to see the signal, says Pedalino. Before that, he spent “thousands of hours” in a basement laboratory looking at “flat lines and noise”, he says.
The team’s superposition is ten times bigger than the previous record. That’s according to a measure known as ‘macroscopicity’, which combines mass with how long the quantum state lasts and how separated the states are. However, this doesn’t mean it’s the largest mass ever put into a superposition, says Rubino. In 2023, another team put a 16-microgram vibrating crystal into a superposition — but that was only over a distance of two billionths of a nanometre.
Scaling up further will not be easy, says co-author Stefan Gerlich, also at the University of Vienna. More-massive particles have shorter wavelengths, which make it harder to distinguish quantum predictions from classical ones. However, Gerlich says that 15 years ago, he thought today’s experiment was “not possible”.
The team is also working on putting biological matter through the same experimental set-up. Some viruses are a similar size to the clusters, but they tend to be more fragile and can fragment during flight, which makes the experiment harder to do — although not impossible. “I think that it’s not so far out of reach anymore,” says Pedalino.
Although a virus is not considered to be alive, experiments with biological matter “would move the entire quantum interference into a new regime,” he adds.
This article is reproduced with permission and was first published on January 21, 2026.
