Researchers have smashed a new record in quantum mechanics, creating the largest ever ‘superposition’ – a strange state in which an object can exist in multiple locations at the same time.
To achieve this feat, a team at the University of Vienna put clusters of more than 7,000 sodium atoms into a superposition at locations 133 nanometres apart. That may not sound like much, but it’s more than 20 times the width of the nanoparticles themselves – a bit like a ping-pong ball being in two places around 80 centimetres apart, simultaneously.
The findings were published in the journal Nature.
Quantum mechanics describes how the Universe behaves at very small scales, where particles don’t follow the neat, predictable rules we see in everyday life. One of its strangest features is superposition, which was made famous by Erwin Schrödinger’s thought experiment involving a cat in a sealed box.
In Schrödinger’s scenario, a radioactive atom has a random chance of decaying, linked to a mechanism that releases poison and kills the cat if it does. Until someone opens the box to check, quantum theory says the atom exists in a superposition of decayed and not-decayed states – meaning the cat must be treated as both alive and dead at the same time.
This sounds ridiculous, because it is. We know cats are not both alive and dead at once, nor do everyday objects exist in multiple states simultaneously. Yet, in principle, quantum mechanics places no strict size limit on where superposition should stop working – which is exactly the tension Schrödinger was trying to expose.
“If you ask a quantum physicist, we assume standard quantum mechanics is universal,” Sebastian Pedalino, lead author of the new study and a PhD researcher at the University of Vienna, told BBC Science Focus. “There shouldn’t be any fundamental limits from the basic theory.”
So why don’t larger objects behave this way?
In practice, quantum effects are extremely fragile. Larger objects constantly interact with their surroundings, which rapidly destroys their delicate quantum states in a process known as decoherence.
But some physicists have proposed that something more fundamental might be going on. So-called 'collapse theories' suggest that beyond certain sizes or masses, quantum mechanics itself may start to break down, forcing objects to behave classically even in perfect isolation.
Such an interpretation would, in short, mean that our current understanding of quantum mechanics is incomplete and needs – scientifically – fudging.
Without experiments, there is no way to know which explanation is right. That’s where this new result comes in. By combining the mass of the object, how far apart the superposed positions are, and how long the state lasts, physicists can calculate a single score called ‘macroscopicity’ – a measure of how strongly an experiment tests the limits of quantum mechanics.
By that yardstick, this experiment beats previous records more than tenfold.
“We’ve tested quantum mechanics again,” Pedalino said. “We’ve proven that it still holds at this mass and size scale… Fundamentally, quantum mechanics does not state any size or mass limits, and that's what we tried to probe with this experiment.”
Pulling this off was a monumental technical challenge that took years to develop. The experiment relies on a two-metre-long machine known as an interferometer inside a vacuum chamber around six metres in length.
First, the researchers generate a beam of sodium nanoparticles and send it through three ‘gratings’ made not from solid material, but from ultraviolet laser light. Where the light is intense, particles are removed from the beam; where it is weak, they pass through – creating a kind of invisible mesh.
The first grating narrows the particle’s position, which, thanks to the uncertainty principle, makes its motion spread out like a wave. By the time it reaches the second grating, that wave is spread across multiple gaps at once – meaning the particle must be treated as taking several paths simultaneously. That is the superposition.
“You don't know which of the slits the particle or wave function went through – you have to assume it went through all of them, which makes up the interference pattern,” Pedalino said.
The third grating acts as a scanner. By sliding it across the beam and counting how many particles get through at each position, the team can map out the interference pattern – the tell-tale signature that the particles really were behaving like waves, not individual particles.
The sensitivity required to pull this off is mindboggling. The chamber must be kept at ultra-high vacuum to avoid collisions with stray gas molecules. The particles have to be cold enough that they don’t shed atoms mid-flight. Even the rotation of the Earth had to be corrected for, because it is enough to smear out the fragile pattern during the particle’s 10-millisecond journey through the device.
After years of seeing nothing but flat lines, the breakthrough finally came late one night in spring 2024.
“It was like looking for a needle in a haystack,” Pedalino said. “In the beginning, you don’t know if it’s noise. But then we found the interference signal. Once you have it, you can optimise, and it gets easier. We ended up with really nice sinusoidal curves. It was very relieving – and beautiful.”
Now the team are aiming even higher. They are already working on sending biological particles through the same setup. Small viruses, for example, are in roughly the same mass range as the sodium clusters used here.
Doing so would open up new ways of probing the physical properties of complex biological particles using exquisitely sensitive quantum techniques, Pedalino said.
And what if quantum mechanics keeps passing every test, no matter how large the objects get?
“It would mean standard quantum mechanics works,” Pedalino said. “It’s kind of boring: it works. But philosophically, it means quantum mechanics is universal.”
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