Large objects, including cats, don’t show quantum effects. But physicists aren’t sure where the size limit lies. Credit: Getty
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 January1, 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.
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