2,000 "Schrodinger's Cats" break record for large-scale quantum superposition

The world of quantum mechanics, where particles can be in two places at once or entangled with each other across vast distances, sounds spooky to us living in the macroscopic world of classical physics. But where exactly the boundary between the two lies is still a mystery. Now physicists have blurred the line more than ever before, with a new experiment showing that massive molecules containing up to 2,000 atoms can exist in two places simultaneously.

The discovery was made using an advanced version of an experiment that’s been conducted countless times over the last 200 years – the double slit experiment. It was through this experiment that scientists came to understand the duality of light as both particles and waves.

The experiment sounds fairly simple. Light is beamed towards a surface that has two slits cut into it, and another surface behind it that the light ends up projected onto. If light was made up of only conventional particles, then the pattern on the rear surface would just appear in the shape and size of the slits. But waves of light bounce off each other like ripples in water, creating a kind of tiger-stripe pattern on the surface.

But the strangest thing is that even when the experiment is done with individual photons (or particles of light), the same striped pattern appears. Somehow, these photons don’t seem to be taking just one path as they might be expected to, but are traversing all of them at once and interfering with themselves.

This phenomenon is known as quantum superposition, and it’s most famously illustrated by Schrödinger’s Cat. In this thought experiment, a cat hidden in a box is neither alive nor dead, but exists as both at the same time. When the box is opened, this superposition collapses into one state or the other.

By the same token, it’s been said that if detectors were set up at the slits, so they were measuring which path the light was taking, the striped patterns would disappear. The fuzziness of the outcome clears up as soon as it’s measured.

But superposition only seems to apply in the quantum realm – as objects get bigger, it gets harder for this phenomenon to occur, and by the time you get up to the macroscopic scale it seems to disappear entirely. Even Schrödinger’s Cat needs a quantum link – the story often goes that there’s a radioactive atom in the box too, and the cat’s survival hinges on whether the atom decays or not.

So where is the limit? What’s the biggest object that can be in two places at once? Scientists at the Universities of Vienna and Basel are now closer to an answer, after conducting the double slit experiment with the largest molecules tested so far.

The previous record involved molecules containing over 800 atoms, but the team on the latest study managed to extend that up to 2,000 atoms.

The researchers specially designed molecules that were massive but still stable enough to be fired at the dual slits, forming a molecular beam in an ultra-high vacuum. Measuring the end result required a newly-designed matter-wave interferometer with a 2-m (6.6-ft) baseline.

Sure enough, these molecules were found to exist in a quantum superposition state and made to interfere with each other like ripples, for up to seven milliseconds at a time. That pushes the boundary closer to our macroscopic world than ever before, blurring the line between quantum and classical physics.

“Our experiments show that quantum mechanics, with all its weirdness, is also amazingly robust, and I’m optimistic that future experiments will test it on an even more massive scale,” says Yaakov Fein, lead author of the study.

The research was published in the journal Nature Physics.

Source: University of Vienna