The Biggest Quantum Breakthrough Yet—Literally

In a remarkable new experiment, physicists have demonstrated quantum superposition, a state in which a particle is two separate places at once, in molecules.
Molecules are the largest particle ever observed to behave this way.
The researchers hope to test increasingly massive particles to further define the limits of quantum superposition.

Physicists have long struggled with a perplexing conundrum: How do we reconcile what we see in the quantum world with what we don’t in the classical world? In a phenomenon called quantum superposition, particles have been shown to shift between particle-like and wave-like states, meaning they’re in two places at once.

But this phenomenon hasn’t been observed with more massive objects—it’s only been seen in the smallest particles, such as atoms, photons, and electrons. That’s beginning to change.

Now, physicist Markus Arndt of the University of Vienna and an international team of researchers have demonstrated quantum superposition in molecules, the largest particles ever tested, according to research published yesterday in the journal Nature.

“The experiments we do show that very complex things can be prepared in states that you would never believe existed if you saw them for a billiard ball, a man, or a car,” Arndt tells Popular Mechanics. “But why should nature be different on a small scale? Is quantum physics not valid at the macroscopic scale?”

In the double slit experiment, which was first conducted in 1801, a wave of light is shot toward a barrier with two slits in it. The pattern of interference, which shows how these particles interact, is revealed on a screen behind the barrier. When light waves pass through the slits, they interact like ripples in water and leave a tiger-striped pattern on the screen at each point where they intersect.

But when particles of light, or photons, pass through alternating slits in the barrier, they strangely leave the same striped pattern. This suggests all of the photon’s possible paths may interfere with each other, even though only one single path is taken.

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This latest experiment proves that these comparatively mammoth molecules, a synthetic concoction of elements, each of which contained as many as 2,000 atoms, behave the same way. But the transition between states can be really complex, so the experiment called for sturdy particles. They “had to be massive, stable and yet fly in a directed beam,” Arndt says.

To ensure their experiment would work, the scientists built a special interferometer, called the Long-Baseline Universal Matter-Wave Interferometer, or LUMI. They used a green laser beam to propel the molecules into a tube, which shot them toward a series of slotted barriers to reveal patterns in a screen behind. In a sense, the design Arndt and his team use is a souped-up version of the double slit experiment.

With a baseline length of 2 meters, it’s the longest interferometer ever built and is specially tuned to compensate for a number of technical challenges, such as the Coriolis effect, which can interfere with the sensitive equipment.

This push and pull between what we know of the quantum and classical worlds has perplexed physicists for nearly a century. “Quantum superposition and interference are cornerstones of quantum physics,” says Arndt. “And yet, we never find ourselves in such states that we colloquially describe as an object being in two positions at once.”