A Quantum Leap in the Classical World

Physicists have long struggled with a perplexing conundrum: Why do tiny particles such as atoms, photons, and electrons behave in ways that bacteria, bees, and bowling balls do not? In a phenomenon called quantum superposition, for example, individual units (say, of light) exist in two states at once. They are both waves and particles, only settling on one or the other if you specifically test for it.

This is not something that will happen to an object like your desk. It won’t turn solid when you set your coffee cup on it, or liquid if you try to drink it. Superposition has only been observed in the smallest units of matter, which made physicist Markus Arndt of the University of Vienna curious about where the line is. Does quantum weirdness stop at some particular size? If so, which?

To find out, Arndt and his team created a souped-up version of the famed double-slit experiment (see below), which can show whether individual particles are also behaving like waves. Then they worked their way toward increasingly massive objects. The synthetic molecules Arndt’s colleagues at the University of Basel in Switzerland developed for the study are the largest particles ever tested in such an experiment. Each contained as many as 2,000 atoms, according to research published in the journal Nature Physics. The molecules, which have a mouthful of a chemical formula (C707H260F908N16S53Zn4), “had to be massive, stable, and yet volatile enough to fly in a directed beam,” Arndt says.

Next, the scientists built a special instrument, a macromolecule interferometer called the Long-Baseline Universal Matter-​Wave Interferometer, or LUMI. With a baseline length of two meters, it’s the longest macromolecule interferometer ever built and is specially tuned to compensate for a number of technical challenges (for example, gravity and the rotation of the Earth).

Inside the interferometer, the team used a nanosecond laser pulse of light to propel the molecules through an ultra-high vacuum tube, which shot them toward a series of slotted barriers to reveal patterns in a screen behind. To Ardnt’s delight, the mammoth molecules created the same interference pattern as smaller objects. Though they were particles, they were also acting like waves.

In 1801, physicist Thomas Young conducted the first double-slit experiment, shooting a beam of light toward a barrier with two slits in it. Instead of forming two lines on a screen behind the barrier—in the same way that particles might—the beam formed a pattern of interference as if a wave had been pushed through the two slits. In 1908, Geoffrey Ingram Taylor repeated the experiment using a single photon. Even though the photon was a single particle, the wave interference pattern still appeared. That was strange enough, but then it got really weird: When scientists tracked the individual particles as they move through the slits, the monitored particles abandoned their wave-like state and showed up as two separate lines on the screen. It’s as if they knew they were being watched.

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

In the hunt for a connection between the quantum and classical world, Arndt aims to push the limits even further, testing larger and more massive particles. “Why not see how far you can go?” physicist Herman Batelaan of the University of Nebraska–Lincoln, who was not involved in the study, tells Popular Mechanics. “It’s a beautiful motivation to do this work.”

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