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How Big Can Quantum Weirdness Get?

The principle of superposition is usually thought to apply to the tiniest particles, but physicists are making it scale up.
​Image: ​Tuncay/Flickr

​The thing in physics that seems to allow real things to exist as indeterminate thing-spaces—where some thing might in fact be many things—is known as a superposition. In cat terms, this would be where Schrodinger's poor feline is still unobserved in the box, with equal probabilities of being alive and dead, as a superposition of aliveness and deadness. ​It's both, really. IRL.

And yet, day to day, things seem pretty normal, and we don't go about our lives imagining the world beyond our currently-observed landscape as an indeterminate blurscape. Cats are reasonably alive or dead, and trees in a forest make a sound. Good. That's a good thing. It's when we get down to the level of individual particles that things get weird, and we don't have to worry about that very much because we're not particles, or, rather, we're vast collections of particles and can exist as well-behaved averages of particle states.

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The boundary between the realm of the very small, where things might exist in superpositions of many things, and our world of the very big and the very organized is not quite as settled as we'd like. Where's the limit? This is the question posed by Andrea Alberti and colleagues at the University of Bonn in Germany (and many others, of course): How big is too big? Could macroscopic objects (like cats!) find themselves in superpositions too? Hopefully not.

A couple of years ago, some physicists at the University of Vienna ​demonstrated that quantum decoherence and nonlocality (here and there) could be achieved using relatively huge molecular structures (compared to electrons, at least) filtered through the natural gratings of algae cell walls. Before that, physicists had demonstrated the same possibility using individual atoms, which are, again, relatively huge.

These things can be shown using the classic double-slit experiment, in which "large molecules are sent through a double slit and made to interfere with themselves," ​writes Tushna Commissariat at Physics World. Interference implies wavelike behavior, which is really the root of the superposition concept: waves and wavefunctions, where some thing is manifested as a sort of probablistic blur rather than the point-like existence we're used to (here is here, now is now).

The challenge facing Alberti and co. is in falsifying what's called macrorealism, which is just a pair of properties that can be reasonably said to define the macroscopic world (in contrast to the quantum world). First, for macrorealism to hold, it must not be possible to put macrorealistic objects in superimposed states. Second, macrorealistic objects can be measured without being influenced. That's the big thing in quantum systems—measuring the system collapses the system, turning a wave into a boring old point.

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"The [superposition] principle has been the source of heated discussions since the inception of quantum theory," Alberti and team ​write in the journal Physical Review X. "The central question of the long-standing debate is about the physical origin of the observed 'definiteness' of macroscopic physical objects. In fact, while it is widely accepted that microscopic systems can live in superposition states, the fact that in a physical apparatus individual measurements always yield single, definite outcomes has so far eluded a comprehensive explanation."

So, to test macrorealism for a given atom or molecule, Alberti and her group took a single cesium atom and put it in a superposition of superfine spin states; then, they excited it by sort of "smearing" it between a pair of optical beams, and, finally, used a fluorescent bath to illuminate the atom's position through time. Through the experiment, as the excited particle stumbles around in what they call a "quantum walk," three measurements are made, with the crucial one being the middle, which, if found to have disturbed the system, would indicate that the system is behaving macrorealistically.

Through many runs of the experiment, Alberti and co. found that the atom behaved in a way that broke the macrorealistic rules, which means it was following quantum rules and achieving a proper superposition of different states. "Our results rigorously excludes (i.e., falsifies) any explanation of quantum transport based on classical, well-defined trajectories," they conclude.

An atom is still an atom and not a cat, but any scaling up of quantum effects only adds to the intuitive strangeness of the superposition concept. Atoms have size, the universe builds things with them—whole giant things.

"Almost a century after the quantum revolution in science, it's perhaps surprising that physicists are still trying to prove the existence of superpositions," George Knee writes ​in an Americal Physical Society viewpoint. "The real motivation lies in the future of theoretical physics. Fledgling theories of macrorealism may well form the basis of the next generation 'upgrade' to quantum theory by setting the scale of the quantum-classical boundary. Thanks to the results of this experiment, we can be sure that the boundary cannot lie below the scale at which the cesium atom has been shown to behave like a wave."