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Quantum Physics' Wave-Particle Duality Enigma Bleeds Into the Classical World

New experiments demonstrate one of the defining oddities of quantum mechanics scaling up to the size of atoms.
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A group of Australian physicists has demonstrated for the first time that even atoms, relatively massive particles hundreds of million times larger than electrons (allowing for electrons to take up space at all, anyhow), exhibit the strangest properties of wave-particle duality—that is, when a particle acts as both a deterministic point and a probabilistic wave, depending on how and when we observe it. More specifically, the researchers performed a version of John Wheeler's canonical "delayed choice" thought experiment, which attempts to pin down when exactly a quantum particle "decides" whether it's going to act like a wave or point.

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The Australian group, led by optical physicist Andrew Truscott, describe their latest experiments in the current issue of Nature Physics.

So, the classic demonstration of quantum oddness goes like this. Particles are fired from an emitter at some barrier with two slits cut into it, side by side. Behind the slits is a surface that records where the particles arrive. In this basic configuration, the particles behave as waves rather than point-like, deterministic objects.

We know this because the final barrier, where the particles are detected, reveals an interference pattern characteristic of two waves meeting. As waves, individual particles don't choose one slit or the other, they pass through both, as we'd expect. On the other side, the waves that passed through each slit recombine. The effect is in essence of a particle interfering with itself, which is really strange, and leads to the popular interpretation that a single particle is somehow in many places at once.

As Richard Feynman famously quipped, wave-particle duality is the "mystery that cannot go away."

To really demonstrate wave-particle duality, the double-slit experiment is modified such that there's a measurement apparatus placed on the emitter side of the slits. The particle is then measured before it goes through the slits, in which case the interference pattern vanishes and the particle just acts like a particle, registering in one place at one time, as expected. The conclusion is that measurement—observation, generally—forces the probabilistic particle-wave to "choose" where it actually is, confining it to a discrete point. The nature of this choice is a deep question.

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For example, when does a particle choose where it will be at a given time? Does it always know, or have some complete knowledge of the experiment beforehand? Or does it only know at the moment of measurement? How important is the observer, really? In Wheeler's thought experiment, and the current IRL experiments, the decision of whether or not to measure the particle is made only after the particle is emitted.

"How does light know when to display wave-like or particle-like properties?," writes Drexel University's Allyson O'Brien in a pretty good paper explaining the whole thing. "One popular answer to this was that light can 'sense' what the experiment is attempting to measure. Based on the initial 'feel,' light would decide whether or not it will display wave-like or particle-like behavior before entering the experiment. This hypothesis is aptly named the conspiracy theory."

So, what happens if we try to fake the particle out, setting up the experiment such that the particle is going to be measured at one of the slits, and then, after the particle has been emitted, not actually making the measurement? If the interference pattern is seen, and the particle has passed through both slits as a wave, then it would have had to have made a new decision about its behavior, "sensing" the new state of the apparatus. It's as if the particle rewrites its own past, re-deciding its own fate.

Imagine that double-slit experiment, but with the possibility of one slit being blocked. If both are open, and an interference pattern results, clearly we're observing wave-like behavior and we can surmise that the particle decided to act like a wave when it was first emitted. If only one slit is open, and there is no interference pattern, we can surmise that the particle decided to be a particle again at the outset.

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The weirdness is what happens when the experimenter suddenly closes or opens a slit, and it's here that we should be able to see when exactly a particle becomes one or the other.

It's only been recently that Wheeler's thought experiment has been realized IRL. It's manifested not by actual slits, but by using an interferometer. A particle is fired first at a beam-splitter, where there is a 50-50 chance of it going in one way or the other. In this configuration, detectors can be set up in such a way to confirm these roughly equal chances. Half the particles will register at one, the other half at the other.

The experiment can be varied such that a second beam-splitter is introduced randomly, which has the effect of recombining the two paths. A particle behaving as a wave will choose both paths, as it would choose both slits, with the result being interference. A group of French physicists led by Institut d'Optique's Alain Aspect achieved this setup in 2007 with photons, observing that, bizarrely, if the second beam splitter is introduced even after a particle passes the first splitter—forcing the photon to choose a path—an interference pattern will be observed. It's as if the particle in the future is re-determining its own past to be wave-like: retrocausality.

That's a lot to take in given that we've barely mentioned the current experiments, but it's worth it to get the full (or fuller) sense of what's going on.

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"Recent advances in the trapping and cooling of atoms has led to the ability to readily observe wavelike phenomena with particles that have mass, such as the interference between two Bose–Einstein condensates," Truscott and his group write.

The catch, however, has been that experiments using heavy particles have usually meant that more than one particle is required to be in the apparatus at a time, which makes the whole idea moot since it's kind of hard to say "a single particle chose one path" for more than one object. The new work achieves the experiment with individual particles for the first time.

"Here we use atoms, which is an important distinction, since atoms have many internal degrees of freedom," the Australians write. "This allows coupling to the external environment through, for example, the atom's sensitivity to magnetic and electric fields. Moreover, an atom has significant mass, which allows strong coupling to gravitational fields. These interactions of the atom with its environment are required for the appearance of decoherence; thus, in this sense an atom can be thought of as a more classical particle than a photon."

"As such, our experiment tests Wheeler's ideas in a regime in which it has never been tested," Truscott and co. add.

Their findings aren't too surprising: the atoms made their particle-wave decision seemingly at the point of measurement, which is not the same thing as retro-causality, though that remains an unlikely alternative explanation. The interesting thing is mostly a defining aspect of the quantum world of the unimaginably tiny bleeding into our seemingly very cut and dried macro-scale world of complete determinism.

"Of course, in this kind of thing there is no more real surprise, but it's a beautiful achievement," Aspect, who led the initial Wheeler experiments with photons, told Physics World. "The fact that you can master single atoms with this degree of accuracy may be useful in quantum information."