The only mystery ...

The figure pictures the simplest example of parity games. Alice (A) flicks a certain number of marbles towards Bob (B), with the aim of learning whether the number of twisted tubes is even or odd. The players need four ordinary classical marbles to complete the task. In contrast, already two quantum marbles would suffice. Credit: © Borivoje Dakić

Richard Feynman is a hero of yours truly. Brilliant, irreverent, equipped with a wicked sense of puckish humor, he was a giant in physics without question, especially with his unique sense to ask questions of profound significance. 

As Richard Feynman famously put it, "the double slit experiment is absolutely impossible to explain in any classical way and has in it the heart of quantum mechanics. In reality, it contains the only mystery."

One of the most striking features of quantum mechanics is the superposition principle. This principle can be most easily illustrated via the double-slit experiment, which involves a particle that is sent through a plate pierced with two slits. According to our common everyday intuitions, one might expect the particle to always pass either through one slit, or through the other. However, quantum mechanics implies that the particle can in a certain sense pass through both slits at the same time, that is, it can be in a superposition of two locations at the same time. This possibility underlies the phenomenon of quantum interference, i.e. the striking wave-like behavior exhibited by quantum particles. Now, is there a way to quantify the degree to which quantum particles can be de-localized? Does quantum theory allow particles to traverse more than two paths at the same time? In order to understand these questions, physicists have analyzed "multi-slit experiments,", which differ from the double-slit experiment only in the number of slits: for example, a triple-slit experiment involves a particle sent through three slits.

The mystery deepens ...

One may think that if a quantum particle can pass through two slits at the same time, it should also be able to simultaneously pass through three, four, or any number of slits. Surprisingly, it was immediately noted that any pattern obtained in multi-slit experiments can be explained by the particle always passing through at most two slits at the same time. Even though this feature is mathematically fully understood, the following questions remain unanswered: is there a physical reason for the apparent asymmetry between the double-slit experiment and multi-slit experiments? What underlies this somewhat arbitrary limitation on the "delocalization" of quantum particles?

Something's afoot. - Sherlock Holmes