At the Ludwig-Maximilian University of Munich, the basement of the physics building is connected to the economics building by nearly half a mile’s worth of optical fiber. It takes a photon three millionths of a second—and a physicist, about five minutes—to travel from one building to the other. Starting in November 2015, researchers beamed individual photons between the buildings, over and over again for seven months, for a physics experiment that could one day help secure your data.
Their immediate goal was to settle a decades-old debate in quantum mechanics: whether the phenomenon known as entanglement actually exists. Entanglement, a cornerstone of quantum theory, describes a bizarre scenario in which the fate of two quantum particles—such as a pair of atoms, or photons, or ions—are intertwined. You could separate these two entangled particles to opposite sides of the galaxy, but when you mess with one, you instantaneously change the other. Einstein famously doubted that entanglement was actually a thing and dismissed it as “spooky action at a distance.”
Over the years, researchers have run all sorts of complicated experiments to poke at the theory. Entangled particles exist in nature, but they’re extremely delicate and hard to manipulate. So researchers make them, often using lasers and special crystals, in precisely controlled settings to test that the particles behave the way prescribed by theory.