Two particles which interact with each other — like two photons passing through a beam splitter, for example — can sometimes remain connected, instantaneously sharing their physical states no matter how great the distance which separates them.
This connection is known as quantum entanglement, and is part of the branch of physics called quantum mechanics, a description of the way the world works at the level of atoms and particles that are even smaller.
Quantum mechanics says that at these very tiny scales, some properties of particles are based entirely on probability. In other words, nothing is certain until it happens.
Albert Einstein did not entirely believe that the laws of quantum mechanics described reality. He and other theoretical physicists postulated that there must be some hidden variables at work, which would allow quantum systems to be predictable.
In 1964, however, John Bell published the idea that any model of physical reality with such hidden variables also must allow for the instantaneous influence of one particle on another.
While Einstein proved that information cannot travel faster than the speed of light, two entangled particles can still affect each other when they are far apart, even on other sides of the Universe, according to Bell.
Today, while Bell entanglement is being harnessed in practical applications like quantum computing and cryptography, it has never been captured in a single image.
“The image we’ve managed to capture is an elegant demonstration of a fundamental property of nature, seen for the very first time in the form of an image,” said Dr. Paul-Antoine Moreau, a team member and the first author of a paper published in the journal Science Advances.
Dr. Moreau and colleagues devised a system which fires a stream of entangled photons from a quantum source of light at ‘non-conventional objects’ — displayed on liquid-crystals materials which change the phase of the photons as they pass through.
The researchers set up a super-sensitive camera capable of detecting single photons which would only take an image when it caught sight of both one photon and its entangled ‘twin,’ creating a visible record of the entanglement of the photons.
“It’s an exciting result which could be used to advance the emerging field of quantum computing and lead to new types of imaging,” Dr. Moreau said.
Paul-Antoine Moreau et al. 2019. Imaging Bell-type nonlocal behavior. Science Advances 5 (7): eaaw2563; doi: 10.1126/sciadv.aaw2563