Conjurers frequently appear to make balls jump between upturned cups. In quantum systems, where the properties of an object, including its location, can vary depending on how you observe them, such feats should be possible without sleight of hand. Now this startling characteristic has been demonstrated experimentally, using a single photon that exists in three locations at once.
Despite quantum theory's knack for explaining experimental results, some physicists have found its weirdness too much to swallow. Albert Einstein mocked entanglement, a notion at the heart of quantum theory in which the properties of one particle can immediately affect those of another regardless of the distance between them. He argued that some invisible classical physics, known as "hidden-variable theories", must be creating the illusion of what he called "spooky action at a distance".
A series of painstakingly designed experiments has since shown that Einstein was wrong: entanglement is real and no hidden-variable theories can explain its weird effects.
But entanglement is not the only phenomenon separating the quantum from the classical. "There is another shocking fact about quantum reality which is often overlooked," says Aephraim Steinberg of the University of Toronto in Canada.
