Why does quantum mechanics work so well for microscopic objects, yet macroscopic objects are described by classical physics? This question has bothered physicists since the development of quantum theory more than 100 years ago. Researchers at Delft University of Technology and the University of Vienna have now devised a macroscopic system that exhibits entanglement between mechanical phonons and optical photons. They tested the entanglement using a Bell test, one of the most convincing and important tests to show a system behaves non-classically.
Ever since its inception more than 100 years ago, physicists realized that quantum theory might be in conflict with some of the basic axioms of classical physics. In particular, the principles in question are if information can be exchanged faster than the speed of light (called 'locality'), and whether physical quantities exist regardless of whether they are observed or not (called 'realism').Albert Einstein once famously asked Abraham Pais, his biographer, if he really thought the moon only existed when he looked at it.
A heated debate between Einstein and Niels Bohr on this conflict of axioms in the 1930sstarteddecades long research on the correlations between quantum systems. This phenomenon, called quantum entanglement, quickly crystallized as one of the key predictions of quantum mechanics. Work by John Bell in the 1960s opened up a route to test these principles experimentally, which added new and exciting results to the debate. Most quantum experiments performed to date, however, deal with either one or a relatively small number of particles.