Eur. Phys. J. Special Topics 227, 2069–2083 (2019)
https://doi.org/10.1140/epjst/e2018-800049-4

Review

Bell non-locality in macroscopic systems

¹ Centre for Quantum and Optical Science, Swinburne University of Technology, Melbourne, Victoria 3122, Australia
² School of Physics and Astronomy, University of Glasgow, Glasgow G12 8QQ, UK

^a e-mail: bdalton@swin.edu.au

Received: 12 April 2018
Received in final form: 18 July 2018
Published online: 13 February 2019

Abstract

The categorization of quantum states for composite systems as either separable or entangled, or alternatively as Bell local or Bell non-local states based on local hidden variable theory is outlined, focusing on simple bipartite systems. The significance of states demonstrating Bell non-locality for settling the long standing controversy between the Copenhagen interpretation of the quantum measurement process involving “collapse of the wave-function” and the alternative interpretation based on pre-existing hidden variables is emphasized. Although experiments demonstrating violations of Bell locality in microscopic systems have now been carried out, there is current interest in finding Bell non-locality in quantum systems on a macroscopic scale, since this is a regime where a classical hidden variable theory might still apply. Progress towards finding macroscopic quantum states that violate Bell inequalities is reviewed. A new test for Bell non-locality that applies when the sub-system measured quantities are spin components with large outcomes is described, and applied to four mode systems of identical massive bosons in Bose-Einstein condensates.