Non-locality from nothing: paper in Nature Communications

Researchers at CQT and their collaborators have shown how to conjure something from nothing in the quantum world. Their work, published this week in Nature Communications, shows that quantum states with boringly classical behaviour on their own can be 'activated' in a group to display quantum theory's bizarre non-locality. The result may provide new clues to physicists struggling to understand where non-locality comes from.

Non-locality is the scientific description for one of the ways that quantum physics defies our intuition. It's what Albert Einstein referred to as "spooky action at a distance"—the apparent ability of quantum particles to coordinate their actions over vast distances without signals travelling between them. In modern research, non-locality is viewed as a resource for quantum technologies including cryptographic systems and random number generators. That provides a driver for understanding and learning to manipulate non-locality.

CQT's Daniel Cavalcanti, Mafalda Almeida and Valerio Scarani, and their collaborator Antonio Acin from the Institute of Photonic Sciences in Barcelona, Spain, started by considering a quantum state of some number of particles. They suppose that these particles have correlations explainable with classical physics—they do not show any non-locality. The researchers then imagine creating multiple copies of the same quantum state, overlapping to form a network. Surprisingly, the network can display non-local correlations even though it was built from local states."Each state has nothing of the property but together they have something," says Daniel.

Underlying the work is a fundamental question about the difference between non-locality and another quantum phenomenon, entanglement. Particles that are linked inseparably in a single quantum state are described as entangled. Non-locality can't exist without entanglement, but entanglement doesn't have to lead to non-locality, a split that has physicists puzzled.

The states that Daniel and his colleagues consider are entangled but local. Finding that these states become non-local when multiple copies are made rekindles the sense of connection between entanglement and non-locality. It may point the way towards a deeper understanding of how these phenomena are related.

The paper proves that activation of non-locality is possible for every quantum state that is is one-way entanglement distillable, plus some others. The researchers now want to test whether it's possible to activate the non-locality of any entangled-but-local quantum state. For further details see "Quantum networks reveal nonlocality", Nature Communications 2, 184 (2010); arXiv:1010.0900.