07 November 2019 Violation of Bell inequalities at Ising Quantum Critical Points

Artistic illustration of non-locality and quantum critical points

ICFO researchers report on the investigation of nonlocal correlations at quantum critical points. Bell inequality experiments are one of the most visible proofs of how quantum physics departs from classical physics. Entanglement is a necessary feature used in these experiments to produce the violation of such inequalities and in doing so, researchers use entangled particles that are put to test to observe and measure quantum correlation states.

Even though entanglement is the key to these experiments, temperature plays a major role since such quantum correlations are very fragile against thermalization. Thermalization seems to be detrimental to entanglement except at a special instance of equilibrium states, where multipartite entanglement is stabilized at all possible length scales. These instances in which this occurs are known as Quantum Critical points (QCPs).

At such so-called quantum-critical points, quantum entanglement develops at all length scales, leading the system in a thermodynamic regime with no analog in classical systems. To what extent do quantum-critical points evade a description in terms of classical physics?

In a study recently published in the Physical Review Letters, ICFO researchers Angelo Piga, Albert Aloy, Maciej Lewenstein and Irénée Frérot have shown that certain quantum-critical points cannot admit a description in terms of a whole class of classical theories. In order to do so, they relied on the concept of local-hidden-variable models, first introduced by John Bell 50 years ago to pinpoint the radically non-local predictions of quantum mechanics. Namely, they showed that the correlations between the microscopic constituents of the system violate a Bell inequality, which excludes the possibility to capture them within classical models, however exotic they are.

This fundamental insight into the structure of quantum-critical points opens a new window on a most-challenging topic for condensed-matter physics. For instance, certain models of high-Tc superconductivity rely on the presence of a quantum-critical point. The published paper indicates that describing the correlations in these systems cannot rely on effective classical models, but requires a quantum-mechanical description in its full glory.