28 November 2019 Witnessing multipartite entanglement depth

Schematic representation of an Entanglement Depth Witness

A team of researchers from ICFO, MPQ, and the Polish Academy of Sciences demonstrate that non-local correlations can quantify the strength of entanglement in a many- body system. Entanglement is one of the most characteristic phenomena of quantum physics. The simplest and most studied form of entanglement is the bipartite case, in which two subsystems form a quantum composite (e.g., two entangled particles). However, systems with more than two particles can exhibit entanglement in a whole plethora of ways, presenting a much richer and challenging case. Contrary to the bipartite case, multipartite entanglement admits a hierarchy of definitions depending on the strength of the correlations between the subsystems forming the quantum system. Therefore, as the quantum system becomes larger, it becomes more challenging to characterize the system.

In two simultaneous studies recently published in Physics Review Letters and Physics Review A, ICFO researchers Albert Aloy, Flavio Baccari, ICREA Professor at ICFO Antonio Acín and Maciej Lewenstein, in collaboration with former ICFO researchers Jordi Tura from Max-Planck-Institut für Quantenoptik and Remigiusz Augusiak from the Polish Academy of Sciences, provide a method to classify the different degrees of multipartite entanglement in a computationally and experimentally tractable way.

In their works, the researchers present a method to derive Device-Independent Witnesses of Entanglement Depth (DIWEDs) from Bell inequalities. Such DIWEDs quantify the strength of entanglement on a quantum many-body system by observing Bell non-local correlations. The difficultly of this approach lies in how one can derive these DIWEDs from Bell inequalities. The researchers have been able to come up with an elegant solution that reduces the issue to an efficient optimization problem. In particular, their methodology finds the maximal amount of Bell non-local correlations that can be achieved given any quantum system that has at most k particles entangled. This provides a hierarchy of certification bounds, since such values can only be surpassed by quantum systems that have more than k particles entangled.

In addition, they have also been able to show how the DIWEDs can be rewritten in terms of collective measurements, and then apply these DIWEDs on existing experimental data in order to certify an entanglement depth of 15 particles in a Bose-Einstein condensate of 480 particles.