Dichotomy of entanglement depth for symmetric states

Abstract

Entanglement depth characterizes the minimal number of particles in a system that are mutually entangled. For symmetric states, we show that there is a dichotomy for entanglement depth: an $N$-particle symmetric state is either fully separable, or fully entangled---the entanglement depth is either $1$ or $N$. This property is even stable under non-symmetric noise. We propose an experimentally accessible method to detect entanglement depth in atomic ensembles based on a bound on the particle number population of Dicke states, and demonstrate that the entanglement depth of some Dicke states, for example the twin Fock state, is very stable even under a large arbitrary noise. Our observation can be applied to atomic Bose-Einstein condensates to infer that these systems can be highly entangled with the entanglement depth that is of the order of the system size (i.e. several thousands of atoms).

We thank Geza Toth for bringing Refs. [16-18] into our attention. More comments are welcome