Observing Quantumness: computable and experimentally useful measures of Entanglement

I discuss information-theoretic methods to estimate quantum correlations in many-body systems without performing expensive full state reconstruction.

Entanglement quantification, rather than mere detection, is believed to require reconstructing the full spectrum of quantum states, via experimentally challenging measurement sets that increase exponentially with the system size. This is troubling, because, for example, quantifying Entanglement is a way to establish that a device is truly quantum.

Here, I report an efficient theoretical method to evaluate quantum correlations with limited computational resources. Specifically, I present tight lower and upper limits to the bipartite Entanglement of a completely unknown quantum state in terms of global and local purities. The bounds are universally valid, because they allow for detection of quantum correlations in both pure and mixed states of any dimension. Also, they are analytically computable. Further, they are experimentally friendly, as their exact form is a function of directly observable quantities. Indeed, some of the reported theoretical results have already been experimentally validated in optical setups.