Entanglement 25 years after Quantum Teleportation: testing joint measurements in quantum networks

TL;DR

This paper reviews 25 years of entanglement research, focusing on the less understood role of entanglement in joint measurements within quantum teleportation and networks, highlighting detection challenges.

Contribution

It emphasizes the poorly understood nature of entanglement in joint measurements and illustrates this through quantum network scenarios.

Findings

Joint measurements' quantumness remains elusive to current detection tools.

Entanglement in joint measurements differs from entanglement in teleportation channels.

Understanding of entanglement in measurement processes is limited.

Abstract

Twenty-five years after the invention of quantum teleportation, the concept of entanglement gained enormous popularity. This is especially nice to those who remember that entanglement was not even taught at universities until the 1990's. Today, entanglement is often presented as a resource, the resource of quantum information science and technology. However, entanglement is exploited twice in quantum teleportation. First, entanglement is the `quantum teleportation channel', i.e. entanglement between distant systems. Second, entanglement appears in the eigenvectors of the joint measurement that Alice, the sender, has to perform jointly on the quantum state to be teleported and her half of the `quantum teleportation channel', i.e. entanglement enabling entirely new kinds of quantum measurements. I emphasize how poorely this second kind of entanglement is understood. In particular, I use quantum networks in which each party connected to several nodes performs a joint measurement to illustrate that the quantumness of such joint measurements remains elusive, escaping today's available tools to detect and quantify it.