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

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.Comment: Feature paper, Celebrating the Silver Jubilee of Teleportation (7 pages). V2 (March'19): Many typos corrected (sorry) and a few comments adde

How far can one send a photon?

The answer to the question {\it How far can one send a photon?} depends heavily on what one means by {\it a photon} and on what one intends to do with that photon. For direct quantum communication the limit is of about 500 km. For terrestrial quantum communication, near future technologies based on quantum teleportation and quantum memories will soon enable quantum repeaters that will turn the development of a world-wide-quantum-web (WWQW) into a (highly non-trivial) engineering problem. For Device Independent Quantum Information Processing, near future qubit amplifiers (i.e. probabilistic heralded amplification of the probability amplitude of presence of photonic qubits) will soon allow demonstrations over a few tens of km.Comment: Proceedings of QCMC 2014, Hefei. 6 pages Correction of an annoying typ

Optical Communication Without Photons

I analyse a recent quantum communication protocol by Salih et al. that allows one to communicate without any particle carrying the information from the sender to the receiver. I show how this can equally be achieved using classical communication.Comment: 2 page