How far can one send a photon?

ABSTRACT

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.