How to use entanglement for long-distance or free-space quantum communication

Entanglement, once called "spooky action at a distance" by Einstein, is the phenomenon in which the quantum states of separated particles cannot be described independently. This puzzling phenomenon is widely exploited in the quantum physicist's toolbox, and is a key resource for applications in secure quantum communication over long distances and quantum cryptography protocols. Unfortunately, entangled particles are easily disturbed by their surroundings, and their entanglement is readily dimini...