Entanglement

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 diminished by the slightest interaction with the environment.