Sending signals through fiber optic cable is reliable and fast, but because of internal absorption and other effects, they will lose photons—which is a problem when the number of photons being sent is small. This is of particular concern in quantum networks, which typically involve a small number of entangled photons. Direct transmission through free space (vacuum or air) experiences less photon loss, but it’s very difficult to align a distant receiver perfectly with the transmitter so that photons arrive at their destination.
