Currently, in a telecoms network that is connected by fiber-optic cable the photons that are transmitted tend to be absorbed within a few kilometers by the material the cable is made out of. In order to avoid deterioration of the signal, repeaters are established at regular intervals to amplify it. Similar problems will occur with efforts towards quantum communications and the development of a quantum internet. Tomoyuki Horikiri and colleagues at Yokohama National University are tackling this issue by developing a new source of entangled photons. Their findings were published on August 12 in Communications Physics.
A pair of particles, or qubits, becomes entangled when the quantum state of each of them is inescapably connected to the quantum state of the other particle. Therefore, a measurement performed on one qubit will always be correlated to a measurement on the other qubit, regardless of the distance.
This entanglement, described in pop-science explanations as "spooky action at a distance," is key to any quantum communication infrastructure or quantum internet of the future.
Exploiting this spooky phenomenon, researchers can use entangled photons to transfer information between two locations. The sender has half the entangled photons and the receiver has the other half. The two users, for example, can establish a random secret bit string for encryption by the shared entanglement.