The potential of quantum communications lies in the phenomena called "entanglement." It involves entangling two or more subatomic particles, like photons, which are connected even at vast distances. Moreover, these subatomic particles have correlated behaviors, such as polarization and spin. And if you know the state of one particle, you know the state of the other connected particle, no matter how far away it is. Thus, quantum entanglement portends huge advances for communications and cryptography.
Recently, NIST researchers used two, time-synchronized and distant quantum-networked nodes to entangle two photons at a third node. The results were reported in White Rabbit-assisted Quantum Network Node Synchronization with Quantum Channel Coexistence and published by the Conference on Lasers and Electro-Optics in May 2022.
Entangling two photons is much easier in a lab than when using two quantum nodes kilometers apart and connected by fiber network. The two photons must arrive at the third node within a fraction of the time the photons need for entanglement; depending on conditions, this can be several nanoseconds, picoseconds, or even femtoseconds.
To synchronize the two nodes, NIST researchers used an Ethernet-based time transfer system called "White Rabbit," with a "leader rabbit' and "follower rabbit" at the two nodes. The White Rabbit system repeatedly helped synchronize the two nodes to within four picoseconds and enabled photon entanglement with greater than 90 percent probability. Additionally, the experiments showed that quantum and White Rabbit signals can coexist on the same fiber.