Quantum Network

A quantum network connects physically separate quantum nodes by distributing entanglement between them, typically using photons travelling through optical fibre or free space. It cannot work like a classical network, because the no-cloning theorem forbids copying an unknown quantum state — so a conventional amplifier or repeater cannot boost a fading qubit without destroying it. The workaround is the quantum repeater, which chops a long link into shorter segments, establishes entanglement across each segment, and then uses entanglement swapping to stitch the segments into one end-to-end entangled pair. Making this work requires matter-photon interfaces that reliably convert a stationary qubit (a trapped ion, a nitrogen-vacancy centre, a neutral atom) into a flying photonic qubit and back. A classical communication channel is always needed alongside the quantum one, since swapping and teleportation protocols depend on sending measurement outcomes. Remote entanglement has been demonstrated over metropolitan distances, but entanglement generation rates and fidelities remain orders of magnitude below what practical use demands. Note that quantum key distribution is a communication-security application of these ideas, not a computing one.