Quantum communication

Quantum communication transmits or distributes quantum states and the correlations they carry between separated systems. An optical photon's polarization or an excitation shared between stationary qubits can serve as a carrier. Loss and noise are represented by a quantum channel and can degrade the transmitted state or its entanglement.

Quantum teleportation transfers an unknown state using shared entanglement, a local measurement and a classical message. The classical message is necessary, so the protocol does not send usable information faster than light. The no-cloning theorem prevents repairing an arbitrary unknown state by simply making redundant copies, motivating quantum error correction and carefully designed communication protocols.