= 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.
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