Quantum mechanics has radically changed our classical conception of
information and communication. The elementary unit of information, the
"bit", is replaced by the "qubit" which is a superposition of zero and one.
Several qubits can be entangled among each other so that each qubit alone is
neither zero nor one, nor any superposition of it. If qubits are used to encode
messages then these "quantum messages" do not behave like ordinary ones.
They cannot be compressed without loss of information, and the process of
reading disturbs their content. Such apparent weaknesses, however, give rise
to...