 in the state |1), the post-measurement state is the pure state /VB) = |1)B Either measurement in subsystem Quantum information theory A, thus, results in the collapse of subsystem B into a pure state. The same conclusion strictly applies if subsystem B is measured first. Thus, in all possible cases, a measure- ment of one subsystem collapses the other into a pure state, which, as we know, contains zero information. As we have seen in Chapter 18, quantum entanglement, as a characteristic property of the Bell-EPR states, can be used for the purpose of quantum teleportation. See more illustrations of the effect of quantum entanglement in the exercises. Quantum communication channel and Holevo bound We may now conceive a quantum version of Shannon's communication channel (Chapter 11), with the purpose of transmitting information from an originator entity to a recipient entity. Here, the words "communication" and "transmission" should not be interpreted in the engineering meaning of sending information through a wire from one point to another over some distance. Instead, the strict meaning is that of a message being communicated in some coded form, from an originator to a recipient, regardless of the physical transmission means, the overall channel being a quantum system. Let me now clarify how such a channel may be operated. As in the classical case, the information is encapsulated into a message X, which represents a succession of symbols (or letters, or codewords), X, to be encoded by the originator from a possible alphabet (or code) of size N. The message symbols are associated with a probability Px. After passing through a physical "transmission pipe," the symbols are then decoded by the recipient, to restore the classical information therein. In the quantum case, however, the information is to be carried by quantum states, which will now be referred to by their density operators Px. From the simplest perspective, the operation of encoding consists, for the originator, of the "preparation" of the quantum states,