  Classical and quantum information theory are simply explained. To be more
specific it is clarified why Shannon entropy is used as measure of classical
information and after a brief review of quantum mechanics it is possible to
demonstrate why the density matrix is the main tool of quantum information
theory. Then von Neumann entropy is introduced and with its help a great
difference between classical and quantum information theory is presented:
quantum entanglement. Moreover an information theoretic interpretation of
quantum measurement is discussed. Data compression, error correction and noisy
channel transmission are simply demonstrated for both classical and quantum
cases. Finally using the above theory quantum cryptography is reviewed and the
possibility of a commercial device realizing it is explored.
