  We discuss quantitatively the complementarity of information transmitted by a
quantum system prepared in a basis state in one out of several different
mutually unbiased bases (MUBs). We obtain upper bounds on the information
available to a receiver who has no knowledge of which MUB was chosen by the
sender. These upper bounds imply a complementarity of information encoded via
different MUBs and ultimately ensure the security in quantum key distribution
protocols.
