  This article is an introduction to quant-ph/0302092. We propose to quantify
how "quantum" a set of quantum states is. The quantumness of a set is the
worst-case difficulty of transmitting the states through a classical
communication channel. Potential applications of this measure arise in quantum
cryptography, where one might like to use an alphabet of states most sensitive
to quantum eavesdropping, and in lab demonstrations of quantum teleportation,
where it is necessary to check that entanglement has indeed been used.
