Quantum ensembles, as generalizations of quantum states, are a universal instrument for describing the physical or informational status in measurement theory and communication theory because of the ubiquitous presence of incomplete information and the necessity of encoding classical messages in quantum states. The interrelations between the constituent states of a quantum ensemble can display more or less quantum characteristics when the involved quantum states do not commute because no single classical basis diagonalizes all these states. This contrasts sharply with the situation of a single quantum state, which is always diagonalizable. To quantify these quantum characteristics and, in particular, to more clearly understand the possibilities of secure data transmission in quantum cryptography, based on certain prototypical quantum ensembles, we introduce some figures of merit quantifying the quantumness of a quantum ensemble, review some existing quantities that are interpretable as measures of quantumness, and investigate their fundamental properties such as subadditivity and concavity. Comparing these measures, we find that different measures can yield different quantumness orderings for quantum ensembles. This reveals the elusive and complex nature of quantum ensembles and shows that no unique measure can describe all the fundamental and subtle properties of quantumness.
