  In this study, a distinctive feature of quantum computation (QC) is
characterized. To this end, a seemingly-powerful classical computing model,
called "stochastic ensemble machine (SEnM)," is considered. The SEnM runs with
an ensemble consisting of finite copies of a single probabilistic machine,
hence is as powerful as a probabilistic Turing machine (PTM). Then the
hypothesis--that is, the SEnM can effectively simulate a general circuit model
of QC--is tested by introducing an information-theoretic inequality, named
readout inequality. The inequality is satisfied by the SEnM and imposes a
critical condition: if the hypothesis holds, the inequality should be satisfied
by the probing model of QC. However, it is shown that the above hypothesis is
not generally accepted with the inequality violation, namely, such a simulation
necessarily fails, implying that PTM $\subseteq$ QC.
