  In a recent work by Novo et al. (Sci. Rep. 5, 13304, 2015), the invariant
subspace method was applied to the study of continuous-time quantum walk
(CTQW). The method helps to reduce a graph into a simpler version that allows
more transparent analyses of the quantum walk model. In this work, we adopt the
aforementioned method to investigate the optimality of a quantum walk search of
a marked element on a complete multi-partite graph. We formulate the eigenbasis
that would facilitate the transport between the two lowest energy eigenstates
and demonstrate how to set the appropriate coupling factor to preserve the
optimality.
