  The multipartite unitary gates are called genuine if they are not product
unitary operators across any bipartition. We mainly investigate the
classification of genuine multipartite unitary gates of Schmidt rank two, by
focusing on the multiqubit scenario. For genuine multipartite (excluding
bipartite) unitary gates of Schmidt rank two, there is an essential fact that
their Schmidt decompositions are unique. Based on this fact, we propose a key
notion named as singular number to classify the unitary gates concerned. The
singular number is defined as the number of local singular operators in the
Schmidt decomposition. We then determine the accurate range of singular number.
For each singular number, we formulate the parametric Schmidt decompositions of
genuine multiqubit unitary gates under local equivalence. Finally, we extend
the study to three-qubit diagonal unitary gates due to the close relation
between diagonal unitary gates and Schmidt-rank-two unitaries. We start with
discussing two typical examples of Schmidt rank two, one of which is a
fundamental three-qubit unitary gate, i.e., the CCZ gate. Then we characterize
the diagonal unitary gates of Schmidt rank greater than two. We show that a
three-qubit diagonal unitary gate has Schmidt rank at most three, and present a
necessary and sufficient condition for such a unitary gate of Schmidt rank
three. This completes the characterization of all genuine three-qubit diagonal
unitary gates.
