Quantum Multiple-Valued Decision Diagrams Containing Skipped Variables

Quantum multiple-valued decision diagrams (QMDD) are data structures that enable efficient representation and manipulation of unitary matrices representing reversible and quantum circuits. QMDDs are of the form of a rooted, directed, acyclic graph with initial and terminal vertices where each vertex is annotated with a variable name representing a circuit line. Directed paths from the intial to a terminal vertex can be represented as a sequence of variable names in the order in which they appear in the path. The existence of QMDD paths that do not contain all variable names, or "skipped variables", has direct ramifications in the formulation of synthesis and other algorithms for reversible and quantum logic. Skipped variables are generally rare and tend to appear in quantum circuits that are intended to operate using superimposed values on the control lines. We have found that a unitary matrix representing a circuit whose QMDD contains skipped variables is likely to exhibit a specific anomaly when decomposed into a cascade of unitary matrices using the Reck-Zeilinger-Bernstein-Bertani algorithm.