Technical session I: VLSI and MEMS
A garbage output is an output that is needed to change an irreversible gate to a reversible one and are not used to the input to the other gates.
2.3 Quantum cost
All reversible 1 × 1 and 2 × 2 gates have unity quantum cost. As every reversible gate is a combination of 1 × 1 or 2 × 2 quantum gate, so the quantum cost of a reversible gate is the numbers of quantum gate (Thapliyal et al. 2010).