  Quantum-dot Cellular Automata (QCA) is one of the emerging nanotechnologies,
promising alternative to CMOS technology due to faster speed, smaller size,
lower power consumption, higher scale integration and higher switching
frequency. Also, power dissipation is the main limitation of all the nano
electronics design techniques including the QCA. Researchers have proposed the
various mechanisms to limit this problem. Among them, reversible computing is
considered as the reliable solution to lower the power dissipation. On the
other hand, adders are fundamental circuits for most digital systems. In this
paper, Innovation is divided to three sections. In the first section, a method
for converting irreversible functions to a reversible one is presented. This
method has advantages such as: converting of irreversible functions to
reversible one directly and as optimal. So, in this method, sub-optimal methods
of using of conventional reversible blocks such as Toffoli and Fredkin are not
used, having of minimum number of garbage outputs and so on. Then, Using the
method, two new symmetric and planar designs of reversible full-adders are
presented. In the second section, a new symmetric, planar and fault tolerant
five-input majority gate is proposed. Based on the designed gate, a reversible
full-adder are presented. Also, for this gate, a fault-tolerant analysis is
proposed. And in the third section, three new 8-bit reversible
full-adder/subtractors are designed based on full-adders/subtractors proposed
in the second section. The results are indicative of the outperformance of the
proposed designs in comparison to the best available ones in terms of area,
complexity, delay, reversible/irreversible layout, and also in logic level in
terms of garbage outputs, control inputs, number of majority and NOT gates.
