Design of a 4-bit adder using reversible logic in quantum-dot cellular automata (QCA)

Both quantum-dot cellular automata (QCA) and reversible logic are emerging technologies that are promising alternatives to overcoming the scaling and heat dissipation issues, respectively, in the current CMOS designs. Here, the fundamentals of QCA and reversible logic are studied; the feasibility of incorporating reversible logic in QCA designs is also demonstrated. Based on two existing designs, an improved version of the reversible gates, namely the Feynman Gate and the Toffoli Gate, were implemented in QCA technology using QCADesigner. The proposed design of the QCA-based Feynman Gate is faster by ½ cycle as compared to the existing design; while the proposed Toffoli Gate has the same latency as the existing design but it is readily to be cascaded into a more complex design. A 4-bit ripple carry adder in QCA is then designed using the proposed Feynman and Toffoli gates to realize a reversible QCA full adder. This 4-bit QCA adder with reversible logic consists of 2030 QCA cells, has a latency of 7 clock cycles and 8 garbage outputs.