A reversible logic gate differs from the conversational logic gate in many ways, such as equal number of input-output, its reverse operation and bijective input/output mapping. In this paper, a new class of 4 × 4 conservative-reversible logic gate named the equalizer SWAP gate (ESG) has been proposed. This gate is self-invertible and performs SWAP operation when two of its control inputs are logic either 0 or 1. The all-optical design of the ESG using a 'semiconductor optical amplifier (SOA) assisted Sagnac interferometer' with a counter-propagating signal passing through the SOA has been shown with a mathematical model in this paper. Quantum realization of the ESG has been carried out to construct its unitary matrix. Also, the matrix has been established through numerical simulation. Some complex reversible logic circuits, viz. n-bit XOR/XNOR, full-adder circuit, n-bit binary-to-gray and gray-to-binary conversion, using the ESG unit have also been shown. Simulated outputs, logical complexity, quantum realization, optical cost and delay of the proposed circuit have been analyzed, and are compared with other reversible logic gates.

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