Reversible logic has comprehensive applications in communications, quantum computing, low power VLSI design, computer graphics, cryptography, nanotechnology, and optical computing. It has received significant attention in low power dissipating circuit design in the past few years. While several researchers have inspected the design of reversible logic units, there is not much reported works on reversible subtractors. In this paper we proposed the quantum equivalent circuit for SRK gate and we have computed the quantum cost of SRK gate. We also showed that how SRK gate can work singly as a half-subtractor circuit. It is being tried to design the circuit optimal in terms of number of reversible gates, number of garbage outputs, number of constant inputs, and quantum cost with compared to the existing circuits. At last we proposed an implementation of the new full-subtractor circuit based on SRK gate. All the designs have nanometric scales.