Design of Ripple Carry Adder using Quantum Cellular Automata

Quantum Cellular Automata (QCA) is one of the emerging nanotechnologies in which it is possible to implement reversible logic gates. QCA makes it possible to achieve high performance beyond the limits of existing CMOS technology. QCA does not have to dissipate its signal energy during transition. Further, there is no movement of electrons from one QCA cell to the other, there is no current flow. Thus, QCA has no dissipation in signal propagation. Fredkin gate can be used to design D Latch, DET flip flops and Master slave flip flops. The factors such as area, delay, power and the complexity of the design is much increased. Hence QCA technology can be implemented in order to increase the performance. In my project based on QCA computing, ripple carry adder (RCA) can be designed with high performance when compared with that of CMOS technology. In terms of testability, LFSR can be used to generate test patterns which are used as input vectors (0s and 1s) for ripple carry adder. Keywords— QCA and Fredkin Gate INTRODUCTION Conservative logic is a logic family that exhibits the property that there are an equal number of 1s in the outputs as in the inputs. Conservative logic can be reversible in nature or may not be reversible in nature. Reversibility is the property of circuits in which there is one toone mapping between the inputs and the output vectors, that is for each input vector there is a unique output vector and vice-versa. Conservative logic is called reversible conservative logic when there is a one-to-one mapping between the inputs and the outputs vectors along with the property that there are equal number of 1s in the outputs as in the inputs. Conservative logic circuits are not reversible, if one-to-one mapping between the inputs and the outputs vectors is not preserved. Researchers have proved that if the computation is performed in an irreversible manner, each bit of information lost will produce KTln2 Joules of heat energy. From a thermodynamic point of view, it is also proved that kTln2 energy dissipation would not occur, if a computation is carried out in reversible way. I. QCA TECHNOLOGY QCA is one of the nanotechnologies in which it is possible to implement reversible logic gates [2], [3]. QCA makes it possible to achieve circuit densities and clock frequencies beyond the limits of existing CMOS technology [4], [5]. In QCA, computing logic states of 1 and 0 are represented by the position of the electrons inside the QCA cell. Thus, when the bit is flipped from 1 to 0 there is no actual discharging of the capacitor as in conventional CMOS. Hence, QCA does not have to dissipate all its signal energy during transition. Further, propagation of the polarization from one cell to another is because of interaction of the electrons in adjacent QCA cells. As there is no movement of electrons from one QCA to another. Fig. 1 (a) Quantum Dot and (b) QCA cell. II. BASICS OF QCA The conservative logic gates are implemented in the QCA nanotechnology, thus we are also providing the introductory material on QCA computing. A QCA cell is a coupled dot system in which four dots are at the vertices of a square. The cell has two extra electrons that occupy the diagonals within the cell due to electrostatic repulsion. Fig. 1(a) and (b) shows the four quantum dots in a QCA cell,and the implementation aof l QCA cell, respectively. The basic QCA device is the MV or majority gates, which is represented as F = AB + BC + AC, where F is the majority of the inputs A, B, and C. Another important gate in QCA is the INV. There can be many ways of designing the QCA INV, one of which is shown in Fig.1(d). In QCA computing, signal transfer is made through wires that are of two types: 1) binary wire and 2) INV chain. The binary wire is shown in Fig. 1(e). The INV chain is shown in Fig. 1(f). In QCA, when a binary wire crosses the INV chain, there is no interaction between the two; hence, the signals in the INV chain and binary wire can pass over each other. Fig.2(a) Majority Voters B Praveenkumar et al, / (IJCSIT) International Journal of Computer Science and Information Technologies, Vol. 5 (6) , 2014, 7998-8002