QCA Based Efficient Toffoli Gate Design and Implementation for Nanotechnology Applications

— The construct of computing is the field of ever improving and advancing systems and architectures. One of the emerging archetype that has made its ground as a research area is the concept of reversible logic in computing. Reversible logic suggests the systems with zero heat dissipation since the computational models in this logic incorporate the concept of reversibility at a ground level thereby satisfying the work and heat relations according to the laws of physics. In this paper we have first of all presented a detailed insight about the reversible logic and the perspectives related to it. Further the design of reversible logic circuits using the new emerging nanotechnology called Quantum-dot Cellular Automata (QCA) has been envisaged. These two paradigms together hold the potential of realizing ultra-low power systems and architectures. In this paper, the most common and popular reversible gate called as the Toffoli Gate has been considered and designed using this new QCA technology. This paper presents the approach of using cell to cell interaction in designing the reversible circuits which result in efficient implementations. The necessary comparisons with the literature are drawn and it is explicitly shown that the proposed designs are highly optimized as compared to existing counterparts.

the counterparts of the basic physical principles are achieved and incorporated in the theory a better match is possible between theoretical and application correspondent.
For any system to be reversible its building blocks need to be individually reversible. This means that at the basic level we can talk about a reversible gate. For reversible structures the number of inputs and outputs has to be always equal since without that the one to one mapping which is the second important consideration, cannot be achieved. Whenever achieving reversibility is a target some source and sink lines need to be added to construct an invertible function. These include some constant inputs, the inputs which have a sole function of achieving the invertible function and the garbage outputs, the outputs which are not used further in computation. The efficient systems aim for a reduction in the number of constant inputs and garbage outputs. The design optimization parameters in the reversible circuits include these constant inputs, garbage outputs and the quantum cost assessment. Quantum cost is defined as the number of primitives needed for the design of the particular circuit. In reversible logic optimizing the quantum cost is identified as the most important and most challenging task. This emerging concept of reversibility with its feature of low power dissipation forms the centre of attraction for the VLSI designers since CMOS VLSI is suffering the issue of power dissipation at the nano scale levels [3][4][5]. The quantum mechanical effects which lead to the failure of the CMOS devices in the deep submicron ranges can be countered by reversible logic since the quantum mechanical systems are inherently reversible in nature. This calls for extensive research in the areas like reversible design. The issue however remains of the designing of these circuits. A new design paradigm if successfully used in designing reversible circuits can lead to the ultra-low power systems which is the need of the hour to continue with the technology advancements. We discuss this new nanotechnology based cellular automata in the next section.
II. QCA ARCHITECTURE Quantum-dot Cellular Automata (QCA) is identified as the new upcoming nanotechnology for the design of the digital circuits [6].The need for such a technology arises from the fact that as the scaling of conventional CMOS is taken down to the nano regime, the behavior of the device can no longer be classically governed. The effect of quantum mechanics cannot be neglected in this range and needs to be taken into consideration. The conventional technology models however fail to consider the quantum effects and thus are unable to explain the phenomenon occurring at the nano scale. This new technology called QCA uses the quantum effects to its advantage and thus is more suitable for the nanotechnology based applications. This technology calls for a departure from the conventional current and voltage paradigms and is governed by the laws of physics such as coulombic repulsions and the quantum mechanical tunneling. The basic concept of QCA architectures lies in two facts. Firstly, due to coulombic repulsions the electrons tend to take up the positions where they face minimum repulsion from each other and secondly the tunneling controls the alignment of these electrons. The structures in QCA are the cellular automata type designs where the basic component repetitions form all the circuitry. The fundamental element in QCA is the QCA cell which is a square structure as shown in the figure 1. This cell consists of four areas for the localization of the trapped electrons and there are two tunneling junctions due to which the electrons can change the alignment. The potential wells (areas of localization) can created by quantum dots or metal islands. As is evident from the figure there are four places at which the electrons can reside. Due to coulombic repulsion between the trapped electrons they tend to take up the positions with minimum repulsion. For the shown structure of the cell this calls for two possible types of alignments called as the two polarizations of the cell. This is as shown in figure 1. Since in QCA we deal with the digital designs the two polarizations represent the two possible digital states i.e. the binary zero and the binary one. This basic cell forms the building block for all the design architectures using the QCA nanotechnology [7,8]. When we talk about the conventional CMOS or any other technology the circuits and interconnections are two different class of elements but in QCA the basic cell forms both the circuits as well as the interconnections. The alignment of the cells adjacent to each other results in the formation of a binary wire which is used for the transmission of the information from one point to another. The inverter and the majority voter along with the binary wire form the basic blocks for designing any kind of complex architectures in the QCA. The three blocks are shown in the figure 2.
The working mechanism of the inverter is based on the opposite alignment of the cell at the right hand side due to which the alignment of electrons changes the polarization to achieve the stable state. Thus the output from such a structure is opposite as compared to the input. In the case of majority gate the central cell governs the output which in turn depends on the three inputs of the majority gate. The polarization of the central cell is that of the majority of the inputs since that results in a stable state. Besides the basic blocks another very important consideration in the QCA design methodologies is the clocking mechanism involved in these circuits. The clocking in QCA governs the tunneling of the electrons between the potential wells which may be the quantum dots or islands as discussed earlier. In QCA one clock cycle consists of four clock phases. Each phase is characterized by either lowering or raising of the barrier. The first phase is the switch phase in which the tunneling barriers are raised such that the cell attains a particular polarization. The next is the hold phase in which the barriers are maintained at a high level and the cell is in a position to effect the polarization of the adjacent cells. Further the other two phases are the release and the relax phase in which the barriers are lowered such that the cell again attains the null polarization. The clocking to the QCA circuits is provided by the underlining CMOS or carbon nanotube wiring to provide the necessary electric field for the alignment of electrons in particular polarization in a particular cell. Besides the clocking in QCA, another important criteria is the crossing over of the wires which result in the designing of efficient and optimized designs. A number of crossover schemes have been applied for the QCA designs. The most common are the coplanar and multilayer crossovers. Logical clock zone based crossovers have also been envisaged in the literature [14][15][16][17].
III. PROPOSED TOFFOLI GATE QCA DESIGN When dealing with reversible logic which we have discussed in the first section of this paper, the basic concept is to achieve bijective functions. This is achieved in the case of reversible gates. A number of reversible gates have been proposed in literature and a number of applications of the designs are highlighted [9-12, 18, 19]. However design of reversible gates using the new QCA technology is still an emerging topic and researchers are working towards obtaining the optimized designs of the gates and circuits in the QCA technology. The need to do so is to achieve efficiency in all the aspects, be it the ultra-low power dissipation or the ultra-low size of the devices. If the reversible gates find optimal realization in QCA the achievement of the above two targets can be changed from just a theoretical concept to the applicable paradigm. One of the most oldest and popular reversible gate is the Toffoli gate. This gate satisfies the conditions of reversibility which are the equal number of input and output, the one to one mapping between the input and output, no fan out and no feedback.      The graphical representation of the comparisons of proposed gate with the Toffoli Gate designs in literature are also shown in figures 6-9.

IV. CONCLUSION
This paper presents an overview of the concept of reversibility with focus on its utility and tendency to revolutionize the circuit designing paradigms. The combination of reversible logic and the Quantum-dot Cellular Automata Nanotechnology which has emerged as a hot area of research holds the potential to replace the conventional transistor based design architectures. A brief review of the QCA technology and the laws governing the QCA designing has been presented. Further the optimized Toffoli gate implementation in QCA has been proposed and comparisons are drawn with the existing implementations in the literature. It is shown both graphically and numerically that the proposed designs are highly efficient in terms of cost, area and delay as compared to existing counterparts.