2. Background of QCA

In 1993, Craig Lent [16] proposed a new concept called quantum-dot cellular automata (QCA). This emerging technology has made a direct deviation to replace conventional CMOS technology based on silicon [17]. QCA generally uses arrays of coupled quantum dots in order to implement different Boolean logic functions. QCA or quantum-dot cellular automata as its name is pronounced uses the quantum mechanical phenomena for the physical implementation of cellular automata. In the general case, conventional digital technologies require a range of voltages or currents to have logical values, whereas in QCA technology, the position of the electrons gives an idea of the binary values [18]. The advantages of this technology are [19] especially given in terms of speed (range of terahertz), density (50 Gbits/cm<sup>2</sup> ) [20] and in terms of energy or power dissipation (100 W/cm2 ).

QCA is based essentially on a cell. Each cell represents a bit by a suitable charge configuration as shown in Figure 1. It consists of four quantum dots and two electrons charge. Under the effect of the force of Colombian repulsion, the two electrons can be placed only in two quantum sites diametrically opposite.

3. Clocking in QCA

Figure 1. Basic QCA cell.

Relax, and Hold.

shown in Table 1.

Clocking is an important term in QCA design. In order to propagate the information through QCA without any random adjustments of QCA cells, it is necessary to make a clock to guarantee the same data putting from input to the output. According to Figure 3, timing in QCA is obtained by clocking in four distinct periodic phases [21, 22] namely Switch, Release,

Figure 2. Operations of a QCA wire propagation by application of logic 0 and logic 1 1 to a QCA cell at the input.

Quantum Dot Electron

Nanoarchitecture of Quantum-Dot Cellular Automata (QCA) Using Small Area for Digital Circuits

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P = -1 P = 1

P = -1 P = 1

Logic 0

> Logic 1

Based on the position of the potential barrier, the arrays of QCA cells in each phase have different polarizations. There are four phases, and every phase has its own polarizations as

From Table 1, during the "Switch" phase of the clock, the QCA cell begins without polarization and switches to polarized state while the potential barrier has been raised from low to high. In the "'Hold" phase, the polarization state is preserved as the preceding phase and the potential barrier is high. From the "'Release" phase, the potential barrier is lowered and the cells become unpolarized. In "Relax" phase, the potential barrier remains lowered and the cells keep at nonpolarized state. This phase, the cells are ready to switch again. This way information is propagated in QCA circuits by keeping the ground-state polarization all the time. Figure 4 illustrates the polarizations

and interdot barriers of the QCA cells in each of the QCA clock zones.

Logic 0

> Logic 1

A QCA cell is composed of four points with one electron each in two of the four points occupying diametrically opposite locations. The question that arises in this case is why do electrons occupy quantum dots of opposite or diagonal corner To answer this question, it is enough to have an idea about the principle of the repulsion of Coulomb, which is less effective with respect to the electrons when they are in adjacent quantum dots. The points are coupled to one another by tunnel junctions.

Thus, the internal effect of the cell highlights two configurations possible; each one will be used to represent a binary state "0" or "1." A topology of QCA is a paving of cells QCA. The interaction between the cells makes it possible to transmit information which gives the possibility of replacing physical interconnection of the devices. The information (logic 0 or logic 1) can propagate from input to the output of the QCA cell only by taking advantage of the force of repulsion as shown in Figure 2.

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> Logic 1

Logic

1

Figure 2. Operations of a QCA wire propagation by application of logic 0 and logic 1 1 to a QCA cell at the input.
