Quantum Dot cellular Automata (QCA) is an emerging, promising alternative to CMOS technology that performs its task by encoding binary information on electronic charge configuration of a cell. All circuit based on QCA has an advantages of high speed,
American Journal of Engineering Research (AJER) 2014
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American Journal of Engineering Research (AJER)
eISSN : 23200847 pISSN : 23200936 Volume03, Issue12, pp8792 www.ajer.org Research Paper Open Access
A Novel Design of Half Subtractor using Reversible Feynman Gate in Quantum Dot cellular Automata
Rubina Akter
1
,Nasrin Jahan
2
, Md. Mamunur Rashid Shanta
3
, Anik Barua
4
1, 4
(Department of ICT, MawlanaBhasani Science and Technology University, Bangladesh)
2
(Department of TE, University of South Asia, Bangladesh)
3
(Department of EEE, East West University, Bangladesh)
Received Manuscript: XXXXXXXX Accept Manuscript: XXXXXXXX Published Manuscript: XXXXXXXX
ABSTRACT:
Quantum Dot cellular Automata (QCA) is an emerging, promising alternative to CMOS technology that performs its task by encoding binary information on electronic charge configuration of a cell. All circuit based on QCA has an advantages of high speed, high parallel processing, high integrityand low power consumption. Reversible logic gates are the leading part in Quantum Dot cellular Automata. Reversible logic gates have an extensive feature that does not lose information. In this paper, we present a novel architecture of half subtractor gate design by reversible Feynman gate. This circuit is designedbased on QCA logic gates such as QCA majority voter gate, majority AND gate, majority OR gate and inverter gate. This circuit will provide an effective working efficiency on computational units of the digital circuit system.
Keywords

AndOrInverter (AOI), Basic QCA, Majority Voter (MV), QCA Logic Gate, Reversible Feynman Gate.
I.
INTRODUCTION
The paradigm of Quantum Dot cellular Automata is a revolutionary approach to molecularscale computing. Using the charge configuration of nano structures Quantum Dot cellular Automata presents binary information on the current switching devices. For several decades the size of electronics semiconductor device and operating currents has been reduced. Fundamental problems arising from scaling such as quantum mechanical effects and severe power dissipations assist the continued development toward devices on the nanometer scale [1]. Logic design with quantum dots is one of the most recent technologies being researched which allows scaling to continue to atomistic dimensions. Using the current switching Quantum Dot cellular Automata uses the charge configuration of a set of quantum dots to present binary information on molecular scale computing [2]. QCA cell is the basic building block of QCA devices. A QCA cell consists of several quantum dots with two mobile electrons [3, 4]. The charge that takes place on which corner of the cell, the place is known as the quantum dot. Coulombic repulsion between like charge forces electrons in the same cell to occupy dots which maximize their separation. In QCA the electron of a cell can tunnel between a dot to dot. The two mobile electrons in a cell take place as a diagonal pattern of the coulombic interaction between them [5]. Hence the four dotted QCA cells can be used represent the binary information. Due to the electrostatic repulsion the two extra electrons take their position in the four dots. These two free electrons create only two stable positions. These
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two stables positions known as 1 and +1 polarity or boolean values 0 and 1 respectively [6]. The presence of clocking zones is a unique feature of QCA based circuits. There are four phases of clocking zones such as the switch, hold, release and relax [7]. Now a day nanotechnology is one of the most wanted research field and Quantum Dot Cellular Automata offers a revolutionary approach to computing at nano level. Parallely, reversible logic is getting more and more conspicuous technology which offers better working performance in QCA technology. Previously researchers have addresseda number of studies on reversible logic gates and their implementation such as described in [8, 9, 10]. H. cho and j. Earl Swartzlander described adder and multiplier design that are optimized in terms of quantum cost, delay and garbage output [11]. A revolutionary design of a shift register and its operation is described in [5]. H. Thapliyal and N. Ranganathan proposed the design of binary and bcd adder circuits based on reversible logic gates. These designs are very effective in terms of reversible gates, garbage output and quantum cost [12]. I. Hanninen and J. Takala emphasis a binary adders design on QCA which is very important in arithmetic logic unit. The main parts is the implementation of logical devices and the reversible logic using QCA is shown in [13, 14]. There are many reversible logic gates and these gate are important for their reversibility characteristics. Among them Feynman gate is one of the most important reversible logic gates. In this paper using the reversible Feynman gate we design a Half Subtractor circuit. This design is very useful for digital signal processing (DSP), optical computing, cryptography etc. This paper is apportioned into five sections. Section I describes about QCA, reversible logic gate and their implementation in various logical circuits. Section II provides a brief overview about the QCA fundamental logic units. Section III presents our proposed circuit using QCA technology. Section IV shows the simulated waveform and discuss about the simulation results of the proposed circuit. Finally, section V summed up the beneficial perspective of the presented circuit.
II.
QCA
R
EVIEW
Quantum Dot cellular Automata is an outstanding nanotechnology which is used for its better performance than CMOS technology. We describe the basic building block of QCA cells in the previous section. A QCA cell is shown in figure 1. This figure is considered as a square with four dots as its corners. The cell is to consist of two extra electrons which can tunnel between cell dots [1, 15]. Figure 1: Structure of a QCA cell and its binary logic The QCA wire is basic QCA logic element which is consist of a set of QCA cells. Due to the electrostatic repulsion signal propagates from input to output through the QCA wire. The QCA wire is a horizontal row of QCA cells [16]. A normal QCA wire is shown in figure 2.
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Figure 2: QCA Wire Another basic QCA logic elementis QCA majority Voter (MV) gate. The 3input majority gate is composed of five cells. The three cells act as input cell, the center cell is known as device cell and the remaining cell is known as output cell. The center cell performs the calculations. Figure 3 shows a three input majority gates where the input cells are labeled as A,B,C and the output cell labeled as OUT. The Boolean function of this three input majority gate is, OUT (A, B, C)=Maj (A, B, C)= AB+BC+CA. The three input majority gate works as 2input AND gate having a fixed polarity of zero and its output is ab. The three input majority gate works as 2input OR gate having a fixed polarity of 1 and its output is a+b. Figure 3: QCA Majority Voter (MV) gate Inverter gate is a basic building logic element in QCA. For making a complete logical set is needed the inverter gate. Combinational logic circuits need a complete logical set which is fully depends on the Majority Voter gate and the inverter gate. In the inverter gate the input polarization splits and converts to the opposite polarization at the inverter output [17]. Figure 4 shows a QCA inverter gate.
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Figure 4: QCA Inverter gate III.
P
ROPOSED
C
IRCUIT AND
P
RESENTATION
There are many reversible logic gates and these are extensively important to QCA technology. In this section we will present our proposed Halfsubtractor circuit which is implemented by reversible Feynman gate. 1.
Feynman gate (FG) Figure 5 shows a 2 x 2 Feynman gate. The input vector is I (A, B) and the output vector is O (P, Q) and the relation between input and output is given by P=A, Q =A
⊕
B
.
Figure 5: Feynman gate In this paper we design a reversible Half Subtractor circuit by Feynman gate using QCA Technology. Let A and B are two binary numbers. The half subtractor performs AB operation. Table 2 shows the truth table of the half subtractor. The output of the XOR gate produces the difference between A and B. The output of the AND gate A'B produces a Borrow. Thus, the output function will be Borr = A'B, Diff = A
⊕
B. Table 1: Truth Table of Half Subtractor Figure 6 shows QCA layout structure of the Halfsubtractor design using reversible Feynman Gate. In this figure the input cells are labeled with A and B and the outputs are labeled as Y2 and Y3. The circuit performs the Boolean functions Y3 = Borr = A'B, Y2 = Diff = A
⊕
B.
Input Output A B Borr Diff
0 0 0 0 0 1 1 1 1 0 0 1 1 1 0 0
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Figure 6: Simulated waveforms for Half Subtractor gate by Feynman gate
IV.
R
ESULT AND
D
ISCUSSION
Our proposed Halfsubtractor circuit functionally simulated using the QCA Designer 2.0.3. The following parameters are used for a Bistable Approximation: cell size=18nm, number of samples=12800, convergence tolerance=0.0000100, radius of effect=65.000000nm, relative permittivity=12.900000, clock high=9.800000e022J, clock low=3.800000e
–
023J, clock shift=0, clock amplitude factor=2.000000, layer separation=11.500000, Temperature=1.000000,Reluxation time=1.000000e015, Time step 1.000000e016 and Dot Diameter=5.0000. Most of the above mentioned parameters are default values in QCA Designer. Figure 7 shows the input output waveforms of our proposed circuit. In this Figure, the input signals are A, B and the output signals are Y3 and Y2. The input is mapping to output as Y2 =A
⊕
B and borrow Y3=
A'B
. Figure 7: Simulated waveforms for HalfSubtractor gate by Feynman gate