QCA Material

8/13/2019 QCA Material

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Quantum cellular automata:

Introduction:

For the past 30 years, microelectronics industries have succeeded in making huge leaps

in increasing the speed and reducing the power requirements of electronic devices. According

to Moore’s law, number of devices integrated in a chip doubles every 18-24 months.

International Technology Roadmap for Semiconductors (ITRS) reported that this trend is

expected to continue until 2020. Beyond that the physical and power density restrictions would

restrict further scaling of integrated circuits in modern-day manufacturing technology of CMOS.

In order to further increase device density, microelectronics industries have to change from the

established prototype to a new prototype which can get over these effects i.e. Quantum-dot

Cellular Automata (QCA)

QCA-Cell, a basic device, enables high device densities and also can function at THz

frequencies by consuming low power. QCA cell contains electrons that help in encoding binary

information.

The position of electrons is decided by Columbic interaction between electrons and this

facilitates signal transmission and thus the logic state of a cell.

It depends on the Columbic interaction between device cells. A QCA cell consists of four

charge containers or “dots”, placed at the corners of a square.

QCA Cell

QCA cell enables both computation and transmission of information

QCA cell consists of four Quantum-dots in a square array & two electrons.

The quantum dots are coupled through potential barriers and the electrons can tunnel

through the barriers depending on their heights.

The potential barrier heights are controlled by external voltage.

If the potential barriers are low the electrons can easily tunnel through them. If they are

raised, the electrons cannot tunnel and the cell is latched.

The cell has two minimum state arrangements.

These relative positions of the electrons in a cell are used to encode the binary states ‘0’

and ‘1’. The encoded states of a cell are represented by the polarizations. Fig. 1 shows the

cells with polarization P=+1, P=-1 and P=0 which encodes binary information 1, 0 and

null states respectively.



Polarisation of a qca cell is influenced by using an 1) external electric potential or 2) by the

neighboring cells.

How do you achieve correct room temperature operation of QCA

individual cells?

1) The gap between energy levels in a single quantum-dot must be greater than the thermal

noise.

2) The difference between the two lower energy states and the third higher energy state in a

QCA cell must be greater than the thermal noise.

QCA w ire(inverter ch ain):

QCA cells arranged in a linear fashion act as a wire . A QCA wire functions by transferring columbic interactions from one cell to the next as

shown in Fig. 8.

Formation of QCA wire (Mechanism)

by placing the QCA cells in a linear fashion.

Columbic interaction between the adjacent cells ->polarization of a cell -> transmission

of information along the array of cells

Fig. 8. QCA Cells in a linear arrangement acting as a wire.

Input Output

Fig. 7. QCA Cell Polarization used to encode binary information

P=-1 P=+1 P=0

Binary ‘0’ Binary ‘1’ Null state



The input cell is strongly polarized in one direction. Thus it acts as a driver to the other

cells in the NULL state.

QCA inv erter

In a wire if the cells are displaced by 45° from their path then the structure acts as a QCA

inverter Fig. 10.

In this structure, the gate attains the ground state when oppositely polarized input and output

wires are connected with 45O displacement. In order to avoid signal integrity issues, a different structure

shown in Fig. 10 is commonly used as inverter.

The signal is divided into two and added back to a single wire with a 45O displaced cell.

QCA Major i ty g ate:

In a majority gate three input cells vote on the polarization of a single cell and the majority

wins. The majority gate can be used to create AND gate and OR gate Fig. 11.

Fig. 10. QCA Inverter.

Input

01

Fig. 10. Inverter with 45o displaced wire.

Input

Output

Fig. 9. 45O rotated Cells in a linear arrangement acting as aninverted wire.

Input Output

Input A 1





Example

Metallic islands of gold clusters

Arrangement is via self organization

Dimension of single island is in nanometer

Appl icat ion:

Quantum computer.

Solving nonlinear differential equations in QCA network.

Associative information storage system.

Image processing.

Documents

QCA Material