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It has always happened in human history that when we thought that things were perfect and nothing better can be done or the thing will suffice us forever some problems unforeseen came up same is the case with CMOS (Complementary Metal Oxide Semiconductors), now researchers are looking beyond CMOS to QCA (Quantum-dot Cellular Automata), a technique which was formulated out of the applications of quantum mechanics. Talking about QCA it is an ingenious technique to fabricate new generation processors and other combinational and sequential circuits. The eye-catching and commercially useful properties of QCA that set it apart from any other technique is its extremely low power consumption in meV, small size going up to nanometer scale, high frequency operation (THz) and elimination of short channel effect. Quantum computing aims to use QCA as the building block for upcoming devices. In this paper, we have improved on the design of the Toffoli gate which is a reversible universal gate. We have successfully improved it on parameters such as cell count by more than 20%, total area by 34%, cell area by 29% and we have maintained latency and complexity as the previous optimal designs even by having less cell count. These improvements will help reduce the size of the chip and reduce power dissipation and thus improve efficiency and be compliant with the ever-increasing computational ability and downscaling requirements.

Quantum-dot cellular automata (QCA) is a new and adroit technology currently under extensive research for post-complementary metal-oxide-semiconductor era VLSI chip design. QCA has promised more reliable, fault-tolerant and secure chip designs. In addition, while analyzing QCA circuits for power and energy dissipation, promising results have been reported that suggest that QCA circuits dissipate significantly less energy and operate very close to the Shannon–von Neumann–Landauer limit. Security is another concern that has led to the development of QCA-based security systems such as physically unclonable functions and true random number generators. In this paper, a survey of different fault-tolerant and QCA-based security circuits is provided, along with a discussion of critical design aspects and parameters of QCA technology.

Quantum-dot cellular automata (QCA) are a promising, novel nanoscale technology that allow the design of integrated circuits with high speed, low power consumption, and high density. Because of this potential benefit, QCA are chosen as a viable alternative to complementary metal-oxide semiconductor (CMOS) technology. In this paper, we have provided a comprehensive review of various types of digital circuits and modules in QCA nanotechnology. We have discussed circuits such as XOR/XNOR, half and full adder, multiplexers and demultiplexers, comparators, flip-flops, arithmetic and logical unit, and random-access memory. We have shown how these circuits are designed using various methodologies such as different types of cross-overs, multi-layer designs, or using cell-to-cell interaction method with their corresponding advantages and overheads. These logical circuits are compared on the basis of various parameters including cell area, total area, latency, number of cells, energy dissipation, and complexity and are explained starting from the design which is having larger cell count to the current efficient design present in terms of the above parameters.