Complementary metal-oxide semiconductors (CMOSs) with low area requirement, power dissipation, and reliability characteristics are integrated into nanosystems for better performance. However, quantum dot cellular automata are improved alternatives to CMOS that help analyze a feasible alternative system with similar capabilities.
Study: Quantum dot cellular automata using a one-bit comparator for QCA gates. Image Credit: Untitled Title/Shutterstock.com
In an article recently published in the journal Materials Today: Proceedings, researchers constructed a one-bit comparator using quantum dot cellular automata nanotechnology. Due to the absence of crossovers, these circuits were simple to build. The new designs reported in this work showed performance improvement between 74.81 to 99.87%, suggesting their efficiency in terms of cell count, quantum cost, area, and delay time.
The constructed designs could be applied in various digital logics that call for low area and power requirements. Simulations performed on quantum dot cellular automata designer E Tool to emulate one-bit comparator circuits showed that the designed circuit required an area of 0.02 square micrometer and 16 quantum dot cellular automata cells, with a delay of 0.5 clocks.
Quantum Dot Cellular Automata Cell
Designing very large-scale integration (VLSI) involves two major issues based on their scaling and time reduction for computation. However, quantum dot cellular automata technology can overcome these problems at the nanoscale.
The fundamental element of quantum dot cellular automata technology involves four dots with two free electrons in a square cell. A quantum dot cellular automata cell is a building block for quantum dot cellular automata gates. The three basic gates, inverter gate, exclusive-OR gate (XOR) gate and main gate, are the building blocks to construct quantum dot cellular automata multiplexers-like logic circuits.
A quantum dot cellular automata cell is an imaginary square area with two electrons and four electronic locations that allows data computation and transmission. The dots of a quantum dot cellular automata cell are possible locations to find electrons. The quantum mechanical tunnel barriers enable the tunneling of electrons based on the system state. Moreover, electrons occupy well-separated parts of a quantum dot cellular automata cell due to Coulombic repulsion, thus maintaining the system at the lowest energy state. The binary states 0 or 1 encode the position of an electron in a cell, termed cell polarization.
A single-bit comparator compares two equivalent bits. For two single-bit numbers, the comparator has two inputs and three outputs. A conventional comparator can compare and produce desired output with a minimum of two variables. In contrast, the proposed one-bit comparator compared two voltage signals and produced an output when the following three different conditions were approached, for two different inputs (A and B): A = B, A > B and A < B.
One-Bit Comparator for Quantum Dot Cellular Automata Gate
In the present study, the proposed quantum dot cellular automata comparator circuit consisted of two one-bit inputs and three one-bit outputs.
In the developed one-bit quantum dot cellular automata comparator circuit, the majority gates were used to implement AND gates. Consequently, one input in majority gates was set as logic “0”. The first design of one-bit quantum dot cellular automata comparator circuit necessitated an area of 0.03 square micrometers and 38 quantum dot cellular automata cells. The second design was a viable option with an area size of 0.02 square micrometers and 20 quantum dot cellular automata cells.
The one-bit comparator design was comparatively small and accommodated lower cell than its two-bit counterpart. Consequently, the latency of the circuit was greater, and the circuits were constructed without crossovers, reducing the quantum cost. Furthermore, the third design, with an area of 0.02 square micrometers accommodated 16 cells.
The simulation tool, quantum dot cellular automata designer 2.0.3 was utilized to simulate the proposed quantum dot cellular automata comparator circuit and check the correctness of the design. The results revealed that the accurate outputs of the one-bit comparator circuit were obtained after 0.5 clock cycles. Moreover, an area of 0.03 square micrometers and 38 cells were required to construct a one-bit quantum dot cellular automata comparator circuit. The designed one-bit comparator had area and cost advantages over other comparators.
In summary, the quantum dot cellular automata technology is an emerging technology that enables the construction of nanoscale digital circuits. The comparator circuits have an essential role in digital circuits. A quantum dot cellular automata comparator circuit accommodating 16 quantum cells was constructed and tested for efficiency, and the results showed less area requirement, no crossover, and low quantum cost with minimal power dissipation.
The logic gates: XNOR, main, and inverter were used as building blocks to construct the quantum dot cellular automata one-bit comparator circuit. Simulations on the quantum dot cellular automata designer 2.0.3 helped confirm the one-bit comparator system’s functionality. The results revealed that an area of 0.02 square micrometers and the 16 quantum dot cellular automata cell were optimal requirements for designing a one-bit comparator circuit.
Moreover, the simulations showed a 0.5 clock cycle delay. The overall results revealed that the designed one-bit comparator circuit had a lower area requirement, low cell count, and less clock cycle delay. Thus, compared to previously reported quantum dot cellular automata circuits, the newly designed quantum dot cellular automata comparator circuit improved efficiency, cell count, delay, and quantum cost.
Begum, AY., Balaji, M., Satyanarayana,V. (2022). Quantum dot cellular automata using a one-bit comparator for QCA gates, Materials Today: Proceedings. https://www.sciencedirect.com/science/article/pii/S2214785322044340?via%3Dihub