System Level Simulation of Quantum-Dot Cellular Automata Computer Circuits

Quantum-dot cellular automata (QCA) is an emerging transistor-less field-coupled nanocomputing (FCN) approach to ultra-scale nanochip integration, enabling molecular electronics. In QCA, electron tunnelling between quantum dots enables switching, while electrostatic repulsion drives electrons to opposite positions within the four-dot cell, thereby encoding binary configuration. Current QCA circuit designs are either irreversible or logically reversible. This thesis introduces three innovative design methods for building QCA circuits, including logically and physically reversible, partially reversible, and hybrid design methods. The core component of the logically and physically reversible design method, is an innovative reversible majority gate. This method was applied to construct combinational circuits, sequential flip-flops, and more advanced computing circuits, an arithmetic logic unit and multiplexers. Simulation results demonstrate that the logically and physically reversible design method produces functional QCA combinational, sequential, and more sophistecated circuits, with exceptional energy efficiency improvement of up to 97%. The innovative partially reversible majority gate, is the key component of the partially reversible design method. This method was used to build half-adder circuit. Simulation results of the proposed partially reversible half-adder show that the partially reversible design approach enhances speed by up to 67%, reduces circuit cost by up to 77%, compared to the reversible design method, and improves energy efficiency by up to 86% compared to conventional irreversible circuits. The hybrid design method integrates reversible, irreversible, and partially reversible majority gates, offering greater control over the level of circuit reversibility. This approach was utilized to design four distinct QCA half-adder circuits, each employing a specific combination of these three types of majority gates. Simulation results indicate that increasing circuit reversibility tends to increase cost while reducing energy dissipation. Conversely, reducing circuit reversibility can lower costs but results in higher energy dissipation. The present research endeavour has produced six publications in total: five journal articles and one conference paper.