The use of molecules to enact quantum cellular automata (QCA) cells has been proposed as a new way for performing electronic logic operations at sub-nm dimensions. A key question that arises concerns whether chemical or physical processes are to be exploited. The use of chemical reactions allows the state of a switch element to be latched in molecular form, making the output of a cell independent of its inputs, but costs energy to do the reaction. Alternatively, if purely electronic polarization is manipulated then no internal latching occurs, but no power is dissipated provided the fields from the inputs change slowly compared to the molecular response times. How these scenarios pan out is discussed by considering calculated properties of the 1,4-diallylbutane cation, a species often used as a paradigm for molecular electronic switching. Utilized are results from different calculation approaches that depict the ion either as a charge-localized mixed-valence compound functioning as a bistable switch, or else as an extremely polarizable molecule with a delocalized electronic structure. Practical schemes for using molecular cells in QCA and other devices emerge.

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Quantum-dot cellular automata (QCA) is a new paradigm in nanoelectronics, where binary information is represented by charge configuration in cells. Ideal QCA logic gates are thought to be dissipationless, since there is no intercell charge transfer and no current flows out of cells. This work presents that these gates dissipate energy and compare energy consumptions of conventional QCA logic gates in electrostatic and thermodynamic approaches. The results show that increasing the number of inputs, concentration of the geometry and the unbalanced numbers of '0' and '1' output states in the gate's truth table add to the energy dissipation of a QCA gate.

Molecular quantum-dot cellular automata (mQCA) has received considerable attention in nanoscience. Unlike the current-based molecular switches, where the digital data is represented by the on/off states of the switches, in mQCA devices, binary information is encoded in charge configuration within molecular redox centers. The mQCA paradigm allows high device density and ultra-low power consumption. Digital mQCA gates are the building blocks of circuits in this paradigm. Design and analysis of these gates require quantum chemical calculations, which are demanding in computer time and memory. Therefore, developing simple models to probe mQCA gates is of paramount importance. We derive a semi-classical model to study the steady-state output polarization of mQCA multidriver gates, directly from the two-state approximation in electron transfer theory. The accuracy and validity of this model are analyzed using full quantum chemistry calculations. A complete set of logic gates, including inverters and minority voters, are implemented to provide an appropriate test bench in the two-dot mQCA regime. We also briefly discuss how the QCADesigner tool could find its application in simulation of mQCA devices.

Design for scalability is one of the paramount aspects of nanotechnology. The authors present a new architecture for scalable gates in two-dot molecular quantum-dot cellular automata paradigm. Quantum chemical calculations are performed on multiple molecules that are arranged to carry out logic operations at the nanoscale. Compared to previous multi-input gates in four-dot quantum-dot cellular automata framework, the number of used redox centres has been significantly reduced in the presented gates.