Signal Energy in Quantum-Dot Cellular Automata Bit Packets

Quantum-dot cellular automata is a novel paradigm for computing at the nanoscale. Cells are the basic computing element in quantum-dot cellular automata and function as structured charge containers rather than as current switches. Computing with quantum-dot cellular automata is enabled by quantum-mechanical tunneling and Coulomb interactions. The use of molecules as cells to realize quantum-dot cellular automata may make possible nanometer-scale devices and ultra-high device densities without excessive heat dissipation. Molecular quantum-dot cellular automata can be clocked using an external electric field. A time-varying clock can be used to drive data flow through layouts of cells. Together, the clock and the device layout define a computational architecture where data flows through the circuitry in the form of bit packets. Here we analyze the energetics of QCA bit packets. We find a heuristic model based on cell-cell interactions works well. Bit packet energies in general scale with the packet length. It may, however, be possible to design a cell geometry so that the energy is packet-length independent. Fan-out and fan-in can be understood as investing energy from the clock in the signal, and then returning the energy back to the clock.