Switching Error Modes of QCA Circuits

The Quantum-dot Cellular Automata (QCA) model offers a novel nano-domain computing architecture by mapping the intended logic onto the lowest energy configuration of a collection of QCA cells, with two possible ground states for each cell. A four phased clocking is used to keep the computations at the ground state throughout the circuit. Computing errors in QCA circuits can arise due to the failure of the clocking scheme to switch portions of the circuit to its new ground state with change in input. To study these switching errors we need to consider low-energy state configurations of QCA circuits. However, current QCA simulators compute just the ground state configuration of a QCA arrangement. In this paper, we offer an efficient method, based on graphical probabilistic models, to compute the N-lowest energy modes of a clocked QCA circuit. The overall low-energy, excited, spectrum of multiple clocking zones is constructed by concatenating the excited spectra of the individual clocking zones. We demonstrate the use of this error model by comparing different designs of wire crossings.