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Quantum-dot Cellular Automata: Computation with Real-world Molecules
Alessandro P. Bramanti, Alejandro Santana-Bonilla and Ross Rinaldi

Quantum-dot Cellular Automata (QCA) are digital computing machines based on electrostatic and quantum effects. Their basic unit, the cell, is a bistable memory element containing mobile charges. Computation ensues from electrostatic interaction among neighboring cells. QCA are conceived to be implemented at the nanoscale: molecular-scale cells, in particular, would make them operable at ambient temperature. Designing suitable molecular unit is then common ground for chemists and computer scientists, and a shaky one. The classical QCA paradigm, in fact, imposes many, often implicit but severe constraints on the geometry and energy behavior of cells, not easily fulfilled by realistic systems. Here, a general model is developed to include the several symmetry-breaking characteristics of real-world molecules. The consequences of such non-idealities on the computational behavior of molecular QCA are investigated. Some are found very critical to computation; some other configurations, however apparently odd, might surprisingly prove of some usefulness. Anyway, future chemical design of suitable QCA molecular units cannot neglect these findings.

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