Molecule Based Materials for Quantum Cellular Automata: A Short Overview and Challenging Problems

Abstract

In this review we shortly summarize in an accessible way the physical and nanoscale material science aspects of the promising field of molecular quantum cellular automata (QCA). QCA is a revolutionary paradigm in quantum electronics with promising application in the conceptually new computing scheme which can be referred to as “computing with molecules”. This scheme of devices have vitally important advantages compared with conventional schemes of quantum computing in which the binary information is stored in the eigenvectors of a two‐level quantum system and therefore their practical application is strongly limited by the requirement of coherence. Molecular QCA promise nanometer‐scale units with ultra‐high device densities, as well as room temperature operation with extremely small heat release. Molecular materials provide at the same time options to control the key properties of the active molecules by chemical means. Hereunder we summarize the background of the electronic and vibronic problems in QCA cells represented by the tetrameric mixed‐valence (MV) complexes and organic molecules proposed for implementation as four‐site molecular QCA. We discuss the basic model of the molecular cell that include the following ingredients: 1) intracell Coulomb repulsion energy of the electrons in different electronic distributions among the redox sites; 2) electron transfer between redox sites which changes electronic distributions; 3) interaction of the itinerant electrons with molecular vibrations which is referred to as the vibronic interactions; 4) intercell Coulomb interaction by mean of which the binary information is transmitted from cell to cell. On the basis of this model we will study the cell polarization, adiabatic picture of the switching cycle describing interaction between the “input” and “output” molecular cells, non‐linear cell‐cell response function whose shape describes the action of molecular logic gates. Along with developed and current problems, we discuss new challenging trends of research in this fascinating area.

Controllable Electron Transfer in Mixed-Valence Bridged Norbornylogous Compounds:Ab InitioCalculation Combined with a Parametric Model and Through-Bond and Through-Space Interpretation

“…Finally, one should mention the direct observation in the scanning tunneling microscopy experiments of the electron localization at the single-molecule scale in view of the comparative role of the through-bond and through-space coupling in mixed-valence molecules. This aspect of the ET phenomenon is important in view of the development of the area of molecular quantum cellular automata that has been already mentioned (see refs − ).…”

“…By arranging the cells, one can create various logical devices such as quantum wires, inverters, majority logic gates, and integrated circuits. It has been demonstrated that the functional properties of molecular MV cells such as cell–cell response are closely related to the ET processes and vibronic coupling. − To date, a number of organic and inorganic MV systems have been proposed as promising candidates to obtain molecular cells for quantum cellular automata. − …”

In Quest of Molecular Materials for Quantum Cellular Automata: Exploration of the Double Exchange in the Two-Mode Vibronic Model of a Dimeric Mixed Valence Cell

“…As a further development of the concept of QCA based on quantum dots, a new fruitful idea of usage molecules as cells was proposed (see discussion in Reference [6] and a short overview in [7]). As natural candidates act as molecular cells, the dimeric MV molecules with one mobile excess electron and tetrameric MV molecules with two excess electrons have been proposed, as these bistable systems can encode the binary information [6].…”

Zwitterionic Mixed-Valence Species for the Design of Neutral Clocked Molecular Quantum-Dot Cellular Automata

“…Some prerequisites must be met, in particular: a two-dot QCA compound must have (i) a stable mixed-valence state with a rather large comproportionation constant; (ii) a moderate coupling between the two redox centers (rapid exchange of the charge between the two sites is necessary); and less importantly, (iii) the capability of functionalization for surface binding in ordered arrays. Large comproportionation constants and moderate coupling between the two redox centers are typically observed for partially localized Class-II systems according to the Robin–Day classification of MV compounds. , Several two-dot and four-dot organic and organometallic QCA candidate MV compounds have been proposed either theoretically − or experimentally, − more or less successfully for QCA applications.…”