The quantum-dot cellular automata (QCA) paradigm is a revolutionary approach to molecular-scale computing which represents binary information using the charge configuration of nanostructures in lieu of current switching devices. The basic building-block of QCA devices is the QCA cell. Electrostatic interaction between neighboring cells allows the design of QCA wires, logic devices and even simple microprocessors. The geometry of molecular six-dot QCA cells enables the clocking of QCA devices via an electric field generated by a layout of clocking wires. Thus, precise control over the timing and direction of data flow in QCA circuits is possible. The design of QCA circuits now lies not only in the logic structure of the cells, but also in the layout of clocking wires. We discuss the clocking of QCA devices and connect layout to architecture.Kqvords-architecture; clocking; defect tolerance; molecular QCA I.

Molecular quantum-dot cellular automata is a computing paradigm in which digital information is encoded by the charge configuration of a mixed-valence molecule. General-purpose computing can be achieved by arranging these compounds on a substrate and exploiting intermolecular Coulombic coupling. The operation of such a device relies on nonequilibrium electron transfer (ET), whereby the time-varying electric field of one molecule induces an ET event in a neighboring molecule. The magnitude of the electric fields can be quite large because of close spatial proximity, and the induced ET rate is a measure of the nonequilibrium response of the molecule. We calculate the electric-field-driven ET rate for a model mixed-valence compound. The mixed-valence molecule is regarded as a two-state electronic system coupled to a molecular vibrational mode, which is, in turn, coupled to a thermal environment. Both the electronic and vibrational degrees-of-freedom are treated quantum mechanically, and the dissipative vibrational-bath interaction is modeled with the Lindblad equation. This approach captures both tunneling and nonadiabatic dynamics. Relationships between microscopic molecular properties and the driven ET rate are explored for two time-dependent applied fields: an abruptly switched field and a linearly ramped field. In both cases, the driven ET rate is only weakly temperature dependent. When the model is applied using parameters appropriate to a specific mixed-valence molecule, diferrocenylacetylene, terahertz-range ET transfer rates are predicted.

In the molecular quantum-dot cellular automata (QCA) paradigm clocking wires are used to produce an electric field which is perpendicular to the device plane of surface-bound molecules and is sinusoidally modulated in space and time. This clocking field guides the data flow through the molecular QCA array. Power is dissipated in clocking wires due to the non-zero resistance of the conductors. We analyze quantitatively the amount of power dissipated in the clocking wires and find that in the relevant parameter range it is fairly small. Dissipation in the molecular devices themselves will likely dominate the energy budget. Quantum-dot cellular automata (QCA)QCA is a novel approach to computing at the nanoscale [1, 2] which is enabled by quantum-mechanical tunneling and simple Coulomb interactions but does not rely on the flow of current [3]. Information is encoded in the charge configuration of cells comprised of quantum dots. Coulomb interaction between cells creates device-device coupling that has been shown to support general-purpose computation.QCA operation has been demonstrated in several materials systems at low temperatures . Circuit-level QCA devices composed of metal tunnel junctions have been fabricated. Inverters, logic gates, and shift registers have been demonstrated. These experiments have also demonstrated fan-out, power gain, and significant error tolerance in QCA systems [20,26,28]. The major drawback is the need for cryogenic operation.Molecular QCA offers the promise of nanometer-scale devices with accompanying ultra-high device densities as well as room-temperature operation, yet without suffering the crippling heat dissipation which prevents achieving such densities in current-switching technologies [29][30][31]. Room temperature operation at the molecular scale has been predicted theoretically [32] and confirmed experimentally [33].Small assemblies of QCA cells operate properly as circuits by simply relaxing to the ground state configuration. Large circuits require clocked operation [34]. Clocking is effected by controllably varying the relative energies of cell charge configurations. This has been demonstrated in (cryogenic) metal-dot cells. Clocking in the QCA paradigm provides the mechanism for power gain, which is crucial for the restoration of weakened signals, and minimizes power dissipation [28,35] by enabling quasi-adiabatic switching.In metal-dot and semiconductor-dot QCA clocking signals can be directly applied to the individual QCA dots. For molecular devices, this would be impractical. However, it has been shown that molecular QCA devices attached to a surface can be clocked using an electric field, E z (x, y, t), perpendicular to the surface, that varies smoothly in space and time [36]. Locally, this field is usually simply a traveling sinusoidal wave which slides across the surface. This time-varying field guides data flow through arrays of cells. It does not affect the value of the data itself, which is contained in the cellular state and moves by cell-cell coupling

Quantum-dot cellular automata (QCA) is a lowpower, non-von-Neumann, general-purpose paradigm for classical computing using transistor-free logic. An elementary QCA device called a "cell" is made from a system of coupled quantum dots with a few mobile charges. The cell's charge configuration encodes a bit, and quantum charge tunneling within a cell enables device switching. Arrays of cells networked locally via the electrostatic field form QCA circuits, which mix logic, memory and interconnect. A molecular QCA implementation promises ultra-high device densities, high switching speeds, and roomtemperature operation. We propose a novel approach to the technical challenge of transducing bits from larger conventional devices to nanoscale QCA molecules. This signal transduction begins with lithographically-formed electrodes placed on the device plane. A voltage applied across these electrodes establishes an in-plane electric field, which selects a bit packet on a large QCA input circuit. A typical QCA binary wire may be used to transmit a smaller bit packet of a size more suitable for processing from this input to other QCA circuitry. In contrast to previous concepts for bit inputs to molecular QCA, this approach requires neither special QCA cells with fixed states nor nanoelectrodes which establish fields with single-electron specificity. A brief overview of the QCA paradigm is given. Proof-of-principle simulation results are shown, demonstrating the input concept in circuits made from two-dot QCA cells. Importantly, this concept for bit inputs to molecular QCA may enable solutions to or provide insights into other challenges to the realization of molecular QCA, such as the demonstration of molecular device switching, the read-out of molecular QCA states, and the layout of molecular QCA circuits.

We consider the effects of interaction with the environment on decoherence in quantum-dot cellular automata (QCA). We model the environment as a Coulombically interacting random assembly of quantum double-dots. The time evolution of our model system þ environment is unitary and maintains one coherent state. We explicitly calculate the reduced density operators for the system and for the environment from the full coherent state. From the reduced density matrix of the system, we calculate the coherence vector and the Von Neumann entropy. The entanglement of system and environmental degrees of freedom lead to decoherence, which drives the system into the Zurek pointer states. The quantum information lost by the system, quantified by the entropy, is present in the quantum mutual information between the system and the environment. We explore the competition between environmental decoherence and system dynamics. For even a modest environmental interaction, the pointer states are the QCA information-bearing degrees of freedom, so that environmental decoherence, while destructive of quantum information, tends to stabilize QCA bit information. V