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Quantum-dot cellular automata (QCA) is a scheme for molecular electronics in which information is transmitted and processed through electrostatic interactions between charges in an array of quantum dots. QCA wires, majority gates, clocked cell operation, and (recently) true power gain between QCA cells has been demonstrated in a metal-dot prototype system at cryogenic temperatures. Molecular QCA offers very high device densities, low power dissipation, and ways to directly integrate sensors with QCA logic and memory elements. A group of faculty at Notre Dame has been working to implement QCA at the size scale of molecules, where room-temperature operation is theoretically predicted. This paper reviews QCA theory and the experimental measurements in metal-dot QCA systems, and describes progress toward making QCA molecules and working out surface attachment chemistry compatible with QCA operation.

The amine functionality of the linker on the dinuclear complex [trans-Ru(dppm)(2)(Ctbd1;CFc)(NCCH(2)CH(2)NH(2))][PF(6)] reacts with Si-Cl bonds of a chlorinated, highly B doped Si (111) surface to yield Si-N surface-complex bonds. The surface bound complex is constrained to a near vertical orientation by the chain length of the linker as confirmed by variable angle XPS. Oxidation of the dinuclear complex with ferrocenium ion or electrochemically generates a stable, biased Fe(III)-Ru(II) mixed-valence complex on the surface. Characterization of the array of surface bound complexes with spectroscopic as well as electrochemical techniques confirms the presence of strongly bound, chemically robust, mixed-valence complexes. Capping the flat array of complexes with a minimally perturbing mercury electrode permits the equalization of the Fe and Ru energy wells by an applied electric field. The differential capacitance of oxidized and unoxidized bound complexes is compared as a function of voltage applied between the Hg gate and the Si. The results show that electron exchange between the Fe and Ru sites of the array of dinuclear mixed-valence complexes at energy equalization generates a fluctuating dipole that produces a maximum in the capacitance versus voltage curve for each complex-counterion combination present. Passage through the capacitance maximum corresponds to switching of the molecular quantum cellular automata (QCA) cell array by the electric field from the Fe(III)-Ru(II) configuration to the Fe(II)-Ru(III) configuration, thereby confirming that molecules possess an essential property necessary for their use as elements of a QCA device.

Quantum-dot cellular automata (QCA) is an approach to computing that eliminates the need for current switches by representing binary information as the configuration of charge among quantum dots. For molecular QCA, redox sites of molecules serve as the quantum dots. The Coulomb interaction between neighboring molecules provides device-device coupling. By introducing clocked control of the QCA cell, power gain, reduced power dissipation, and computational pipelining can be achieved. We present an ab initio analysis of a simple molecular system, which acts as a clocked molecular QCA cell. The intrinsic bistability of the molecular charge configuration results in dipole or quadrupole fields that couple strongly to the state of neighboring molecules. We show how clocked control of the molecular QCA can be accomplished with a local electric field.

Quantum-dot cellular automata (QCA) involves representing binary information with the charge configuration of closed cells comprised of several dots. Current does not flow between cells, but rather the Coulomb interaction between cells enables computation to occur. We use this system to explore, quantitatively and in a specific physical system, the relation between computation and energy dissipation. Our results support the connection made by Landauer between logical reversibility and physical reversibility. While computation always involves some energy dissipation, there is no fundamental lower limit on how much energy must be dissipated in performing a logically reversible computation. We explicitly calculate the amount of energy that is dissipated to the environment in both logically irreversible “erase” and logically reversible “copy-then-erase” operations carried out in finite time at nonzero temperature. The “copy” operation is performed by using a near-by QCA cell which plays the role of Maxwell's demon. The QCA shift register can then be viewed as a sequence of copy-then-erase operations where the role of the demon cell shifts down the line.

An introduction to the operation of quantum-dot cellular automata (QCA) is presented, along with recent experimental results. QCA is a transistorless computation paradigm that addresses the issues of device density and interconnection. The basic building blocks of the QCA architecture, such as AND, OR, and NOT are presented. The experimental device is a four-dot QCA cell with two electrometers. The dots are metal islands, which are coupled by capacitors and tunnel junctions. An improved design of the cell is presented in which all four dots of the cell are coupled by tunnel junctions. A noninvasive electrometer is presented which improves the sensitivity and linearity of dot potential measurements. The operation of this basic cell is confirmed by an externally controlled polarization change of the cell.

The notion of quantum-dot cellular automata (QCA) as a possible replacement paradigm for conventional transistor-based logic is reviewed. Experiments using metal tunnel structures demonstrating a functional QCA cell are presented. It is shown that single electron tunnelling transistors used as electrometers verify proper switching of the QCA cell. Additionally, we provide evidence for a QCA cell switching frequency of 14 MHz, and a calculated upper limit of more than 5 GHz.

Quantum-dot cellular automata (QCA) is a paradigm for connecting nanoscale bistable devices to accomplish general-purpose computation. The idea has its origins in the technology of quantum dots, Coulomb blockade, and Landauer’s observations on digital devices and energy dissipation. We examine the early development of this paradigm and its various implementations.

We describe a paradigm for computing with interacting quantum dots, quantum-dot cellular automata (QCA). We show how arrays of quantum-dot cells could be used to perform useful computations. A new adiabatic switching paradigm is developed which permits clocked control, eliminates metastability problems, and enables a pipelined architecture.

We examine the dynamic behavior of quantum cellular automata, arrays of artificial quantum‐dot cells that can be used to perform useful computations. The dynamics of the array can be solved directly, retaining the full many‐electron degrees of freedom only for small array sizes. For larger arrays, we develop several approximate techniques for reducing the size of the basis set required. We examine the effect of intercellular quantum correlations on the switching response. Several important examples of switching behavior are solved using the techniques developed.

We examine the possible implementation of logic devices using coupled quantum dot cells. Each quantum cell contains two electrons which interact Coulombically with neighboring cells. The charge distribution in each cell tends to align along one of two perpendicular axes, which allows the encoding of binary information using the state of the cell. The state of each cell is affected in a very nonlinear way by the states of its neighbors. A line of these cells can be used to transmit binary information. We use these cells to design inverters, programmable logic gates, dedicated AND and OR gates, and non‐interfering wire crossings. Complex arrays are simulated which implement the exclusive‐OR function and a single‐bit full adder.

We present an experimental demonstration of power gain in quantum-dot cellular automata (QCA) devices. Power gain is necessary in all practical electronic circuits where power dissipation leads to decay of logic levels. In QCA devices, charge configurations in quantum dots are used to encode and process binary information. The energy required to restore logic levels in QCA devices is drawn from the clock signal. We measure the energy flow through a clocked QCA latch and show that power gain is achieved.

The authors formulate a new paradigm for computing with cellular automata (CAS) composed of arrays of quantum devices-quantum cellular automata. Computing in such a paradigm is edge driven. Input, output, and power are delivered at the edge of the CA array only; no direct flow of information or energy to internal cells is required. Computing in this paradigm is also computing with the ground state. The architecture is so designed that the ground-state configuration of the array, subject to boundary conditions determined by the input, yields the computational result. The authors propose a specific realization of these ideas using two-electron cells composed of quantum dots. The charge density in the cell is very highly polarized (aligned) along one of the two cell axes, suggestive of a two-state CA. The polarization of one cell induces a polarization in a neighboring cell through the Coulomb interaction in a very non-linear fashion. Quantum cellular automata can perform useful computing. The authors show that AND gates, OR gates, and inverters can be constructed and interconnected.

We present the experimental characterization of a leadless (floating) double-dot system and a leadless quantum-dot cellular automata cell, where aluminum metal islands are connected to the environment only by capacitors. Here, single electron charge transfer can be accomplished only by the exchange of an electron between the dots. The charge state of the dots is monitored using metal islands configured as electrometers. We show improvements in the cell performance relative to leaded dots, and discuss possible implications of our leadless design to the quantum-dot cellular automata logic implementation.

Quantum-dot Cellular Automata (QCA) is a promising architecture which employs quantum dots for digital computation. It is a revolutionary approach which addresses the issues of device density and power dissipation. With a dot size of 20 nm an entire full adder would occupy only one square micron, and the power delay product is as low as a few kT. A basic QCA cell consists of four quantum dots coupled capacitively and by tunnel barriers. The cell is biased to contain two excess electrons within the four dots, which are forced to opposite "corners" of the four-dot system by Coulomb repulsion. These two possible polarization states of the cell represent logic "0" and "1". Properly arranged, arrays of these basic cells can implement Boolean logic functions. We present experimental results from functional QCA devices built of nanoscale metal dots defined by tunnel barriers.

An introduction to the operation of quantum-dot cellular automata is presented, along with recent experimental results. Quantum-dot cellular automata (QCA) is a transistorless computation paradigm that addresses the issues of device density and interconnection. The basic building blocks of the QCA architecture, such as AND, OR, and NOT are presented. The experimental device is a four-dot QCA cell with two electrometers. The dots are metal islands, which are coupled by capacitors and tunnel junctions. An improved design of the cell is presented in which all four dots of the cell are coupled by tunnel junctions. The operation of this basic cell is confirmed by the externally controlled polarization change of the cell.

This paper presents an overview of the electronic implementation of quantum-dot cellular automata (QCA). QCA is a computing paradigm that encodes and processes information by the position of individual electrons. This opens the possibility of dense, ultra-low power devices. Resent results are presented from QCA cells implemented using metal-dots, as well as investigations toward molecular and silicon QCA devices.

We report an experimental demonstration of a logic cell for quantum-dot cellular automata (QCA). This nanostructure-based computational paradigm allows logic function implementation without the use of transistors. The four-dot QCA cell is defined by a pair of series-connected double dots, and the coupling between the input and the output double dots is provided by lithographically defined capacitors. We demonstrate that, at low temperature, an electron switch in the input double dot induces an opposite electron switch in the output double dot, resulting in a polarization change of the QCA cell. Switching is verified from the electrometer signals, which are coupled to the output double dot. We perform theoretical simulations of the device characteristics and find excellent agreement with theory.

In this work, we study the dynamics of molecular quantum-dot cellular automata (QCA) devices by combining the technique of quantum chemistry and coherence vector formalism. First, we apply the quantum chemistry ab initio calculation on the single molecule, and construct a simple model Hamiltonian for each QCA cell based on the calculation. For a three-dot molecule discussed in this work, we show a 3×3 Hamiltonian is sufficient to describe its switching behavior. The second step is to use the model Hamiltonian to calculate the coherence vector evolved as time, from which we will investigate the dynamic behavior of various QCA devices and circuits. Finally, we discuss the structure-functionality relation between the molecular structure and circuit dynamics.

An introduction to quantum-dot cellular automata (QCA) is presented along with experimental implementations. QCA is a transistorless nanoelectronic computation paradigm that addresses the issues of device and power density, which are becoming increasingly important in the electronics industry. Scaling of CMOS is expected to come to an end in the next 10-15 years, with perhaps the most important limiting problem being the power density and the resulting heat generated. QCA offers the possibility of circuitry that dissipates many orders of magnitude less power than CMOS, is scalable to molecular dimensions, and provides the power gain necessary to restore signal levels.

Quantum-dot cellular automata (QCA) is a strategy in which binary data is represented by charge configuration within a multi-dot cell. Data is transmitted to nearest neighbors by the Coulombic interaction. An electric field acts as a clock and imposes directionality on circuits. We have explored the connection between logical reversibility and physical reversibility in the context of a QCA system, explicitly calculating the energy dissipated by performing an erasure as a function of the time over which it is performed [1]. Further, we present a Bennett-style clocking scheme to implement reversible computation that is natural to the circuits to minimize the amount of information that is erased. Molecular QCA may provide a practical implementation of reversible computing

In this report, we describe the fabrication and experimental demonstration of fanout in QCA. Fanout is important as it is necessary for complex digital logic circuits and is essential for generating compact designs, as multiple cells can be then driven by a single driver cell. Fanout in QCA is also a direct demonstration of power gain in QCA circuits. The device is realized using metal islands (as quantum dots) and multiple tunnel junctions (MTJs) fabricated using Dolan bridge technique (Fulton, 1987). The circuit consists of three latches, with the latch in the first stage (L1) capacitively coupled to the two latches of the second stage (L2 and L3). The goal of the experiment is to switch L2 and L3 simultaneously using L1 as an input driving both L2 and L3. Each latch is formed by three quantum dots with the middle dot being connected to the end dots by MTJs

This paper reports on the dynamic behavior of power dissipation in a clocked QCA majority gate. A distributed clocking scheme is employed in the QCA array to form a "computation wave" which moves smoothly across the circuit. The quantum dynamical calculation is done with coherence vector formalism with dissipation incorporated so that we can see power flowing to the environment, and also to and from the clocking circuit

Quantum-dot cellular automata (QCA) is an approach to computing which eliminates the need for transistors by representing binary digits as charge configurations rather than current levels. Coulomb interactions provide device-device coupling without current flow. Clocked control of the device allows power gain, control of power dissipation, and pipelined computation. Molecular QCA uses redox sites of molecules as quantum dots. We present an ab initio analysis of a simple molecular system which acts as a clocked molecular QCA cell.

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 device 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.

We discuss the fundamental limits of computing using a new paradigm for quantum computation, cellular automata composed of arrays of coulombically coupled quantum dot molecules, which we term quantum cellular automata (QCA). Any logical or arithmetic operation can be performed in this scheme. QCA's provide a valuable concrete example of quantum computation in which a number of fundamental issues come to light. We examine the physics of the computing process in this paradigm. We show to what extent thermodynamic considerations impose limits on the ultimate size of individual QCA arrays. Adiabatic operation of the QCA is examined and the implications for dissipationless computing are explored. >

A coherence vector formalism is used to describe quantum computing with quantum-dot cellular automata, and the realizations of basic quantum gates are also discussed.

We discuss a novel nano-electronic computing paradigm in which cells composed of interacting quantum dots are employed in a locally-interconnected architecture. We develop a network-theoretic description in terms of appropriate local state variables in each cell.

An overview is given of the QCA architecture, along with a summary of experimental demonstrations of QCA devices. Quantum-dot cellular automata (QCA) is a transistorless computation paradigm that addresses such challenging issues as device and power density. The basic building blocks of the QCA architecture, such as AND, OR gates and clocked cells have been demonstrated and are presented. The quantum dots used in the experiments are metal islands that are coupled by capacitors and tunnel junctions. An improved design of the cell is presented in which all four dots of the cell are coupled by tunnel junctions. A noninvasive electrometer is presented which improves the sensitivity and linearity of dot potential measurements. The operation of this basic cell is confirmed by an externally controlled polarization change of the cell.

Presents an experimental demonstration of two novel clocked QCA devices - a QCA latch and a QCA shift register. We demonstrate the operation of the devices, and discuss sources and methods of lowering the digital errors in QCA clocked devices.

Discusses an experiment that demonstrates power gain in a quantum-dot cellular automata (QCA) shift register. Power gain is essential in any electronic system for the restoration of logic levels. The clock signal plays an important role in providing power gain in QCA devices as it can be used as a source of energy for the system. We discuss how this can be achieved in a QCA shift register and experimentally demonstrate a power gain greater than unity.

A simple model quantum dot cell containing two electrons is analyzed as a candidate for quantum cellular automata implementations. The cell has eigenstates whose charge density is strongly aligned along one of two directions. In the presence of the electrostatic perturbation due to a neighboring cell, the ground state is nearly completely aligned (polarized) in one direction only. The polarization is a highly nonlinear function of the perturbing electrostatic fields and shows the strong bistable saturation important for cellular automation function.

Quantum-dot cellular automata (QCA), arrays of coupled quantum-dot devices, are proposed for quantum computing. The notion of coherent QCA (CQCA) is introduced in order to distinguish QCA applied to quantum computing from classical digital QCA. Information is encoded in the spatial state of the electrons in the multidot system. A line of CQCA cells can work as a quantum register. The basic single- and multi-qubit operations can be realized by pulses given to the cell electrodes.

This AASERT award augmented and provided additional support for the parent grant entitled "Quantum Cellular Automata" (QCA), ONR grant N00014-93-1-1084. Initially, the main focus of this research was the study of Coulombic coupling effects in semiconductor nanostructures for applications in quantum cellular automata. We considered both III-V and Si-based semiconductor systems. In later years, the AASERT award supported work on QCA realizations in Coulomb-blockade metal-dot systems, which were successful in demonstrating the basic QCA switching operation. This latter work has contributed in large measure to the visibility and success of the QCA concept. Recently, AASERT-supported students have explored the applicability of QCA systems for quantum computing.

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 + 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.

We establish a method for exploring the dynamics of molecular quantum-dot cellular automata (QCA) devices by hierarchically combining the techniques of quantum chemistry with the nonequilibrium time-dependent coherence vector formalism. Single QCA molecules are characterized using ab initio quantum chemistry methods. We show how to construct a simple model Hamiltonian for each QCA cell based on parameters extracted from the ab initio calculation. The model Hamiltonian captures well the relevant switching behavior and can then be used to calculate the time-dependent coherence vector, including thermal and nonequilibrium behavior. This enables us to explore dynamic behavior and power dissipation for various QCA devices and circuits.

Quantum-dot cellular automata (QCA) may provide a novel way to bypass the transistor paradigm to form ultrasmall computing elements. In the QCA paradigm information is represented in the charge configuration of a QCA cell, which maps naturally to a binary model. Molecular QCA implementations are being explored where the quantum dots in the cell take the form of redox centers in a molecule. Clocking has proved important in QCA cells synthesized so far. Here we examine a method to clock molecular QCA cells which are assembled at an interface. The clocking signals in this scheme originate from wires buried below the QCA surface. We present a simplified model of this clocking method and examine its behavior as a molecular shift register.

We study the operation of quantum cellular automata (QCA) devices in the presence of stray charge. The operation of linear arrays of QCA cells, called binary wires, relies on Coulombic interaction between the cells, which is affected by the presence of such stray charge. The position of the charge determines whether or not the devices function properly, and it is possible to determine the “forbidden” region near the array in which the presence of stray charge causes device failure. We calculate this forbidden region by directly diagonalizing the Hamiltonian for the system including the stray charge. We find that the QCA binary wire is unaffected by stray charge at a distance greater than the intercellular repeat distance of the wire.

Devices based on the quantum-dot cellular automata (QCA) computational approach (Lent et al, 1993) use interacting quantum dots to encode and process binary information. In this transistorless approach to computation, logic levels are represented by the configurations of single electrons in coupled quantum-dot systems. In the last few years, significant progress has been made towards the realization of basic QCA elements. However, in these devices, power gain needed for the operation of large QCA arrays was not possible since the only source of energy was the signal input. Recent theoretical work (Lent and Tougaw, 1997) proposed clocked control of the QCA circuitry. Clocked controlled QCA systems have many advantages such as power gain, reduced power dissipation, and pipelined architectures. The original theoretical work applied only to semiconductor implementation of clocked QCA arrays, but recently a scheme for clocked control of metallic QCA cells was proposed (Toth and Lent, 1999; Korotkov and Likharev, 1998). Here an extra dot placed between the two dots of the QCA half-cell acts as a tunable barrier controlled by the clock signal. We present the experimental demonstration of a clocked QCA cell. The device consists of two capacitively coupled half-cells, where each half-cell consists of three micron-size Al islands separated by tunnel junctions, and four electrometers to measure the charge state of the half-cells. The half-cells are leadless, with no DC connection to the environment.