Quantum-dot Cellular Automata Wire Research Articles

A general and efficient clocking scheme for majority logic in quantum-dot cellular automata

Quantum-dot cellular automata (QCA) is a nanotechnology with characteristics of high speed, high density, and low power consumption to be one of the strong alternatives to replace traditional CMOS in the future in theory. Its primitive is a majority gate, instead of AND/OR logic in CMOS, which can reduce the complexity of a system and thus its overhead. In this paper, a general and efficient clocking scheme to facilitate the design of circuits using majority gates in QCA is proposed as their scale extends. Each clock zone in this scheme is an octagon adjacent to the other three zones. This results in four input and two output directions in clock zones 0 and 2 to design circuits with majority gates, two input and four output directions in clock zones 1 and 3 to improve the flexibility of circuit routing. Besides, this scheme utilizes the flexible feedback paths for sequential circuits, equivalent clock zones, and elastic QCA wires for efficiently wiring circuits. This paper then takes a one-bit full adder and an SR-latch as examples to show the generality and efficiency of the proposed clocking scheme in designing combinational and sequential circuits, respectively. An arithmetic unit is also taken as an example to illustrate its high expansibility in designing larger circuits. Experimental results show that the proposed clocking scheme is more suitable for QCA circuit design in contrast to previous clocking schemes.

Clocked molecular quantum-dot cellular automata circuits tolerate unwanted external electric fields

Quantum-dot cellular automata (QCA) may provide low-power, general-purpose computing in the post-CMOS era. A molecular implementation of QCA features nanometer-scale devices and may support ∼THz switching speeds at room-temperature. Here, we explore the ability of molecular QCA circuits to tolerate unwanted applied electric fields, which may come from a variety of sources. One likely source of strong unwanted electric fields may be electrodes recently proposed for the write-in of classical bits to molecular QCA input circuits. Previous models have shown that the input circuits are sensitive to the applied field, and a coupled QCA wire can successfully transfer the input bit to downstream circuits despite strong applied fields. However, the ability of other QCA circuits to tolerate an applied field has not yet been demonstrated. Here, we study the robustness of various QCA circuits by calculating their ground state responses in the presence of an applied field. To do this, a circuit is built from several QCA molecules, each described as a two-state system. A circuit Hamiltonian is formed and diagonalized. All pairwise interactions between cells are considered, along with all correlations. An examination of the ground state shows that these QCA circuits may indeed tolerate strong unwanted electric fields. We also show that circuit immunity to the dominant unwanted field component may be obtained by choosing the orientation of constituent molecules. This suggests that relatively large electrodes used for bit write-in to molecular QCA need not disrupt the operation of nearby QCA circuits. The circuits may tolerate significant electric fields from other sources as well.

Triangular Quantum-Dot Cellular Automata Wire for Standard Ternary Logic

Quantum-dot cellular automata (QCA) is a computational paradigm that is based on encoding information in terms of the configuration of charge among quantum dots without requiring current switches. Mostly, QCA cells have been realized by employing the electron switching in square quantum-dot cells. In this paper, a triangular metal-dot cell with a single electron is proposed to represent ternary information. Then, an inverter chain of the proposed triangle cells is suggested through juxtaposing the cells. Additionally, wire crossing and wire fan-out are the two inseparable subjects of QCA wire. A wire-crossing method is presented for the proposed ternary wire. Wire fan-out is also dealt with in this paper by an innovative approach. The correctness of operation of proposed cells and wires are thoroughly verified by applying the physical relations.

Design and simulation of quantum-dot cellular automata serial decimal pipelined processor based on Turing machine model

The quantum-dot cellular automata (QCA) nanoscale computer technology is promising to overcome the limits of the microelectronic CMOS technology. Because the leading role of QCA wires, the serial data transfer/processing is preferable. The financial, Internet of Things, and control computer applications require direct processing of decimal information without representation and conversion errors. Because a QCA wire can be considered as a virtual tape with written binary symbols, a special version of Turing machine model can be used for a QCA computer implementation. Design of a novel QCA serial decimal pipelined processor based on the Turing machine model is presented. The processor uses the run-time tape reconfiguration for arithmetic processing of decimal operands encoded in the 5-bit Johnson-Mobius code. The proposed design demonstrates significant hardware simplification.

Population congestion in 3-state quantum-dot cellular automata

The behavior of quantum-dot cellular automata (QCA) networks is typically understood through considering polarization-like interactions with energies arising from the agreement or disagreement of the defined polarization states of neighboring QCA devices. It is known that additional interactions are present in 3-state molecular QCA that alter the required clocking fields needed for a device operation. Recent efforts in implementing logic gates using patterned dangling bonds (SiDBs) on hydrogen passivated silicon reveal significant challenges arising from similar effects. The necessary applied electrical potential needed to increase the population of an SiDB is strongly dependent on the current population of its neighbors, an effect we term congestion. It is unclear whether the strength of these interactions may pose an obstacle for future applications of SiDBs as a nanoscale QCA architecture. In this work, we investigate 3-state QCA in the regime in which congestion is significant and determine the extent to which such effects can be mitigated for SiDB devices. We propose that while SiDB-based QCA wires may be achievable depending on limitations of inter-dot tunneling, higher density devices such as majority gates may need to be replaced by more architecture specific implementations unless net-neutral variants of SiDB QCA devices can be demonstrated.

Design and implementation of finite state machine using quantum-dot cellular automata

Moore's law states that the number of transistors per square inch on integrated circuits has doubled approximately every two years; this is true for CMOS based VLSI circuit design. Quantum-dot cellular automata (QCA) replaces CMOS based VLSI technology. The assembly of quantum dots replaces transistors which is said to be 'quantum-dot cellular automata', an emerging nanotechnology in the field of quantum electronics. Such type of circuit can be used in many digital applications and has an advantage of reduced area utilisation. Quantum mechanics and cellular automata are together said to be QCA. QCA technology has advantages like small size and high speed. CMOS technology uses transistors to create a logic gates but in QCA technology, logic gates and wires are created by using QCA cells. The basic logic gates like AND, OR, inverter, majority gates are implemented. Many combinational and sequential circuits are designed by using these basic gates. This paper aims at the design of finite state machines and its use in vending machine and traffic light controller were discussed. The circuit was designed and the functionality of those was verified using QCADesigner tool.

Characterisation of a bis‐ferrocene molecular QCA wire on a non‐ideal gold surface

Field-coupled nanocomputing represents one of the possible proposals for the post-Complementary Metal Oxide Semiconductor (CMOS) scenario. Contrarily to standard technology, the information is not propagated using electron transport, but via field interactions among single elements. The molecular quantum-dot cellular automata (QCA) is one of the most promising implementations; redox centres of oxidised molecules are used to concentrate charges. Coulomb interaction between redox centres enables the information propagation. The necessity for charge transport is overcome, entailing very low power consumption notwithstanding the nanometric size and the very high expected operating frequency. Nevertheless, as the Coulomb interaction strongly depends on the distance between charges, the position of each molecule plays a relevant role in the interaction. This work investigates the information propagation of a possible molecular wire thiolated on a non-ideal gold surface, where the intermolecular distance may vary due to the surface roughness and possible defects introduced in the formation of the molecular wire. The efficiency of the information transfer is analysed with the aim of providing significant constrains and suggestions for future fabrication steps.

Design of novel carry save adder using quantum dot-cellular automata

Complex adder designs in Quantum Dot-Cellular Automata (QCA) are primary focus of researchers on lowering cell-count, delay and QCA gates. The cell count, area and delay of two input adders such as Brent-Kung, Ladner-Ficher, Han-Carlson and Kogg-Stone are at least doubles when three numbers are added, compared to adding two numbers. Besides, the interconnected QCA wires for these adders decrease the addition time. To eliminate this problem, a new 4-bit Carry Save Adder (CSA) is proposed in this paper. To design the CSA circuit, a novel design of full adder circuit using 5-input majority gate (MV) has been proposed. The proposed design has around 11.76% improvement in cell count and 33.33% improvement in latency compared to the existing QCA full-adders. Besides, when three binary numbers are added, CSA is far more advantageous compared to similar addition using two input prefix adders, in terms of QCA cell count, area and clock delay. Compared to 4-bit traditional adders, proposed CSA has around 80% decreases in overall circuit cost. The QCA layout of CSA is the first one of its kind. An improved QCA layout of a 4-bit ripple carry adder (RCA) is also proposed in this paper. The RCA has around 7.6% improvement in cell count, 33.33% improvement in latency and around 5.56% improvement in overall cost over compared to the existing layouts.

An Optimal design of 2-to-4 Decoder circuit in coplanar Quantum-dot cellular automata

Quantum-dot Cellular Automata (QCA), an emergent nanotechnology which promises to deals with the limitations of CMOS technology. This technology offers very low power operation of digital systems with high speed. This paper represents an efficient 2-to-4 decoder, which has been designed with the help of three-input majority voters and successfully implemented in QCA. The design is optimized by considerably reducing the number of cell counts and QCA wire crossings. The proposed decoder is more robust and enjoys single layer wire crossing, via clock phasing, which requires only one type of cell. The results shows that the proposed decoder circuit performs equally well compared to existing decoder designs and performs better in case of previous coplanar decoder designs with enable input functionality in all aspects. Our design achieves 41% and 28% improvement in number of cell counts and 43% and 42% area improvements in comparison to the existing five input and three input based majority gates decoder designs respectively, which have presented in this paper. QCA Designer tool is used to validate the layout of the proposed designs.

Quantum-dot cellular automata serial decimal processing-in-wire: Run-time reconfigurable wiring approach

The quantum-dot cellular automata (QCA) technology is promising to overcome the limits of CMOS technology for perspective computers. A chain of QCA is used as a wire. Logic gates are simply implemented by a cross pattern of QCA. Because the leading role of QCA wires, the serial data transfer/processing is preferable. The growing market of financial, Internet-based, and automatic control computer applications requires a binary-coded decimal data encoding for direct processing of decimal information without representation and conversion errors. The 5-bit decimal Johnson–Mobius encoding and radical departure from Boolean logic allow using a delay element, implemented by short length of QCA wire, as a function element. The previous published by author serial decimal QCA arithmetic designs demonstrate hardware simplification in comparison with traditional designs. The paper presents novel serial decimal adder and adder/subtractor designs used the run-time reconfigurable wiring approach, which results in further significant QCA hardware simplification.

Design of QCA based Programmable Logic Array using decoder

A novel cost effective design of Programmable Logic Array (PLA) is proposed by recursive use of XOR gate, which is used to design 2×4, 3×8 and 4×16 decoders. The 4×16 decoder is coupled with an OR-Array to implement the proposed PLA using Quantum-dot Cellular Automata (QCA). The design is made effective by substantially reducing QCA wire crossing and cell count. A comparative study shows the minimization of cell count and clock-cycle delay of the XOR and Decoders. The PLA is utilized to design an efficient and delay effective 2 bit full adder.

An optimal design of full adder based on 5-input majority gate in coplanar quantum-dot cellular automata

Quantum-dot cellular automata (QCA) is one of the alternative technologies that enable nanoscale circuit design with high performance and low power consumption features. In this aspect, QCA wire crossing is a challenging task in the coplanar QCA fabrication, as defects appear to be inherent due to two cell types in single layout structure. This work showcases an extensive structural and power analysis of previous 5-input majority gates. It has been found that the existing 5-input majority gates are not power efficient and the structures are not well optimized. To overcome this, we have proposed a new low-complexity coplanar 5-input majority gate, which consumes less power compared to prior designs. To evaluate the usefulness of proposed gate a new one bit full adder circuit is presented. The proposed full adder is more robust and enjoys single layer wire crossing, via clock phasing, which requires only one type of cell. The results show that the proposed full adder performs equally well compared to existing multilayer designs and performs better in case of previous coplanar full adder designs in all aspects. Our design achieves 20% improvement in cell count and consumes 7% less area in comparison to the best single layer design. QCADesigner tool is used to validate the layout of the proposed designs and QCAPro power estimator tool is used to evaluate the power dissipation of all considered designs.

Delay-based processing-in-wire for design of QCA serial decimal arithmetic units

Quantum-dot cellular automata (QCA) technology is now considered to be one of the prospective technologies for a nanocomputer creation. The physical properties of QCA and its expanding range of computer applications make it expedient to use the novel paradigm of nanocomputer architecture: serial decimal storage-transfer-processing. The delay-based encoding of decimal digits allows the use a delay element as a main element of QCA serial arithmetic units. The simple implementation of the delay element by a short length of QCA wire results in reduction of complexity and of the area required for a QCA circuit. The theoretical basics of delay-based processing-in-wire and design examples of QCA serial decimal arithmetic units are presented.

A Novel Presentation of Toffoli Gate in Quantum-dot Cellular Automata (QCA)

Quantum dot Cellular Automata (QCA) is one of the emerging nanotechnologies, promising alternative to CMOS technology due to faster speed, smaller size, lower power consumption, higher scale integration and higher switching frequency. The basic element in QCA is majority gate. This paper, present two different design layout of Toffoli gate based on QCA logic gates: majority voter gate (MV), QCA wire and inverter gate. QCADesigner, a common QCA layout design and verification tool is employed to verify and simulate the proposed Toffoli Gate (TG). The simulation result confirmed the correctness of the proposed circuits. The proposed circuit has a promising future in constructing of nano-scale low power exhausting information processing system and can stimulate higher digital applications in QCA. General Terms Nano Technology, Quantum dot Cellular Automata (QCA).

Design and simulation of novel adder/subtractors on quantum-dot cellular automata: Radical departure from Boolean logic circuits

The quantum-dot cellular automata (QCA) technology is now considered as one of the perspective technologies for a nanocomputer creation. The physical properties of QCA make it expedient to use serial data transfer/processing. The use of decimal notation and decimal arithmetic in QCA computing eliminates the decimal-to-binary conversion errors and processor time loses for these conversions, and also simplifies the software and increases the computer reliability. The 5-bit decimal Johnson-Mobius encoding simplifies decimal processing algorithms and allows error detection. The paper presents two original serial decimal adder/subtractor designs and a simple change-of-level Johnson-Mobius code error detector design on QCA. The radical departure from traditional parallel Boolean logic processing allows the use of a delay element implemented by short length of QCA wire as a function element. Such approach results in QCA decimal hardware simplification.

Stray Charge in Quantum-dot Cellular Automata: A Validation of the Intercellular Hartree Approximation

The authors analyze the effect of stray charges near a line of quantum-dot cellular automata (QCA) cells. Considering both the ground-state polarization and the excitation energy of the system, it is determined that there is a 129-nm-wide region surrounding a QCA wire where a stray charge will cause the wire to fail. This calculation is the result of a full-basis-set simulation of a four-cell line. A comparison is made between cells with parallel-spin electrons and those with antiparallel spin electrons, showing that they yield essentially identical results. Therefore, the added complexity of accounting for antiparallel spins does not yield superior simulation results. Finally, a comparison is made between the full-basis calculations and the results of the same calculation using the intercellular Hartree approximation (ICHA). The similarity of these two results demonstrates that the ICHA method is a valid tool for studying the effect of stray charges in larger systems.

Probabilistic Defect Analysis Model for Quantum dot Cellular Automata Design at Analytical Phase

The advantage of defect analysis on Quantum dot Cellular Automata(QCA) is that defects can be predict (which are probable to arise during fabrication phase) at analytical phase of QCA design. Since QCA is probabilistic in nature, the probability theory is introduced here to analyze the defect/fault tolerance at gate level of QCA design. We proposed a Bayesian network based Probabilistic Defect Analysis Model (PDAM) to analyze the defect at analytical phase of QCA design. Proposed model is applied over QCA wire, three input Majority voter, Five Input Majority voter and the result is compared with QCADesigner to justify the importance of PDAM approach over exhaustive simulation process with QCADesigner.

Analysis of missing and additional cell defects in sequential quantum-dot cellular automata

The defect characterization of sequential devices and circuits, implemented by molecular quantum-dot cellular automata (QCA), is analyzed in this paper. A RS-type flip–flop is first introduced; this flip–flop takes into account the timing issues associated with the adiabatic switching of this technology and its requirements. It is then shown that a D-type flip–flop can be constructed with an embedded QCA wire which extends over multiple clocking zones. The logic-level characterization of both flip–flop devices is provided. A single additional and missing cell defect model is assumed for molecular implementation. For sequential circuits, defect characterization is pursued. It is shown that defects affect the functionality of basic QCA devices, resulting mostly in unwanted inversion and majority voter acting as a wire at logic level. In this paper, it is shown that a device-level characterization of the defects and faults can be consistently extended to a circuit-level analysis.

Thermodynamic Behavior of Molecular-Scale Quantum-Dot Cellular Automata (QCA) Wires and Logic Devices

Quantum-dot cellular automata (QCA) offers a new paradigm for molecular electronics, a paradigm in which information transmission and processing depend on electrostatic interactions between charges in arrays of cells composed of quantum dots. Fundamental questions about the operational temperature and functional gain of devices built from molecular-scale QCA cells are addressed in this paper through a statistical-mechanical model based on electrostatic interactions. The model provides exact solutions for the thermodynamic constraints on operation of small arrays of cells (up to 15). An Ising approximation dramatically reduces the computational task and allows modeling of the thermodynamic behavior of semi-infinite QCA wires. The probability of getting the correct output from a QCA device for a given input depends on temperature, cell size, cell-cell distance, effective dielectric constant of the medium, and the number of cells in the array. Using parameters derived from molecular candidates for QCA cells, the statistical-mechanical model predicts that majority gates should give correct output at temperatures of up to 450 K, while wires of thousands to millions of QCA cells are predicted to operate as functional devices at room temperature.

Quantum‐Dot Cellular Automata at a Molecular Scale

Abstract: 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.