Quantum-dot Cellular Automata Systems Research Articles

Towards nanoscale fault-tolerant logical circuits using proposed robust majority voter in quantum-dot cellular automata technology

The occupied area, power consumption, and delay are the most crucial and critical factors in constructing integrated circuits. Due to the reduced occupied area, highly low power consumption, and extremely high speed of quantum-dot cellular automata (QCA) technology, it is one of the finest alternatives to complementary metal–oxide–semiconductor (CMOS) technology for nanoscale construction of circuits. On the other hand, fault tolerance becomes crucial in QCA due to the inherent sensitivity of quantum dots to various sources of errors and faults. These errors can arise from environmental disturbances, manufacturing imperfections, thermal fluctuations, and other factors. The presence of defects or faults can significantly impact the functionality and accuracy of QCA systems, leading to incorrect computation or signal corruption. To address these challenges, fault-tolerant structures are designed in QCA systems. These structures are specifically engineered to detect, tolerate, and mitigate the effects of faults, thereby enhancing the reliability and robustness of QCA-based computation. Fault-tolerant designs aim to ensure that the system can continue to operate correctly even in the presence of defects or faults. In QCA, proposed a fault-tolerant majority gate is necessary to ensure reliable computation in the presence of defects or faults. The fault-tolerant majority gate is a fundamental component in digital logic circuits, and it plays a crucial role in performing computations. It takes multiple input signals and produces an output based on the majority of those inputs. In classical computing, the majority gates are typically implemented using transistors. Therefore, this paper introduces a new and efficient fault-tolerant 3-input majority voter (FT MV3) using 11 simple and rotated cells in the QCA technology, which is 100% and 90.47% tolerant against single-cell and double-cell omission defects. The recommended FT MV3 gate verification is confirmed using some physical proofs. Afterward, to illustrate the performance of the introduced gate, three fault-tolerant computational circuits, including multiplexer, adder and ALU, are presented using the introduced FT MV3 gate. The comparison of the proposed fault tolerant ALU to the best coplanar design shows a 28.80% and 34.01% reduction of cell count and occupied area, respectively. All circuits are simulated using QCADesigner 2.0.3 software.

A New Design of a 3 × 3 Reversible Circuit Based on a Nanoscale Quantum-Dot Cellular Automata

Quantum-dot cellular automata (QCA) is the best-suggested nanotechnology for designing digital electronic circuits. It has a higher switching frequency, low-power expenditures, low area, high speed and higher scale integration. Recently, many types of research have been on the design of reversible logic gates. Nevertheless, a high demand exists for designing high-speed, high-performance and low-area QCA circuits. Reversible circuits have notably improved with developments in complementary metal–oxide–semiconductor (CMOS) and QCA technologies. In QCA systems, it is important to communicate with other circuits and reversible gates reliably. So, we have used efficient approaches for designing a [Formula: see text] reversible circuit based on XOR gates. Also, the suggested circuits can be widely used in reversible and high-performance systems. The suggested architecture for the [Formula: see text] reversible circuit in QCA is composed of 28 cells, occupying only 0.04[Formula: see text][Formula: see text]m2. Compared to the state-of-the-art, shorter time, smaller areas, more operational frequency and better performance are the essential benefits of the suggested reversible gate design. Full simulations have been conducted with the utilization of QCADesigner software. Additionally, the proposed [Formula: see text] gate has been schematized using two XOR gates.

Mixed-valence realizations of quantum dot cellular automata

This article reviews recent progress in the design and theoretical investigation of molecular implementations of quantum-dot cellular automata (QCA) for field-coupled nanocomputing applications. QCA is a classical computing paradigm based on Coulombic quantum interactions between nanoscale subunits consisting of two or more quantum dots. Shrinking these dots to the molecular scale maximizes device density and permits operation at ambient temperatures. The essential feature of molecular QCA systems is the presence of two or more coupled redox centers separated by spacers. In this work candidate systems ranging from simple organic molecules to self-assembled multi-center mixed-valence organometallic complexes are surveyed, and some of the challenges remaining to be faced both in theoretical understanding and practical implementation are discussed.

Synchronization in Quantum-Dot Cellular Automata Circuits and Systems

Signal synchronization of large scale Quantum-dot Cellular Automata (QCA) circuits is one of the most complex QCA design challenges. More specifically, the QCA circuits synchronization problem, especially in the large circuits, is characterized as rather complex due to technology constraints. In this paper, by extensively analyzing the most important properties of the signal synchronization problem in QCA circuits, we propose an efficient design methodology to tackle the problem, based on the well-known from computer science, Firing Squad Synchronization Problem (FSSP). Comparing FSSP with the QCA circuits synchronization problem many similarities can be found. Among the numerous FSSP's algorithmic solutions in literature, the Mazoyer algorithm has proven to be the most efficient one. In this paper, a novel design and implementation in QCA technology of this algorithm is presented. Moreover, by the appropriate modification of the Mazoyer algorithm, we are able to propose a generic synchronization design methodology for QCA circuits and systems. This method is enhanced by a novel freezing technique, that makes it applicable to any QCA circuit and system as manifested by our corresponding simulation results. The proposed synchronization methodology is a universal design tool, that can be applied to exiting designs without increasing the complexity.

An Energy-Aware Model for the Logic Synthesis of Quantum-Dot Cellular Automata

Quantum-dot cellular automata (QCA) are an emerging field-coupled nanotechnology with remarkable performance and energy efficiency. In order to enable the exploration of this technology, we propose a model for the logic synthesis of QCA circuits that, for the first time, considers and abstracts all main physical aspects—in particular, energy dissipation. To this end, we review in detail how energy is dissipated in QCA cells and present a corresponding environment that allows for the estimation of the energy dissipation with respect to any specific set of technology parameters. Based on that, we derive a model for logic synthesis. A case study confirms the accuracy of the proposed model and reveals that interconnections have a significant impact in this technology—motivating a more rigorous consideration. These findings eventually provide the basis for a new generation of synthesis approaches at the logic level that are explicitly dedicated to QCA systems.

Exponentially Adiabatic Switching in Quantum-Dot Cellular Automata

We calculate the excess energy transferred into two-dot and three-dot quantum dot cellular automata systems during switching events. This is the energy that must eventually be dissipated as heat. The adiabaticity of a switching event is quantified using the adiabaticity parameter of Landau and Zener. For the logically reversible operations of WRITE or ERASE WITH COPY, the excess energy transferred to the system decreases exponentially with increasing adiabaticity. For the logically irreversible operation of ERASE WITHOUT COPY, considerable energy is transferred and so must be dissipated, in accordance with the Landauer Principle. The exponential decrease in energy dissipation with adiabaticity (e.g., switching time) distinguishes adiabatic quantum switching from the usual linear improvement in classical systems.

Novel 8-bit reversible full adder/subtractor using a QCA reversible gate

Conventional digital circuits consume a considerable amount of energy. If bits of information remain during logical operations, power consumption decreases considerably because the data bits in reversible computations are not lost. The types of reversible gate used in quantum computations are quantum-dot cellular automata (QCA), nuclear magnetic resonance, and optical computations. QCA systems offer low power consumption, high density, section regularity and support new devices designed for nanotechnology. A QCA-based reversible gate has minimal delay, complexity and considering the potential quality of a QCA pipeline, computes at maximum speed. The present study designed a novel $$3 \times 3$$3×3 reversible gate that is universal and testable. A reversible logic gate is also designed based on the majority gate in the QCA and as a QCA reversible (QR) gate. A new 8-bit reversible full adder/subtractor based on the QR gate in QCA with a minimum number of cells and area combines both designs for implementation of a reversible full adder/subtractor in QCA. The performance of these gates was compared with testable Fredkin and Toffoli gates and improved performance of logic functions in terms of complexity and delay over those of the Fredkin and Toffoli gates.

A signal distribution grid for quantum-dot cellular automata

Coplanar wire crossing has been a major challenge for quantum-dot cellular automata systems since their development. Several possible solutions have been presented, but they have either relied on non-adjacent cell interactions or have required switching time that scales with the number of inputs or outputs. In this paper, the authors present a signal distribution grid that enables multiple parallel crossings, while doing so with only adjacent cell interactions, a constant time for signal distribution regardless of the number of inputs or outputs, and regularly shaped and contiguous clocking regions that will be relatively easier to fabricate. The utility of this device is demonstrated by the design of a one-bit full adder that meets all of the listed requirements.

A Signal Distribution Network for Sequential Quantum-Dot Cellular Automata Systems

The authors describe a signal distribution network for sequential systems constructed using the Quantum-dot cellular automata (QCA) computing paradigm. This network promises to enable the construction of arbitrarily complex QCA sequential systems in which all wire crossings are performed using nearest neighbor interactions, which will improve the thermal behavior of QCA systems as well as their resistance to stray charge and fabrication imperfections. The new sequential signal distribution network is demonstrated by the complete design and simulation of a two-bit counter, a three-bit counter, and a pattern detection circuit.

A Scalable Signal Distribution Network for Quantum-Dot Cellular Automata

The authors describe a signal distribution network (SDN) for quantum-dot cellular automata (QCA) devices. This network allows the distribution of a set of inputs to an arbitrary number of combinational functions, overcoming the challenges associated with wire crossings that have faced QCA systems for many years. As an additional benefit, the proposed SDN requires only four distinct clock signals, regardless of the number of inputs or outputs, and those clock signals each repeat a very simple pattern. Furthermore, this network is highly scalable, completing the distribution of inputs to an arbitrary number of distributed signals and an arbitrary number of outputs in 4 - 2 clock cycles. To illustrate its operation, the authors apply the SDN to a two-input/one-output exclusive OR operation, a three-input/two-output full adder, and a four-input/four-output multiplier.

Design of dual-edge triggered flip-flops based on quantum-dot cellular automata

Quantum-dot cellular automata (QCA) technology has been widely considered as an alternative to complementary metal-oxide-semiconductor (CMOS) due to QCA’s inherent merits. Many interesting QCA-based logic circuits with smaller feature size, higher operating frequency, and lower power consumption than CMOS have been presented. However, QCA is limited in its sequential circuit design with high performance flip-flops. Based on a brief introduction of QCA and dual-edge triggered (DET) flip-flop, we propose two original QCA-based D and JK DET flip-flops, offering the same data throughput of corresponding single-edge triggered (SET) flip-flops at half the clock pulse frequency. The logic functionality of the two proposed flip-flops is verified with the QCADesigner tool. All the proposed QCA-based DET flip-flops show higher performance than their SET counterparts in terms of data throughput. Furthermore, compared with a previous DET D flip-flop, the number of cells, covered area, and time delay of the proposed DET D flip-flop are reduced by 20.5%, 23.5%, and 25%, respectively. By using a lower clock pulse frequency, the proposed DET flip-flops are promising for constructing QCA sequential circuits and systems with high performance.

Reliability Impact of N-Modular Redundancy in QCA

Nanoelectronic systems are extremely likely to demonstrate high defect and fault rates. As a result, defect and/or fault tolerance may be necessary at several levels throughout the system. Methods for improving defect tolerance, in order to prevent faults, at the component level for quantum-dot cellular automata (QCA)1 have been studied. However, methods and results considering fault tolerance in QCA have received less attention. In this paper, we present an analysis of how QCA system reliability may be impacted by using various N-modular redundancy (NMR) schemes. Our results demonstrate that using NMR in QCA can improve reliability in some cases, but can harm reliability in others.

Improving Line-Based QCA Memory Cell Design Through Dual Phase Clocking

This paper describes a line-based, quantum-dot cellular automata (QCA) memory cell design that is synchronized by a dual-phase clocking scheme. In line-based QCA memory cells, data bits are stored oscillating along QCA lines. The best known line-based memory cell implementation requires three new clocking zones in addition to the four clocking zones defined by the conventional QCA clocking scheme and utilizes three parallel clocking zones per cell. The proposed memory cell requires only two new clocking zones and utilizes two parallel clock zones per memory cell; permitting less CMOS circuity for clock design and denser QCA system implementations. Furthermore, read throughput is improved to one operation per clock cycle (from one read per two clock cycles). Simulations with the QCADesigner simulator are performed to verify the functionality of the proposed QCA memory cell.

A Serial Memory by Quantum-Dot Cellular Automata (QCA)

Quantum-dot Cellular Automata (QCA) has been widely advocated as a new device architecture for nanotechnology. QCA systems require extremely low power, together with the potential for high density and regularity. These features make QCA an attractive technology for manufacturing memories in which the paradigm of memory-in-motion can be fully exploited. This paper proposes a novel serial memory architecture for QCA implementation. This architecture is based on utilizing new building blocks (referred to as tiles) in the storage and input/output circuitry of the memory. The QCA paradigm of memory-in-motion is accomplished using a novel arrangement in the storage loop and timing/clocking; a three-zone memory tile is proposed by which information is moved across a concatenation of tiles by utilizing a two-level clocking mechanism. Clocking zones are shared between memory cells and the length of the QCA line of a clocking zone is independent of the word size. QCA circuits for address decoding and input/output for simplification of the Read/Write operations are discussed in detail. An extensive comparison of the proposed architecture and previous QCA serial memories is pursued in terms of latency, timing, clocking requirements, and hardware complexity.

Two-Dimensional Schemes for Clocking/Timing of QCA Circuits

At nanoscale, quantum-dot cellular automata (QCA) defines a new device architecture that permits the innovative design of digital systems. Features of these systems are the allowed crossing of signal lines with different orientation in polarization on a Cartesian plane, the potential of high throughput due to efficient pipelining, fast signal switching, and propagation. However, QCA designs of even modest complexity suffer from the negative impact due to the placement of long lines of cells among clocking zones, thus resulting in increased delay, slow timing, and sensitivity to thermal fluctuations. In this paper, different schemes for clocking and timing of the QCA systems are proposed; these schemes utilize 2D techniques that permit a reduction in the longest line length in each clocking zone. The proposed clocking schemes utilize logic-propagation techniques that have been developed for systolic arrays. Placement of QCA cells is modified to ensure correct signal generation and timing. The significant reduction in the longest line length permits a fast timing and efficient pipelining to occur while guaranteeing a kink-free behavior in switching.

HDLQ

Emerging technologies have attracted a substantial interest in overcoming the physical limitations of CMOS as projected at the end of the Technology Roadmap; among these technologies, quantum-dot cellular automata (QCA) relies on different and novel paradigms to implement dense, low power circuits and systems for high-performance computing. As applicable to existing technologies, a hierarchical process can be utilized to facilitate the design of QCA circuits. Tools and methodologies both at system and physical levels are required to support all design phases. This article presents an HDL model to describe QCA “devices” (also referred elsewhere in the technical literature as building blocks, i.e., majority voter, inverter, wire, crossover) and facilitate the evaluation of their design. This tool, referred to as HDLQ, allows a designer to verify the logic characteristics of a QCA system, while supporting within a design environment different operational mechanisms (such as fault injection) and the unique features of QCA (such as bidirectionality and timing/clocking partitioning). The applicability of this design environment to various memory circuits for logic and timing verification is presented in detail. Various defective conditions for kinks due to thermodynamic effects and permanent faults due to manufacturing defects are considered for injection.

A Line-Based Parallel Memory for QCA Implementation

Quantum-dot cellular automata (QCA) has been widely advocated as a new device architecture for nanotechnology. Using QCA, the innovative design of digital systems can be achieved by exploiting the so-called capability of processing-in-wire, i.e., signal manipulation proceeds at the same time as propagation. QCA systems require low power together with the potential for high density and regularity. These features make QCA an attractive technology for manufacturing memories in which the in-wire paradigm can be exploited for storage purposes. This paper proposes a novel parallel memory architecture for QCA implementation. This architecture is based on storing information on a QCA line by changing the direction of signal flow among three clocking zones. Timing of these zones requires two additional clocks to implement a four-step process for reading/writing data to the memory. Its operation has been verified by simulation. It is shown that the requirements for clocking, number of zones, as well as the underlying CMOS circuitry are significantly reduced compared with previous QCA parallel architectures.

Partitioning and placement for buildable QCA circuits

Quantum-dot Cellular Automata (QCA) is a novel computing mechanism that can represent binary information based on spatial distribution of an electron charge configuration in chemical molecules. In this article, we present the first partitioning and placement algorithm for automatic QCA layout. We identify several objectives and constraints that will enhance the buildability of QCA circuits. The results are intended to: (1) define what is computationally interesting and could actually be built within a set of predefined constraints, (2) project what designs will be possible as additional constructs become realizable, and (3) provide a vehicle that we can use to compare QCA systems to silicon-based systems.

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.

Practical issues in the realization of quantum-dot cellular automata

Several practical issues in the development and operation of quantum-dot cellular automata (QCA) cells and systems are discussed. The need for adiabatic clocking of QCA systems and modeling of electrostatic confinement of quantum dots are presented. Experimental data on dot coupling and applications to QCA detectors in a 2-dimensional electron gas (2DEG) are presented. We report a charge detection scheme where we observe strong modulation in the detector signal, in addition to the detector exhibiting minimal effect on the dot being measured. With this investigation, we demonstrate these two key components required for QCA in AlGaAs/GaAs materials, namely dot coupling and charge-state detection.