Quantum-dot Cellular Automata Devices Research Articles

Test generation algorithm for QCA circuits targeting novel defects and its corresponding fault models

Considering the scaling limitations of current Complementary Metal Oxide Semiconductor (CMOS) technology, Quantum-dot-Cellular Automata (QCA) is emerging as one of the alternatives. QCA being at the molecular scale, defects are more likely to occur in it. Therefore, substantial development of QCA-oriented defects, its corresponding fault models and test generation is required. In this paper, a test generation algorithm for a QCA combinational circuit is proposed. The FAN (A Fanout Oriented) test generation algorithm is extended for QCA. The proposed Automatic Test Pattern Generator (ATPG) for QCA targets Single Stuck at Fault (SSF) set produced by novel Multiple Missing Cells (MMC) defects. The proposed ATPG is based on the QCA-oriented test generation properties and guided by proposed testability measures.The MCNC benchmark circuits are synthesized into QCA using proposed synthesis algorithms to check the effectiveness of the proposed ATPG. The ATPG is developed using C++ and tested on MCNC benchmark circuits. Further, ATPG-generated test vectors are validated at the QCA device level to demonstrate their correctness. The QCADesigner-E tool is used for the device-level implementation of the MCNC benchmark circuit.

Ab initio studies of counterion effects in molecular quantum-dot cellular automata.

Molecular quantum-dot cellular automata (QCA) is a low-power computing paradigm that may offer ultra-high device densities and THz-speed switching at room temperature. A single mixed-valence (MV) molecule acts as an elementary QCA device known as a cell. Cells coupled locally via the electrostatic field form logic circuits. However, previously-synthesized ionic MV molecular cells are affected by randomly-located, nearby neutralizing counterions that can bias device states or change device characteristics, causing incorrect computational results. This ab initio study explores how non-biasing counterions affect individual molecular cells. Additionally, we model two novel neutral, zwitterionic MV QCA molecules designed to avoid biasing and other undesirable counterionic effects. The location of the neutralizing counterion is controlled by integrating one counterion into each cell at a well-defined, non-biasing location. Each zwitterionic QCA candidate molecule presented here has a fixed, integrated counterion, which neutralizes the mobile charges used to encode the device state.

Designing boron-cluster-centered zwitterionic Y-shaped clocked QCA molecules

Quantum-dot cellular automata (QCA) is a nanoscale, transistor-less device technology. A single molecule may provide an elementary QCA device known as a cell. Molecular redox centers function as quantum dots, and the configuration of mobile charge on the dots encodes device states useful for classical computing. Molecular QCA may support ultra-high device densities and THz-scale switching speeds at room temperature. An applied electric field may be used to clock molecular QCA, providing power gain to boost weakened signals, as well as quasi-adiabatic device operation for minimal power dissipation in QCA devices and circuits. A zwitterionic, Y-shaped, three-dot molecule may function as a field-clocked QCA cell. We focus on the design of a counterion built into the center of the cell. Ab initio computations demonstrate that choice of counterion determines the number of mobile charges for encoding the device state on the three quantum dots. We use or as the central counterionic linker for two different Y-shaped, three-dot QCA molecules. While both molecules support the desired device states, the number of trapped charges in the counterion determines the number of mobile holes on the molecular quantum dots. This, in turn, determines whether the device state is encoded by a hole or an electron. This choice of encoding determines how the molecular QCA cell responds to a clocking field. The two counterions studied here lead to two QCA molecules with opposite responses to the clock, similar to the complementary responses of PMOS and NMOS transistors to gated voltage control.

Investigating multiple defects on a new fault-tolerant three-input QCA majority gate

Quantum-dot cellular automata (QCA) are a new technology used to fabricate digital circuits on the nanoscale in place of CMOS technology, which has limitations in device density. QCA devices are low in power consumption and high in speed due to their structure. Although some defects may occur during chemical fabrication, QCA gates and circuits can be designed to be fault-tolerant. The majority gate is most often used in QCA circuits; thus, many papers have investigated different structures for it and tried to design a fault-tolerant gate against only one defect. We have proposed a new structure for a three-input majority gate with a good percentage of truth output despite the multiple defects that may take place concurrently. A full adder is then designed using the proposed majority gate to demonstrate the degree of fault tolerance of QCA circuits made of fault-tolerant gates.

Low-Energy Eigenspectrum Decomposition (LEED) of Quantum-Dot Cellular Automata Networks

The design and understanding of quantum-dot cellular automata (QCA) networks has been largely influenced by limitations in the approximation methods used in common design tools. In some cases, such limitations have led to unrealistic selections of clock zones which are not feasible for nanoscale QCA implementations given current fabrication constraints on clocking electrodes. A better understanding of the behaviour of larger QCA networks of perhaps tens to hundreds of QCA devices is needed. One approach is by investigating the low energy spectrum; however, diagonalization of the system Hamiltonian even in the 2-state approximation is impractical beyond 20 or so devices. In this work, we present a methodology for understanding the spectrum of the full network in terms of contributions from components of the network. We show that important features of the low energy spectrum can be attributed to specific critical components, and present one scheme for decomposing the network into these components. In addition, we address the question of computing the low energy spectrum of large QCA networks. A method based on basis reduction which naturally emerges from the component decomposition is successfully applied to a 49 cell XOR gate with results compared against a density matrix renormalization group implementation.

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.

Limits of adiabatic clocking in quantum-dot cellular automata

Ultimate bounds on the maximum operating frequency of networks of Quantum-dot Cellular Automata (QCA) devices have yet to be established. We consider the adiabaticity of such networks in the two-state approximation where clocking is achieved via modulation of the interdot tunneling barriers. Estimates of the maximum operating frequency that would allow a 99% probability of observing the correct logical output are presented for a subset of the basic components used in QCA network design. Simulations are performed both in the coherent limit and for a simple dissipative model. We approach the problem of tunnel-based clocking from the perspective of quantum annealing and present an improved clocking schedule allowing for faster operation. Using an analytical solution for driven QCA wires, we show that the maximum operating frequency in the coherent limit falls off with the square of the wire length, potentially limiting the size of clocked regions.

Test Pattern Generator for MV-Based QCA Combinational Circuit Targeting MMC Fault Models

Quantum-dot-Cellular Automata (QCA) is emerging as one of the alternatives for Integrated Circuit Technology considering the scaling limitations of current Complementary Metal Oxide Semiconductor (CMOS) technology. Being at molecular scale, defects are more likely to occur in QCA devices. Therefore, the substantial development of QCA-oriented defects, its corresponding fault models and test generation is required. This paper addresses the fault model caused by multiple missing cells defect and test generation for these faults for QCA circuits. We have shown that the single missing cell considerations are not enough. Correspondingly, in this paper, testing properties for the detection of fault caused by multiple missing cell defects in QCA devices mainly Majority Voter (MV) are proposed. Even though there are many advanced ATPGs available for CMOS technology, to demonstrate very special MV-based properties with reference to the test pattern generation process and to open the path for advanced combinational and sequential ATPGs specific to MV-based QCA circuits, the extension of basic Automatic Test Pattern Generator is proposed here. The proposed test generation algorithm is guided by the extended Sandia Controllability Observability Analysis Program (SCOAP) testability measures especially for QCA logic primitives.

Using the full quantum basis set to simulate quantum-dot cellular automata devices

Quantum-dot cellular automata (QCA) are a leading example of field-coupled nanocomputing (FCN) devices. All FCN devices rely on local field interactions among nanoscale building blocks that are arranged in patterns to perform useful calculations. Many methods have been introduced to simulate these devices, but not all of those methods are guaranteed to give correct results. The only method that is certain to give the correct results is constructing and diagonalizing the Hamiltonian operator using a full-basis set for the entire device. Not only does this give all stable states of the device (necessarily including the ground state), it also allows the detection of non-polarized states of QCA and other FCN cells. Unfortunately, simulating moderately complex devices using the full-basis set has been prohibitively expensive, both in storage space and in calculation time. The authors present a new method for calculating the eigenstates of a full-basis set Hamiltonian and use this method to demonstrate that at least one previously published QCA device does not, in fact, operate as it was simulated and described in the previous work. Identifying an error in a previously published device using a heretofore computationally infeasible simulation shows the value of using the exact full-basis calculation in the simulation of QCA devices.

Multiple Missing Cell Defect Modeling for QCA Devices

Considering the limitations of CMOS technology, the Quantum-dot Cellular Automata (QCA) is emerging as one of the alternatives for Integrated Circuit (IC) Technology. A lot of work is being carried out for design, fabrication and testing of QCA circuits. In this paper, we have worked on defect analysis, fault models development and deriving various properties for QCA Majority Voter (MV) to effectively generate the test patterns for QCA circuits. It has been shown that unlike CMOS technology, single missing cell consideration is not enough for QCA technology. We have presented that the Multiple Missing Cell (MMC) defect, which is very natural at nanoscale, causes the sizable difference in functionality compared to Single Missing Cell consideration described in literature, and hence, must be considered while test generation. The proposed MMC is supported by exhaustive simulation results as well as kink energy based mathematical analysis. Further, Verilog fault models are proposed which can be used for the functional, timing verification and activation of faults caused by MMC defect. The effect of MMC on output is analyzed in stand-alone MV as well as when MV is a part of circuit. At the end, we have proposed the test properties of MV when being used as MV itself, as AND gate or OR gate. These properties may be further helpful in development of test generation algorithms.

Clock Topologies for Molecular Quantum-Dot Cellular Automata

Quantum-dot cellular automata (QCA) is a low-power, non-von-Neumann, general-purpose paradigm for classical computing using transistor-free logic. Here, classical bits are encoded on the charge configuration of individual computing primitives known as “cells.” A cell is a system of quantum dots with a few mobile charges. Device switching occurs through quantum mechanical inter-dot charge tunneling, and devices are interconnected via the electrostatic field. QCA devices are implemented using arrays of QCA cells. A molecular implementation of QCA may support THz-scale clocking or better at room temperature. Molecular QCA may be clocked using an applied electric field, known as a clocking field. A time-varying clocking field may be established using an array of conductors. The clocking field determines the flow of data and calculations. Various arrangements of clocking conductors are laid out, and the resulting electric field is simulated. It is shown that that control of molecular QCA can enable feedback loops, memories, planar circuit crossings, and versatile circuit grids that support feedback and memory, as well as data flow in any of the ordinal grid directions. Logic, interconnect and memory now become indistinguishable, and the von Neumann bottleneck is avoided.

Electric-Field Inputs for Molecular Quantum-Dot Cellular Automata Circuits

Quantum-dot cellular automata (QCA) is a low-power, 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 room-temperature 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.

QCAPUF: QCA-based physically unclonable function as a hardware security primitive

Physically unclonable functions (PUFs) are increasingly used as innovative security primitives to provide the hardware authentication and identification as well as the secret key generation based on unique and random variations in identically fabricated devices. Security and low power have appeared to become two crucial necessities to modern designs. As an emerging nanoelectronic technology, a quantum-dot cellular automata (QCA) can achieve ultra-low power consumption as well as an extremely small area for implementing digital designs. However, there are various classes of permanent defects that can happen during the manufacture of QCA devices. The recent extensive research has been focused on how to eliminate errors in QCA structures resulting from fabrication variances. By a completely different vision, to turn this disadvantage into an advantage, this paper presents a novel QCA-based PUF (QCAPUF) architecture to exploit the unique physical characteristics of fabricated QCA cells in order to produce different hardware fingerprint instances. This architecture is composed of proposed logic and interconnect blocks that have critical vulnerabilities and perform unexpected logical operations. The behaviour of QCAPUF is thoroughly analysed through physical relations and simulations. Results confirm that the proposed QCAPUF has state of the art PUF characteristics in the QCA technology. This paper will serve as a basis for further research into QCA-based hardware security primitives and applications.

Design of image steganographic architecture using quantum-dot cellular automata for secure nanocommunication networks

The objective of this article is to introduce an architecture that performs image steganography. The architecture is built on quantum dot cellular automata (QCA) technology. The reason behind increasing use of QCA device is that it provides low device density with small amount of power dissipated. To encode message inside an image, Least Significant Bit (LSB) based encoder/decoder steganography circuit has been proposed. To design this encoder/decoder circuit, a new QCA XOR gate has been designed. The proposed QCA XOR gate has low device density and less cell count compare to other existing circuits. The proposed circuit also requires less amount of power. The Mean Square Error (MSE), Peak Signal to Noise Ratio (PSNR), and Structural Similarity Index Method (SSIM) are used to measure the embedded information is in the acceptable range or not. The design accuracy is confirmed through simulation results and power analysis performed.

Fan-out constraints in quantum dot cellular automata circuit design

In this paper, fan-out constraints are demonstrated for robust and reliable quantum dot cellular automata (QCA) device design. These QCA devices can be used to implement large and complex circuits without affecting the expected result. The tile-based QCA nanostructures are more robust and reliable. In this paper, the Hopfield artificial neural network (HANN) model is proposed to find out the robustness and the reliability of QCA nanostructure devices. This proposed HANN model demonstrates that the tile nanostructures of QCA devices are more robust and reliable for driving multiple fan-outs from a particular QCA device. The Kink energy was used in computing the polarity when considering multiple fan-outs. The tile three-input majority voter, the triple fan-out butterfly tile, the multiple fan-out tile structure and the five-input majority voter are the most robust and reliable structures for solving the fan-out problem in QCA and are described in this paper by the proposed HANN model.

Reversible nanorouter using QCA for nanocommunication

In reversible nanocomputing, an emerging area of research is quantum dot cellular automata (QCA)-based design of cost-effective reversible nanocircuits. In a digital nanocommunication system, the router is the heart of data transmission. At nanoscale, for data communication the device density and energy dissipation of the circuit are key issues. A reversible circuit exhibits low energy dissipation or ideally no energy dissipation. This paper presents the design of a Fredkin gate- and a BJN gate-based design of reversible 4:1 multiplexer and 1:4 demultiplexer based on QCA with quantum costs of 15 and 30 respectively and garbage output of six and ten respectively. The proposed reversible multiplexer and demultiplexer are used to achieve a reversible nanorouter in QCA for the first time, with a quantum cost of 40 and garbage output of 16. The proposed reversible nanorouter could be used to achieve a nanocommunication system in the near future at the nanoscale low energy level. A QCA device is used to implement the proposed reversible circuits, to achieve a low-energy nanoscale circuit. The comparison of simulation consequences of proposed circuits with theoretical knowledge established the functional efficiency of the circuit.

Defect‐tolerance analysis of fundamental quantum‐dot cellular automata devices

Quantum-dot cellular automata (QCA) is a burgeoning technology at the nano-scale range, with the potential for lower power consumption, smaller size and faster speed than conventional complementary metal–oxide semiconductor-based technology. Because of its ultra-density integration and its inherent physical properties, fault-tolerance is an important property to consider in the research and manufacture of QCA. In this paper, one type of defect, in which displacement and misalignment occur coinstantaneously, is investigated in detail on rudimentary QCA devices (majority voter (MV), inverter, wire) with QCADesigner. Another MV with rotated cells is also proposed, and it is more robust than the original one. Simulation results present the defect-tolerance of these devices, that is, the maximum precise region the defective cell can be moved moreover, with correct logical function. These conclusions have a meaningful guiding significance for QCA physical implementation and fault-tolerance research.

Defect and Temperature Effects on Complex Quantum-Dot Cellular Automata Devices

The authors present an analysis of the fault tolerant properties and the effects of temperature on an exclusive OR (XOR) gate and a full adder device implemented using quantum-dot cellular automata (QCA) structures. A Hubbard-type Hamiltonian and the Inter-cellular Hartree approximation have been used for modeling, and a uniform random distribution has been implemented for the simulated dot displacements within cells. We have shown characteristic features of all four possible input configurations for the XOR device. The device performance degrades significantly as the magnitude of defects and the temperature increase. Our results show that the fault-tolerant characteristics of an XOR device are highly dependent on the input configurations. The input signal that travels through the wire crossing (also called a crossover) in the central part of the device weakens the signal significantly. The presence of multiple wire crossings in the full adder design has a major impact on the functionality of the device. Even at absolute zero temperature, the effect of the dot displacement defect is very significant. We have observed that the breakdown characteristic is much more pronounced in the full adder than in any other devices under investigation.

Are QCA cryptographic circuits resistant to power analysis attack?

Quantum-dot cellular automata (QCA) technology is expected to offer fast computation performance, high density, and low power consumption. Thus, researchers believe that QCA may be an attractive alternative to CMOS for future digital designs. Side channel attacks, such as power analysis attacks, have become a significant threat to the security of CMOS cryptographic circuits. A power analysis attack can reveal the secret key from measurements of the power consumption during the encryption and decryption process. As there is no electric current flow in QCA technology, the power consumption of QCA circuits is extremely low when compared to their CMOS counterparts. Therefore, in this paper an investigation into both the best and worst case scenarios for attackers is carried out to ascertain if QCA circuits are immune to power analysis attack. A QCA design of a submodule of the Serpent cipher is proposed. In comparison to a previous design, the proposed design is more efficient in terms of complexity, area, and latency. By using an upper bound power model, the first power analysis attack of a QCA cryptographic circuit is presented. The simulation results show that even though the power consumption is low, it can still be correlated with the correct key guess, and all possible subkeys applied to the Serpent submodule can be revealed in the best case scenario. Therefore, in theory QCA cryptographic circuits would be vulnerable to power analysis attack. However, the security of practical QCA devices can be greatly improved by applying a smoother clock. Moreover, in the worst case scenario, the design of logically reversible QCA circuits with Bennett clocking could be used as a natural countermeasure to power analysis attack. Therefore, it is believed that QCA could be a niche technology in the future for the implementation of security architectures resistant to power analysis attack.

Reliability and Performance Evaluation of QCA Devices With Rotation Cell Defect

As a novel nanotechnology, quantum-dot cellular automata (QCA) can achieve dense packages due to the extremely small size of quantum dots. However, fabrication defects and fault rates in the nanotechnology are expected to be quite high. In this paper, the behaviors of basic QCA devices in the presence of cell rotation are thoroughly analyzed in order to study their rotation defect tolerances and determine the ranges of allowable rotation angles. Rotation effect of QCA cell is modeled by using modified coherence vector formalism and rotation angle. The performances of five basic QCA devices with rotation cell defect are simulated under various cell sizes. The results show that different QCA devices have different rotation angle tolerances. The inverter is the weakest structure while the straight wire is the most reliable structure if measured by the smallest angle that impacts success rate, and the bigger cell size has a negative effect on allowable rotation angle range. More analysis results show that the diagonal cell and the cell close to the output perform the worst rotation tolerance on the inverter and the interconnect, respectively. Furthermore, the power gain of rotation defect device is discussed and the finding is provided that moderate rotation error can be restored at the location that is three cells away from the defect.