Design of quantum-dot cellular automata circuits using cut-set retiming

As a potential alternative to CMOS technology, QCA provides an interesting paradigm in both communication and computation. However, QCAs unique four-phase clocking scheme and timing constraints present serious timing issues for interconnection and feedback. In this work, a cut-set retiming design procedure is proposed to resolve these QCA timing issues. The proposed design procedure can accommodate QCAs unique characteristics by performing delay-transfer and time-scaling to reallocate the existing delays so as to achieve efficient clocking zone assignment. Cut-set retiming makes it possible to effectively design relatively complex QCA circuits that include feedback. It utilizes the similar characteristics of synchronization, deep pipelines and local interconnections common to both QCA and systolic architectures. As a case study, a systolic Montgomery modular multiplier is designed to illustrate the procedure. Furthermore, a nonsystolic architecture, an S27 benchmark circuit, is designed and compared with previous designs. The comparison shows that the cut-set retiming method achieves a more efficient design, with a reduction of 22%, 44%, and 46% in terms of cell count, area, and latency, respectively.

Bit erasure analysis of binary adders in quantum-dot cellular automata

As a post-CMOS technology, the incipient Quantum-dot Cellular Automata technology has various advantages. A key aspect which makes it highly desirable is low power dissipation. One method that is used to analyse power dissipation in QCA circuits is bit erasure analysis. This method has been applied to analyse previously proposed QCA binary adders. However, a number of improved QCA adders have been proposed more recently that have only been evaluated in terms of area and speed. As the three key performance metrics for QCA circuits are speed, area and power, in this paper, a bit erasure analysis of these adders will be presented to determine their power dissipation. The adders to be analysed are the Carry Flow Adder (CFA), Brent-Kung Adder (B-K), Ladner-Fischer Adder (L-F) and a more recently developed area-delay efficient adder. This research will allow for a more comprehensive comparison between the different QCA adder proposals. To the best of the authors' knowledge, this is the first time power dissipation analysis has been carried out on these adders.

A First Step Towards Cost Functions for Quantum-dot Cellular Automata Designs

Quantum-dot cellular automata (QCA) is potentially a very attractive alternative to CMOS for future digital designs. Circuit designs in QCA have been extensively studied. However, how to properly evaluate the QCA circuits has not been carefully considered. To date, metrics and area-delay cost functions directly mapped from CMOS technology have been used to compare QCA designs, which is inappropriate due to the differences between these two technologies. In this paper, several cost metrics specifically aimed at QCA circuits are studied. It is found that delay, the number of QCA logic gates, and the number and type of crossovers, are important metrics that should be considered when comparing QCA designs. A family of new cost functions for QCA circuits is proposed. As fundamental components in QCA computing arithmetic, QCA adders are reviewed and evaluated with the proposed cost functions. By taking the new cost metrics into account, previous best adders become unattractive and it has been shown that different optimization goals lead to different “best” adders.

Cost-Efficient Decimal Adder Design in Quantum-dot Cellular Automata

Applications that cannot tolerate the loss of accuracy that results from binary arithmetic demand hardware decimal arithmetic designs. Binary arithmetic in Quantum-dot cellular automata (QCA) technology has been extensively investigated in recent years. However, only limited attention has been paid to QCA decimal arithmetic. In this paper, two cost-efficient binary-coded decimal (BCD) adders are presented. One is based on the carry flow adder (CFA) using a conventional correction method. The other uses the carry look ahead (CLA) algorithm which is the first QCA CLA decimal adder proposed to date. Compared with previous designs, both decimal adders achieve better performance in terms of latency and overall cost. The proposed CFA-based BCD adder has the smallest area with the least number of cells. The proposed CLA-based BCD adder is the fastest with an increase in speed of over 60% when compared with the previous fastest decimal QCA adder. It also has the lowest overall cost with a reduction of over 90% when compared with the previous most cost-efficient design.

Facile Synthesis of Anatase TiO2 Quantum-Dot/Graphene-Nanosheet Composites with Enhanced Electrochemical Performance for Lithium-Ion Batteries

A facile method to synthesize well-dispersed TiO₂ quantum dots on graphene nanosheets (TiO₂-QDs/GNs) in a water-in-oil (W/O) emulsion system is reported. The TiO₂/graphene composites display high performance as an anode material for lithium-ion batteries (LIBs), such as having high reversible lithium storage capacity, high Coulombic efficiency, excellent cycling stability, and high rate capability. The excellent electrochemical performance and special structure of the composites thus offer a way to prepare novel graphene-based electrode materials for high-energy-density and high-power LIBs. 

Competition between memory-keeping and memory-erasing decoherence channels

We study the competing effects of simultaneous Markovian and non-Markovian decoherence mechanisms acting on a single spin. We show the existence of a threshold in the relative strength of such mechanisms above which the spin dynamics becomes fully Markovian, as revealed by the use of several non-Markovianity measures. We identify a measure-dependent nested structure of such thresholds, hinting at a causality relationship among the various non-Markovianity witnesses used in our analysis. Our considerations are then used to argue the unavoidably non-Markovian evolution of a single-electron quantum dot exposed to both intrinsic and Markovian technical noise, the latter of arbitrary strength. 

Tip-enhanced fluorescence imaging of quantum dots

We have imaged the fluorescence from a single quantum dot cluster using an apertureless scanning near-field optical microscope. When a sharp gold tip is brought within a few nanometers from the sample surface, the resulting enhancement in quantum dot fluorescence in the vicinity of the tip leads to a resolution of about 60 nm. We determine this enhancement of the fluorescence to be about fourfold in magnitude, which is consistent with the value expected as a result of competition between fluorescence quenching and electromagnetic field enhancement. (C) 2005 American Institute of Physics.

Projected equations of motion approach to hybrid quantum/classical dynamics in dielectric-metal composites

We introduce a hybrid method for dielectric-metal composites that describes the dynamics of the metallic system classically whilst retaining a quantum description of the dielectric. The time-dependent dipole moment of the classical system is mimicked by the introduction of projected equations of motion (PEOM) and the coupling between the two systems is achieved through an effective dipole-dipole interaction. To benchmark this method, we model a test system (semiconducting quantum dot-metal nanoparticle hybrid). We begin by examining the energy absorption rate, showing agreement between the PEOM method and the analytical rotating wave approximation (RWA) solution. We then investigate population inversion and show that the PEOM method provides an accurate model for the interaction under ultrashort pulse excitation where the traditional RWA breaks down.

QCA Systolic array design

Quantum-dot Cellular Automata (QCA) technology is a promising potential alternative to CMOS technology. To explore the characteristics of QCA and suitable design methodologies, digital circuit design approaches have been investigated. Due to the inherent wire delay in QCA, pipelined architectures appear to be a particularly suitable design technique. Also, because of the pipeline nature of QCA technology, it is not suitable for complicated control system design. Systolic arrays take advantage of pipelining, parallelism and simple local control. Therefore, an investigation into these architectures in QCA technology is provided in this paper. Two case studies, (a matrix multiplier and a Galois Field multiplier) are designed and analyzed based on both multilayer and coplanar crossings. The performance of these two types of interconnections are compared and it is found that even though coplanar crossings are currently more practical, they tend to occupy a larger design area and incur slightly more delay. A general semi-conductor QCA systolic array design methodology is also proposed. It is found that by applying a systolic array structure in QCA design, significant benefits can be achieved particularly with large systolic arrays, even more so than when applied in CMOS-based technology.

Fluorescence enhancement and energy transfer in apertureless scanning near-field optical microscopy

We investigate the mechanisms for fluorescence enhancement and energy transfer near a gold tip in apertureless scanning near-field optical microscopy. Using a simple quasi-static model, we show that the observed enhancement of fluorescence results from competition between enhancement and quenching, and is dependent on a range of experimental parameters. We find good qualitative agreement with the results of measurements of the effect of both sharp and blunt tips on quantum dot fluorescence, and provide a demonstration of tip-enhanced fluorescence imaging with 60 nm resolution.

Transferring entanglement to the steady state of flying qubits

The transfer of entanglement from optical fields to qubits provides a viable approach to entangling remote qubits in a quantum network. In cavity quantum electrodynamics, the scheme relies on the interaction between a photonic resource and two stationary intracavity atomic qubits. However, it might be hard in practice to trap two atoms simultaneously and synchronize their coupling to the cavities. To address this point, we propose and study entanglement transfer from cavities driven by an entangled external field to controlled flying qubits. We consider two exemplary non-Gaussian driving fields: NOON and entangled coherent states. We show that in the limit of long coherence time of the cavity fields, when the dynamics is approximately unitary, entanglement is transferred from the driving field to two atomic qubits that cross the cavities. On the other hand, a dissipation-dominated dynamics leads to very weakly quantum-correlated atomic systems, as witnessed by vanishing quantum discord.