Ravi K. Kummamuru

Operation of a quantum-dot cellular automata (QCA) shift register and analysis of errors

Quantum-dot cellular automata (QCA) is a digital logic architecture that uses single electrons in arrays of quantum dots to perform binary operations. A QCA latch is an elementary building block which can be used to build shift registers and logic devices for clocked QCA architectures. We discuss the operation of a QCA latch and a shift register and present an analysis of the types and properties of errors encountered in their operation.

Experimental demonstration of a leadless quantum-dot cellular automata cell

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.

Experimental demonstration of clocked single-electron switching in quantum-dot cellular automata

A device representing a basic building block for clocked quantum-dot cellular automata architecture is reported. Our device consists of three floating micron-size metal islands connected in series by two small tunnel junctions where the location of an excess electron is defined by electrostatic potentials on gates capacitively coupled to the islands. In this configuration, the middle dot acts as an adjustable Coulomb barrier allowing clocked control of the charge state of the device. Charging diagrams of the device show the existence of several operational modes, in good agreement with theory. The clocked switching of a single electron is experimentally demonstrated and advantages of this architecture are discussed.

Power gain in a quantum-dot cellular automata latch

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.

Clocked quantum-dot cellular automata shift register

The quantum-dot cellular automata (QCA) computational paradigm provides a means to achieve ultimately low limits of power dissipation by replacing binary coding in currents and voltages with single-electron switching within arrays of quantum dots (‘‘cells’’). Clocked control over the cells allows the realization of power gain, memory and pipelining in QCA circuits. We present an experimental demonstration of a clocked QCA two-stage shift register (SR) and use it to mimic the operation of a multi-stage SR. Error-bit rates for binary switching operations in a metal tunnel junction device are experimentally investigated, and discussed for future molecular QCAs. 2003 Elsevier Science B.V. All rights reserved.

Experimental demonstration of a latch in clocked quantum-dot cellular automata

We present an experimental demonstration of a latch in a clocked quantum-dot cellular automata (QCA) device. The device consists of three floating micron-size metal dots, connected in series by multiple tunnel junctions and controlled by capacitively coupled gates. The middle dot acts as an adjustable barrier to control single-electron tunneling between end dots. The position of a switching electron in the half cell is detected by a single-electron electrometer. We demonstrate “latching” of a single electron in the end dots controlled by the gate connected to the middle dot. This ability to lock an electron in a controllable way enables pipelining, power gain and reduced power dissipation in QCA arrays.

Clocked quantum-dot cellular automata devices: experimental studies

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.

Quantum-dot cellular automata: introduction and experimental overview

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.

Fanout in quantum dot cellular automata

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

Quantum-dot cellular automata

An introduction to of quantum-dot cellular automata (QCA) is presented. 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. QCA cells are scalable to molecular dimensions and initial measurements have demonstrated single-electron switching within a molecule.