Quantum dot logic advancesBy Eric Smalley, Technology Research News
Efforts to make computers using quantum dots, which are microscopic specks of material that behave like atoms, have taken two steps forward with the development of a switch that needs no leads to the external environment and a clocking function that controls the switch.
In theory, quantum dots can be grouped into cells that in turn can be combined to form the logic gates, memory units and wires that are the building blocks of computers. Computer components based on these cells would be much faster and smaller and would use less power than those made with today's semiconductor technology.
Researchers at Notre Dame are developing Quantum-dot Cellular Automata (QCA), a computer architecture based on quantum dot cells. They have demonstrated QCA cells that are completely isolated from the environment and the clocking function that could move and store bits in a QCA-based computer. The research shows how to make QCA-based computers should researchers learn how to make quantum dots precisely enough to build the devices.
The cells developed at Notre Dame use a stand-in for quantum dots. "These are actually metallic islands that are playing the role of quantum dots," said Gregory L. Snider, an associate professor of electrical engineering at Notre Dame and one of the principal researchers on the QCA project. "They have the same basic properties [required for QCA] that quantum dots have."
A cell based on four metallic islands is 3,000 to 4,000 nanometers across. In theory, a cell based on quantum dots would be less than 100 nanometers across.
The Notre Dame researchers have succeeded in building a QCA cell out of four metallic islands that are physically isolated from each other and the environment, said Ravi K. Kummamuru, a graduate student at Notre Dame and researcher on the project. Previous QCA cells included electrical leads used to control the gates on the islands, he said.
"This is as close as it gets to the ideal QCA cell," he said. "The electron occupancy of the cell is controlled by gates that are capacitively coupled." This means that the gate on an island is controlled by a nearby, but physically separate object with an electrical charge. In the case of a QCA cell, the nearby object is a gate on an adjacent island.
When the gates are open, electrons are free to jump across to adjacent islands. The four islands in the leadless QCA cell are arranged in a square. When two of the islands contain an electron, the electrons move to diagonally opposite islands because electrons in neighboring islands repel each other. If a gate opens, allowing an electron to move to a neighboring island, the other electron in the cell moves to the island in the opposite corner.
This configuration of four metallic islands has two possible arrangements of two islands containing electrons and two remaining empty. Those two states can serve as the ones and zeros of binary computing, forming a switch.
The movement of electrons within a cell can also trigger movement of electrons in adjacent cells, allowing bits to be transferred across cells.
"You have a line of these cells and you give an input to the first cell, which causes it to switch in one direction. That switching, because of the coupling between each cell, propagates along the QCA line," said Kummamuru.
Moving bits around a computer also requires a clocking function so components can be synchronized. The Notre Dame researchers have done this by adding two clocking islands to the four data islands. They have fashioned a prototype half cell of three islands to demonstrate the concept.
"The middle [island] is used to control the flow of electrons from the top [island] to the bottom [island]," said Kummamuru. "A clocking signal is applied to the center [island], so you can control the flow using a clock. By making it controllable by a clock, it's basically possible to pipeline the QCA architecture and make digital logic possible."
The clocking function also serves to lock cells, making QCA memory possible, said Snider.
QCA-based computing "is an extremely challenging thing to implement. It's very difficult experimentally to actually pull this off," said Eric Snow, research physicist and head of the nanostructures section of the Electronic Science and Technology Division at the Naval Research Laboratory.
One of the biggest challenges facing anyone seeking to make QCA-based computers is being able to produce quantum dots precisely enough and consistently enough to make QCA cells on demand. Researchers at Notre Dame and elsewhere are working on molecular implementations of quantum dots, Snow said. That would mean using chemistry techniques, which are potentially faster, cheaper and more consistent than the semiconductor techniques used today.
"Given some revolution happens [in quantum dot fabrication], you want to form the foundations for [QCA] computing," said Snow. "You have to put the research in first to see if it's even worth the effort to go in that direction." The Notre Dame researchers have put "heroic efforts into those steps," he added.
Useful devices based on the QCA architecture are probably 15 years away, said Snider.
Snider and Kummamuru's colleagues on the research were Islamshah Amlani, Alexei O. Orlov, Gary H. Bernstein, Craig Lent, Rajagopal Ramasubramaniam and Geza Toth. They published their work on leadless quantum-dot cellular automata cells in the July 31, 2000 issue and their work on clocked QCA in the July 10, 2000 issue of the journal Applied Physics Letters.
The QCA research was funded by the Defense Advanced Research Projects Agency, the National Science Foundation and the Office of Naval Research.
Timeline: 15 years
TRN Categories: Integrated Circuits; Semiconductors and Materials
Story Type: News
Related Elements: Photos, Diagrams, Technical paper "Experimental demonstration of a leadless quantum-dot cellular automata cell" in Applied Physics Letters July 31, 2000, Technical paper "Experimental demonstration of clocked single-electron switching in quantum-dot cellular automata" Applied Physics Letters July 10, 2000
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