Quantum Dot Computing

Introduction:
Quantum dots are nanoscopic metallic or semiconductor devices which can hold a well defined number of electrons. Quantum dots have been built which can hold an excess charge of just a single electron. Today these devices are typically 30 nanometres across. In the future they are likely to be made even smaller.

Professors Craig Lent and Wolfgang Porod of the University of Notre Dame in Indiana, USA, have proposed a way of using arrays of these quantum dots to replace conventional transistor based logic circuits. They propose that a number of these dots be combined to produce a cell with switching capabilities. Two or more neighbouring cells then interact to produce logic gates and connecting wires. They call this proposal Quantum-dot Cellular Automata (QCA).

QCA follows the well established rules of Boolean logic and at the same time offers the potential of extreme increases in circuit density and speed as well as reduced power usage. For dots of size 10nm, a circuit that would conventionally require 30 transistors occupies an area of less than 1.5µm2

Quantum-dot Cellular Automata:
In the QCA each cell is made from 4 quantum dots. Two of these dots carry an excess electric charge. Due to electric repulsion between them, the charges tend to move to the dots at opposite corners of the cell. Adjacent cells then interact such that all cells try to adopt the same configuration of charged dots. By adopting matching configurations they naturally reduce the energy state of the system. In this way, lines of cells can act as wires which transmit information without any electrons actually moving along the wire.

Logic gates such as NOT, AND and OR, can also be constructed from arrays of cells. And from these gates complete logic circuits can be built.

Experimental Setup:
In experiments funded by DARPA and NSF, the scientists at Notre Dame have been trying to prove the technical feasibility of building quantum dot cells. In September 1998 they reported the successful demonstration of switching in a single cell.

In theory, QCA could be implemented using semiconductors, metal tunnel junctions, or even individual molecules. At Notre Dame metal tunnel junctions were used because they were the easiest to fabricate. Six dots were used, forming a 4 dot cell and 2 probe dots. These were built using a combination of optical and electron beam lithography as well as metal evaporation technologies.

The experiments found that the switching frequency of the cells was 14MHz at the experimental temperature of 10mK. The theoretical frequency at room temperature is 5GHz. Further research is now underway towards the building of larger arrays of cells.

Magnetic Quantum Dots:
Many researchers around the world are now investigating the properties of quantum dots. In August 1999, researchers in Germany and California built an array of quantum dots which relied on the use of magnetic fields. These were the smallest semiconductor rings ever to support a measurable electric current. The rings were only 50nm across and could contain only one or two electrons. A perpendicular magnetic field was applied so as to confine the electrons to moving around the wire in only one direction. Using a technique that involved spraying two atomic layers onto a semiconductor surface, the researchers made as many as 1011 of these quantum dots.

Links:
Notre Dame University: www.nd.edu/~qcahome
QWEST, University of California: www.quest.ucsb.edu
LM University, Munich: www.nanoscience.uni-muenchen.de