Crossbar circuits (sometimes called crossbar latches) allow to create logic functions without the use of transistors. The main component of the crossbar circuit is a network of nanoscale switches that can be activated by applying a voltage to the corresponding crossing point. Due to the very small size of these nanowires, defects and imperfections are unavoidable. Random fluctuations make it essentially impossible to create a perfect structure from elements with the size of a few atoms. The crossbar circuit must therefore have a built in redundancy. Novel architectural designs and coding theory techniques are being used to compensate for these physical imperfections.
An overview of the groups working on crossbar architectures can be found in Scientific American 11/2005, p. 80. The following table is taken ffrom this article and the corresponding links are added for further reading.