Single-Phosphorus Transistor

In work that could help advance the development of quantum computers, researchers have created a transistor that consists of a single atom positioned precisely between two electrodes in a silicon substrate. Quantum computers could use certain algorithms to perform some calculations and simulations not possible on current computers, such as the Schrödinger equation for large molecules.

Quantum computing specialist Michelle Y. Simmons of the University of New South Wales, in Australia, and coworkers prepared the transistor. They used scanning tunneling microscopy, lithography, and phosphine chemistry to place, with single-lattice-site spatial accuracy, an individual phosphorus atom between electrodes in a silicon device (Nat. Nanotechnol., DOI: 10.1038/nnano.2012.21). Such precise positioning hadn’t been achieved before.

Single-atom transistors may represent the ultimate density in integrated circuits. But creating such transistors is painstaking, and the feasibility of making practical devices that comprise millions or billions of them is not yet known. The fact that the phosphorus transistor operates only at temperatures close to absolute zero also limits everyday applications for now.

Nevertheless, the phosphorus transistor represents a step toward quantum computers. Quantum computers would achieve greater power and speed by encoding information in qubits, which adopt more states than just the two (0 and 1) in conventional computer bits. Precise atom positioning would be required to interrogate the information in qubits accurately.

Device modeler Asen Asenov of the University of Glasgow believes the experimentation is “ground-breaking.” Molecular device fabricator Robert A. Wolkow of the University of Alberta believes that Simmons’ group and others reported substantially similar results earlier. Some of the study’s simulations have technical deficiencies, Asenov adds.

Quantum computing expert Bruce E. Kane of the University of Maryland notes that the one-atom transistor is not currently practical for conventional devices and doesn’t carry out quantum operations either. But he calls the work “an experimental and engineering tour de force” and believes Simmons’ group now has the requisite tools to begin building quantum computers “that would go beyond the current state of the art.”