July 10, 2008 — Understanding electron interaction is crucial to bring quantum computers to fruition, and now, researchers have determined that carbon nanotubes are a useful tool toward that end. Carbon nanotubes are ideal for studying spin-based, two-atom systems because of their long spin coherence time, say researchers at Hitachi Cambridge Laboratory (Cambridge, England) and the Nano-Science Center, Niels Bohr Institute, University of Copenhagen (Copenhagen, Denmark). Post-doc Henrik Jørgensen and colleagues created a so-called “double quantum dot” by placing two 50 nm titanium electrodes on the carbon nanotube to act as source and drain, along with three central top-gate electrodes of aluminum oxide and titanium. A small voltage applied across the device created two strongly coupled quantum dots with a clear shell structure of electrons. Surface plots of current as a function of voltage across the device confirmed shell occupation numbers where the quantum dots formed in series. The device exhibited inelastic cotunneling via the singlet and triplet states, which can be used to probe splitting of those states. Such manipulation of quantum dots could be the key to creating a quantum bit, the cornerstone of quantum computing.