Credit: © 2008 Nature Physics

Many aspects of quantum physics only become apparent when we can control small numbers of electrons. Quantum dots — structures where electrons are confined in all three dimensions — are ideal systems in which to take this approach. This is demonstrated by the artificial molecules made from a pair of quantum dots in a carbon nanotube by Henrik Ingerslev Jørgensen and co-workers1 at the Hitachi Cambridge Laboratory and the Niels Bohr Institute.

The carbon nanotube was grown on a highly doped silicon substrate with a top layer of silicon dioxide, after which source and drain electrodes were added, followed by three gate electrodes. By carefully altering voltages, the researchers showed that parts of the nanotube under two of the gate electrodes behaved like quantum dots with shells of either four or eight electrons. They then used magnetic field spectroscopy to alter the energy states of the electrons, and to observe the electrons quantum tunnelling from one quantum dot to the other.

Moreover, the energy difference between singlet states (in which two electrons have a total spin of zero) and triplet states (in which the total spin is one) was measured, and the result was found to be in agreement with theoretical predictions.