Credit: © 2006 Trauzettel, Bulaev, Loss, Burkard

In a spin quantum bit (qubit), digital logic states are defined by the relative orientations of the spins. Placing one spin on each of two quantum dots and controlling their relative orientations amounts to storing information. There is interest in making spin qubits out of graphene quantum dots because the spin polarization (up or down) in carbon-based materials decays more slowly compared with others, meaning longer-term information storage.

However, there needs to be an energy difference between the parallel and antiparallel orientations (so the spins can be prepared in a unique configuration) and this is not easily obtained in graphene. Now, a theoretical study from the University of Basel suggests that if the opposite edges of a graphene sheet are ‘cut’ along a particular direction, it is possible to obtain an energy difference between the parallel and antiparallel spin configurations.

Bjorn Trauzettel and co-workers1 propose a device consisting of a graphene ribbon in which two regions are electrically isolated from one another by a voltage on the electrode between them. They show not only that a qubit can be realized between two spins on these quantum dot ‘strips’, but that coupling can occur over long distances, an important requirement for increasing the error tolerance in quantum computing.