Quantum computers are expected to perform much faster than classical computers because they exploit the ability of quantum bits to exist in a superposition of different states. A classical bit can only ever take on a value of '0' or '1', but a quantum bit or 'qubit' can have any combination of these two values. However, real-world qubits must satisfy stringent requirements to take these ideas from the blackboard to the motherboard.

Daniel Loss, a theoretical physicist from the University of Basel in Switzerland, has been exploring the performance of different types of qubits for a number of years. Now, on page 312 of this issue, Loss and colleagues in Basel and the University of Valencia in Spain suggest that qubits based on molecular magnets could have advantages over other approaches (Nature Nanotech. 2, 312–317; 2007).

They study a polyoxometalate molecule in which two vanadium oxide (VO)2+ groups are separated by a core of molybdenum oxide. Each (VO)2+ group has an electron spin that can point up or down, or in a superposition of both directions, so the molecule plays host to two spin qubits, with the interaction between them depending on the number of electrons on the core. The spins do not interact with each other when there are an even number of electrons on the core, but if an electron tunnels on or off the core, leaving an odd number, they will start to interact.

The length of time over which the interaction, or 'gate', is turned on determines the final state of the spins. This graph shows how a figure of merit known as the gate fidelity is predicted to vary as a function of time (x axis) and the strength of the interaction between the spins (y axis). The maxima are indicated in yellow, with the circle indicating the highest calculated fidelity of 0.99.

The calculations were performed for a single molecule with electrons tunnelling from the core to a scanning probe tip and back, but Loss and co-workers believe that their system could be scaled to large numbers of qubits by combining molecular self assembly techniques with a crossbar electrode geometry.