Quantum computers will have the power to solve important problems that are intractable to the classical computers that exist today. However, a useful fault-tolerant computer will need to have a very large number of qubits. One estimate is that a computer to factorise a 600-digit number (which is probably beyond any classical computer that will ever exist) would need more than 100 million qubits. This is far larger than present-day quantum computers.

No-one knows what technology the first large quantum computers will use, but one approach is to exploit the long experience of the semiconductor industry by using electron spin qubits. We have worked with two kinds of spin qubits, realised in different semiconductors: gallium arsenide and carbon nanotubes.

Because spin qubits are very sensitive to their environment, these devices also explore subtle quantum effects in electrical transport, such as hyperfine interaction and spin-orbit coupling. These devices also require us to develop important experimental tools, such as nanofabrication, sensitive measurement, and algorithmic control of experiments.

A carbon nanotube qubit device. The nanotube is suspended between two electrical contacts (green). Nearby gate electrodes (yellow) create electric fields that trap a pair of electrons along the nanotube. To make this nanotube visible in the electron microscope, it has been coated with a thin layer of metal. The uncoated nanotube is less than 5 nm wide.