At present single-electron transistors (SETs) and superconducting quantum interference devices (SQUIDS) are the most promising candidates for read-out devices in solid-state quantum computers. SETs work well with large impedances and SQUIDS with small impedances. However, both run into problems at intermediate impedances, of about 1 megaohm.

The Helsinki team has built a “Bloch oscillating transistor” consisting of three junctions. The first is a Josephson junction in which two superconducting layers are separated by a thin insulating layer. The second is a “normal” tunnel junction and the third, a large resistance. The Josephson junction measures less than 100 nanometres across.

The researchers inject a base current made up of single electrons into one side of the Josephson junction and find that a “supercurrent” of Cooper pairs emerges on the other side. Cooper pairs form when the electrons in a superconducting material overcome their mutual repulsion as a result of their interactions with vibrations of the crystal lattice.

The device works by setting up Bloch oscillations in the Josephson junction. Normally, Bloch oscillations only occur in the ground state, E₀. However, electrons can also tunnel from E₀ to E₁ in a process known as Zener tunnelling, and the device will only work if the electrons can be made to “relax” back down to the ground state.

The researchers achieve this by injecting a current of “quasiparticles” into the normal junction, which then allows relaxation between the two energy levels. The team observed a current gain of 30 and a power amplification of 5 in their device.

The resistance in the circuit simply acts as “island” that suppresses unwanted fluctuations in the system.