The majority of the research work of the Quantum Nanoelectronics Group is based on Josephson junctions. The Josephson junction was predicted by Brian Josephson in 1962, for which he won the Nobel Prize for Physics in 1973. In its most general form it consists of two superconductors separated by a thin layer of something which is not superconducting. The interference between the wavefunctions of the superconducting condensate on either side of this non-superconducting barrier leads to a variety of interesting quantum phenomena, many of which have been (or can be) exploited in applications.

The technology for fabricating Josephson junctions has come a long way since the 1960's. The first junctions were made of soft materials such as lead. These junctions tended to degrade significantly when thermally cycled between room temperature and the operating temperature of around 4 Kelvin. In the early 1980's a more robust technology based on niobium was developed. There is now a major world-wide research effort in developing a new high-speed logic family (known as RSFQ logic) with predicted switching speeds of up to 100 GHz, based on niobium Josephson junctions.

Following the discovery of the high-temperature cuprate superconductors in 1986 (for which Bednorz and Müller were awared the Nobel Prize for Physics in 1987) many researchers tried to develop Josephson junctions based on these materials. Unfortunately the short coherence length (which is a consequence of the high transition temperature) makes the development of a high T_c Josephson technology very difficult. Our group at UCL (amongst others) have therefore looked at a novel type of high T_c Josephson junction architecture which actually exploits the low coherence length. These so-called "intrinsic" Josephson junctions were first demonstrated in single crystals by our collaborators in Erlangen in 1992. With a view to potential applications (particularly as THz oscillators) we have extended this work to thin films. We now routinely fabricate intrinsic junctions with dimensions as small as 1.5 nm x 100 nm x 250 nm, using focussed ion-beam patterning.

Interest in the quantum properties of Josephson junctions has been rekindled over the last few years by the prospect of developing a solid-state quantum computer. The two quantum conjugate variables in a Josephson junction are N, the number of electron pairs on the electrode of a junction, and f, the phase difference between the wavefunctions on either side of the barrier. Proposals have been made for quantum computers in which the qubits are based on N and f. Demonstrations of the superposition of distinct quantum states (i.e. the creation of a Schrödinger's Cat) in both types of Josephson junction have also recently been performed.