The quantum dynamics of Josephson junctions with controlled current-phase relationships

There is rapidly expanding international activity on the development of electronic circuits which can be used as artificial atoms in that they possess a set of well defined quantum states. Such circuits are known as quantum bits (qubits) with applications ranging from quantum computing to measurement standards. These quantum states are extremely fragile (decohere readily), are most easily established at very low temperatures (<1K) and can be destroyed by sources of electronic noise in the environment or associated with the materials themselves. Superconducting materials have proved to be very sucessful for qubit fabrication and progress has been rapid with a number of approaches being demonstrated in recent years based upon Josephson junctions in superconducting loops.Qubit designs with significantly improved immunity to decoherence would extremely important to this field. There have been a number of designs for environmentally decoupled qubits based upon unconventional Josephson devices known as pi junctions. There are two main types of these: superconducting tunnel junctions which contain a ferromagnetic layer between the tunnel barrier and one electrode (SFIS structures) and junctions with a conventional superconductor as one electrode and a d-wave superconductor as the other (SND structures). The aim of this proposal is to carry out the key experiments which test their suitability for qubit applications. The two systems share many features common and so by investigating both in parallel, we will be able employ common experimental approaches, evaluate their relative merits and differences and to determine the overall feasibility of qubit designs which rely on pi junctions. The experiments will involve optimising both technologies for qubit applications, determining what junction designs give the most suitable pi junction behaviour, investigating potential intrinsic noise sources associated with the devices, investigating whether the junctions themselves develop well defined quantum states at low temperature and finally, using the results of the previous experiments to select a suitable qubit design and investigate its properties.The applicants are researchers at the Universities of Cambridge and Birmingham with a strong track record of the fabrication of superconducting devices from metallic and oxide heterostructures and measurements at temperatures below 1K, who are thus well placed to make a major impact in this area.