The basis for superconducting electronics is the Josephson junction, a thin insulating barrier that separates two superconducting leads and
thereby behaves as a tunneling barrier. These junctions are the non-linear circuit element inside superconducting quantum interference
devices, low temperature microwave electronics, and superconducting quantum computers. The width of this barrier can be as thin as two nanometers and the electronic properties of such junctions are strongly dependent on the morphology of the barrier, both at the interfaces of the superconducting leads and within the metal oxide itself. Using a range of computational techniques we analyse how the junctions are formed and how their electrical response depends on the junction microstructure.Our results provide new insights into the influence of fabrication conditions on the electrical response of metal-oxide barriers and the resulting performance of quantum technologies constructed from them.
Incoherent quantum charge tunneling forms the basis of semiconductor devices. However it is well known from atomic and optical work, that coherent tunneling can be significantly faster and in principle, coherent transfer is dissipationless, i.e. the charge being transferred does not exchange heat with the environment. Furthermore, adiabatic controls are inherently robust provided the adiabaticity criterion is satisfied. Adiabatic control methods are therefore preferable to ensure high-fidelity operation despite increased operating time. We describe recent work towards understanding charge transfer mechanisms based on adiabatic passage techniques with all-electrical controls in three-, five- and seven-dot systems based on Coherent Tunneling Adiabatic Paassage (CTAP). We derive analytical values for the important eigenstates and the adiabaticity criteria for these schemes. These analytical results allow us to make comments regarding the scalability of these schemes to realistic quantum networks.
Quantum-dot Cellular Automata (QDCA) provide an interesting experimental system with which to study the interaction of a charge-based two-level system with the surrounding solid-state environment. This is particularly important given the recent interest in solid-state quantum computers using charge-qubits. We show that many of the properties of these qubits, coupling strength, tunnelling time, relaxation rate and dephasing time can be estimated using a QDCA cell like structure. Calculations are performed for the case of buried donors in silicon but the results are equally applicable to other forms of charge-qubit.