Quantum Computing with Superconducting Structures

In a quantum computer the information is stored in arrays of quantum two-level systems; these qubits generalize the notion of the well known bit in a classical computer. Execution of a quantum algorithm involves quantum gates, unitary operations rotating individual qubits and entangling them pairwise. Superconducting solid-state qubits are promising candidates for the hardware implementation of scalable quantum information processors; quantum fluctuations are introduced through small-capacitance Josephson junctions and the frustrating drive is introduced through a gate potential (charge-qubit) or a magnetic flux (phase- or flux qubit). We have proposed various designs for the solid-state implementation of qubits based on superconducting structures: We have shown how to build a quiet qubit using superconductors with d-wave symmetry and have suggested a first design for topological computing based on a quantum Josephson junction array. In our most recent proposal we suggest to exploit the symmetry properties of a tetrahedral structure (four equal islands with pairwise symmetric coupling) in order to emulate a spin-1/2 system in zero field with quadratic noise stability and generically large quantum fluctuations, an ideal starting point for a qubit. We have investigated a generic decoherence channel in superconducting qubits which is due to phonon radiation in the Josephson junctions arising from the piezoelectric coupling between the dynamic superconducting phase and the junction insulator. Our recent interest is in the further understanding and development of the tetrahedral qubit, in particular the effect of (weak) symmetry breaking on its properties and the implementation of simple experiments testing the coherent operation of the device.