Low-bandwidth control scheme for an oscillator stabilized Josephson qubit

We introduce a new flux-based Josephson junction circuit for which quantum operations are realized by low-bandwidth, nearly adiabatic magnetic-flux pulses. Coupling to the fundamental mode of a superconducting transmission line permits a stabilization of the rotation angle of the quantum operation against flux noise. A complete scheme for one-qubit rotations, and high-visibility Ramsey-fringe oscillations, is given. We show that high visibility depends on passing through a portal in the space of applied fluxes, where the width of the portal is proportional to the ramp-up rate of the flux pulse. Initial measurements of such a qubit show a measured visibility of 60 percent and a coherence time of greater than 35 ns. The fundamental mode frequency of the transmission line was 1.54 GHz.