  We introduce a new 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.
