  We report on the design, fabrication and characterization of superconducting
coplanar waveguide resonators with nanoscopic constrictions. By reducing the
size of the center line down to 50 nm, the radio frequency currents are
concentrated and the magnetic field in its vicinity is increased. The device
characteristics are only slightly modified by the constrictions, with changes
in resonance frequency lower than 1% and internal quality factors of the same
order of magnitude as the original ones. These devices could enable the
achievement of higher couplings to small magnetic samples or even to single
molecular spins and have applications in circuit quantum electrodynamics,
quantum computing and electron paramagnetic resonance.
