  We present the realization and characterization of a novel-concept
interferometer, the superconducting quantum interference proximity transistor
(SQUIPT). Its operation relies on the modulation with the magnetic field of the
density of states of a proximized metallic wire embedded in a superconducting
ring. Flux sensitivities down to $\sim 10^{-5} \Phi_0$Hz$^{-1/2}$ can be
achieved even for a non-optimized design, with an intrinsic dissipation ($\sim
100$ fW) which is several orders of magnitude smaller than in conventional
superconducting interferometers. Our results are in agreement with the
theoretical prediction of the SQUIPT behavior, and suggest that optimization of
the device parameters would lead to a large enhancement of sensitivity for the
detection of tiny magnetic fields. The features of this setup and their
potential relevance for applications are further discussed.
