  We characterize a niobium-based superconducting quantum interference
proximity transistor (Nb-SQUIPT) built upon a Nb-Cu-Nb SNS weak link. The
Nb-SQUIPT and SNS devices are fabricated simultaneously in two separate
lithography and deposition steps, relying on Ar ion cleaning of the Nb contact
surfaces. The quality of the Nb-Cu interface is characterized by measuring the
temperature-dependent equilibrium critical supercurrent of the SNS junction. In
the Nb-SQUIPT device, we observe a maximum flux-to-current transfer function
value of about 55 nA/\Phi_0 in the sub-gap regime of bias voltages. This
results in suppression of power dissipation down to a few fW. The device can
implement a low-dissipation SQUIPT, improving by up to two orders of magnitude
compared to a conventional device based on an Al-Cu-Al SNS junction and an Al
tunnel probe (Al-SQUIPT).
