  An important feature of strong correlated electron systems is the tunability
between interesting ground states such as unconventional superconductivity and
exotic magnetism. Pressure is a clean, continuous and systematic tuning
parameter. However, due to the restricted accessibility introduced by
high-pressure devices, compatible magnetic field sensors with sufficient
sensitivity are rare. This greatly limits the detections and detailed studies
of pressure-induced phenomena. Here, we utilize nitrogen vacancy (NV) centers
in diamond as a powerful, spatially-resolved vector field sensor for material
research under pressure at cryogenic temperatures. Using a single crystal of
BaFe2(As0:59P0:41)2 as an example, we extract the superconducting transition
temperature (Tc), the local magnetic field profile in the Meissner state and
the critical fields (Hc1 and Hc2). The method developed in this work will
become a unique tool for tuning, probing and understanding quantum many body
systems.
