  Single nitrogen-vacancy (NV) centers are widely used as nanoscale sensors for
magnetic and electric fields, strain and temperature. Nanoscale magnetometry
using NV centers allows for example to quantitatively measure local magnetic
fields produced by vortices in superconductors, topological spin textures such
as skyrmions, as well as to detect nuclear magnetic resonance signals. However
one drawback when used as magnetic field sensor has been that an external bias
field is required to perform magnetometry with NV centers. In this work we
demonstrate a technique which allows access to a regime where no external bias
field is needed. This enables new applications in which this bias field might
disturb the system under investigation. Furthermore, we show that our technique
is sensitive enough to detect spins outside of the diamond which enables
nanoscale zero- to ultralow-field nuclear magnetic resonance.
