  High-sensitivity detection of microscopic magnetic field is essential in many
fields. Good sensitivity and high spatial resolution are mutually contradictory
in measurement, which is quantified by the energy resolution limit (ERL). Here
we report that a sensitivity of 0.5 ${\bf{nT/\sqrt{Hz}}}$ at the nanoscale is
achieved experimentally by using nitrogen-vacancy defects in diamond with
depths of tens of nanometers. The achieved sensitivity is substantially
enhanced by integrating with multiple quantum techniques, including
real-time-feedback initialization, dynamical decoupling with shaped pulses,
repetitive readout via quantum logic. Our magnetic sensors will shed new light
on searching new physics beyond the standard model, investigating microscopic
magnetic phenomena in condensed matters, and detection of life activities at
the sub-cellular scale.
