We present a sensitive diamond quantum sensor with a magnetic field
sensitivity of 9.4±0.1 pT/Hz​ in a near-dc frequency range
of 5 to 100~Hz. This sensor is based on the continuous-wave optically detected
magnetic resonance of an ensemble of nitrogen-vacancy centers along the [111]
direction in a diamond (111) single crystal. The long T2∗​∼2 μs in our diamond and the reduced intensity noise in
laser-induced fluorescence result in remarkable sensitivity among diamond
quantum sensors. Based on an Allan deviation analysis, we demonstrate that a
sub-picotesla field of 0.3~pT is detectable by interrogating the magnetic field
for a few thousand seconds. The sensor head is compatible with various
practical applications and allows a minimum measurement distance of about 1~mm
from the sensing region. The proposed sensor facilitates the practical
application of diamond quantum sensors.Comment: 8 pages, 5 figure