Diamond quantum magnetometer with dc sensitivity of < 10 pT Hz^-1/2 toward measurement of biomagnetic field

We present a sensitive diamond quantum sensor with a magnetic field sensitivity of 9.4 pm 0.1 mathrm{pT/sqrt{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^ast sim 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.