We present a spin-exchange relaxation-free vector magnetometer with suppressed $1/f$ probe noise, achieved by applying a small dc bias field and a comb of magnetic dc $\pi$ pulses along the pump direction. This results in a synchronous orthogonal ac response for each of its two sensitive axes. The magnetometer is particularly well suited to applications such as biomagnetism in which the signal to be measured carries a dominant component of its power at low frequencies. The magnetometer reaches a technical noise floor of 8.4 fT ${\mathrm{Hz}}^{-1/2}$ ($\hat{\mathbf{x}}$) and 11 fT ${\mathrm{Hz}}^{-1/2}$ ($\hat{\mathbf{y}}$) at 0.01 Hz. A single-axis dc spin-exchange relaxation-free (SERF) magnetometer sharing the same experimental apparatus attains 61 fT ${\mathrm{Hz}}^{-1/2}$ at the same frequency. A noise minimum of 1.1 fT ${\mathrm{Hz}}^{-1/2}$ ($\hat{\mathbf{x}}$) and 2.0 fT ${\mathrm{Hz}}^{-1/2}$ ($\hat{\mathbf{y}}$) is reached by the magnetometer at 10 Hz, compared to 0.7 fT ${\mathrm{Hz}}^{-1/2}$ at 25 Hz for a dc SERF magnetometer.

• Received 27 February 2018
• Revised 5 January 2019

DOI:https://doi.org/10.1103/PhysRevApplied.11.034040