An ensemble of nitrogen-vacancy (NV) centers in diamond is a fascinating candidate to realize a sensitive magnetic field sensor. In particular, since the axes of the NV centers are distributed along four directions, a collection of measurement data from NV centers with different axes provides information on the vector components of a magnetic field. However, in the conventional approach, the low measurement contrast of NV centers limits the sensitivity of vector magnetic field sensing. Recently, to overcome this problem, multi-frequency control of the NV centers has been proposed. The key idea is that the four types of NV centers with different axes are simultaneously controlled by multi-frequency microwave pulses. Here, we demonstrate vector magnetic field sensing with an ensemble of NV centers in diamond via such multi-frequency control with pulsed-type measurements. We use Hahn echo pulses and extract information on the vector components of an applied AC magnetic field. We find that the sensitivity of diamond-based vector field sensing with multi-frequency control is better than that with single-frequency control for every vector component of a magnetic field.Magnetic field sensors have significant applications in chemistry, biology, and medical science. For example, in electron spin resonance, which is a widely used technique in chemistry, magnetic field sensors play an important role in obtaining information about the electron spin. Magnetoencephalography is a clinical technique for measuring electrical activity in the brain via magnetic field sensing, thereby providing information about brain function. Magnetic resonance imaging allows examination of the internal structure of the human body based on magnetic field information. For the magnetic field sensors used in these contexts, sensitivity and spatial resolution are essential parameters to quantify performance, and much effort has been devoted to measuring weak magnetic fields in local regions. 1-3
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