Fig. 1. Working principle of NV centers in diamond. (a) Lattice structure of the NV centers, in which N atoms are bonded to adjacent vacancies. Four axes exist in an NV center. (b) Magnetic measurement principle of the NV centers. Microwave (MW) and AC signals are conducted to the diamond surface through a home-made radio frequency antenna and coil. Bias magnetic field with magnetic induction intensity B0 is provided by a large homogeneous permanent magnet. The fluorescence obtained by 532 nm laser excitation is conducted downward by a light waveguide to a photodiode, whereby the signal of the magnetic field to be measured is converted into an electrical signal. (c) Fine energy level distribution of NV electron. The triplet states of the ground and excited states are denoted as ³A₂ and ³E, respectively. ³A₂ consists of three subenergy levels ms=0,±1. The states of ms=0 and ms=-1 are encoded as sensors, which are initialised and read by a 532 nm laser. On the right, the intersystem crossing (ISC) through the metastable states ¹E and ¹A₁ is showed

Fig. 2. Overall experimental setup of the NV centers magnetometer. (a) NV centers magnetometer experimental setup, with the blue dashed box indicating the optical system and its data processing system, the black dashed box indicating the microwave system, and the red dashed box indicating the signal transmission system to be measured. The microwave RF antenna will output frequency modulated microwaves through the microwave amplifier, with a resonant frequency center of 2.82 GHz. Diamond is placed at the center of microwave antenna. (b) Detailed setup of the optical system of the NV centers magnetometer, where the coil transmitting the signal to be measured is rotated horizontally center on the diamond. Permanent magnet is used to provide a bias magnetic field, with a magnetic induction intensity of approximately 10mT

Fig. 3. MSK signal modulation and demodulation. (a) MSK signal modulation. Similar to the I/Q frequency modulation of microwave signals, MSK signals also need to be divided into I and Q orthogonal signals for modulation. The MSK signal can be obtained by dividing original binary data into odd and even symbols, mixing them separately, and adding them together. (b) MSK signal demodulation involves dividing the MSK signal into two orthogonal signals and performing frequency mixing, low-pass filtering, sampled quantizer encoding before obtaining the original binary signal through parallel-serial conversion and differential decoding

Fig. 4. MSK signal demodulation with a magnetic induction intensity of 30 μT. (a) Actual measured MSK carrier signal, with carrier frequency fc=10kHz; (b) I and Q mixed branch signals, where the orange lines represent the binary signal obtained from the mixed signal after low-pass filtering and sampled quantizer encoding; (c) demodulated binary signal obtained from the I and Q mixing branch binary signal through differential decoding (Comparing the demodulated binary signal with the original signal gives a bit error rate is 0)

Fig. 5. Relationship between decoding bit error rate of MSK signal and signal strength (Bit error rate is mainly limited by system noise around 10 kHz)

Fig. 6. NV centers four-axis ODMR spectra and their noise power spectral density. (a) The first-order differential spectra of the four axes of the NV centers showing NV1, NV2, NV3, and NV4 from left to right; (b) the red curve is the first-order differential spectra of NV3, the orange curve is its slope curve, and the blue dashed line is the curve fitted to its linear section, which records the linear operating section of the NV3 axis ( The section is approximately from 2.662 GHz to 2.664 GHz, according to which the linear dynamic range can be converted as 35.68 μT); (c) noise power spectral density of the four axes of the NV centers, representing the sensitivity of each axis of the NV centers when used as an antenna

Fig. 7. Angular relationship between the four axes of the NV centers and the emitting coil to be measured ( Each of the four axes of the NV centers makes an angle of 109.47°, and makes an angle of 35.26°with the horizontal plane. The top left corner is the top view of the diamond top surface crystal direction[100], in this situation the NV four axes are perpendicular to each other)

Fig. 8. Four-axis radiation patterns of NV centers. (a) Ideal NV centers four-axis radiation patterns; (b) measured NV centers four-axis radiation patterns