Very low frequency (VLF) refers to radio frequencies within the range of 3 to 30kHz, characterized by long wavelengths and strong penetration capabilities. These VLF waves can propagate effectively between the surface of the Earth and the ionosphere, enabling intercontinental communication. Moreover, their ability to penetrate through obstacles like seawater and rock makes them ideal for communication in underwater and underground environments. Traditional VLF communication systems utilize large, ground-based receiving antennas, which are often bulky and impractical in modern airborne and satellite-based observation systems. Consequently, an increasing demand existed for miniaturized, portable antennas. Various compact antennas have been developed to meet this need, including electrically small antennas such as loop, ferrite rod, whip, and magnetoelectric (ME) antennas. Nitrogen-vacancy (NV) centers in diamond, recognized for their high sensitivity in magnetic sensing, exhibit significant potential as VLF receivers. These NV centers offer key advantages, such as directional sensitivity to magnetic fields and wide-frequency bandwidth, making them suitable candidates for antenna applications. These properties make NV centers particularly suitable for use as antennas. This study investigates the integration of NV centers in diamond for communication modulation signal reception in the VLF range, aiming to create a miniaturized vector communication receiving antenna. Unlike superconducting quantum interference devices (SQUIDs) or atomic magnetometers, which rely on magnetic sensing but require specific thermal conditions (either high or low temperatures) to maintain high sensitivity, NV centers can operate effectively without such conditions. This unique characteristic highlights their immense potential for a wide array of applications, including VLF signal detection and quantum magnetometry.

This study developed a diamond NV center vector magnetometer comprising three main components: (1) an optical system for laser emission and fluorescence collection from the NV centers, paired with a data processing system; (2) a microwave system to provide modulated microwaves required for demodulation; and (3) an emission system for transmitting test signals. Using this configuration, the vector magnetometer detected fluorescence signals from the NV centers excited by a 532 nm laser in conjunction with continuous wave microwave modulation, enabling the detection of 10 kHz VLF radio waves.

The magnetometer was used to receive and decode minimum shift keying (MSK) modulated signals in the VLF band to validate the demodulation capability of typical VLF signals. The original binary signals were recovered through a series of steps, including mixing, low-pass filtering, and sampled quantizer encoding. To further analyze the vector characteristics of the antenna, a coil emitting the test signal was rotated around the diamond NV center. This approach enabled the measurement of the response of the antenna to the vector information of the VLF signals and facilitated the plotting of the radiation patterns of the antenna.

The signals received by the diamond NV center magnetometer are successfully demodulated and decoded into their original binary form using a process that involves mixing, low-pass filtering, and sampled quantizer encoding (Fig. 4). Experimental results reveal that when the amplitude of the alternating magnetic field coupled to the NV center exceeds 17 μT, the bit error rate (BER) remains below 1%. When the amplitude reaches 30μT or higher, the system demonstrates the ability to receive communication signals continuously without errors (Fig. 5).

The performance of the antenna can be characterized by its noise power spectral density and dynamic range. At 10 kHz, the noise power spectral densities for the four NV axes (NV1, NV2, NV3, and NV4) are 14.35, 26.50, 16.03, and 24.80 nT/Hz^1/2, respectively. The dynamic range for NV3 is measured at 35.68 μT (Fig. 6). Furthermore, based on the vector characteristics of the four NV axes, the radiation patterns of the antenna are plotted, and the angular sensitivity is quantified. The calculated angular resolution is 0.2°, with an equivalent angular noise power spectral density of 19.01×10-3(°)/Hz1/2 (Fig. 8).

In this study, we designed an NV center magnetometer with a significantly reduced antenna volume compared to traditional coils. Under 532nm laser irradiation, the VLF vector antenna, composed of a diamond with a volume of 4.5mm3, detected radio waves at a frequency of 10 kHz, achieving an optimal sensitivity of 14.35nT/Hz1/2. This antenna successfully received the magnetic component of VLF radio waves using this magnetometer as a sensor and demodulated real-time VLF MSK-modulated signals at the μT level with a low bit error rate, confirming its potential as a low-frequency communication signal receiver. Leveraging the vector characteristics of the diamond NV center, the four NV axes, fixed at specific angles to each other, are employed to plot the radiation patterns of the four axes under specific calibration and measurement field conditions. This setup verified the ability of the antenna to detect the direction of the communication signal source. The resulting angular resolution is 0.2°, with an equivalent angular noise power spectral density of 19.01×10^-3 (°)/Hz^1/2.