NV color center fluorescence signal measurement based on FPGA architecture

Nitrogen-vacancy (NV) quantum defects in diamond are sensitive detectors of magnetic fields. Then, the existing sensing technologies based on diamond NV centers rely on bulky and expensive instruments, which must be replaced by a more compact design. Here we combine NV quantum sensing technology with integrated circuit technology to create a compact and easy-to-operate platform. Using the existing integrated circuit technology, we integrate the lock-in amplifier, microwave generator and TTL modulator together. with this platform, we can obtain a magnetic sensitivity of 5.72μT/Hz 1/2.


1.Inroduction
The excellent ability of diamond NV color center in magnetic detection has aroused interest.NV color center has been widely used in magnetometry, electrometry and ion detection [1][2][3].The diamond NV center has spin selectivity, and its electron spin state can be judged by the change of fluorescence intensity, that is, the spin spectrum of NV center can be detected by optical means.This characteristic is the key to the application of diamond NV color center in the field of quantum sensing.
The optical properties of the NV color center are as follows.The energy level structure of NV color center is shown in Fig. 1.Its ground state is a spin triplet 3 A2 [4], including ms=0 and ms= ± 1.Under the continuous pumping of 532 nm laser [5], the electron spin state of NV color center will be polarized to the ground state of ms = 0, and the fluorescence signal is the maximum.The microwave frequency is changed while the laser is continuously pumped.When the microwave frequency reaches the resonant frequency of 2.87GHz, the ms = 0 state transitions to the ms = ± 1 state.ms = ± 1 state due to the ISC process [6], the intensity of the radiated fluorescence is far less than the NV color center of the initial state ms = 0, and the fluorescence intensity is greatly reduced.

Electronic system implementation
The picotesla magnetic field sensitivity has been realized in the laboratory [7], which is dependent on the spin polarization of NV color and the efficient fluorescence collection ability.Traditional NV magnetic measurement design based on optical detection of magnetic resonance(ODMR) involves bulky and discrete devices, which NV color center magnetometers in practical applications.The NV-ODMR experimental platform includes optical system, microwave system, control and readout system.Figure 2 illustrates the overall system architecture.The optical system is composed of various optical devices.The microwave system block mainly includes microwave source, power amplifier, radiation structure TTL modulation module.The control and readout system includes a FPGA control module, digital lock-in amplifier, a high-speed and high-precision ADC.As shown in Fig. 3, it is the physical diagram of the self-built FPGA-based NV color center ODMR experimental platform.

digital lock-in amplifier
The key and difficult point in the design of digital lock-in amplifier is how to obtain the mixed DC signal and calculate the amplitude.The low-pass filter uses the IIR structure and uses the Butterworth analog filter approximation.The design parameters are : fs=100 KHz，fp=100 Hz，fs=1 KHz，Rp = -3 dB，Rs = -30 dB.Using the cordic algorithm to calculate the final amplitude.Fig. 4 is the block diagram of the digital lock-in amplifier system designed by this system.

microwave source
Based on ADI 's phase-locked loop chip ADF4351, a microwave signal source is designed to generate 2.5GHz-3.0GHzRF signal.Fig. 5 shows the phase noise simulation results of the microwave module designed by this system.

TTL modulator
AD9708 is used to generate 0-5V analog square wave, which is given to HMC284 to generate a modulation signal.The function of the modulation module is to extract the effective signal of the system from the low frequency noise to the high frequency, and to process the subsequent digital lock-in amplifier.

3.conclusion
We proposed a NV color center magnetometer based on FPGA, and manufactured the basic components of the NV-ODMR experiment--digital lock-in amplifier, microwave source, and TTL modulator.Figure 6 shows the ODMR spectrum obtained by the experimental platform.The shot-noise-limited sensitivity of the system can be expressed as [8]: The magnetic field measurement is carried out on the integrated platform with a sensitivity of

Figure 4 .
Figure 4. Structure diagram of digital lock-in amplifier