A method and system for resonance frequency tracking control of diamond nitrogen-vacancy color centers

Abstract

This invention discloses a method and system for resonant frequency tracking and control of diamond nitrogen-vacancy color centers, belonging to the field of quantum sensing technology. The method includes: setting an initial center frequency output from a microwave source; during the tracking and control process, recording the changes in fluorescence signals generated by the diamond nitrogen-vacancy color centers under microwave frequency modulation, and demodulating to obtain the full-amplitude demodulated amplitude. R The system determines whether the value is less than the dead zone threshold; if so, the center frequency of the microwave source output remains unchanged; otherwise, the demodulation amplitude is based on the full amplitude value. R Calculate the microwave frequency adjustment Δ f The center frequency of the microwave source is determined based on the phase frequency characteristics and the real-time demodulation phase difference Δφ during the current frequency tracking control process. f The adjustment direction is 0; the adjustment amount Δ is based on the microwave frequency. f The center frequency of the microwave source is adjusted by regulating the direction of the modulation to achieve resonant frequency tracking; the phase frequency characteristic refers to the characteristics of the demodulated spectral lines on both sides of the resonant frequency. This enhances the robustness and accuracy of the resonant frequency tracking method.

Description

Technical Field

[0001] This invention belongs to the field of quantum sensing technology, and more specifically, relates to a method and system for tracking and controlling the resonant frequency of nitrogen vacancy color centers in diamond. Background Technology

[0002] Quantum sensing technology refers to the measurement of a physical quantity using quantum systems, quantum properties, or quantum phenomena. Among these, diamond nitrogen-vacancy (NV) centers possess extremely high magnetic sensitivity at room temperature, exceptional spatial resolution, and a large dynamic range. These characteristics make them promising for broad industrial applications under relaxed operating conditions, including but not limited to: current sensing, biomolecular sensing, medical diagnostics, and electromagnetic imaging.

[0003] Optical Detection Magnetic Resonance (ODMR) is an effective method for obtaining the resonance frequencies of the NV centers in diamond. ODMR can be performed in continuous-wave (CW) mode, which is characterized by the simultaneous application of a continuously acting pump laser and an excitation microwave (MW) field to initialize and manipulate the spin states of the NV centers. The ODMR spectrum can be obtained by scanning the microwave frequency. Compared with pulsed ODMR, CW-ODMR is advantageous for detecting low-frequency (DC-kHz range) AC magnetic fields and does not require complex calibration. In CW-ODMR measurements, frequency modulation (FM) and demodulation methods are typically used to improve the signal-to-noise ratio (SNR). This method induces synchronous fluorescence oscillation by frequency modulation of the microwaves and uses a lock-in amplifier to demodulate the fluorescence to obtain the demodulated ODMR spectrum.

[0004] Based on frequency modulation and demodulation methods, the amplitude changes of the ODMR demodulated spectrum can be continuously monitored at a fixed microwave center frequency. The resonant frequency shift can be calculated using pre-calibrated conversion coefficients (such as slope), and the magnitude of the measured magnetic field can be inferred. This method does not require changing the microwave center frequency and has high measurement speed. However, it is limited by the approximately linear region of the demodulated spectrum, resulting in a limited dynamic range for magnetic field measurement. To overcome these limitations, the Resonance Frequency Tracking (RFT) method has been proposed and applied in some research areas. Utilizing the zero-crossing characteristic of the ODMR demodulated spectrum at the resonant frequency, the real-time demodulated signal can be used as an error signal to construct a closed-loop control system. By adjusting the microwave center frequency through closed-loop tracking control to track the resonant frequency in real time, the desired demodulated output of the system is made zero.

[0005] However, current RFT methods primarily use demodulated in-phase ( I or orthogonal signals Q This serves as the error signal for the RFT method. However, I and Q The signals themselves contain sine or cosine components of the phase difference between the modulating signal and the reference signal. Fluctuations in the phase difference may interfere with the tracking process of the resonant frequency, reduce the accuracy of the measurement results, and weaken the overall robustness of the system.

[0006] Therefore, traditional frequency tracking control methods are susceptible to fluctuations in the phase difference between the modulation signal and the reference signal, leading to poor tracking performance or even tracking misalignment. Summary of the Invention

[0007] To address the shortcomings of related technologies, the present invention aims to provide a resonant frequency tracking control method and system for diamond nitrogen vacancy color centers. This method aims to solve the problem that traditional frequency tracking control methods suffer from poor tracking performance or even tracking misalignment due to the influence of phase difference fluctuations between the modulation signal and the reference signal, thereby improving the robustness of the resonant frequency tracking method.

[0008] To achieve the above objectives, in a first aspect, the present invention provides a method for tracking and controlling the resonant frequency of nitrogen-vacancy color centers in diamond, comprising:

[0009] S1. Obtain the change in fluorescence signal generated by the frequency modulation of the microwave source on the nitrogen vacancy color center in diamond during the frequency tracking control process, and take the resonance frequency corresponding to the resonance absorption peak of the maximum slope point in the demodulated spectrum of the fluorescence signal as the initial center frequency of the microwave source output during the frequency tracking control process.

[0010] S2. Demodulate the fluorescence signal to obtain the demodulated amplitude, and determine whether it is less than the dead zone threshold; if yes, do not change the center frequency of the microwave source output; if no, calculate the microwave frequency adjustment amount Δ. f ;

[0011] S3. Determine the center frequency of the microwave source based on the phase frequency characteristics and the real-time demodulation phase difference Δφ during the current frequency tracking control process. f The adjustment direction of 0; according to the microwave frequency adjustment amount Δ f The center frequency of the microwave source is adjusted by adjusting the direction of the adjustment for resonance frequency tracking; wherein, the phase frequency characteristic is the characteristic of the demodulated spectral line on both sides of the resonance frequency;

[0012] S4. Calculate and output the sensor value for the frequency tracking control process; keep the microwave source at the current center frequency, and return to S2 to continue the next round of frequency tracking control.

[0013] Optionally, the phase frequency characteristics include: a first phase frequency characteristic, a second phase frequency characteristic, a third phase frequency characteristic, and a fourth phase frequency characteristic;

[0014] The first phase frequency characteristic is that the demodulated phase to the left of the resonance frequency is smaller than the demodulated phase to the right of the resonance frequency, and the phase difference shows a positive phase jump at the resonance frequency;

[0015] The second phase frequency characteristic is that the demodulated phase to the left of the resonance frequency is greater than the demodulated phase to the right of the resonance frequency, and the phase difference shows a negative phase jump at the resonance frequency;

[0016] The third phase frequency characteristic is the demodulation phase to the left of the resonance frequency, which is ( -180°, 180°- The demodulation phase to the right of the resonant frequency is ;in, and Both represent the preset phase error range. , ,in, , All are set phase values ​​greater than 0, determined based on the system's phase error;

[0017] The fourth phase frequency characteristic is the demodulated phase to the left of the resonance frequency. The demodulated phase to the right of the resonant frequency is ( -180°, 180°- ),in, and Both represent the preset phase error range. , ,in, , All are set phase values ​​greater than 0, determined based on the system's phase error.

[0018] Optionally, the microwave frequency adjustment amount Δ f The calculation methods include: Method 1 or Method 2;

[0019] The first method includes: combining the conversion coefficient between the demodulation amplitude and the off-resonance frequency. m and full amplitude demodulated signal R Calculate the adjustment amount Δ of the center frequency of the microwave source. f = R / m Wherein, the conversion coefficient is the slope of the linear fitting of the demodulated spectral line at the resonance frequency point;

[0020] The second method includes: utilizing demodulated signals RThe error signal is used for closed-loop frequency tracking, and the adjustment amount Δ is calculated based on PID control. f .

[0021] Optionally, the method for selecting the conversion coefficient includes:

[0022] A sliding frequency window is set to traverse the demodulated spectrum, and linear fitting is performed within each window to obtain the corresponding fitting slope.

[0023] The maximum value of the fitted slope in all windows is selected as the conversion coefficient between the demodulated amplitude and the off-resonance frequency.

[0024] Optionally, the center frequency of the microwave source is determined based on the phase frequency characteristics and the real-time demodulation phase difference Δφ during the current frequency tracking control process. f The adjustment direction of 0 is determined according to the microwave frequency adjustment amount Δ. f Adjusting the direction of the microwave source to adjust its center frequency for resonant frequency tracking includes:

[0025] When the phase frequency characteristic is the first phase frequency characteristic and the real-time demodulation phase difference Δφ is positive, the center frequency of the microwave source is... f n The direction adjustment is negative, that is... f n = f n-1 -Δ f When the phase frequency characteristic is the first phase frequency characteristic and the real-time demodulation phase difference Δφ is negative, the center frequency of the microwave source is... f n The direction is adjusted to positive, that is... f n = f n-1 +Δ f Where n is the current calculation round, and its value range is a positive integer greater than or equal to 1;

[0026] When the phase frequency characteristic is the second phase frequency characteristic and the real-time demodulation phase difference Δφ is positive, the center frequency of the microwave source is... f n The direction is adjusted to positive, that is... f n = f n-1 +Δ f When the phase frequency characteristic is the second phase frequency characteristic, and the real-time demodulation phase difference Δφ is negative, the center frequency of the microwave source is... f n The direction adjustment is negative, that is... f n = f n-1 -Δ f ;

[0027] When the phase frequency characteristic is the third phase frequency characteristic, the real-time demodulation phase difference Δφ belongs to ( -180°, 180°- When the microwave source center frequency is... f n The direction is adjusted to positive, that is... f n = f n-1 +Δ f When the phase frequency characteristic is the third phase frequency characteristic, the real-time demodulation phase difference Δφ belongs to At that time, the center frequency of the microwave source f n The direction adjustment is negative, that is... f n = f n-1 -Δ f ;

[0028] When the phase frequency characteristic is the fourth phase frequency characteristic, the real-time demodulation phase difference Δφ belongs to At that time, the center frequency of the microwave source f n The direction is adjusted to positive, that is... f n = f n-1 +Δ f When the phase frequency characteristic is the fourth phase frequency characteristic, the real-time demodulation phase difference Δφ belongs to ( -180°, 180°- When the microwave source center frequency is... f n The direction adjustment is negative, that is... f n = f n-1 -Δ f .

[0029] Optionally, the calculation and output of the sensing values ​​for the frequency tracking control process includes:

[0030] Calculate the coarse frequency shift = f n - f r ,in, f n This is the current center frequency of the microwave source. f r This is the initial resonant frequency;

[0031] Calculate minute frequency shifts = R / m ;in,R This represents the current demodulated signal amplitude. m The current conversion factor;

[0032] Based on rough frequency offset and subtle frequency shifts Calculate the total frequency shift and according to Calculate magnetic field strength , as the sensing value; among which, The angle coefficient is introduced by the inconsistency between the magnetic field and the NV axis.

[0033] Optionally, based on the coarse frequency offset δ0 and the fine frequency offset Calculate the total frequency shift ,include:

[0034] When the phase frequency characteristic is the first phase frequency characteristic, and the real-time demodulation phase difference Δφ is positive, the total frequency offset is the difference between the coarse frequency offset and the fine frequency offset, that is... = - When the phase frequency characteristic is the first phase frequency characteristic, and the real-time demodulation phase difference Δφ is negative, the total frequency offset is the sum of the coarse frequency offset and the fine frequency offset, that is... = + ;

[0035] When the phase frequency characteristic is the second phase frequency characteristic, and the real-time demodulation phase difference Δφ is positive, the total frequency offset is the sum of the coarse frequency offset and the fine frequency offset, that is... = + When the phase frequency characteristic is the second phase frequency characteristic, and the real-time demodulation phase difference Δφ is negative, the total frequency offset is the difference between the coarse frequency offset and the fine frequency offset, that is... = - ;

[0036] When the phase frequency characteristic is the third phase frequency characteristic, the real-time demodulation phase difference Δφ belongs to At that time, the total frequency shift is the difference between the coarse frequency shift and the fine frequency shift, that is... = - When the phase frequency characteristic is the third phase frequency characteristic, the real-time demodulation phase difference Δφ belongs to ( -180°, 180°- When the total frequency shift is the sum of the coarse frequency shift and the fine frequency shift, that is... = + ;

[0037] When the phase frequency characteristic is the fourth phase frequency characteristic, the real-time demodulation phase difference Δφ belongs to At that time, the total frequency shift is the sum of the coarse frequency shift and the fine frequency shift, that is... = + When the phase frequency characteristic is the fourth phase frequency characteristic, the real-time demodulation phase difference Δφ belongs to ( -180°, 180°- When the total frequency shift is the difference between the coarse frequency shift and the fine frequency shift, that is... = - .

[0038] Optionally, in the calculation of microwave frequency adjustment Δ f Previously, it also included

[0039] Determine whether the demodulation amplitude is greater than the linear threshold;

[0040] If so, then use Method 1 to calculate the microwave frequency adjustment Δ. f ;

[0041] If not, then use method two to calculate the microwave frequency adjustment Δ. f .

[0042] Optionally, the microwave frequency adjustment Δ can be calculated using method one. f Then, execute step S3 and return to step S2 to continue the next round of frequency tracking control;

[0043] The microwave frequency adjustment Δ is calculated using the PID control calculation method. f Then, proceed with steps S3 and S4.

[0044] Secondly, the present invention also provides a resonant frequency tracking and control system for diamond nitrogen vacancy color centers, comprising: an all-fiber optical circuit subsystem and an electronic subsystem;

[0045] The all-fiber optical path subsystem includes an NV color center sensor head and an optical path structure. The optical path structure is used to transmit laser light to the NV color center sensor head, and the NV color center sensor head is used to perform laser pumping and fluorescence collection on the NV color centers.

[0046] The electronic subsystem includes a microwave source, a lock-in amplifier, and a frequency tracking algorithm module; the microwave source is used to provide a microwave signal to modulate the frequency of the fluorescence signal; the lock-in amplifier is used to demodulate the fluorescence signal; the frequency tracking control module is used to execute the diamond nitrogen-vacancy color center resonance frequency tracking control method described in any one of the first aspects, receive the demodulation signal from the lock-in amplifier, and perform frequency tracking control.

[0047] Compared with the prior art, the above-described technical solutions conceived in this invention can achieve the following beneficial effects:

[0048] 1. This invention provides a method for tracking and controlling the resonant frequency of nitrogen-vacancy color centers in diamond, through orthogonal-phase ( I / Q Demodulation acquires a full-amplitude demodulated signal that is immune to phase difference changes. R This is used as the error signal for resonant frequency tracking, and the full-amplitude demodulated signal is used. R Calculate the microwave frequency adjustment Δ f This invention introduces... I / Q The phase difference information Δφ obtained from demodulation calculation, combined with the phase frequency characteristics of the demodulated spectrum on both sides of the resonance frequency obtained from pre-sweep, determines the center frequency of the microwave source. f The adjustment direction of 0 solves the problem that the control direction cannot be determined by the full-amplitude demodulated signal. On the one hand, this frequency tracking control method is not affected by the phase difference fluctuation between the fluorescence modulation signal and the frequency modulation (FM) reference signal, thus enhancing the robustness and accuracy of the control. On the other hand, based on the phase frequency characteristics of the demodulated spectrum on both sides of the resonance frequency, the acceptable phase difference fluctuation margin of the system is improved by introducing phase adjustment control.

[0049] 2. This invention provides a resonant frequency tracking control method for nitrogen-vacancy color centers in diamond. To avoid unnecessary adjustment disturbances after frequency stabilization, this invention sets a dead-zone threshold, stopping the tracking operation when the system frequency approaches the resonant frequency, effectively reducing control disturbances. A linear threshold is set for the demodulated spectrum, fully utilizing the demodulated spectrum to achieve frequency tracking control with large adjustment ranges. When the demodulated amplitude is within the linear threshold range, a linear control method (such as PID control) is used to achieve real-time tracking of the resonant frequency; when the demodulated amplitude is outside the linear threshold range, a preset nonlinear conversion coefficient is used. m Calculate the adjustment amount Δ of the center frequency of the microwave source f This control strategy fully utilizes the characteristics of the detuning spectral line, allowing linear control methods (such as PID control) to operate only in the linear region, which is beneficial for improving control accuracy and achieving high-precision measurement. Furthermore, since the NV color center possesses multi-parameter sensing capabilities for external factors such as magnetic fields, temperature, and electric fields, the frequency tracking control method of this application can be effective in these scenarios. Therefore, the method of this application is of great significance for quantum sensing systems for magnetic fields and other physical parameters, and is expected to significantly improve their stability and reliability. Attached Figure Description

[0050] Figure 1 The diamond NV color center crystal structure involved in the embodiments of the present invention.

[0051] Figure 2 This refers to the diamond NV color center crystal energy level structure involved in the embodiments of the present invention;

[0052] Figure 3 The swept-frequency ODMR spectrum provided in the embodiments of the present invention;

[0053] Figure 4 This is a simplified model of a quantum sensing system provided in an embodiment of the present invention;

[0054] Figure 5 This is a flowchart illustrating a method for tracking and controlling the resonant frequency of nitrogen-vacancy color centers in diamond, provided in an embodiment of the present invention.

[0055] Figure 6 A flowchart illustrating another method for tracking and controlling the resonant frequency of nitrogen-vacancy color centers in diamond, provided in an embodiment of the present invention.

[0056] Figure 7 This is a schematic diagram of frequency modulation and demodulation provided in an embodiment of the present invention;

[0057] Figure 8 A comparison of different demodulated signals provided in embodiments of the present invention;

[0058] Figure 9 This is a schematic diagram of the frequency tracking control amplitude-phase joint discrimination method provided in an embodiment of the present invention. Detailed Implementation

[0059] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention. Furthermore, the technical features involved in the various embodiments of this invention described below can be combined with each other as long as they do not conflict with each other.

[0060] The following description, in conjunction with a preferred embodiment, illustrates the content involved in the above embodiments.

[0061] See Figure 1 and Figure 2 The embodiments of the present invention are based on Figure 1 and Figure 2 The crystal structure and energy level structure of diamond NV centers are shown. This embodiment uses a quantum system composed of diamond NV centers as an example to perform quantum sensing using its energy level structure and quantum effects. An NV center is a point defect in the diamond lattice structure, consisting of a nitrogen atom and an adjacent vacancy, as shown in the diagram. Figure 1As shown. After capturing additional electrons from its surroundings, it typically exists in a negatively charged state (NV-). The NV- color center possesses two unpaired electrons, forming a state with a total spin quantum number of... S A spin system with a spin of 1. Therefore, the ground state at the NV⁻ center ( 3 A 2) and excited state ( 3 E All of them exhibit a spin triplet state, and the spin state can be determined by the spin quantum number ( m s = 0, -1, +1) means, for example Figure 2 As shown. Due to the magnetic dipole-magnetic dipole interaction, m s = 0 and degeneracy m s = ±1 spin state from approximately D Separated by zero-field splitting (ZFS) at 2.87 GHz. Optically pumped to excited state ( 3 E The NV color center decays into the ground state. 3 A 2) It will emit fluorescence. During this process, some of the spin states are... m s The NV color center with a value of ±1 will transition to an intermediate metastable singlet state between the excited state and the ground state via an inter-system crossing (ISC) pathway, and then fall back to the ground state from the metastable singlet state. This process is a non-radiative transition and causes the spin state to change. m s =0. Therefore, continuous laser pumping is beneficial for initializing the NV color center to 0. m s =0 spin state, producing higher fluorescence intensity. Furthermore, microwave excitation at the resonant frequency will cause spin transitions from... m s = 0 spin state transition m s =±1 spin states, thus reducing fluorescence intensity (compared to initialization). These characteristics can be detected by measuring the ODMR spectrum, and then the magnetic field can be measured by monitoring the resonance frequency.

[0062] See Figure 3 The ODMR spectrum after the initial frequency sweep in this embodiment of the invention is shown in the figure. Because the magnetic field induces the spin state... m s = ±1 occurs during separation, therefore from m s = 0 state to m s = +1 and from ms = 0 state to self m s The resonance frequencies of the -1 states are different, so the external magnetic field can be calculated by using the resonance frequency difference in the split absorption peaks in the ODMR spectrum.

[0063] See Figure 4 The embodiments of the present invention can be applied to, but are not limited to, […]. Figure 4 The diagram illustrates a resonant frequency tracking and control system for diamond nitrogen-vacancy color centers, using the quantum sensing system for diamond NV color centers in this embodiment as an example. The quantum sensing system includes an all-fiber optical path subsystem and an electronic subsystem.

[0064] The all-optical subsystem includes: a laser source, a single-mode fiber, a coupler, an attenuator, a multimode circulator, and an NV color center sensor head. The NV color center sensor head is used for laser pumping and fluorescence collection of the NV color centers.

[0065] The electronic subsystem includes a microwave source, a lock-in amplifier, and a frequency tracking algorithm module. The microwave source provides microwave signals for excitation and also features frequency modulation capabilities, enabling frequency modulation of the fluorescence signal. The lock-in amplifier demodulates the signal, and the output demodulated signal serves as an error signal for the closed-loop tracking system, sent to the frequency tracking algorithm module as input parameters for calculating the microwave source center frequency adjustment. The frequency tracking control module executes the diamond nitrogen-vacancy color center resonant frequency tracking control method, receiving the demodulated signal from the lock-in amplifier for frequency tracking control. The calculated resonant frequency offset is used to adjust the center frequency of the microwave source. The lock-in amplifier, frequency tracking algorithm, and microwave source control unit are integrated within a field-programmable gate array (FPGA).

[0066] In addition, a bias magnetic field is used for initial separation of the resonance peaks to avoid crosstalk between peaks.

[0067] See Figure 5 This invention provides a method for tracking and controlling the resonant frequency of nitrogen-vacancy color centers in diamond. This method achieves highly robust real-time tracking of the resonant frequency of NV color centers through demodulation amplitude-phase joint discrimination. The resonant frequency tracking and control method includes the following steps:

[0068] S1. Obtain the change in fluorescence signal generated by the frequency modulation of the microwave source on the nitrogen vacancy color center in diamond during the frequency tracking control process, and take the resonance frequency corresponding to the resonance absorption peak of the maximum slope point in the demodulated spectrum of the fluorescence signal as the initial center frequency of the microwave source output during the frequency tracking control process.

[0069] S2. Demodulate the fluorescence signal to obtain the demodulated amplitude, and determine whether it is less than the dead zone threshold; if yes, do not change the center frequency of the microwave source output; if no, calculate the microwave frequency adjustment amount Δ. f ;

[0070] S3. Determine the center frequency of the microwave source based on the phase frequency characteristics and the real-time demodulation phase difference Δφ during the current frequency tracking control process. f The adjustment direction of 0; according to the microwave frequency adjustment amount Δ f The center frequency of the microwave source is adjusted by adjusting the direction of the adjustment for resonant frequency tracking; wherein, the phase frequency characteristic is the characteristic of the demodulated phase on both sides of the resonant frequency.

[0071] S4. Calculate and output the sensor value for the frequency tracking control process; keep the microwave source at the current center frequency, and return to S2 to continue the next round of frequency tracking control.

[0072] The following steps are included before step S1:

[0073] The system powers on and begins operation; initialization is performed, including configuring the laser source, microwave source, and other system configurations; frequency sweeping is executed while frequency modulation is enabled.

[0074] In step S1, based on the sweep frequency curve results, the host computer sets the resonance frequency corresponding to the resonance absorption peak near the maximum slope in the sweep frequency demodulation spectrum as the initial microwave source output center frequency, that is, the initial microwave center frequency is set to be equal to the resonance frequency.

[0075] The program enables the resonant frequency tracking control mode, and the measurement process begins.

[0076] During frequency tracking control, the microwave source acts on the fluorescence signal generated by the nitrogen-vacancy color center in diamond. The frequency tracking control module detects the demodulated amplitude obtained by demodulating the fluorescence signal from the lock-in amplifier output. If the demodulated amplitude is less than the dead zone threshold, the center frequency output by the microwave source is not changed, i.e., the frequency deviation Δ f The value is set to 0, and the demodulated amplitude of the lock-in amplifier output is continuously monitored; if the demodulated amplitude is greater than or equal to the dead zone threshold, the microwave frequency adjustment Δ is calculated. f .

[0077] The direction of the phase difference jump near the resonant frequency is determined based on the swept demodulation spectrum and phase frequency response curve monitored by the lock-in amplifier. The phase frequency characteristics of the system on both sides of the resonant frequency of the ODMR spectrum can be summarized into the following four cases:

[0078] The first phase frequency characteristic is that the demodulated phase to the left of the resonant frequency (frequency less than the resonant frequency) is less than the demodulated phase to the right of the resonant frequency (frequency greater than the resonant frequency), and the phase difference shows a positive phase jump at the resonant frequency.

[0079] The second phase frequency characteristic is that the demodulated phase to the left of the resonant frequency is greater than the demodulated phase to the right of the resonant frequency, and the phase difference shows a negative phase jump at the resonant frequency.

[0080] The third phase frequency characteristic, the demodulated phase to the left of the resonant frequency is ( -180°, 180°- The demodulation phase to the right of the resonant frequency is The phase difference at the resonant frequency is from ( -180°, 180°- Jump to ;in, and Both represent the preset phase error range. , ,in, , All are set phase values ​​greater than 0, determined based on the system's phase error. This applies to situations where the reference signal and modulation signal are in phase.

[0081] The fourth phase frequency characteristic, the demodulated phase to the left of the resonant frequency is The demodulated phase to the right of the resonant frequency is ( -180°, 180°- The phase difference at the resonant frequency causes a phase jump from Jump to ( -180°, 180°- ),in, and Both represent the preset phase error range. , ,in, , All are set phase values ​​greater than 0, determined based on the system's phase error. This applies to cases where the reference signal and modulation signal are out of phase.

[0082] Among them, due to the periodicity of the phase calculation function, the third and fourth phase frequency characteristics are special cases that exist in the first and second phase frequency characteristics.

[0083] The microwave frequency adjustment amount Δ f The calculation methods include: Method 1 or Method 2;

[0084] The first method includes: combining the full-amplitude demodulated signal. R The conversion coefficient between demodulated amplitude and off-resonance frequency m Calculate the adjustment amount Δ of the center frequency of the microwave source. f = R / m ;

[0085] The second method includes: utilizing demodulated signals R The error signal is used for closed-loop frequency tracking, and the adjustment amount Δ is calculated based on PID control. f .

[0086] Simultaneously, the conversion coefficient between the demodulated amplitude and the off-resonance frequency is calculated; this coefficient represents the slope of the linear fit of the demodulated spectral lines near the resonance frequency. Specifically, this includes:

[0087] A sliding frequency window is set to traverse the demodulated spectrum, and linear fitting is performed within each window to obtain the corresponding fitting slope.

[0088] The maximum value of the fitted slope in all windows is selected as the conversion coefficient between the demodulated amplitude and the off-resonance frequency.

[0089] The specific steps for adjusting the center frequency of the microwave source in step S3 include:

[0090] When the phase frequency characteristic is the first phase frequency characteristic and the real-time demodulation phase difference Δφ is positive, the center frequency of the microwave source is... f n The direction adjustment is negative, that is... f n = f n-1 -Δ f When the phase frequency characteristic is the first phase frequency characteristic and the real-time demodulation phase difference Δφ is negative, the center frequency of the microwave source is... f n The direction is adjusted to positive, that is... f n = f n-1 +Δ f Where n is the current calculation round, and its value range is a positive integer greater than or equal to 1;

[0091] When the phase frequency characteristic is the second phase frequency characteristic and the real-time demodulation phase difference Δφ is positive, the center frequency of the microwave source is... f n The direction is adjusted to positive, that is... f n = f n-1 +Δ f When the phase frequency characteristic is the second phase frequency characteristic, and the real-time demodulation phase difference Δφ is negative, the center frequency of the microwave source is... f n The direction adjustment is negative, that is... f n = f n-1 -Δ f ;

[0092] When the phase frequency characteristic is the third phase frequency characteristic, the real-time demodulation phase difference Δφ belongs to At that time, the center frequency of the microwave source f n The direction adjustment is negative, that is... f n = f n-1 -Δ f When the phase frequency characteristic is the third phase frequency characteristic, the real-time demodulation phase difference Δφ belongs to ( -180°, 180°- When the microwave source center frequency is... f n The direction is adjusted to positive, that is... f n = f n-1 +Δ f ;

[0093] When the phase frequency characteristic is the fourth phase frequency characteristic, the real-time demodulation phase difference Δφ belongs to At that time, the center frequency of the microwave source f n The direction is adjusted to positive, that is... f n = f n-1 +Δ f When the phase frequency characteristic is the fourth phase frequency characteristic, the real-time demodulation phase difference Δφ belongs to ( -180°, 180°- When the microwave source center frequency is... f n The direction adjustment is negative, that is... f n = f n-1 -Δ f .

[0094] Based on the microwave frequency adjustment amount Δ f After adjusting the center frequency of the microwave source for resonant frequency tracking, and adjusting the direction, the method further includes:

[0095] The process of calculating the sensor value includes:

[0096] Calculate the coarse frequency shift = f n - f r ,in, f n This is the current center frequency of the microwave source. f r This is the initial resonant frequency;

[0097] Calculate minute frequency shifts = R / m ;in, R This represents the current demodulated signal amplitude. m The current conversion factor;

[0098] Based on rough frequency offset and subtle frequency shifts Calculate the total frequency shift and according to Calculate magnetic field strength , as the sensing value; among which, The angle coefficient is introduced by the inconsistency between the magnetic field and the NV axis.

[0099] Specifically, when the phase frequency characteristic is the first phase frequency characteristic and the real-time demodulation phase difference Δφ is positive, the total frequency offset is the difference between the coarse frequency offset δ0 and the fine frequency offset δ1, i.e., δ = δ0 - δ1; when the phase frequency characteristic is the first phase frequency characteristic and the real-time demodulation phase difference Δφ is negative, the total frequency offset is the coarse frequency offset. and subtle frequency shifts The sum of = + ;

[0100] When the phase frequency characteristic is the second phase frequency characteristic and the real-time demodulation phase difference Δφ is positive, the total frequency offset is a coarse frequency offset. and subtle frequency shifts The sum of = + When the phase frequency characteristic is the second phase frequency characteristic, and the real-time demodulation phase difference Δφ is negative, the total frequency offset is a coarse frequency offset. and subtle frequency shifts The difference, that is = - ;

[0101] When the phase frequency characteristic is the third phase frequency characteristic, the real-time demodulation phase difference Δφ belongs to At that time, the total frequency shift is the difference between the coarse frequency shift and the fine frequency shift, that is... = - When the phase frequency characteristic is the third phase frequency characteristic, the real-time demodulation phase difference Δφ belongs to ( -180°, 180°- When the total frequency shift is the sum of the coarse frequency shift and the fine frequency shift, that is... = + ;

[0102] When the phase frequency characteristic is the fourth phase frequency characteristic, the real-time demodulation phase difference Δφ belongs to At that time, the total frequency shift is a coarse frequency shift. and subtle frequency shifts The sum of = + When the phase frequency characteristic is the fourth phase frequency characteristic, the real-time demodulation phase difference Δφ belongs to ( -180°, 180°- When the total frequency shift is a coarse frequency shift, the total frequency shift is... and subtle frequency shifts The difference, that is = - .

[0103] It also includes: determining whether the tracking control remains enabled; if no frequency tracking control termination signal appears, then continue executing step S2; otherwise, end the tracking adjustment process.

[0104] Based on the above embodiments, another alternative embodiment is described below. Figure 6 The resonant frequency tracking control method includes the following steps:

[0105] Step 1: Power on the system and start running; initialize and complete system configuration such as laser source and microwave source configuration; while enabling frequency modulation, perform frequency sweep operation.

[0106] Step 2: Based on the frequency sweep curve results, the host computer sets the resonant frequency corresponding to the resonant absorption peak near the maximum slope as the initial microwave source output center frequency, that is, initially setting the microwave center frequency equal to the resonant frequency, and simultaneously obtaining the phase frequency characteristic curve and conversion coefficient. m .

[0107] Step 3: Enable the resonant frequency tracking control mode in the program, and the measurement process begins.

[0108] Step 4: During the frequency tracking control process, the microwave source acts on the fluorescence signal generated by the nitrogen-vacancy color center in the diamond. The frequency tracking control module detects the demodulated amplitude obtained by demodulating the fluorescence signal from the lock-in amplifier output. If the demodulated amplitude is less than the dead zone threshold, the center frequency output by the microwave source is not changed, i.e., the frequency deviation Δ f If the value is 0, proceed to step 5 to continue detecting the demodulated amplitude of the lock-in amplifier output; if the demodulated amplitude is greater than or equal to the dead zone threshold, proceed to step 7 to calculate the microwave frequency adjustment Δ. f .

[0109] Step 5, the output of the solved sensor value remains unchanged.

[0110] Step 6: The frequency tracking control module detects the real-time demodulated amplitude of the lock-in amplifier output and determines whether the demodulated amplitude is greater than the linear threshold. If the demodulated amplitude is greater than the linear threshold, proceed to step 7; otherwise, proceed to step 9.

[0111] Before performing step 7, the direction of phase difference jump near the resonant frequency is determined based on the swept demodulation spectrum and phase frequency characteristic curve monitored by the lock-in amplifier. The phase frequency characteristics of the system on both sides of the resonant frequency include four cases, as described in the previous embodiment, and will not be repeated here.

[0112] Simultaneously, the conversion coefficient between the demodulated amplitude and the frequency deviation is calculated. This coefficient represents the slope of the linear fit of the demodulated spectral lines near the resonance frequency. Specifically, this can be achieved by traversing the demodulated spectral lines through a sliding frequency window and performing linear fitting within each window to obtain the fitting slope. Furthermore, the maximum value of the fitting slope across all windows is taken as the final conversion coefficient.

[0113] Step 7: Calculate the microwave frequency adjustment Δ using Method 1. f = R / m The specific content is the same as that in the above embodiments.

[0114] Step 8: Control the microwave center frequency adjustment direction based on the real-time phase polarity, according to the microwave frequency adjustment amount Δ. f The center frequency of the microwave source is adjusted to track the resonant frequency by adjusting the direction; the specific steps are the same as those in the above embodiment. After completing step 8, return to step 4 to continue the next round of frequency tracking control.

[0115] Step 9, using the demodulated signal R The error signal is used for closed-loop frequency tracking, and the adjustment amount Δ is calculated based on PID control. f .

[0116] Step 10: Control the microwave center frequency adjustment direction based on the real-time phase polarity, and adjust the microwave frequency by the amount Δ. f Change the center frequency control word of the microwave source, and drive the microwave source to perform resonant frequency tracking according to the center frequency control word. The specific content is the same as step 8.

[0117] Step 11: Calculate and output the sensor value. The specific steps are the same as those in the above embodiment.

[0118] Step 12: Determine if tracking control remains enabled. If no frequency tracking control termination signal appears, continue to step 4; otherwise, end the tracking adjustment process.

[0119] See Figure 7This invention demonstrates the entire process of fluorescence intensity oscillation and demodulation caused by microwave frequency modulation. Specifically,

[0120]

[0121]

[0122]

[0123]

[0124] in It is the amplitude of the reference signal. It is the phase of the reference signal. It is the amplitude of the modulated signal. It is the phase of the reference signal, and arctan2 is an extension of the function arctan, whose range is... Instead .

[0125] See Figure 8 The full-amplitude demodulated signal used in the embodiments of the present invention R It is based on demodulated signal I and Q Calculated. Compared to the demodulated signal. I or Q Full-amplitude demodulated signal R It exhibits better signal strength. More importantly, it is unaffected by phase difference fluctuations. When performing resonant frequency tracking, the full-amplitude demodulated signal... R It is immune to phase difference changes. These characteristics enhance the robustness of the control system and are beneficial for improving the system's measurement performance.

[0126] See Figure 9In this embodiment of the invention, when resonant frequency tracking control is performed and the phase difference between the modulated fluorescence signal and the reference signal is 90°, the demodulation amplitude-frequency characteristic curves and phase-frequency characteristic curves near the resonant frequency are shown. The solid lines represent the demodulation amplitude-frequency characteristic curves and phase-frequency characteristic curves of the lock-in amplifier output under the current magnetic field, and the dashed lines represent the demodulation amplitude-frequency characteristic curves and phase-frequency characteristic curves of the lock-in amplifier output after the measured magnetic field changes. It can be observed that when the center frequency of the microwave source is equal to the NV color center magnetic resonance frequency, the demodulation output of the lock-in amplifier is zero, and the phase-frequency characteristic curve shows a positive jump at this point as the frequency increases. When the magnetic field changes, the resonant frequency changes, and the demodulation amplitude-frequency characteristic curve and phase-frequency characteristic curve of the lock-in amplifier show an overall shift. If the center frequency of the microwave source output signal is not changed, the demodulation output is not zero at this time. In addition, the phase difference will also become negative. At this time, the adjustment amount of the microwave center frequency can be calculated according to the demodulation amplitude-phase joint discrimination method of resonant frequency tracking, and the adjustment direction can be indicated, thereby realizing real-time tracking of the resonant frequency.

[0127] The embodiments of the present invention introduce I / Q The phase difference information Δφ obtained from demodulation calculation, combined with the phase frequency characteristics of the demodulated spectrum on both sides of the resonance frequency obtained from pre-sweep, determines the center frequency of the microwave source. f The adjustment direction of 0 solves the problem that the control direction cannot be determined by the full-amplitude demodulated signal. On the one hand, this frequency tracking control method is not affected by the phase difference fluctuation between the fluorescence modulation signal and the frequency modulation (FM) reference signal, thus enhancing the robustness and accuracy of the control. On the other hand, based on the pre-obtained phase frequency characteristics of the demodulated spectrum on both sides of the resonant frequency, the acceptable phase difference fluctuation margin of the system is improved by introducing phase adjustment control. This solves the problem that traditional frequency tracking control methods are affected by the phase difference fluctuation between the modulation signal and the reference signal, resulting in poor tracking performance or even tracking misalignment, and improves the robustness of the resonant frequency tracking method.

[0128] Those skilled in the art will readily understand that the above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for tracking and controlling the resonant frequency of nitrogen-vacancy color centers in diamond, characterized in that, include S1. Obtain the change in fluorescence signal generated by the frequency modulation of the microwave source on the nitrogen vacancy color center in diamond during the frequency tracking control process, and take the resonance frequency corresponding to the resonance absorption peak of the maximum slope point in the demodulated spectrum of the fluorescence signal as the initial center frequency of the microwave source output during the frequency tracking control process. S2. Demodulate the fluorescence signal to obtain the demodulated amplitude, and determine whether it is less than the dead zone threshold; if yes, do not change the center frequency of the microwave source output, and execute S4; if no, calculate the microwave frequency adjustment amount Δ. f And execute S3-S4; S3. Determine the center frequency of the microwave source based on the phase frequency characteristics and the real-time demodulation phase difference Δφ during the current frequency tracking control process. f The adjustment direction of 0; according to the microwave frequency adjustment amount Δ f The center frequency of the microwave source is adjusted by adjusting the direction of the adjustment for resonance frequency tracking; wherein, the phase frequency characteristic is the characteristic of the demodulated spectral line on both sides of the resonance frequency; S4. Calculate and output the sensor values ​​for the frequency tracking control process; keep the microwave source at the current center frequency, and return to S2 to continue the next round of frequency tracking control; The phase frequency characteristics include: a first phase frequency characteristic, a second phase frequency characteristic, a third phase frequency characteristic, and a fourth phase frequency characteristic; The first phase frequency characteristic is that the demodulated phase to the left of the resonance frequency is smaller than the demodulated phase to the right of the resonance frequency; The second phase frequency characteristic is that the demodulated phase to the left of the resonant frequency is greater than the demodulated phase to the right of the resonant frequency; The third phase frequency characteristic is the demodulation phase to the left of the resonance frequency, which is ( -180°, 180°- The demodulation phase to the right of the resonant frequency is ;in, and Both represent the preset phase error range. , ,in, , All are set phase values ​​greater than 0, determined based on the system's phase error; The fourth phase frequency characteristic is the demodulated phase to the left of the resonance frequency. The demodulated phase to the right of the resonant frequency is ( -180°, 180°- ),in, and Both represent the preset phase error range. , ,in, , All are set phase values ​​greater than 0, determined based on the system's phase error.

2. The method as described in claim 1, characterized in that, The microwave frequency adjustment amount Δ f The calculation methods include: Method 1 or Method 2; The first method includes: combining the conversion coefficient between the demodulation amplitude and the off-resonance frequency. m and full amplitude demodulated signal R Calculate the adjustment amount Δ of the center frequency of the microwave source. f = R / m Wherein, the conversion coefficient is the slope of the linear fitting of the demodulated spectral line at the resonance frequency point; The second method includes: using a full-amplitude demodulated signal. R The error signal is used for closed-loop frequency tracking, and the adjustment amount Δ is calculated based on PID control. f .

3. The method as described in claim 2, characterized in that, The method for selecting the conversion coefficients includes: A sliding frequency window is set to traverse the demodulated spectrum, and linear fitting is performed within each window to obtain the corresponding fitting slope. The maximum value of the fitted slope in all windows is selected as the conversion coefficient between the demodulated amplitude and the off-resonance frequency.

4. The method as described in claim 3, characterized in that, The center frequency of the microwave source is determined based on the phase frequency characteristics and the real-time demodulation phase difference Δφ during the current frequency tracking control process. f The adjustment direction of 0 is determined according to the microwave frequency adjustment amount Δ. f Adjusting the direction of the microwave source to adjust its center frequency for resonant frequency tracking includes: When the phase frequency characteristic is the first phase frequency characteristic and the real-time demodulation phase difference Δφ is positive, the center frequency of the microwave source is... f n The direction adjustment is negative, that is... f n = f n-1 -Δ f When the phase frequency characteristic is the first phase frequency characteristic and the real-time demodulation phase difference Δφ is negative, the center frequency of the microwave source is... f n The direction is adjusted to positive, that is... f n = f n-1 +Δ f Where n is the current calculation round, and its value is a positive integer greater than or equal to 1; When the phase frequency characteristic is the second phase frequency characteristic and the real-time demodulation phase difference Δφ is positive, the center frequency of the microwave source is... f n The direction is adjusted to positive, that is... f n = f n-1 +Δ f When the phase frequency characteristic is the second phase frequency characteristic, and the real-time demodulation phase difference Δφ is negative, the center frequency of the microwave source is... f n The direction adjustment is negative, that is... f n = f n-1 -Δ f ; When the phase frequency characteristic is the third phase frequency characteristic, the real-time demodulation phase difference Δφ belongs to ( -180°, 180°- When the microwave source center frequency is... f n The direction is adjusted to positive, that is... f n = f n-1 +Δ f When the phase frequency characteristic is the third phase frequency characteristic, the real-time demodulation phase difference Δφ belongs to At that time, the center frequency of the microwave source f n The direction adjustment is negative, that is... f n = f n-1 -Δ f ; When the phase frequency characteristic is the fourth phase frequency characteristic, the real-time demodulation phase difference Δφ belongs to At that time, the center frequency of the microwave source f n The direction is adjusted to positive, that is... f n = f n-1 +Δ f When the phase frequency characteristic is the fourth phase frequency characteristic, the real-time demodulation phase difference Δφ belongs to ( -180°, 180°- When the microwave source center frequency is... f n The direction adjustment is negative, that is... f n = f n-1 -Δ f .

5. The method as described in claim 1, characterized in that, The calculation and output of the sensor values ​​for the frequency tracking control process includes: Calculate the coarse frequency shift = f n - f r ,in, f n This is the current center frequency of the microwave source. f r This is the initial resonant frequency; Calculate minute frequency shifts = R / m ;in, R This represents the current demodulated signal amplitude. m The current conversion factor; Based on rough frequency offset and subtle frequency shifts Calculate the total frequency shift and according to Calculate magnetic field strength , as the sensing value; among which, The angle coefficient is introduced by the inconsistency between the magnetic field and the NV axis.

6. The method as described in claim 5, characterized in that, Based on the coarse frequency offset δ0 and the fine frequency offset Calculate the total frequency shift ,include: When the phase frequency characteristic is the first phase frequency characteristic, and the real-time demodulation phase difference Δφ is positive, the total frequency offset is the difference between the coarse frequency offset and the fine frequency offset, that is... = - When the phase frequency characteristic is the first phase frequency characteristic, and the real-time demodulation phase difference Δφ is negative, the total frequency offset is the sum of the coarse frequency offset and the fine frequency offset, that is... = + ; When the phase frequency characteristic is the second phase frequency characteristic, and the real-time demodulation phase difference Δφ is positive, the total frequency offset is the sum of the coarse frequency offset and the fine frequency offset, that is... = + When the phase frequency characteristic is the second phase frequency characteristic, and the real-time demodulation phase difference Δφ is negative, the total frequency offset is the difference between the coarse frequency offset and the fine frequency offset, that is... = - ; When the phase frequency characteristic is the third phase frequency characteristic, the real-time demodulation phase difference Δφ belongs to At that time, the total frequency shift is the difference between the coarse frequency shift and the fine frequency shift, that is... = - When the phase frequency characteristic is the third phase frequency characteristic, the real-time demodulation phase difference Δφ belongs to ( -180°, 180°- When the total frequency shift is the sum of the coarse frequency shift and the fine frequency shift, that is... = + ; When the phase frequency characteristic is the fourth phase frequency characteristic, the real-time demodulation phase difference Δφ belongs to At that time, the total frequency shift is the sum of the coarse frequency shift and the fine frequency shift, that is... = + When the phase frequency characteristic is the fourth phase frequency characteristic, the real-time demodulation phase difference Δφ belongs to ( -180°, 180°- When the total frequency shift is the difference between the coarse frequency shift and the fine frequency shift, that is... = - .

7. The method as described in claim 3, characterized in that, The calculation of microwave frequency adjustment Δ f Previously, it also included: Determine whether the demodulation amplitude is greater than the linear threshold; If so, then use Method 1 to calculate the microwave frequency adjustment Δ. f ; If not, then use method two to calculate the microwave frequency adjustment Δ. f .

8. The method as described in claim 7, characterized in that, Calculate the microwave frequency adjustment Δ using Method 1 f Then, execute step S3 and return to step S2 to continue the next round of frequency tracking control; The microwave frequency adjustment Δ is calculated using the PID control calculation method. f Then, proceed with steps S3 and S4.

9. A resonant frequency tracking and control system for nitrogen-vacancy color centers in diamond, characterized in that, include: All-fiber optical circuit subsystem and electronic subsystem; The all-fiber optical path subsystem includes an NV color center sensor head and an optical path structure. The optical path structure is used to transmit laser light to the NV color center sensor head, and the NV color center sensor head is used to perform laser pumping and fluorescence collection on the NV color centers and sense the measured quantity. The electronic subsystem includes a microwave source, a lock-in amplifier, and a frequency tracking algorithm module; the microwave source is used to provide a microwave signal to frequency modulate the fluorescence signal; the lock-in amplifier is used to demodulate the fluorescence signal; the frequency tracking algorithm module is used to execute the diamond nitrogen-vacancy color center resonance frequency tracking control method as described in any one of claims 1-8, receive the demodulated signal from the lock-in amplifier, and perform frequency tracking control.

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