Automatic resonance control for NV magnetometer

Abstract

The invention relates to a method (100) for determining an external magnetic field (B) using an NV magnetometer (1) with automatic resonance control, and to the use of an NV magnetometer (1) of said type and one or more of the NV magnetometers (1) of said type for gradient measurements.

Description

Technical Field

[0001] The invention relates to a method for determining an external magnetic field using an NV magnetometer with automatic resonance control, as well as to an NV magnetometer of said type and the use of one or more NV magnetometers of said type for gradient measurements. Background Art

[0002] The nitrogen vacancy center is one of more than 100 known defects in the diamond lattice. These defects are impurities in the pure carbon lattice of diamond. Nitrogen vacancy centers are also known as NV centers. Quantum sensors such as NV magnetometers exploit the extreme sensitivity of quantum systems relative to their environment and enable the creation of significantly improved sensors. By capturing additional electrons, the NV center forms a microscopic compass via the electron's spin. By precisely manipulating and measuring the state of this NV spin, even the smallest changes in the surrounding magnetic field can be measured. This technique is called NV magnetometry and is based on the interaction between the NV center and the surrounding magnetic field. The presence of a magnetic field manifests itself as a change in the energy of the spin state of the NV center. The influence of the magnetic field can be easily identified from changes in the microwave excitation spectrum of the NV center.

[0003] The decisive advantage of NV magnetometry stems from the known behavior of the NV center spin transition frequency, which provides clear information about the intensity and direction of the magnetic field. This ability to provide self-calibration, quantitative data together with very high reproducibility (each NV center is the same) is a key feature of the NV magnetometer. The main advantage of the NV magnetometer is its high sensitivity to small signals. The sensitivity of the NV magnetometer to the static magnetic field depends in particular on the density of the NV center in the diamond chip, the quantum coherence of the NV center, and the efficiency of the optical readout. Changes in magnetic field, temperature and / or pressure shift the magnetic resonance of the NV center ground state, which is used for the magnetic field measurement of the NV magnetometer. This results in a limited dynamic range and a poor signal-to-noise ratio (SNR) of the NV magnetometer.

[0004] H. Clevenson et al., Appl. Phys. Lett 112, 252406 (2018), disclose a robust, scale-factor-free vector magnetometer that uses a closed-loop frequency locking scheme to simultaneously track Zeeman splitting resonance pairs of nitrogen vacancy centers (NV centers) in diamond. However, it is also desirable to further increase the dynamic range and bandwidth of NV magnetometers with respect to detectable magnetic field changes, as well as to improve the SNR of the measurement, and to enable extension to gradient magnetic field measurements while suppressing temperature and pressure fluctuations. Summary of the invention

[0005] The present invention is based on the object of providing an NV magnetometer which achieves an increased dynamic range and a higher bandwidth with respect to detectable magnetic field changes, as well as a better SNR in the measurement.

[0006] The object is achieved by a method for determining an external magnetic field using an NV magnetometer with automatic resonance control, wherein the NV magnetometer comprises a sensor material having NV centers with corresponding spin states m s = 0, ±1 and for transitions between spin states The resonant frequency f ± , where the resonant frequency f ± The difference f + -f - Depending on the external magnetic field B, the method comprises the following steps:

[0007] -Record single resonance signal V SR-LIA , with the aid of microwave frequency f MW and the carrier frequency f SB The resonant frequency f is determined by transmitting frequency modulated microwave radiation of a frequency band onto the sensor material. ± , where microwave frequency f MW and the carrier frequency f SB is selected such that the frequency bands are directed successively to two magnetic resonances;

[0008] -Record double resonance signal V DR-LIA , the double resonance signal has a disperser form and a resonance frequency f ± The linear range near the microwave frequency f MW and the carrier frequency f SB is chosen so that f MW ±f SB =f ± Applicable, thereby exciting two magnetic resonances simultaneously;

[0009] -According to the demodulated signal V DR-LIA The slope of the linear range was determined as the scalar factor α;

[0010] - In the case of resonant incidence of microwave radiation due to a change in the external magnetic field, the demodulated signal V is detected by means of an evaluation unit DR-LIA Observe the resonant frequency f ± Detuning Δ B =V DR-LIA / α;

[0011] - by including the observed detuning Δ B The controller of the control algorithm is used to control the frequency band f MW ±f SB , so that even if the detuning Δ B After this occurs, the frequency band f MW ±f SBAlso maintained at the NV center, at this detuning Δ B The resonant frequency f exists in the case ± to the extent that

[0012] - by means of the evaluation unit the distance 2*f between the two frequency bands SB and detuning Δ B to determine the external magnetic field B.

[0013] The term "sensor material" refers to a material having NV centres. This is diamond, for example.

[0014] For the spin state m s = 0, ±1 and for transitions between spin states The resonant frequency f ± For more detailed description, please refer to Figure 1 .

[0015] The term "magnetic resonance" refers to the excitation of the higher energy spin states in the NV center, and the resonance frequency f ± During the transition to the ground state, the corresponding radiation is re-emitted. For example, magnetic resonance can be measured in absorption or emission spectroscopy.

[0018] Here, the carrier frequency is a fixed frequency whose amplitude, phase position or frequency changes in the rhythm of the modulation frequency, depending on the modulation method. For example, in frequency modulation, the frequency changes. The carrier frequency is added again when the signal is demodulated.

[0019] The so-called “scalar factor α” corresponds to the demodulated signal V in the linear range around the resonance frequency DR-LIA The slope of the demodulated signal V is , where the demodulated signal has a zero crossing point. DR-LIA The zero crossing of can be shifted in its frequency because the external magnetic field shifts the actual resonant frequency compared to the resonant frequency without the influence of the external magnetic field. This shift is called the resonance detuning Δ B By repeatedly controlling the operating point of the NV magnetometer by means of a control algorithm at the point where the slope of the demodulated signal is steepest (maximum value of α), an improved signal-to-noise ratio of the measurement signal is obtained.

[0020] In this respect, the control algorithm can be installed as a software program on a controller and executed, wherein the controller is connected to the evaluation unit and to the microwave transmitter for emitting microwave radiation in order to excite the magnetic resonance via data or control lines so that the microwave frequency f MW can be determined according to the detuning Δ observed using the evaluation unit BIn this respect, the evaluation unit can be designed as a processor connected to a storage medium so that a corresponding evaluation program can be executed and the evaluated data can be stored.

[0021] In the NV magnetometer according to the invention, an intelligent control algorithm including the hardware configuration required for this purpose ensures that the frequency of the microwave excitation required for the operation of the sensitive sensor remains within the magnetic resonance range.

[0022] By means of the method according to the invention, NV magnetometers can operate with an increased dynamic range with respect to detectable magnetic field changes, higher bandwidth and better SNR in the measurements.The above advantages also enable gradient measurements using one or more such NV magnetometers.

[0023] In one embodiment of the method or the NV magnetometer, controlling comprises the following steps:

[0024] - The demodulated signal V VIA Compare with the rated value r=0;

[0025] - Calculate the deviation e = rV DR-LIA = - V DR-LIA ;as well as

[0026] - Shift the frequency band of the microwave radiation in the opposite direction by Δf with the aid of a controller MW .

[0027] By continuously matching the operating point of the NV magnetometer, the dynamic range of the NV magnetometer is significantly increased. After a change in magnetic field or temperature, which could result in an out-of-resonance range, a time-consuming search for resonance in a wide spectrum is no longer necessary. The bandwidth of the NV magnetometer is increased accordingly.

[0028] In a preferred embodiment of the method or the NV magnetometer, the controller is an integrated software-based controller which, according to the rule Δf MW [n]=K*(e[0]+e[1]+…+e[n]), where K is the proportional factor of the controller and i=0,1,…,n is the iteration step. Here, the proportional factor K is preferably equal to 1 / α.

[0029] In one embodiment of the method or the NV magnetometer, the controller has a control period (Regelperiode) Tc, after which the microwave frequency f is controlled by the controller again. MWControl is performed, wherein the control period Tc is between 1 μs and 10 s, preferably between 0.05 s and 1 s, and more preferably 0.5 s. A short control period Tc enables tracking of rapidly changing magnetic field signals, but also limits the bandwidth of the NV magnetometer because setting the frequency band generates dead time. In addition, a long control period Tc deteriorates the signal-to-noise ratio SNR because the optimal operating point (resonance point / zero crossing point) cannot be fully maintained. Therefore, a control period Tc=0.5 s is particularly preferred.

[0030] In a preferred embodiment of the method or the NV magnetometer, the signal generator generates a microwave having a frequency f MW and another signal generator generates a signal with a carrier frequency f SB , modulation frequency f m and the modulation amplitude f d And a frequency modulated signal with corresponding power, where f SB (t) = f SB +f d *cos(2πf m t), where t is time.

[0031] In a preferred embodiment of the method or of the NV magnetometer, the two signals are frequency mixed by means of a mixer and the microwave radiation emitted to the sensor material comprises two frequency bands f MW+ f SB (t) and f MW -f SB (t), where f SB (t) in f MW -f SB (t) is shifted by π. Preferably, in this regard, the microwave frequency f MW and the carrier frequency f SB is selected so that the two frequency bands are respectively ± resonance so as to excite two magnetic resonances simultaneously.

[0032] In a preferred embodiment of the method or the NV magnetometer, the two frequency bands are emitted at individually set powers. By being able to individually set the powers of the two frequency bands, the difference in the scalar factor of the single resonance spectrum can be minimized, and thus the suppression of temperature and pressure variations in the double resonance signal can be maximized. Here, depending on the embodiment of the hardware configuration of the NV magnetometer, the power can be individually set by means of a programmable attenuator or by matching the power of the signal generator.

[0033] The invention also relates to an NV magnetometer for determining an external magnetic field B, the NV magnetometer being arranged to perform the method for automatic resonance control according to any one of the preceding claims, the NV magnetometer comprising:

[0034] - a sensor material having an NV center with a corresponding spin state m s = 0, ±1 and for transitions between spin states The resonant frequency f ± , where the resonant frequency f ± The difference f + -f - Depends on the external magnetic field B;

[0035] - a microwave transmitter for transmitting a signal based on a microwave frequency f MW and the carrier frequency f SB The frequency band of the microwave radiation emitted to the sensor material is used to excite the resonance frequency f ± The magnetic resonance at ; where the microwave frequency f MW and the carrier frequency f SB is selected to record the single resonance signal V SR-LIA , so that the frequency bands are sequentially directed to the two magnetic resonances, and where the microwave frequency f MW and the carrier frequency f SB is selected to record the double resonance signal V DR-LIA , so that f MW ±f SB =f ± Applicable, thereby exciting two magnetic resonances simultaneously;

[0036] - a laser for emitting continuous wave operation with a wavelength in the range of 510 nm to 540 nm for continuous initialization and readout of the spin states;

[0037] - a suitable detector, preferably an optical detector, for recording the frequency modulated fluorescence from the sensor material;

[0038] - Lock-in amplifier for demodulation of fluorescence, wherein a signal V is generated when one or two magnetic resonances are excited SR-LIA or signal V DR-LIA , the signal V DR-LIA It has a dispersed form and a linear range near the resonant frequency;

[0039] - an evaluation unit for transmitting the demodulated signal V DR-LIA Observe the resonant frequency f ± Detuning Δ B =V DR-LIA / α;

[0040] - a controller for using a detuning Δ B The control algorithm is used to control the frequency band f MW ±f SB , so that even if the detuning Δ BAfter this occurs, the frequency band f MW ±f SB Also maintained at the NV center, at this detuning Δ B The resonant frequency f exists in the case ± within the scope of

[0041] The evaluation unit is set up for the distance 2*f between the two frequency bands. SB and detuning Δ B to determine the external magnetic field B.

[0042] In the NV magnetometer according to the invention, an intelligent control algorithm including the hardware configuration required for this purpose ensures that the frequency of the microwave excitation required for the sensitive sensor operation remains within the range of the magnetic resonance.

[0043] The NV magnetometer according to the present invention achieves an increased dynamic range with respect to detectable magnetic field changes, a higher bandwidth and a better SNR in the measurement. The above advantages also enable gradient measurements using one or more such NV magnetometers.

[0044] In this respect, the sensor material is preferably diamond due to its special properties.

[0045] In one embodiment, the microwave transmitter comprises: a signal generator for generating a microwave having a frequency f MW and another signal generator for generating a signal having a carrier frequency f SB , modulation frequency f m and the modulation amplitude f d And a frequency modulated signal with corresponding power, where f SB (t) = f SB +f d *cos(2πf m t), where t is time.

[0047] In further embodiments, the signal generator is provided for adapting the power of the signal, or the signal generator has an upstream attenuator for individual power adaptation.

[0048] The invention also relates to the use of one or more NV magnetometers according to the invention for gradient measurement. Gradient measurement is the measurement of the components of a gradient field, such as the gradient of a magnetic field. In this context, the vector components are registered simultaneously with two sensors (in this case two NV magnetometers) positioned at a fixed distance apart. Gradient measurement is used in geomagnetism. Gradient measurement requires precise and fast measurements, which can be made possible by the NV magnetometer according to the invention and the corresponding method according to the invention.

[0049] It should be clearly pointed out that in order to improve readability, the expression of "at least" is avoided as much as possible. On the contrary, indefinite articles ("one", "two", etc.) should generally be understood as "at least one, at least two, etc.", unless it is clear from the context that "exactly" means a specified number.

[0050] At this point, it should also be mentioned that in the context of this patent application, the term "particularly" should always be understood to introduce optional, preferred features. Therefore, this expression should not be understood as "specifically" nor as "i.e."

[0051] It will be appreciated that, if applicable, the features of the solutions described above or in the claims may also be combined without departing from the claims in order to achieve individually or cumulatively the advantages and effects that can be achieved in the present case. BRIEF DESCRIPTION OF THE DRAWINGS

[0052] In addition, other features, effects and advantages of the present invention will be explained with reference to the accompanying drawings and the following description. In the various drawings, components that are at least substantially corresponding in their functions are represented by the same reference numerals, wherein these components do not need to be numbered and described in all drawings.

[0053] In the attached picture:

[0054] Figure 1 A schematic representation of the dependence of the spin states and energy levels of NV centers in the sensor material on magnetic field, temperature and pressure as well as resonant and microwave excitation of the spin states is shown;

[0055] Figure 2 Schematic representations of: (a) a signal flow diagram; and (b) frequency portions (bands) of microwave radiation emitted onto a sensor material in a first embodiment of a magnetic field measurement scheme are shown;

[0056] Figure 3 Schematic representations of: (a) a signal flow diagram; and (b) frequency portions (frequency bands) of microwave radiation emitted onto a sensor material in a second alternative embodiment of a magnetic field measurement scheme are shown;

[0057] Figure 4 Schematic representations of: (a) a single resonance signal as a function of resonance detuning due to a magnetic field change (top: before change; bottom: after change); (b) a single resonance signal as a function of resonance detuning due to a pressure and temperature change (top: before change; bottom: after change); and (c) a demodulated signal resulting from the addition of two single signals;

[0058] Figure 5Schematic representations of: (a) a signal flow diagram of a control algorithm of a controller; and (b) resonant detuning of a demodulated signal due to magnetic field variation (left) and backshifting of the control algorithm are shown;

[0059] Figure 6 An embodiment of an NV magnetometer according to the present invention is shown; and

[0060] Figure 7 An embodiment of the method according to the invention is shown. DETAILED DESCRIPTION

[0061] Figure 1 A schematic representation of the dependence of the spin states and energy levels of the NV centers in the sensor material 2 of the NV magnetometer 1 on the magnetic field B, the temperature T and the pressure p, and the microwave frequency f is shown. MW The case of resonance and microwave excitation of the spin state, where f MW+ Indicates the resonance frequency f + The excitation microwave frequency and f MW- Indicates the resonance frequency f - The microwave frequency of the excitation. The ground state spin is represented by m s = 0, and the two excited spin states are represented by m s =1 and m s = -1 means that this depends on the orientation of the spin relative to the external magnetic field. Magnetic resonance The resonant frequency f ± Depends on the magnetic field B, temperature T and pressure p:

[0062] f ± =D(T,p)±γB

[0063] If the two resonant frequencies f are known by subtracting them from each other ± , and the difference is proportional only to the external magnetic field B, then the temperature and pressure fluctuations can be separated from the magnetic field changes:

[0064] f + -f - =2γB

[0065] Here, γ is the gyromagnetic ratio of the NV center in the sensor material 2 .

[0066] Figure 2 Schematic representations of (a) optically detected magnetic resonance under frequency modulated microwave radiation emitted on the NV centre; and (b) demodulated V DR-LIA Signal. The magnetic field measurement solution is based on the following technologies:

[0067] - Lock detection: for the emitted microwave (microwave frequency f MWand the carrier frequency f SB ) is (frequency) modulated and the optically detected magnetic resonance is subsequently demodulated by means of a lock-in amplifier 3. The demodulated signal V of the magnetic resonance DR-LIA has a dispersive form and is linear near the resonant frequency, see Figure 2 a.

[0068] - Scalar factor α: Here, the demodulated signal V must first be determined DR-LIA The slope of the linear range of ΔT is calculated (the "scalar factor"). The microwaves are then emitted in a resonant manner. If the resonant frequency changes due to changes in the magnetic field, temperature or pressure, the detuning relative to the resonant frequency can be evaluated by the signal from the lock-in amplifier 3. When evaluating the difference signal of the two magnetic resonances, the influence of temperature T and pressure p and therefore any (resonance) detuning due to pressure or temperature changes ΔT are eliminated, so that only the influence of the external magnetic field has the following contribution as a (magnetic) detuning: Δ B =V DR-LIA / α.

[0069] Figure 2 Schematic representations of: (a) a signal flow diagram of an NV magnetometer 1 according to the present invention; and (b) frequency portions (bands) of microwave radiation emitted onto a sensor material 2 in a first embodiment of a magnetic field measurement scheme are shown. Figure 2 In a, the signal generator 61 generates a signal with a frequency f MW The signal generator 62 generates a signal having a carrier frequency f SB , modulation frequency f m and the modulation amplitude f d FM signal:

[0070] f SB (t) = f SB +f d cos(2πf m t)

[0071] The signal is then frequency mixed in a power combiner 92 so that the signal emitted by means of the antenna 10 onto the sensor material 2 (eg diamond) contains the following frequency components:

[0072] f MW +f SB +f d cos(2πf m t) and f MW -f SB +f d cos(2πf m t+π)

[0073] As can be seen, the frequency mixing results in a phase difference of π or 180° between the two frequency components. System parameter f MW and f SB are chosen so that the frequency components are each Resonance (see Figure 2 b) The generated magnetic resonance (fluorescence FL) is measured by an optical photodetector 8 and converted into a double resonance signal V by means of a corresponding lock-in amplifier 3 with a mixer 31 and a low-pass filter 32. DR-LIA .

[0074] Figure 3 Schematic representations of: (a) a signal flow diagram of an NV magnetometer 1 according to the present invention; and (b) Figure 2 The configuration shown in FIG. 1 compares the frequency portions (frequency bands) of the microwave radiation emitted to the sensor material 2 , here diamond, in a second alternative embodiment of a magnetic field measurement solution with an alternative hardware configuration of an NV magnetometer 1 . Figure 3 The method shown in a is based on Figure 2 aSame functional principle, but here the power of the frequency bands for magnetic resonance can be set individually. This is achieved by:

[0075] ●Programmable attenuator 7 or

[0076] • By matching the power of the signal generators 61, 62 that generate microwave frequencies.

[0077] The corresponding frequency band is emitted via the corresponding antenna 10 onto the sensor material 2, and the resulting magnetic resonance (fluorescence FL) is measured via the corresponding optical photodetector 8 and converted into a double resonance signal V by means of a corresponding lock-in amplifier 3 with a mixer 31 and a low-pass filter 32. DR-LIA To generate two separate frequency bands, according to Figure 2 The circuit of a comprises a respective power divider 91 in front of a programmable attenuator and a respective power combiner 92 for producing microwave radiation of the respective frequency band emitted by the respective antenna 10. By being able to set the power of the two frequency bands individually, it is possible to make the single resonance spectrum V SR-LIA The difference in the scalar factor α is minimized, and thus the double resonance signal V DR-LIA The suppression of temperature and pressure changes in the process is maximized. Figure 3 The modulation phase in b is indicated as 0 or π above the corresponding frequency portion.

[0078] Figure 4 The following schematic representations are shown: (a) a single resonance signal V SR-LIA According to the resonance detuning Δ caused by the magnetic field change B With microwave frequency f MWChanges (top: before change; bottom: after change); (b) single resonance signal V SR-LIA According to the resonance detuning Δ caused by pressure and temperature changes T With microwave frequency f MW changes (top: before change; bottom: after change); and (c) by two single signals V SR-LIA The demodulated signal V DR-LIA (Double resonance signal). In order to understand the effect of magnetic field, temperature and pressure changes on the demodulated signal V DR-LIA The influence of the magnetic resonance signal V SR-LIA ) is useful. From this we can draw the following conclusions:

[0079] A relative modulation phase of π or 180° between the two frequency components results in a single resonant signal V SR-LIA A vertical mirror image of .

[0080] ●Single resonance signal V SR-LIA As the magnetic field changes, the temperature and pressure changes add constructively resulting in a single resonance signal V SR-LIA The signs of are different, and therefore these single resonance signals add destructively.

[0081] ●Double resonance signal V DR-LIA The detuning Δ relative to the resonant frequency caused by the change in the magnetic field is B It can therefore be expressed as: B =V DR-LIA / α.

[0082] This leads to the following measurement scheme:

[0083] 1. Recording of single resonance spectra of two magnetic resonances V SR-LIA → Determine f + and f - (Resonance frequency during calibration)

[0084] 2. Recording double resonance spectrum V DR-LIA →Determine the scalar factor α

[0085] 3. System parameter f MW and f SB is fixed so that the frequency portion is located at f + and f - → The magnetic field is determined continuously using the following equation:

[0086] 2γB=f + -f - +2V DR-LIA / α

[0087] Figure 5Schematic representation of: (a) signal flow diagram of the control algorithm of the controller; and (b) resonance detuning Δ of the demodulated signal caused by magnetic field change (left) and backshift of the control algorithm B The control algorithm is based on a feedback loop, which Figure 5 This is based on step 3 of the measurement scheme above:

[0088] ●The change of magnetic field causes the resonance detuning Δ B

[0089] ●Resonance detuning Δ B Leading to the change of the output signal S of the control system, see Figure 5 b, left side.

[0090] The output is then compared with the nominal value r=0 and the deviation e=rV is calculated DR-LIA =-V DR-LIA In this regard, the controller 5 performs a discrete time interval, namely every T C Seconds to operate.

[0091] Then, the controller 5 shifts the frequency band in the opposite direction by Δf MW (By matching the system parameters f MW and f SB ), see Figure 5 b, right. An integrated software-based controller 5 is used here, which controls according to the following rules:

[0092] Δf MW [n] = K (e[0] + e[1] + ... + e[n])

[0093] Here, K is the proportional factor of the controller, and i=0,1,…,n is the iteration step size. In this regard, the proportional factor K should be selected as follows to eliminate the deviation (e=0): K=1 / α. The following preferably applies to the control period T C : A short control period enables tracking of rapidly changing magnetic field signals, but it also limits the bandwidth of the NV magnetometer 1 because setting the frequency band creates dead time. In addition, a long control period deteriorates the signal-to-noise ratio because the optimal operating point (resonance point / zero crossing point) cannot be fully maintained. For example, T C =0.5s control cycle. For the output signal of the control system 2γB=f + -f - +2V DR-LIA / α,f ± Indicates the position of the frequency band after the last operation of the controller 5 .

[0094] Figure 6An embodiment of an NV magnetometer 1 according to the invention is shown, comprising: a sensor material 2; a lock-in amplifier 3 having a mixer 31 and a low-pass filter 32; an evaluation unit 4; a controller 5 having a control algorithm executed on the controller; a microwave transmitter 6 having a microwave frequency f MW The signal generator 61 is used for a signal having a carrier frequency f SB , modulation frequency f m and the modulation amplitude f d A signal generator 62 for a frequency modulated signal; a programmable attenuator; and a signal generator for recording a single resonance signal V SR-LIA Furthermore, the NV magnetometer may include a power divider 91, a power combiner 62, and an antenna 10 (not shown here) for emitting microwave radiation onto the sensor material 2 (in this regard, see Figure 2 a and Figure 3 a). The NV magnetometer 1 further comprises a laser 11 for emitting continuous wave light with a wavelength in the range of 510 nm to 540 nm for continuous initialization and readout of the spin states.

[0095] Figure 7 An embodiment of a method 100 according to the invention for determining an external magnetic field using an NV magnetometer 1 with automatic resonance control is shown, wherein the NV magnetometer 1 comprises a sensor material 2 having NV centers with corresponding spin states m s = 0, ±1 and for transitions between spin states The resonant frequency f ± , where the resonant frequency f ± The difference f + -f - Depending on the external magnetic field B, the method comprises the following steps: recording 110 single resonance signals V SR-LIA , with the aid of microwave frequency f MW and the carrier frequency f SB The resonant frequency f is determined by transmitting the frequency modulated microwave radiation of the frequency band to the sensor material 2. ± , where microwave frequency f MW and the carrier frequency f SB is selected so that the frequency band targets two magnetic resonances in sequence; 120 double resonance signals V are recorded DR-LIA , the double resonance signal has a dispersed form and a resonance frequency f ± The linear range near the microwave frequency f MW and the carrier frequency f SB is chosen so that f MW ±f SB=f ± Applicable, thereby simultaneously exciting two magnetic resonances; according to the demodulated signal V DR-LIA The slope of the linear range is determined 130 as a scalar factor α; by means of the evaluation unit 4, the demodulated signal V is determined as a result of the resonant emission of microwave radiation due to the external magnetic field change. DR-LIA Observe the 140 resonance frequency f ± Detuning Δ B =V DR-LIA / α; use includes depending on the observed detuning Δ B The control algorithm of controller 5 is used to control the 150 frequency band f MW ±f SB , so that even if the detuning Δ B After this occurs, the frequency band f MW ±f SB Also maintained at the NV center, at this detuning Δ B The resonant frequency f exists in the case ± and by means of the evaluation unit 4 according to the distance between the two frequency bands 2 * f SB and detuning Δ B To determine 160 the external magnetic field B, wherein, in this respect, the control 150 comprises the following steps: DR-VIA Compare 152 with the setpoint value r=0; calculate 154 the deviation e=rV DR-LIA = - V DR-LIA ; and using the controller 5 to shift the frequency band of the microwave radiation in the opposite direction by 156Δf MW In addition, the further steps and measures of claims 3 to 10 can be implemented in the method 100 according to the invention ( Figure 7 not shown).

[0096] At this point it should be explicitly noted that, if applicable, features of the solutions described above or in the claims and / or drawings may also be combined in order to implement or achieve the described features, effects and advantages in a corresponding cumulative manner.

[0097] It should be understood that the above exemplary embodiment is only an initial embodiment of the present invention. In this regard, the embodiments of the present invention are not limited to the exemplary embodiment.

[0098] List of reference symbols used

[0099] 1NV magnetometer

[0100] 2Sensor material (diamond)

[0101] 3 Lock-in amplifier

[0102] 31 Mixer

[0103] 32 Low pass filter

[0104] 4 Evaluation Units

[0105] 5. Controller

[0106] 6 Microwave transmitter

[0107] 61 for microwave frequency f MW Signal generator for the signal

[0108] 62 for a carrier frequency f SB , modulation frequency f m and the modulation amplitude f d Signal generator for FM signals

[0109] 7 Programmable Attenuators

[0110] 8 is used to record the single resonance signal V SR-LIA Detector (e.g., photodetector)

[0111] 91 Power Divider

[0112] 92 Power Synthesizer

[0113] 10 Antennas (Antenna 1, Antenna 2)

[0114] 11 Laser

[0115] 100 Method for determining an external magnetic field by means of an NV magnetometer with automatic resonance control

[0116] 110 Record single resonance signal V SR-LIA , in order to determine the resonance frequency f by means of frequency-modulated microwave radiation emitted onto the sensor material in a frequency band ±

[0117] 120 Record double resonance signal V DR-LIA , the double resonance signal has a dispersed form and a resonance frequency f ± The linear range near

[0118] 130 is the demodulated signal V DR-LIA Determine the slope of the linear range as a scalar factor α

[0119] 140 In the case of resonant emission of microwave radiation, the demodulated signal V DR-LIA Observe the resonant frequency f ± Detuning Δ B =V DR-LIA / α

[0120] 150 by including the observed detuning Δ BThe controller of the control algorithm is used to control the microwave frequency f MW

[0121] 152 The demodulated signal V LIA Compare with the rated value r = 0

[0122] 154 Calculate the deviation e = rV LIA = - V LIA

[0123] 156 Use a controller to shift the frequency band of microwave radiation in the opposite direction by Δf MW

[0124] 160Determine the external magnetic field B

[0125] α scalar factor

[0126] Gyromagnetic ratio of γNV center

[0127] B External magnetic field

[0128] D(T,P) energy levels depend on pressure and temperature

[0129] Δ B Detuning (resonance) due to magnetic field changes (magnetic detuning)

[0130] Δ T Detuning (resonance) due to pressure or temperature changes

[0131] e deviation e = rV LIA = - V LIA

[0132] f ± For transitions between spin states The resonant frequency

[0133] f MW Microwave frequency

[0134] f MW+ For the resonance frequency f + The microwave frequency of the excitation

[0135] f MW- For the resonance frequency f - The microwave frequency of the excitation

[0136] f SB Carrier frequency

[0137] Δf MW The frequency band of microwave radiation is shifted in the opposite direction by the controller

[0138] FL fluorescence

[0139] L Power

[0140] m s Spin state (0 = ground state; ±1: excited state)

[0141] pPressure

[0142] r Rated value of the controller

[0143] S Output signal of the control system

[0144] T Temperature

[0145] Tc control cycle

[0146] V SR-LIA Single resonance signal

[0147] V DR-LIA Demodulated signal

Claims

1. A method (100) for determining an external magnetic field by means of an NV magnetometer (1) with automatic resonance control, wherein: The NV magnetometer (1) comprises a sensor material (2) having NV centers, wherein the NV centers have corresponding spin states m s = 0, ± 1 and for transitions between the spin states The resonant frequency f ± , where the resonant frequency f ± The difference f + -f - Depending on the external magnetic field B, the method comprises the following steps: - Record (110) single resonance signal V SR-LIA , with the aid of microwave frequency f MW and the carrier frequency f SB The resonance frequency f is determined by transmitting frequency modulated microwave radiation in a frequency band onto the sensor material (2). ± , wherein the microwave frequency f is selected such that MW and the carrier frequency f SB , so that the frequency bands are sequentially directed to two magnetic resonances; - Record (120) double resonance signal V DR-LIA , the double resonance signal has a dispersed form and the resonance frequency f ± The linear range is around , wherein the microwave frequency f is selected MW and the carrier frequency f SB , so that f MW ±f SB =f ± Applicable, thereby exciting two magnetic resonances simultaneously; -The demodulated signal V DR-LIA determining (130) the slope of the linear range as a scalar factor α; - by means of the evaluation unit (4) in the case of a resonant emission of microwave radiation due to a change in the external magnetic field, by means of the demodulated signal V DR-LIA Observe (140) the resonance frequency f ± Detuning Δ B =V DR-LIA / α; - by including the observed detuning Δ B The controller (5) of the control algorithm is used to control (150) the frequency band f MW ±f SB , so that even at the detuning Δ B After the occurrence of the frequency band f MW ±f SB Also maintained at the NV center, at the detuning Δ B The resonant frequency f ± to the extent that - by means of the evaluation unit (4) the distance 2*f between the two frequency bands SB and the detuning Δ B To determine (160) the external magnetic field B.

2. The method (100) according to claim 1, wherein: The control (150) comprises the following steps: - The demodulated signal V DR-VIA Compare with the rated value r=0 (152); - Calculate (154) the deviation e = rV DR-LIA =-V DR-LIA ;as well as - shifting (156) the frequency band of the microwave radiation in the opposite direction by Δf by means of the controller (5) MW .

3. The method (100) according to claim 2, wherein: The controller (5) is an integrated software-based controller (5) which, according to the rule Δf MW [n]=K*(e[0]+e[1]+…+e[n]), where K is the proportional factor of the controller (5) and i=0,1,…,n is the iteration step.

4. The method (100) according to claim 3, wherein: The proportionality factor K=1 / α.

5. The method (100) according to any one of the preceding claims, wherein: The controller (5) has a control period Tc, after which the microwave frequency f is controlled again by the controller. MW The control is performed, wherein the control period Tc is between 1 μs and 10s, preferably between 0.05s and 1s, and more preferably 0.5s.

6. The method (100) according to any one of the preceding claims, wherein: The signal generator (61) generates a microwave having the frequency f MW and another signal generator (62) generates a signal having the carrier frequency f SB , modulation frequency f m and the modulation amplitude f d And a frequency modulated signal with corresponding power, where f SB (t) = f SB +f d *cos(2πf m t), where t is time.

7. The method (100) according to claim 6, wherein: The two signals are frequency mixed by means of a mixer (9) and the microwave radiation emitted onto the sensor material (2) comprises the two frequency bands f MW +f SB (t) and f MW -f SB (t), where f SB (t) in f MW -f SB (t) is phase shifted by π.

8. The method (100) according to claim 7, wherein: The microwave frequency f is selected in this way MW and the carrier frequency f SB , so that the two frequency bands are respectively related to the resonance frequency f ± Resonance.

9. The method (100) according to any one of claims 6 to 8, wherein: The two frequency bands are transmitted at individually set powers.

10. The method (100) according to claim 9, wherein: The power is set individually by means of a programmable attenuator (7) or by matching the power of the signal generators (61, 62).

11. An NV magnetometer (1) for determining an external magnetic field B, the NV magnetometer being arranged to perform the method (100) for automatic resonance control according to any one of the preceding claims, the NV magnetometer comprising: - a sensor material (2) having an NV center with a corresponding spin state m s = 0, ± 1 and for transitions between the spin states The resonant frequency f ± , where the resonant frequency f ± The difference f + -f - Depends on the external magnetic field B; - a microwave transmitter (6) for transmitting the microwave energy by means of a microwave frequency f MW and the carrier frequency f SB The resonant frequency f is excited by transmitting frequency modulated microwave radiation of a frequency band to the sensor material (2) ± The magnetic resonance at the microwave frequency f MW and the carrier frequency f SB is selected to record the single resonance signal V SR-LIA , so that the frequency bands are sequentially directed to two magnetic resonances, and wherein the microwave frequency f MW and the carrier frequency f SB is selected to record the double resonance signal V DR-LIA , so that f MW ±f SB =f ± Applicable, thereby exciting two magnetic resonances simultaneously; - a laser (11) for emitting a continuous wave operation with a wavelength in the range of 510 nm to 540 nm for continuous initialization and readout of the spin state; - a suitable detector (8), preferably an optical detector, for recording the frequency modulated fluorescence from the sensor material (2); - a lock-in amplifier (3) for demodulating the fluorescence, wherein a signal V is generated when one or two magnetic resonances are excited SR-LIA or signal V DR-LIA , the signal V DR-LIA has a dispersed form and the resonant frequency f ± The linear range near - an evaluation unit (4) for determining the demodulated signal V DR-LIA Observe the resonant frequency f ± Detuning Δ B =V DR-LIA / α; - a controller (5) for determining the detuning Δ B The control algorithm is used to control the frequency band f MW ±f SB , so that even at the detuning Δ B After the occurrence of the frequency band f MW ±f SB Also maintained at the NV center, at the detuning Δ B The resonant frequency f ± within the scope of The evaluation unit (4) is configured to evaluate the distance 2*f between the two frequency bands. SB and the detuning Δ B To determine the external magnetic field B.

12. The NV magnetometer (1) according to claim 11, It is characterized in that The microwave transmitter (6) comprises: a signal generator (61) for generating a microwave having the microwave frequency f MW and another signal generator (62) for generating a signal having the carrier frequency f SB , modulation frequency f m and the modulation amplitude f d And a frequency modulated signal with corresponding power, where f SB (t) = f SB +f d *cos(2πf m t), where t is time.

13. The NV magnetometer (1) according to claim 12, It is characterized in that The signal generator (61, 62) is provided for matching the power of the signal, or the signal generator (61, 62) has an upstream attenuator (7) for individual power matching.

14. The NV magnetometer (1) according to any one of claims 11 to 13, It is characterized in that The sensor material (2) is diamond.

15. Use of one or more NV magnetometers according to any one of claims 11 to 14 for gradient measurement.

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