Magnetic field gradiometer

Abstract

The invention relates to a magnetic field gradiometer (1) for determining a magnetic field gradient (Ba-Bb), comprising: at least one excitation light source (5a, 5b) for emitting excitation light (6a, 6b); two spatially spaced-apart measurement regions (3a, 3b) for magnetic field measurement, which have diamond centers, preferably NV centers (4a, 4b), which emit fluorescence light (7a, 7b) when excited with excitation light (6a, 6b); a first detector (8a) for detecting fluorescence (7a) from the first measurement region (3a); a second detector (8b) for detecting fluorescence (7b) from the second measurement region (3b); a first microwave emitter (10a) for applying a first microwave field (9a) to the first measurement region (3a); a second microwave emitter (10b) for applying a second microwave field (9b) to the second measurement region (3b); and an analysis device (11) which is designed to determine a magnetic field gradient (Ba-Bb) on the basis of the detected fluorescence (7a) from the first measurement region (3a) and the detected fluorescence (7b) from the second measurement region (3b). The magnetic field gradiometer (1) comprises a signal generator (12), which is designed to generate: a first microwave signal (fMW1 (t)) for the first microwave emitter (10a), which first microwave signal has at least two frequency components that are phase-shifted by [pi] relative to one another, and a second microwave signal (fMW2 (t)) for the second microwave emitter (10a), which second microwave signal has at least two frequency components that are phase-shifted by [pi] relative to one another; and a second microwave signal (fMW2 (t)) for the second microwave emitter (10b), the second microwave signal having at least two frequency components that are phase shifted by pi relative to each other.

Description

Background Art

[0002] Highly sensitive magnetometers can be used in particular to record magnetic signals of brain or neural activity in a locally resolved manner, so as to control prostheses, exoskeletons or machines using a novel human-machine interface in this way. Since the amplitude of the effective signal of human neural activity may be lower than the ambient magnetic noise, a solution to suppress the ambient noise for the above-mentioned application must be found. Such a solution uses a gradient sensor concept, in which the measurement area of ​​the first magnetometer is directly placed at the magnetic field source, and the measurement area of ​​the second magnetometer is arranged away from the magnetic field source in a manner of a limited distance from the first measurement area. Here, the magnetometer with the second measurement area that is spaced farther apart only records the ambient noise, while the measurement area of ​​the magnetometer that is arranged closer to the signal source also detects the effective signal in addition to the ambient noise. By subtracting the noise signal recorded by the first magnetometer from the signal of the second magnetometer, a signal that only still contains the effective signal and is referred to as the magnetic field gradient in the sense of the present application is obtained. Compared with the magnetic field measurement performed using a single magnetometer, the magnetic field gradiometer can achieve the elimination or suppression of the ambient noise that has the same effect on the two magnetometers.

[0003] Magnetic field gradiometers of the type mentioned at the outset and the corresponding magnetometers are based on the magnetic field-dependent fluorescence changes of color centers in diamond (typically NV centers) in the presence of an alternating magnetic field. More precisely, this involves optically detected magnetic resonance (ODMR). The principle of this detection is described in the article "Nanoscale imaging magnetometry with diamond spins under ambient conditions", Nature 455, 648 (2008) by G. Balasubramian et al. In the following, this principle is explained using the example of NV centers.

[0004] An NV center is a special color center in the diamond lattice consisting of a nitrogen atom and an adjacent vacancy. Within the scope of the present application, an NV center is to be understood more precisely as a negatively charged NV center. The magnetic field gradiometer is also designed for measurements at multiple NV centers (so-called ensemble magnetometrie).

[0005] The energy levels of the NV center include a ground state and an excited state in the form of a triplet state. The corresponding three states of each triplet state differ in their magnetic spin quantum number, namely, m S =-1,0,+1. These m S = ±1 state has a ratio m due to spin-spin interaction. S= 0 state has higher energy. In the absence of a magnetic field, m S = ±1 states are degenerate (if hyperfine structure is neglected) and therefore have the same energy. However, in the presence of a magnetic field, m S The degeneracy of the =±1 state is cancelled in proportion to the magnetic field due to the Zeeman effect.

[0006] By irradiating with excitation light in the green wavelength range, the NV center can be pumped from the ground state to the excited state. The return to the ground state is partly carried out by emitting red fluorescence. The intensity of this fluorescence depends on the m S = Population distribution between -1, 0, +1 states. This population distribution may be affected by the interaction with the resonant microwave field. More precisely, the frequency is f + and f - The microwave field (corresponding to m S =0 and m S = +1 state or m S = -1 state) will result in a decrease in the detected fluorescence intensity.

[0007] The frequency of the microwave field is now varied in order to determine the magnetic field with the aid of a corresponding magnetometer. Depending on the frequency of the microwave field, the fluorescence intensity measured at these frequencies f ± The magnetic field is determined by the location of this minimum.

[0008] Here, the frequency f + and f - By formula

[0009] f ± =D(T,p)±γ NV B0,

[0010] See, for example, the article by F. Stürner et al. “Integrated and portable magnetometer based on nitrogen-vacancy ensembles in diamond”, arXiv:2012.01053. Here, D(T,p) is the m of the triplet ground state. S =0 state and m S = ±1 states, which depends on the temperature T and the ambient pressure p, γ NV is the gyromagnetic ratio of the NV centre, and B0 is the projection of the magnetic field onto the axis of the corresponding NV centre.

[0011] In the case of a magnetic field gradiometer, such a measurement is carried out in two spatially spaced measurement regions, in which an NV centre or other diamond colour centre is arranged in each case. The magnetic field gradient is then determined based on a comparison of the minimum values ​​of the fluorescence from the first and second measurement regions and based on the spacing of these measurement regions (see above).

[0012] However, due to the construction of such a magnetic field gradiometer, a temperature difference and / or a pressure difference may easily occur between the two magnetometers or between the two measurement areas. ± Such temperature or pressure differences may distort the acquired magnetic field gradient.

[0013] In the article “Diamond magnetometer enhanced by ferrite flux concentrators”, Physical Review Research 2, 023394 (2020) by I. Fescenko et al., a magnetometer is described in which the influence of thermal displacements of the energy levels of NV centers is compensated using the double resonance technique. For this purpose, two frequency-modulated microwave signals, which have two frequency components that are phase-shifted relative to each other by π, are combined and irradiated by a microwave transmitter onto a diamond crystal having NV centers. The fluorescence of the NV centers is detected with a photodetector, fed to a phase-locked amplifier and demodulated there with the aid of a reference signal. In this way, an output signal of the phase-locked amplifier is generated, which is proportional to the two frequencies f + With f - The difference between + -f - ) and does not depend on thermal displacement.

[0014] In the article “Robust high-dynamic-range vector magnetometry with nitrogen-vacancy centers in diamond”, Appl. Phys. Lett. 112, 252406 (2018) by H. Clevenson et al., a magnetometer with a wide dynamic range is described. In order to achieve the wide dynamic range, it is proposed to control the carrier frequency of the microwave signal by means of feedback of a phase-locked signal.

[0015] In the article “Diamond Magnetometry and Gradiometry Towards Subpicotesla dc Field Measurement”, Phys. Rev. Applied 15, 064075 (2021) by Chen Zhang et al., a magnetometer and a gradiometer are described, which are particularly combined with a phase-locked detection method to perform continuous wave (cw) ODMR measurements, in which two microwave frequencies of the NV center are excited simultaneously. Summary of the invention

[0016] In contrast, the object of the present invention is to provide a magnetic field gradiometer of the type mentioned in the introduction which can effectively compensate for external disturbing influences, in particular temperature influences and / or pressure influences.

[0017] Subject matter of the invention

[0018] According to a first aspect, the object is achieved by a magnetic field gradiometer of the type described in the opening paragraph, which further comprises: a signal generator, which is configured to generate a first microwave signal for a first microwave transmitter, the first microwave signal having at least two frequency components that are phase-shifted by π relative to each other; and to generate a second microwave signal for a second microwave transmitter, the second microwave signal having at least two frequency components that are phase-shifted by π relative to each other.

[0019] In this aspect of the invention, a microwave signal is generated for each measurement region or each of the two magnetometers, the microwave signal having (at least) two frequency components that are phase-shifted relative to each other by π. In the demodulation process (see below) performed, for example, by means of a lock-in amplifier, the phase shift between the two frequency components or the relative modulation phase of π or 180 ° causes the demodulated single resonance signal to be reflected relative to the vertical axis. The magnetic field variation causes the frequency shift of the single resonance signal with the same sign, i.e., the single resonance signal is added constructively. In the case where temperature and pressure change, the frequency shift of the single resonance signal with different signs is caused to add destructively. Therefore, the demodulated double resonance signal analyzed for determining the magnetic field or magnetic field gradient depends only on the magnetic field variation, and the temperature and pressure influence is compensated.

[0020] In one embodiment, in order to form two frequency components of a first microwave signal that are phase-shifted by π relative to each other, the signal generator has a first mixer that is configured to mix a first frequency-modulated signal and a first oscillator signal; and in order to form two frequency components of a second frequency-modulated microwave signal that are phase-shifted by π relative to each other, the signal generator has a second mixer that is configured to mix a second frequency-modulated signal and a second oscillator signal.

[0021] The first oscillator signal or the second oscillator signal is a signal with a constant oscillation frequency that is mixed with a corresponding frequency modulation signal. The first frequency modulation signal and the second frequency modulation signal are generated by corresponding signal generators of the signal generator, and these frequency modulation signals have corresponding carrier frequencies and modulation components. Due to the frequency mixing, the first microwave signal contains frequency components that are phase-shifted by π relative to each other. Correspondingly, the second microwave signal also contains frequency components that are phase-shifted by π relative to each other due to the frequency mixing. The corresponding oscillation frequency and the carrier frequency of the frequency modulation signal are selected in such a way that the two frequency components that are phase-shifted by π relative to each other are respectively aligned with the transition Resonance.

[0022] In an extended solution, the first frequency modulated signal and the second frequency modulated signal have different carrier frequencies, and / or the first oscillator signal and the second oscillator signal have different oscillation frequencies. Since the magnetic field strengths in the first measurement region and the second measurement region are different, the transitions in the two measurement regions are different. The corresponding frequency f + and f - Correspondingly, it is generally not necessary to select the first carrier frequency and the second carrier frequency and the first oscillation frequency and the second oscillation frequency differently.

[0023] In an alternative embodiment, in order to form the first microwave signal, the signal generator has a first power adder for adding a first frequency-modulated microwave signal having two frequency components shifted in phase by π relative to each other and another first frequency-modulated microwave signal having two frequency components shifted in phase by π relative to each other; and in order to form the second frequency-modulated microwave signal, the signal generator has a second power adder for adding a second frequency-modulated microwave signal having two frequency components shifted in phase by π relative to each other and another second frequency-modulated microwave signal having two frequency components shifted in phase by π relative to each other.

[0024] This embodiment is based on the same working principle as the above-mentioned embodiment. By using the first power adder, the power of the first frequency modulated microwave signal and the other first frequency modulated microwave signal can be different. Correspondingly, by using the second power adder, the power of the second frequency modulated microwave signal and the other second frequency modulated microwave signal can be different. The corresponding frequency modulated microwave signal is located for one of the two magnetic field resonances in the first measurement area / second measurement area respectively, and the other frequency modulated microwave signal is located for the corresponding other magnetic field resonance in the first measurement area / second measurement area. Due to the possibility of pre-given different powers during the excitation of the single resonance, the difference of the scalar factor (see below) of each single resonance spectrum can be reduced when analyzing the corresponding demodulated fluorescence signal. In this way, the suppression of temperature change and pressure change in the demodulated double resonance signal can be enhanced.

[0025] In an advantageous extension, the signal generator is configured to adjust the power of the first frequency-modulated microwave signal and the further first frequency-modulated microwave signal independently of each other; and the signal generator is configured to adjust the power of the second frequency-modulated microwave signal and the further second frequency-modulated microwave signal independently of each other. Advantageously, the power of the respective frequency-modulated microwave signal and the respective further frequency-modulated microwave signal can not only be predetermined independently of each other, but can also be adjusted independently of each other. In this way, during the analysis of the respective demodulated fluorescence signal, the differences between the scalar factors of the individual single resonance spectra can be minimized, and the suppression of temperature changes and pressure changes in the demodulated double resonance signal can be maximized. Independent power regulation can be performed in different ways, as described in detail further below.

[0026] In one embodiment, to form the first frequency-modulated microwave signal, the signal generator has a first mixer for mixing the frequency-modulated signal with the first oscillator signal, and to form the further first frequency-modulated microwave signal, the signal generator has a further first mixer for mixing the frequency-modulated signal with the further first oscillator signal. Correspondingly, to form the second frequency-modulated microwave signal, the signal generator has a second mixer for mixing the frequency-modulated signal with the second oscillator signal, and to form the further second frequency-modulated microwave signal, the signal generator has a further second mixer for mixing the frequency-modulated signal with the further second oscillator signal.

[0027] In this development, a signal generator is usually provided which generates a frequency-modulated signal for all four mixers. In this case, the frequency-modulated signal passes through a power divider which divides the frequency-modulated signal into four power components which are in a predetermined ratio to one another. The respective power component of the frequency-modulated signal is fed to one of the four mixers. The magnitude of the four power components is generally the same, but this is not mandatory.

[0028] In a further development, the signal generator has at least one programmable attenuator for adjustably attenuating a corresponding power component of the frequency modulated signal fed to one of the mixers. In this case, the power component of the frequency modulated signal fed to a corresponding mixer of the four mixers can be individually adjusted by means of the programmable attenuator.

[0029] In a further development, the signal generator comprises: a first oscillator for generating a first oscillator signal, the first oscillator signal preferably having an adjustable power; and a further first oscillator for generating a further first oscillator signal, the further first oscillator signal preferably having an adjustable power. Correspondingly, the signal generator comprises: a second oscillator for generating a second oscillator signal, the second oscillator signal preferably having an adjustable power; and a further second oscillator for generating a further second oscillator signal, the further second oscillator signal preferably having an adjustable power. The power of the first frequency-modulated microwave signal and the further first frequency-modulated microwave signal can be adjusted independently of one another by adjusting the power of the first oscillator or the further first oscillator. The corresponding content applies to the power of the second frequency-modulated microwave signal and the further second frequency-modulated microwave signal.

[0030] In another embodiment, the analysis device has: a first demodulator for forming a first demodulated double resonance signal based on the detected fluorescence of the first measurement area; and a second demodulator for forming a second demodulated double resonance signal based on the detected fluorescence of the second measurement area. As described above, in the magnetic field gradiometer according to the present invention, two magnetic resonances of the NV center are excited and read out simultaneously. Based on the first demodulated double resonance signal / the second demodulated double resonance signal, the magnetic field strength in the first measurement area / the second measurement area can be determined.

[0031] In an extended scheme, the two demodulators are configured as lock-in amplifiers respectively. In this improved scheme, the demodulation of the double resonance signal is carried out by means of a lock-in amplifier. The corresponding lock-in amplifier usually has a mixer fed with the double resonance signal and a reference signal. The signal obtained based on the mixing passes through a low-pass filter, which is also a part of the lock-in amplifier. The demodulated double resonance signal at the output of the low-pass filter has a dispersed form and is linear around the resonance frequency.

[0032] In an extended embodiment, the analysis device is configured to: determine the resonance deviation of the first magnetic field in the first measurement region according to the first demodulated double resonance signal and the resonance deviation of the second magnetic field in the second measurement region according to the second demodulated double resonance signal. In order to determine the resonance deviation, the slope of the linear range of the demodulated double resonance signal ("scalar factor" α) is usually determined. To this end, the corresponding microwave field is first resonantly irradiated. If the resonance frequency changes due to changes in the magnetic field, temperature or pressure, it can be determined according to the demodulated double resonance signal v LIA The detuning or resonance deviation Δ from the resonant frequency is determined as follows: Δ=v LIA / α. Since the resonance deviations in the two single resonance signals due to changes in temperature or pressure add destructively (see above), the double resonance signal v LIA The resonance deviation depends only on the change of the magnetic field in the corresponding measurement area, that is, it is established that: Δ B =v LIA / α.

[0033] Another aspect of the invention relates to a magnetic field gradiometer of the type described in the opening paragraph, wherein the signal generator has a first power adder for forming a first microwave signal for a first microwave transmitter by adding two first frequency-modulated microwave signals; wherein the signal generator has a second power adder for forming a second microwave signal for a second microwave transmitter by adding two second frequency-modulated microwave signals; and wherein the analysis device has: a first power divider for distributing the power of the detected fluorescence from the first measurement area to a first pair of demodulators; and a second power divider for distributing the power of the detected fluorescence from the second measurement area to a second pair of demodulators.

[0034] The magnetic field gradiometer according to the second aspect of the present invention differs from the magnetic field gradiometer according to the first aspect of the present invention in that the double resonance excitation is performed in two different frequency bands, which correspond to the two frequency modulated microwave signals whose powers are added in the corresponding power adders. The information contained in the frequency bands corresponds to the single resonance excitation and can be extracted by means of one of the two demodulators. LIA+ and VLIA- , the resonance deviation V associated with the magnetic field can be determined in the manner described further above LIA+ / α + and V LIA- / α - As described further above, the influence of temperature and pressure fluctuations can be eliminated by difference formation during the determination of the corresponding magnetic field.

[0035] In one embodiment, the signal generator is configured to generate a first frequency modulated microwave signal having different carrier frequencies and a second frequency modulated microwave signal having different carrier frequencies. As described above, the two magnetic field resonances of the NV center can be located independently of each other by selecting different carrier frequencies.

[0036] In all cases described further above, the magnetic field changes can be decoupled from the changes in temperature and pressure by simultaneously exciting and analyzing two magnetic field resonances, without limiting the bandwidth of the magnetic field gradiometer in this case. The methods described further above differ in their technical complexity and in temperature and pressure compensation.

[0037] Further advantages of the invention are apparent from the accompanying drawings and the following description thereof. The features described above and still to be described in detail below can also be applied individually or in multiples in any combination. The embodiments shown and described are not to be understood as an exhaustive enumeration, but rather have an exemplary character for explaining the invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0038] Figure 1 A schematic diagram showing the energy levels of the NV center of a diamond crystal and the relationship between the energy levels and the magnetic field strength, temperature and pressure,

[0039] Figure 2a , Figure 2b shows a schematic representation of a magnetic resonance excited by means of a frequency-modulated microwave signal and a magnetic resonance signal demodulated by means of a lock-in amplifier,

[0040] Figure 3 A schematic representation of an embodiment of a magnetic field gradiometer with two mixers for forming a microwave signal with two frequency components which are phase-shifted relative to one another by π, which frequency components are emitted by corresponding microwave transmitters in order to apply a microwave field to a first measuring region and a second measuring region,

[0041] Figure 4 Show Figure 3 Schematic illustration of the frequency components of the microwave signal of the magnetic field gradiometer,

[0042] Figure 5Schematic diagram showing demodulated single resonance signal and double resonance signal and its changing relationship with the resonance deviation caused by changes in magnetic field and temperature,

[0043] Figure 6 a schematic diagram showing an embodiment of a magnetic field gradiometer having two power adders for adding respectively two frequency modulated microwave signals to a first microwave signal or to a second microwave signal,

[0044] Figure 7 Show Figure 6 Schematic illustration of the frequency components of the microwave signal of the magnetic field gradiometer, and

[0045] Figure 8 A schematic illustration of an exemplary embodiment of a magnetic field gradiometer is shown which has a pair of demodulators for analyzing the fluorescence from a first measurement region and a second measurement region.

[0046] In the following description of the figures, the same reference numerals are used for identical or corresponding components. DETAILED DESCRIPTION

[0047] Figure 1 The energy level diagram of the ground state of the NV center of a diamond crystal, which exists in the form of a triplet state, is shown. The corresponding three states of the triplet state differ in their magnetic spin quantum number, namely, m S =-1,0,+1. These m S = ±1 state has a ratio m due to spin-spin interaction. S = 0 state with higher energy. In the absence of a magnetic field, m S = ±1 states are degenerate (if hyperfine structure is neglected) and therefore have the same energy. In the presence of a magnetic field, m S = ±1 state is cancelled out in proportion to the magnetic field due to the Zeeman effect. Therefore, there are two resonant frequencies f + and f - , corresponding to m S =0 and m S = +1 state or m S = -1 state transition.

[0048] Resonance frequency f ± depends on the magnetic field B, temperature T and pressure p at the corresponding NV center. Figure 1 ):

[0049] f ± =D(T,p)±γB.

[0050] Here, γ represents the gyromagnetic ratio of the NV center, and B represents the projection of the magnetic field onto the corresponding axis of the NV center.

[0051] As can be seen from the above formula, if the two resonant frequencies f ± is known as follows, the fluctuations of temperature T and pressure p can be separated from the magnetic field changes:

[0052] f + -f - =2γB.

[0053] As from Figure 1 It can also be seen that in order to excite these two resonant frequencies f ± And we can use the microwave frequency f MW+ 、f MW- of two microwave fields.

[0054] By irradiating with excitation light in the green wavelength range, the NV center can be Figure 1 The ground state shown is pumped into an excited state that is also in the triplet form. The return to the ground state occurs partly by emission of red fluorescence. The intensity of this fluorescence depends on the m S = Population distribution between the states -1, 0, +1. This population distribution can be influenced by the interaction with the resonant microwave field. More precisely, the frequency is f MW+ and f MW- The microwave field (corresponding to m S =0 and m S = +1 state or m S = -1 The frequency of transitions between states f ±,0 ) will result in the intensity v of the detected fluorescence FL Reduce, such as this can be from Figure 2a As can be seen in the figure, it shows the intensity v of the fluorescence as a function of the frequency f. FL The fluorescence intensity v FL At the corresponding resonant frequency f ±,0 has the minimum value below.

[0055] Magnetic resonance is achieved by using a frequency modulated microwave field MW (t) for positioning, the frequency modulated microwave field has the following form:

[0056] f MW (t) = f MW +f d cos(2πf m t),

[0057] where f MW Indicates the carrier frequency, f d represents the modulation amplitude and f m represents the modulation frequency. The intensity of fluorescence vFL Correspondingly, the modulation amplitude V m and the modulation frequency f m To modulate.

[0058] Figure 2b The signal v generated by means of a demodulator in the form of a lock-in amplifier based on the detected fluorescence is shown. LIA The relationship between the change of frequency f. Figure 2b As can be seen in the figure, the demodulated signal v LIA has a dispersed form and at the corresponding resonant frequency f ±,0 Based on the demodulated signal v LIA The slope α of the linear range can be determined. If the resonant frequency changes due to changes in the magnetic field, temperature and / or pressure, the demodulated signal v LIA To analyze the resonance frequency f ±,0 The resonance deviation (detuning) is established as follows: Δ = v LIA / α.

[0059] The measurement principle described above can be used to simultaneously excite two magnetic resonances of the NV center. Figure 3 A magnetic field gradiometer 1 is described, which is the case in this case.

[0060] The magnetic field gradiometer 1 comprises a first diamond crystal 2a and a second diamond crystal 2b, each having a measurement region 3a, 3b, in which the diamond crystals 2a, 2b are doped with NV centers 4a, 4b. The magnetic field gradiometer 1 also comprises two excitation light sources 5a, 5b, which are used to emit excitation light 6a, 6b in the green wavelength range, respectively. When the NV centers 4a, 4b are excited in the respective measurement regions 3a, 3b, fluorescence 7a, 7b in the red wavelength range is generated, which fluorescence is detected by respective detectors 8a, 8b in the form of photodiodes.

[0061] In order to determine the magnetic field B in the respective measurement area 3a, 3b a , B b In addition to irradiating the corresponding diamond crystals 2a, 2b with excitation light 6a, 6b, corresponding microwave fields 9a, 9b are applied to them, which are generated by associated microwave transmitters 10a, 10b. In order to determine the corresponding magnetic field B a , B b The magnetic field gradiometer 1 has an analysis device 11. The signal generator 12 is used to generate a first microwave signal f for the first microwave transmitter 10a. MW1 (t) and the second microwave signal f for the second microwave transmitter 10b MW2 (t).

[0062] In order to form the first microwave signal f MW1 (t), the signal generator 12 has a first mixer 13a, which is configured to generate a first frequency-modulated signal f SB1 (t) and the first oscillator signal f MW1 To generate the first frequency modulated signal f SB1 (t), the signal generator 12 has a first signal generator 14a. In order to generate the first oscillator signal f MW1 , the signal generator 12 has a first oscillator 15a.

[0063] In order to form the second microwave signal f MW2 (t), the signal generator 12 has a second mixer 13b, which is configured to generate a second frequency-modulated signal f SB2 (t) and the second oscillator signal f MW2 To generate the second frequency modulated signal f SB2 (t), the signal generator 12 has a second signal generator 14b. In order to generate the second oscillator signal f MW2 , the signal generator 12 further has a second oscillator 15b.

[0064] The first signal generator 14a is designed to generate a signal having a carrier frequency f in the form given below: SB1 , modulation frequency f m1 and the modulation amplitude f d1 The first frequency modulated signal f SB1 (t):

[0065] f SB1 (t) = f SB1 +f d1 cos(2πf m1 t).

[0066] Correspondingly, the second signal generator 14b is configured to generate a signal having a carrier frequency f in the form given below: SB2 , modulation frequency f m2 and the modulation amplitude f d2 The second frequency modulated signal f SB2 (t):

[0067] f SB2 (t) = f SB2 +f d2 cos(2πf m2 t).

[0068] The first oscillator 15a generates a first oscillator signal f having a constant first oscillation frequency. MW1 , for simplicity it is represented as f MW1Correspondingly, the second oscillator 15b generates a second oscillator signal f having a constant second oscillation frequency. MW2 , for simplicity it is represented as f MW2 .

[0069] As described above, the first mixer 13a is used to generate a first frequency modulated signal f SB1 (t) and the first oscillator signal f MW1 Mixing is performed to form a first microwave signal f MW1 (t). The first microwave signal f MW1 (t) has the following frequency components after mixing in the first mixer 13a:

[0070] f MW1 +f SB1 +f d1 cos(2πf m1 t) and f MW1 -f SB1 +f d1 cos(2πf m1 t+π)

[0071] Therefore, the first microwave signal f is generated by mixing MW1 (t) two frequency components f d1 cos(2πf m1 t) and f d1 cos(2πf m1 t+π) between the phase difference of π or 180°.

[0072] The above content is correspondingly applicable to the second microwave signal f MW2 (t), the second microwave signal is mixed by the second mixer 13b through the second frequency modulation signal f SB2 (t) and the second oscillator signal f MW2 Therefore, the second microwave signal f MW2 (t) also has two frequency components f d2 cos(2πf m2 t) and f d2 cos(2πf m2 t+π), which have a phase difference of π or 180° relative to each other.

[0073] The first frequency modulated signal f is selected as follows SB1 (t) carrier frequency f SB1 and the first oscillator signal f MW1 The oscillation frequency f MW1 , so that in the first measurement area 3a a magnetic field B is given a In the case of a field strength of , the two frequency components with a phase shift of π correspond to mS =0 and m S = +1 state or m S = -1 state transition between the two resonant frequencies f + 、f - , this is like Figure 4 Correspondingly, the second frequency modulation signal f is selected as follows SB2 (t) carrier frequency f SB2 and the second oscillator signal f MW2 The oscillation frequency f MW2 , so that the m of NV center 4b S =0 and m S = +1 state or m S = Two resonant frequencies f of the transition between the -1 states + 、f - Positioned in the second measurement area 3b.

[0074] exist Figure 3 In the case of the magnetic field gradiometer 1, the magnetic field B should be measured in the first measurement area 3a. a The magnetic field B at the position of the second measurement area 3b is b It is used to measure the ambient magnetic field. By forming a magnetic field gradient B a -B b , i.e. the first magnetic field B in the first measurement region 3a a and the second magnetic field B in the second measurement area 3b b The difference between the two can suppress the noise of the ambient magnetic field. a , B b The field strengths in the corresponding measurement areas 3a, 3b are different, so a first frequency modulated signal and a second frequency modulated signal f are usually required. SB1 (t), f SB2 (t) with different carrier frequencies f SB1 、f SB2 Or the first oscillator signal f MW1 and the second oscillator signal f MW2 With different oscillation frequencies f MW1 、f MW2 .

[0075] As can be seen from FIG. 2 , the analysis device 11 has a first demodulator 16 a for determining the fluorescence of the first measurement region 3 a or a corresponding fluorescence signal v detected by means of the first detector 8 a. FL1 (t) Forming the first demodulated double resonance signal v LIA1 DRCorrespondingly, the analysis device 11 also has a second demodulator 16b for detecting the fluorescence or the corresponding fluorescence signal v from the second measurement region 3b detected by means of the second detector 8b. FL2 (t) Forming the second demodulated double resonance signal v LIA2 DR The two demodulators 16a, 16b are designed as lock-in amplifiers and each have a mixer 17a, 17b and a low-pass filter 18a, 18b. The first mixer 17a is fed with a first microwave signal f MW1 The frequency component f of (t) d1 cos(2πf m1 Correspondingly, the second mixer 17b is fed with a reference signal corresponding to the second microwave signal f MW2 The frequency component f of (t) d2 cos(2πf m2 t) The corresponding reference signal. The corresponding first double resonance signal or the second double resonance signal v LIA1 DR or v LIA2 DR With Figure 2b The known dispersed forms.

[0076] Figure 5 Used to show the effect of changes in magnetic field, temperature and pressure Figure 5 The demodulated double resonance signal v is shown on the right LIA DR The demodulated double resonance signal can be Figure 3 The first demodulated double resonance signal or the second demodulated double resonance signal v LIA1 DR or v LIA2 DR In order to understand the effect of the above variables on the demodulated double resonance signal v LIA DR The impact of Figure 5 The corresponding demodulated single resonance signals v are shown on the left and in the middle. LIA SR Due to the relative modulation phase of π or 180° between these two frequency components of the microwave field, we obtain m S = +1 state or m S = Single resonance signal v of two magnetic resonances in the -1 state LIA SR Vertical reflection.

[0077] If available from Figure 5 The resonance shift Δ caused by the magnetic field change is shown on the left. B So that the single resonance signal v LIA SRshift, which causes constructive addition in the double resonance signal (see Figure 5 The resonance shift Δ caused by the change of temperature or pressure T (or Δ p , not shown) causes a single resonance signal v LIA SR The destructive addition of Figure 5 The point below the middle). Figure 5 As shown on the right, the demodulated double resonance signal v LIA DR Therefore, it depends only on the corresponding magnetic field B in the first measurement area 3a or the second measurement area 3b. a , B b The magnetic field-dependent resonance deviation Δ B If you can get Figure 5 As seen on the right, the demodulated double resonance signal v LIA DR In the range of magnetic resonance, B Correspondingly, the demodulated double resonance signal v LIA DR Taking into account the slope α, the magnetic field-dependent resonance deviation Δ is determined as follows B :

[0078] The above considerations apply to both Figure 3 The first demodulated double resonance signal v of the magnetic field gradiometer 1 is shown LIA1 DR , and also applies to the second demodulated double resonance signal v LIA2 DR The following considerations concern the first double resonance signal v LIA1 DR , and correspondingly for the second double resonance signal v LIA2 DR .

[0079] For the first magnetic field B a The determination of takes place in the analysis device 11 as follows: In a calibration step, the two magnetic resonances are recorded (see Figure 2a or Figure 4 ) in order to determine the resonance frequency f at the time of calibration +,0 and f -,0 In a subsequent step, the first demodulated double resonance signal v is recorded during calibration in the manner described further above. LIA1 DR , and in Δ B = 0, the scalar factor α or the slope is determined. Next, the first frequency modulation signal f is selected or permanently set as follows SB1 (t) carrier frequency fSB1 and the first oscillator signal f MW1 The oscillation frequency f MW1 , so that for the magnetic field B in the first measurement area 3a a The field strength present during calibration, these two frequency components phase-shifted by π and m S =0 and m S = +1 state or m S = -1 state transition between the two resonant frequencies f +,0 、f -,0 Corresponding (see also Figure 4 ).

[0080] After calibration, the first magnetic field B is continuously determined using the following equation: a :

[0081]

[0082] The second magnetic field B is determined in a similar manner b In order to determine the first demodulated double resonance signal or the second demodulated double resonance signal v LIA1 DR or v LIA2 DR To calculate the corresponding magnetic field B a , B b , the analysis device 11 has corresponding computer logic 19a, 19b, which is designed in the form of suitable hardware and / or software.

[0083] Figure 6 A magnetic field gradiometer 1 is shown, which is based on the same measuring principle as Figure 3 The magnetic field gradiometers 1 shown are identical. Figure 6 The magnetic field gradiometer 1 shown is Figure 3 The magnetic field gradiometers shown differ substantially in the design of the signal generator 12, which is used to individually adjust the power of the frequency bands for localizing the respective magnetic resonance. MW1 (t), the signal generator 12 has a first power adder 20a, which is used to add a first frequency modulated microwave signal f having two frequency components with a phase shift of π relative to each other. MW1- (t) and another first frequency modulated microwave signal f having two frequency components shifted in phase by π relative to each other MW1+ (t) is added. In order to form the second frequency modulated microwave signal f MW2 (t), the signal generator 12 also has a second power adder 20b, which is used to add the second frequency modulated microwave signal f having two frequency components with a phase shift of π relative to each other. MW2-(t) and another second frequency modulated microwave signal f having two frequency components shifted in phase by π relative to each other MW2+ (t) Add.

[0084] In order to form the first frequency modulated microwave signal f MW1- (t), the signal generator 12 has a first mixer 13a, which is used to SB (t) and the first oscillator signal f MW1- Correspondingly, in order to form another first frequency modulated microwave signal f MW1+ (t), the signal generator 12 has another first mixer 13a', which is used to modulate the frequency modulated signal f SB (t) and another first oscillator signal f MW1+ Mixing is performed.

[0085] In order to form the second frequency modulated microwave signal f MW2- (t), the signal generator 12 further comprises a second mixer 13b, which is used to SB (t) and the second oscillator signal f MW1- To form another second frequency modulated microwave signal f MW2+ (t), the signal generator 12 further comprises a second mixer 13b', which is used to modulate the frequency modulated signal f SB (t) and another second oscillator signal f MW2+ Mixing is performed.

[0086] FM signal f SB (t) is generated in the signal generator 14 of the signal generator 12. The first oscillator signal f MW1- and the first oscillator signal f MW1+ are generated in the first oscillator 15a and the further first oscillator 15a' respectively. Correspondingly, the second oscillator signal f MW2- and another second oscillator signal f MW2+ are generated in the second oscillator 15b and in the further second oscillator 15b', respectively.

[0087] The signal generator 12 is designed to independently adjust the first frequency-modulated microwave signal f MW1- (t) and another first frequency modulated microwave signal f MW1+ The signal generator 12 is also configured to adjust the power of the second frequency modulated microwave signal f independently of each other. MW2- (t) and another second frequency modulated microwave signal f MW2+ (t) power.

[0088] For this purpose, the frequency modulated signal f SB The power of (t) is divided into four power components of equal magnitude. Next, the frequency modulated signal f is individually attenuated by means of a programmable attenuator 22. SB (t), which is fed to the corresponding mixer 13a, 13a', 13b, 13b', the power component is individually adjusted. Alternatively or additionally, it can be realized that the four oscillators 15a, 15a', 15b, 15b' are designed to generate corresponding oscillator signals f respectively having individually adjustable powers. MW1- 、f MW1+ 、f MW2- 、f MW2+ .

[0089] Figure 7 and Figure 4 The diagram similarly shows the Figure 6 The first microwave signal f generated by the signal generator 12 MW1 (t) is the frequency component. Figure 7 As can be seen in the figure, the first microwave signal f MW1 (t) has two pairs, each of which has two frequency components that are phase-shifted by π relative to each other. MW1- (t) and the second frequency modulated microwave signal f MW1+ The possibility of individually adjusting the frequency of (t) can thus minimize the difference in the scalar factor α of the corresponding single resonance spectra and thus maximize the effect of the double resonance signal v LIA1 DR It should be understood that the above considerations also apply similarly to the second microwave signal f MW2 (t).

[0090] Figure 8 A magnetic field gradiometer 1 is shown, which is connected to Figure 3 and Figure 6 The magnetic field gradiometers shown differ in the design of the signal generator 12 and the evaluation device 11. Figure 8 In the magnetic field gradiometer 1 shown, the signal generator 12 has a first power adder 20a for adding two first frequency modulated microwave signals f MW1+ (t), f MW1- (t) is added to form a first microwave signal f for the first microwave transmitter 10a MW1 (t). Correspondingly, the signal generator 12 has a second power adder 20b for adding the two second frequency modulated microwave signals f MW2+ (t), f MW2- (t) is added to form a second microwave signal f for the second microwave transmitter 10bMW2 (t). The first frequency modulated microwave signal f MW1+ (t), f MW1- (t) is generated in the corresponding first signal generator or the other first signal generator 14a, 14a'. Correspondingly, the second frequency modulated microwave signal f MW2+ (t), f MW2- (t) is generated in the corresponding second signal generator or the further second signal generator 14b, 14b'.

[0091] For the first frequency modulated microwave signal f MW1+ (t), f MW1- (t) Established:

[0092] f MW1± (t) = f MW1± +f d cos(2πf m1± t),

[0093] where f MW1± represents the corresponding carrier frequency, f m1± represents the corresponding modulation frequency and f d represents the modulation amplitude. The two first frequency modulated microwave signals f MW1+ (t), f MW1- (t) carrier frequency f m1± are different from one another in order to excite in each case one of the two magnetic resonances of the NV centres 4 a in the first measurement region 3 a .

[0094] Therefore, the fluorescence signal V detected by the first detector 8a of the magnetic field gradiometer 1 fl1 (t) including information on m1± The analysis device 11 comprises a first power divider 23a for dividing the detected fluorescence signal V from the first measurement area 3a into two different frequency bands. fl1 The power of (t) is distributed to the first pair of demodulators 16a, 16a'. Correspondingly, the second power distributor 23b is used to convert the detected fluorescence signal V from the second measurement area 3b into fl2 The power of (t) is distributed to the second pair of demodulators 16b, 16b'. The demodulated single resonance signal v is generated in the corresponding demodulators 16a, 16a', 16b, 16b'. LIA1+ 、v LIA1- 、v LIA2+ 、v LIA2- , the demodulated single resonance signal can be combined with the above Figure 2b Analyze in the manner described.

[0095] For the first magnetic field B in the first measurement region 3a a The following relationship is produced:

[0096]

[0097] where α 1+ and α 1- represents the corresponding single resonance signal v LIA1+ 、v LIA1- In Δ B = 0. The first magnetic field B a The first operation logic 19a uses the above given formula based on the two demodulated single resonance signals v LIA1+ 、v LIA1- The corresponding content is applicable to the second measurement region 3b according to the two second demodulated single resonance signals v using the second operation logic 19b. LIA2+ 、v LIA2- For the second magnetic field B b of determination.

[0098] In summary, the corresponding magnetic field B can be carried out in a manner decoupled from temperature and pressure changes by means of the magnetic field gradiometer 1 described further above. a , B b Or magnetic field gradient B a -B B The determination of the magnetic field gradiometer 1 is not limited in this case, without limiting the bandwidth of the magnetic field gradiometer 1. Therefore, the full dynamic range of the magnetic field gradiometer 1 can be used. It is also generally not necessary to heat or cool the corresponding diamond crystals 3a, 3b to stabilize their temperature.

Claims

1. A method for determining the magnetic field gradient (B a -B b ) of a magnetic field gradiometer (1), comprising: at least one excitation light source (5a, 5b) for emitting excitation light (6a, 6b); two spatially spaced apart measurement regions (3a, 3b) for magnetic field measurement, said measurement regions having color centers in diamond, preferably NV centers (4a, 4b), which emit fluorescence (7a, 7b) when excited with said excitation light (6a, 6b); a first detector (8a) for detecting the fluorescence (7a) from the first measurement area (3a); a second detector (8b) for detecting the fluorescence (7b) from the second measurement area (3b); A first microwave transmitter (10a) for applying a first microwave field (9a) to the first measurement area (3a); A second microwave transmitter (10b) for applying a second microwave field (9b) to the second measurement area (3b); as well as An analysis device (11) configured to determine the magnetic field gradient (B) based on the fluorescence (7a) detected from the first measurement area (3a) and the fluorescence (7b) detected from the second measurement area (3b) a -B b ), Features A signal generator (12) configured to generate a first microwave signal (f MW1 (t)), the first microwave signal has at least two frequency components that are phase-shifted by π relative to each other; and generating a second microwave signal (f) for the second microwave transmitter (10b) MW2 (t)), the second microwave signal has at least two frequency components that are phase-shifted by π relative to each other.

2. The magnetic field gradiometer according to claim 1, wherein in order to form the first microwave signal (f MW1 The signal generator (12) has two frequency components of the first frequency modulated signal (f) that are phase-shifted by π relative to each other, and the signal generator (12) has a first mixer (13a) configured to generate a first frequency modulated signal (f SB1 (t)) and the first oscillator signal (f MW1 ) for mixing; and wherein in order to form a second frequency modulated microwave signal (f MW2 The signal generator (12) has two frequency components of the second frequency modulated signal (f) that are phase-shifted by π relative to each other, and the signal generator (12) has a second mixer (13b) that is configured to MW2 (t)) and the second oscillator signal (f MW2 ) for mixing.

3. The magnetic field gradiometer according to claim 2, wherein the first frequency modulated signal and the second frequency modulated signal (f SB1 (t), f SB2 (t)) with different carrier frequencies (f SB1 , f SB2 ), and / or wherein the first oscillator signal (f MW1 ) and the second oscillator signal (f MW2 ) have different oscillation frequencies.

4. The magnetic field gradiometer according to claim 1, wherein in order to form the first microwave signal (f MW1 (t)), the signal generator (12) has a first power adder (20a), the first power adder is used to add a first frequency modulated microwave signal (f MW1- (t)) and another first frequency modulated microwave signal (f) having two frequency components shifted in phase by π relative to each other MW1+ (t)) are added; and In order to form the second microwave signal (f MW2 (t)), the signal generator (12) has a second power adder (20b), the second power adder is used to add a second frequency modulated microwave signal (f MW2- (t)) and another second frequency modulated microwave signal (f) having two frequency components shifted in phase by π relative to each other MW2+ (t)) are added.

5. The magnetic field gradiometer according to claim 4, wherein the signal generator (12) is configured to adjust the first frequency-modulated microwave signal (f MW1- (t)) and the further first frequency modulated microwave signal (f MW1+ (t)); and wherein the signal generator (12) is configured to adjust the second frequency modulated microwave signal (f MW2- (t)) and the further second frequency modulated microwave signal (f MW2+ (t)) power.

6. The magnetic field gradiometer according to claim 4 , wherein in order to form the first frequency-modulated microwave signal (f MW1- (t)), the signal generator (12) has a first mixer (13a), the first mixer is used to SB (t)) and the first oscillator signal (f MW1- ) is mixed, and in order to form the further first frequency modulated microwave signal (f MW1+ (t)), the signal generator has another first mixer (13a'), the other first mixer is used to SB (t)) and another first oscillator signal (f MW1+ ) for mixing.

7. The magnetic field gradiometer according to claim 6, wherein the signal generator (12) has at least one programmable attenuator (22) for making the frequency modulation signal (f SB The corresponding power components of the frequency converter (t)) which are fed to the corresponding mixer (13a, 13a') can be adjustably attenuated.

8. The magnetic field gradiometer according to claim 6 or 7, wherein the signal generator (12) has: a signal generator for generating the first oscillator signal (f MW- ), the first oscillator signal preferably having an adjustable power; and for generating the further first oscillator signal (f MW+ ) of a further first oscillator (15a'), wherein the further first oscillator signal preferably has an adjustable power.

9. The magnetic field gradiometer according to claim 1 , wherein the analysis device ( 11 ) comprises a first demodulator ( 16 a ) for determining a fluorescence intensity (v ) of the first measurement region ( 3 a ) based on the detected fluorescence intensity (v FL1 (t)) forms the first demodulated double resonance signal (v LIA1 DR ); and a second demodulator (16b), the second demodulator for detecting the fluorescence (v of the second measurement area (3b) based on the detected fluorescence (v FL2 (t)) forms the second demodulated double resonance signal (v LIA2 DR ).

10. The magnetic field gradiometer as claimed in claim 9, wherein the two demodulators (16a, b) are designed as lock-in amplifiers.

11. The magnetic field gradiometer according to claim 9 or 10, wherein the analysis device (11) is configured to: LIA1 DR ) determines a first magnetic field (B) in the first measurement area (3a) a ) of the resonance deviation (Δ B ); and according to the second demodulated double resonance signal (v LIA2 DR ) determines a second magnetic field (B) in the second measurement area (3b) b ) of the resonance deviation (Δ B ).

12. A magnetic field gradiometer according to the preamble of claim 1, It is characterized in that The signal generator (12) has a first power adder (20a) for adding two first frequency modulated microwave signals (f MW1+ (t), f MW1- (t)) are added to form a first microwave signal (f) for the first microwave transmitter (10a) MW1 (t)); The signal generator (12) has a second power adder (20b) for adding two second frequency modulated microwave signals (f MW2+ (t), f MW2- (t)) are added to form a second microwave signal (f) for the second microwave transmitter (10b) MW2 (t)); and The analysis device (11) comprises: a first power distributor (23a) for distributing the detected fluorescence (v FL1 (t)) to a first pair of demodulators (16a, 16a'); and a second power distributor (23b) for distributing the detected fluorescence (v) from the second measurement area (3b ... FL2 The power of (t)) is distributed to the second pair of demodulators (16b, 16b').

13. The magnetic field gradiometer according to claim 12, wherein the signal generator (12) is configured to generate the first frequency-modulated microwave signals (f MW1+ (t), f MW1- (t)) and the second frequency modulated microwave signal (f) having different carrier frequencies respectively MW2+ (t), f MW2- (t)).

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