Magnetic field gradiometer

Abstract

The present invention relates to a magnetic field gradient (B a -B b The present invention relates to a magnetic field gradiometer (1) for determining the magnetic field gradiometer (1), comprising at least one excitation light source (5a, 5b) for emitting excitation light (6a, 6b), two spatially spaced measurement areas (3a, 3b) for measuring the magnetic field, each having color centers in diamond, preferably NV centers (4a, 4b) that emit fluorescent light (7a, 7b) when excited by the excitation light (6a, 6b), and a first detector (3a) for detecting the fluorescent light (7a) from the first measurement area (3a). (8a), a second detector (8b) for detecting fluorescent light (7b) from the second measurement area (3b), a first microwave radiator (10a) for applying a first microwave field (9a) to the first measurement area (3a), a second microwave radiator (10b) for applying a second microwave field (9b) to the second measurement area (3b), and a magnetic field gradient (B) based on the fluorescent light (7a) detected from the first measurement area (3a) and the fluorescent light (7b) detected from the second measurement area (3b). a -B b The magnetic field gradiometer (1) comprises an evaluation device (11) configured to determine a first microwave signal (f MW1 (t)) for the second microwave radiating portion (10b), and a second microwave signal (f MW2 (t)) and a signal generator (12) configured to generate a

Description

[Technical field]

[0001] The present invention relates to a magnetic field gradiometer for determining a magnetic field gradient, comprising at least one excitation light source for emitting excitation light, two spatially spaced measurement regions for magnetic field measurements having color centers in diamond, preferably NV centers, which emit fluorescent light when excited by the excitation light, a first detector for detecting the fluorescent light from the first measurement region, a second detector for detecting the fluorescent light from the second measurement region, a first microwave emitter for applying a first microwave field to the first measurement region, a second microwave emitter for applying a second microwave field to the second measurement region, and an evaluation device configured to determine a magnetic field gradient based on the fluorescent light detected from the first measurement region and the fluorescent light detected from the second measurement region. [Background technology]

[0002] In particular, high-sensitivity magnetometers can be used to obtain magnetic signals from the brain or neural activity with local resolution in order to exploit innovative human-machine interfaces for the control of prostheses, exoskeletons or machines. Since the amplitude of the useful signal of human neural activity can be below the magnetic ambient noise, solutions must be found to suppress the ambient noise in the above mentioned applications. One such solution is the use of a gradiometric sensor concept, in which the measurement area of ​​a first magnetometer is placed directly at the source of the magnetic field and the measurement area of ​​a second magnetometer is placed at a certain distance away from the first measurement area of ​​the source of the magnetic field. The magnetometer with the more distant second measurement area then records only the ambient noise, while the measurement area of ​​the magnetometer closer to the source detects the useful signal in addition to the ambient noise. Subtraction of the noise signal recorded by the first magnetometer by the signal of the second magnetometer results in a signal containing only the useful signal, which is referred to in the present application as the magnetic field gradient. The use of a magnetic field gradiometer makes it possible to eliminate or suppress ambient noise compared to magnetic field measurements using a single magnetometer, and the same effect can be obtained with either magnetometer.

[0003] The above-mentioned type of magnetic field gradiometer and the corresponding magnetometer are based on the magnetic field-dependent change in the fluorescence of color centers (typically NV centers) in diamond in the presence of an exchange magnetic field. More specifically, then, optical detection of magnetic resonance (ODMR). The principle of such detection is described in Non-Patent Document 1. This principle is explained below using the example of NV centers.

[0004] An NV centre is a special colour centre in the diamond crystal lattice consisting of a nitrogen atom and adjacent defects. More precisely, within the scope of this application, an NV centre is understood to be a negatively charged NV centre. Magnetic field gradiometers can also be made to measure a large number of NV centres (so-called ensemble magnetometers).

[0005] The energy levels of the NV center include a ground state and excited states, each in the form of a triplet. The three states of each triplet have the magnetic spin quantum number m S =-1, 0, +1 are different. m S = ±1 state is due to the spin-spin interaction, m S = 0 state. In the absence of a magnetic field, m S =±1 states are degenerate (when hyperfine structure is ignored), i.e., they have the same energy. Conversely, in the presence of a magnetic field, the Zeeman effect causes m S = The degeneracy of the ±1 state is cancelled.

[0006] Irradiation with excitation light in the green wavelength region can excite the NV center from the ground state to an excited state. The return to the ground state is accompanied in part by red fluorescence. The intensity of this fluorescence is approximately the m S = -1, 0, +1 states. This population distribution can be influenced by the interaction with the resonant microwave field. More precisely, m S = 0 state and m S =+1 or m S = the frequency f corresponding to the transition between the -1 state + and f - In the case of a microwave field having a wavelength of 0.1 nm or less, this results in a decrease in the detected intensity of the fluorescent light.

[0007] The frequency of the microwave field is varied to measure the magnetic field using a suitable magnetometer. Depending on the frequency of the microwave field, the measured intensity of the fluorescent light varies with frequency f ± The magnetic field has local minima, also called "dips" or "dips", when the magnetic field is at a constant magnetic field. The magnetic field is determined by the location of these minima.

[0008] frequency f + and f - is obtained from the following formula: f ± =D(T,p)±γ NV B 0 For example, see Non-Patent Document 2. Here, D(T,p) is the triplet ground state, m S = 0 state and m S = ±1 states and depends on the temperature T and the ambient pressure p. NV is the gyromagnetic ratio of the NV center, and B 0 is the projection of the magnetic field onto the corresponding axis of the NV centre.

[0009] In the case of a magnetic field gradiometer, such measurements are performed in two spatially separated measurement regions, in which the NV or other diamond color centers are located, respectively, and the magnetic field gradient is determined from a comparison of the fluorescence minima from the first and second measurement regions and the distance between the measurement regions (see above).

[0010] However, the structure of such a magnetic field gradiometer is prone to generate temperature and / or pressure differences between the two magnetometers or between the two measurement regions. ± Due to the temperature and pressure dependences of , such temperature or pressure differences can distort the determined magnetic field gradient.

[0011] Non-Patent Document 3 describes a magnetometer in which a double resonance technique is used to compensate for the effects of thermal displacement of the energy levels of the NV centers. For this purpose, two frequency-modulated microwave signals with two frequency components out of phase with each other by π are combined and irradiated by a microwave emitter to a diamond crystal containing NV centers. The fluorescent light of the NV centers is detected by a photodetector and sent to a lock-in amplifier, where it is demodulated using a reference signal. In this way, two frequencies f + and f - The difference (f + -f - ) and is independent of thermal displacement.

[0012] A magnetometer with a wide dynamic range is described in Non-Patent Document 4. In order to achieve a wide dynamic range, it is proposed to control the carrier frequency of the microwave signal using feedback of a lock-in signal.

[0013] Non-Patent Document 5 describes inter alia a magnetometer and a gradiometer in which continuous wave (cw) ODMR measurements are performed in combination with lock-in detection in which two microwave frequencies of the NV centre are excited simultaneously. [Prior art documents] [Non-patent literature]

[0014] [Non-Patent Document 1] "Nanoscale imaging magnetometry with diamond spins under ambient conditions", G. Balasubramian et al., Nature 455,648(2008) [Non-Patent Document 2] "Integrated and portable magnetometer based on nitrogen-vacancy ensembles in diamond", F. Stuerner et al., arXiv:2012.01053 [Non-Patent Document 3] "Diamond magnetometer enhanced by ferrite flux concentrators", I.Fescenko et al., Physical Review Research 2,023394(2020) [Non-Patent Document 4] "Robust high-dynamic-range vector magnetometry with nitrogen-vacancy centers in diamond", H.Clevenson et al.,Appl.Phys.Lett.112,252406(2018) [Non-Patent Document 5] "Diamond Magnetometry and Gradiometry Towards Subpicotesla dc Field Measurement",Chen Zhang et al.,Phys.Rev.Applied 15,064075(2021) Summary of the Invention [Problem to be solved by the invention]

[0015] In contrast, the object of the present invention is to provide a magnetic field gradiometer of the above-mentioned type, which is capable of effectively compensating for disturbance influences, in particular temperature and / or pressure influences. [Means for solving the problem]

[0016] According to a first aspect, this problem is solved by a magnetic field gradiometer of the type mentioned at the outset, which further comprises a signal generator which is adapted to generate a first microwave signal for the first microwave radiator having at least two frequency components which are phase offset from one another by π and a second microwave signal for the second microwave radiator having at least two frequency components which are phase offset from one another by π.

[0017] In this aspect of the invention, a microwave signal is generated for each measurement region or each of the two magnetometers, which microwave signal has (at least) two frequency components, which are phase offset from each other by π. The phase offset of π or 180° between the two frequency components or the relative modulation phase leads to mirroring of the demodulated individual resonance signals with respect to the vertical axis, for example during demodulation using a lock-in amplifier (see below). Magnetic field changes lead to a shift of the frequencies of the single resonance signals with the same sign, i.e. the single resonance signals add constructively. During temperature and pressure changes the frequencies of the single resonance signals shift in different signs, so that the single resonance signals add destructively. The demodulated double resonance signal, which is analyzed for the determination of the magnetic field or the magnetic field gradient, therefore depends only on the magnetic field changes, and the effects of temperature and pressure are compensated for.

[0018] In one embodiment, the signal generator comprises a first frequency mixer configured to frequency mix a first frequency modulated signal and a first oscillator signal to form two frequency components phase offset from each other by π of the first microwave signal, and the signal generator comprises a second frequency mixer configured to frequency mix a second frequency modulated signal and a second oscillator signal to form two frequency components phase offset from each other by π of the second microwave signal.

[0019] The first or second oscillator signal is a signal having a constant oscillation frequency that is mixed with the respective frequency modulation signal. The first and second frequency modulation signals are generated by respective signal generators of the signal generating device, and the modulation signals have respective carrier frequencies and modulation components. Due to the frequency mixing, the first microwave signal contains frequency components that are out of phase with each other by π. Correspondingly, due to the frequency mixing, the second microwave signal also contains frequency components that are out of phase with each other by π. The respective oscillation frequencies and the carrier frequency of the frequency modulation signals are each m, where m is the frequency of the two frequency components that are out of phase with each other by π. S =0⇔±1 transition.

[0020] In a further development, the first and second frequency modulated signals have different carrier frequencies and / or the first oscillator signal and the second oscillator signal have different oscillation frequencies. S = Frequency f corresponding to 0⇔±1 + and f - are typically different in the two measurement regions, and correspondingly, the first carrier frequency and the second carrier frequency, and the first oscillation frequency and the second oscillation frequency, usually need to be selected to be different.

[0021] In an alternative embodiment, the signal generator has a first output summer for adding a first frequency modulated microwave signal having two frequency components phase offset from each other by π and a further first frequency modulated microwave signal having two frequency components phase offset from each other by π to form the first microwave signal, and a second output summer for adding a second frequency modulated microwave signal having two frequency components phase offset from each other by π and a further second frequency modulated microwave signal having two frequency components phase offset from each other by π to form the second microwave signal.

[0022] This embodiment is based on the same functional principle as the above embodiment. By using a first power adder, the power of the first frequency modulated microwave signal and the further first frequency modulated microwave signal can be made different. Correspondingly, by using a second power adder, the power of the second frequency modulated microwave signal and the further second frequency modulated microwave signal can be made different. Each frequency modulated microwave signal corresponds to one of the two magnetic field resonances in the first / second measurement area, respectively, while the further frequency modulated microwave signal corresponds to the other magnetic field resonance in the first / second measurement area, respectively. The ability to set different powers during excitation of the single resonances allows the difference in the scalar coefficients (see below) of the respective single resonance spectra to be reduced during the evaluation of the respective demodulated fluorescence signals. In this way, the suppression of temperature and pressure changes in the demodulated double resonance signal can be strengthened.

[0023] In an advantageous development, the signal generator is configured to adjust the power of the first frequency-modulated microwave signal and the power of 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 power of the further second frequency-modulated microwave signal independently of each other. It is advantageous if the power of each frequency-modulated microwave signal and each further frequency-modulated microwave signal can not only be set independently of each other, but also adjusted independently of each other. In this way, differences in the scalar coefficients of the respective single resonance spectra can be minimized and suppression of temperature and pressure changes of the demodulated double resonance signal can be maximized when evaluating the respective demodulated fluorescence signals. The independent adjustment of the power can be performed by different methods, as will be explained in more detail below.

[0024] In one development, the signal generator comprises a first frequency mixer for frequency mixing the frequency modulated signal with a first oscillator signal to form a first frequency modulated microwave signal, and a further first frequency mixer for frequency mixing the frequency modulated signal with the further first oscillator signal to form a further first frequency modulated microwave signal, correspondingly, the signal generator comprises a second frequency mixer for frequency mixing the frequency modulated signal with a second oscillator signal to form a second frequency modulated microwave signal, and a further second frequency mixer for frequency mixing the frequency modulated signal with the further second oscillator signal to form a further second frequency modulated microwave signal.

[0025] In one development of this, a signal generator is typically provided which generates a frequency modulated signal for all four frequency mixers. In this case, the frequency modulated signal passes through an output divider which distributes the frequency modulated signal to four shared outputs in a predetermined ratio to each other. Each shared output of the frequency modulated signal is fed to one of the four frequency mixers. In principle, the four shared outputs are of equal magnitude, but this is not absolutely necessary.

[0026] In a further development, the signal generator comprises at least one programmable attenuator for adjustable attenuation of each power share of the frequency modulated signal fed to each frequency mixer, in which case the power share of the frequency modulated signal fed to each one of the four frequency mixers can be individually adjusted by means of the programmable attenuator.

[0027] In a further development, the signal generator comprises a first oscillator, preferably with adjustable power, for generating the first oscillator signal and a further first oscillator, preferably with adjustable power, for generating the further first oscillator signal. Correspondingly, the signal generator comprises a second oscillator, preferably with adjustable power, for generating the second oscillator signal and a further second oscillator, preferably with adjustable power, for generating the further second oscillator signal. The power of the first frequency modulated microwave signal and the power of the further first frequency modulated microwave signal can be adjusted independently of each other by adjusting the power of the first oscillator or the further first oscillator. The same applies to the power of the second frequency modulated microwave signal and the power of the further second frequency modulated microwave signal.

[0028] In a further embodiment, the evaluation device comprises a first demodulator for forming a first demodulated double resonance signal from the detected fluorescence light of the first measurement area and a second demodulator for forming a second demodulated double resonance signal from the detected fluorescence light of the second measurement area. As described above, in the magnetic field gradiometer according to the invention, both magnetic resonances of the NV center are excited and read out simultaneously. The strength of the magnetic field in the first / second measurement area can be determined from the first / second demodulated double resonance signals.

[0029] In one development, the two demodulators are each formed as a lock-in amplifier. The demodulation of the double resonance signal is performed in this development by means of a lock-in amplifier. Each lock-in amplifier typically comprises a frequency mixer to which the double resonance signal and a reference signal are fed. The signal resulting from the frequency mixing is passed through a low-pass filter, which is also part of the lock-in amplifier. The demodulated double resonance signal at the output of the low-pass filter is dispersive and linear in the vicinity of the resonance frequency.

[0030] In one development, the evaluation device is formed to determine a magnetic field-dependent resonance deviation of a first magnetic field in a first measurement region based on the first demodulated double resonance signal and to determine a magnetic field-dependent resonance deviation of a second magnetic field in a second measurement region based on the second demodulated double resonance signal. To determine the resonance deviation, typically the slope of the linear range of the demodulated double resonance signal ("scalar coefficient" α) is determined. For this purpose, firstly the respective microwave magnetic field is irradiated at resonance. In case of a change in the resonance frequency due to a change in the magnetic field, in the temperature or in the pressure, the detuning or resonance deviation Δ from the resonance frequency is determined by the demodulated double resonance signal v LIA can be determined as follows: Δ=v LIA / α. Since the resonance deviations due to temperature or pressure changes in the two single resonance signals add destructively (see above), the double resonance signal v LIA The resonance deviation of depends only on the change in the magnetic field in the respective measurement range, i.e., Δ B =v LIA / α is applied.

[0031] An aspect of the invention relates to a magnetic field gradiometer of the type mentioned in the introduction, wherein the signal generator has a first output summer for forming a first microwave signal for a first microwave radiator by adding two first frequency-modulated microwave signals, the signal generator has a second output summer for forming a second microwave signal for a second microwave radiator by adding two second frequency-modulated microwave signals, and the evaluation device has a first output distributor for distributing the power of the fluorescent light detected from the first measurement area to a first pair of demodulators and a second output distributor for distributing the power of the fluorescent light detected from the second measurement area to a second pair of demodulators,

[0032] The magnetic field gradiometer according to the second aspect of the invention differs from the magnetic field gradiometer according to the first aspect of the invention in that the double resonance excitation is performed by two different frequency bands, which correspond to two frequency modulated microwave signals whose outputs are summed in respective output summers. The information contained in the frequency bands, which correspond to single resonance excitations, can each be extracted using one of the two demodulators. The two demodulated single resonance signals v LIA+ Or v LIA- Using the magnetic field induced resonance deviation v LIA+ / α + Or v LIA- / α - can be determined by the method described above. As mentioned above, the formation of the difference makes it possible to eliminate the influence of temperature and pressure variations in the determination of the respective magnetic fields.

[0033] In one embodiment, the signal generator is configured to generate two first frequency modulated microwave signals, each having a different carrier frequency, and to generate two second frequency modulated microwave signals, each having a different carrier frequency, such that the two magnetic resonances of the NV center can be addressed independently of one another by selection of the different carrier frequencies, as described above.

[0034] In all the above further cases, by simultaneously exciting and evaluating both magnetic resonances, the magnetic field changes can be decoupled from the temperature and pressure changes without limiting the bandwidth of the magnetic field gradiometer. The above further methods differ in their technical complexity and in the degree of temperature and pressure compensation.

[0035] Further advantages of the present invention are shown in the drawings and the following description. Likewise, the above-mentioned and further enumerated features can be used individually or in any combination. The shown and described embodiments are not to be construed as an exhaustive list, but rather have an exemplary character for the explanation of the present invention. [Brief description of the drawings]

[0036] [Figure 1] FIG. 2 shows a schematic diagram of the energy levels of an NV centre in a diamond crystal and their dependence on magnetic field strength, temperature and pressure. [Figure 2a] FIG. 1 shows a schematic diagram of magnetic resonance excited with a frequency modulated microwave signal and the magnetic resonance signal demodulated by a lock-in amplifier. [Figure 2b] FIG. 1 shows a schematic diagram of magnetic resonance excited with a frequency modulated microwave signal and the magnetic resonance signal demodulated by a lock-in amplifier. [Diagram 3] FIG. 13 is a schematic diagram illustrating an embodiment of a magnetic field gradiometer having two frequency mixers for forming a microwave signal having two frequency components, phase offset from each other by π, for radiating via respective microwave radiators in order to apply a microwave magnetic field to first and second measurement regions. [Figure 4] FIG. 4 is a diagram illustrating the frequency components of a microwave signal of the magnetic field gradiometer of FIG. [Diagram 5] FIG. 2 shows a schematic diagram of demodulated single and double resonance signals and their dependence on resonance deviations due to changes in magnetic field and temperature. [Figure 6] FIG. 2 shows a schematic diagram of an embodiment of a magnetic field gradiometer having two output summers for summing two frequency modulated microwave signals for a first or second microwave signal, respectively. [Figure 7] FIG. 7 is a diagram illustrating the frequency components of a microwave signal of the magnetic field gradiometer of FIG. 6. [Figure 8] FIG. 2 shows a schematic diagram of an embodiment of a magnetic field gradiometer comprising a pair of demodulators for evaluating fluorescent light from first and second measurement regions. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0037] In the following description of the drawings, the same reference numerals are used for the same or corresponding parts.

[0038] FIG. 1 shows the energy levels of the ground state of the NV center of a diamond crystal, which exists as a triplet. The three triplet states have magnetic spin quantum numbers m S =-1, 0, +1 are different. m S = ±1 state is due to the spin-spin interaction, m S = 0 state. In the absence of a magnetic field, m S = ±1 states are degenerate (if hyperfine structure is ignored) and therefore have identical energies. In the presence of a magnetic field, the Zeeman effect causes m S The degeneracy of the m = ±1 states is broken. S = 0 state and m S =+1 state, or m S There are two resonant frequencies corresponding to the transition between the -1 and -2 states, f = -1 and + and f - exists.

[0039] resonance frequency f ± depends on the magnetic field B, temperature T, and pressure p at the location of each NV centre. The following applies (see Fig. 1): f ± =D(T,p)±γB where γ represents the gyromagnetic ratio of the NV centre and B represents the projection of the magnetic field onto the corresponding axis of the NV centre.

[0040] As can be seen from the above equation, both resonant frequencies f ± If is known, the variations in temperature T and pressure p can be separated from the changes in the magnetic field. f + -f - =2γB

[0041] As can be seen in Figure 1, there are two resonant frequencies f ± For excitation of the corresponding microwave frequency f MW+ ,f MW- Two microwave magnetic fields having

[0042] By irradiating the NV center with excitation light in the green wavelength region, it is possible to pump it from the ground state shown in Figure 1 to an excited state, which is also in the triplet form. The return to the ground state is partially accompanied by the emission of red fluorescent light. The intensity of this fluorescent light is proportional to the m S = -1, 0, +1 states. This population distribution is influenced by the interaction with the resonant microwave magnetic field. More precisely, at frequency f MW+ and f MW- In the case of a microwave field of m S = 0 state and m S The frequency of the transition between the mS=+1 and mS=-1 states, f ±,0 Correspondingly, the fluorescent light v FL This is because the detected intensity of the fluorescent light v FL This can be seen in FIG. 2a, which shows the intensity of the fluorescent light v as a function of the frequency f. FL are the respective resonant frequencies f ±,0 It is smallest at

[0043] The magnetic resonance response is a frequency modulated microwave magnetic field f MW This is done using (t). f MW (t)=f MW +f d cos(2πf m t) where f MW is the carrier frequency, f d is the modulation amplitude, f m denotes the modulation frequency. The intensity of the fluorescent light v FL corresponds to the modulation amplitude f d and modulation frequency f m and modulated appropriately.

[0044] Figure 2b shows the signal v generated by a lock-in amplifier-type demodulator from the detected fluorescence light. LIA , depending on the frequency f. As can be seen in FIG. 2b, the demodulated signal v LIA is a distributed type, and each resonant frequency f±,0 The slope of the linear region, α, is the slope of the demodulated signal v LIA If the resonant frequency changes due to magnetic field, temperature, and / or pressure changes, the demodulated signal v LIA Based on the resonant frequency f ±,0 We can determine the resonance deviation (detuning) Δ from v LIA / α is applied.

[0045] The measurement principle described above can be used to simultaneously excite both magnetic resonances of the NV center, the magnetic field gradiometer 1 for which is described below in connection with FIG.

[0046] The magnetic field gradiometer 1 comprises a first diamond crystal 2a and a second diamond crystal 2b, each of which has a measurement area 3a, 3b in which the diamond crystal 2a, 2b is doped with NV centers 4a, 4b. The magnetic field gradiometer 1 also comprises two excitation light sources 5a, 5b, each of which is used to emit excitation light 6a, 6b in the green wavelength range. When the NV centers 4a, 4b are excited in the respective measurement areas 3a, 3b, fluorescent light 7a, 7b is generated in the red wavelength range, which is detected by respective detectors 8a, 8b in the form of photodiodes.

[0047] The magnetic field B in each measurement area 3a, 3b a ,B b To determine the strength of each magnetic field B, each diamond crystal 2a, 2b is exposed to a respective microwave field 9a, 9b generated by an associated microwave emitter 10a, 10b in addition to being irradiated with an excitation light 6a, 6b. a ,B b The signal generator 12 generates a first microwave signal f for the first microwave radiator 10a. MW1 (t) and a second microwave signal f for the second microwave radiating unit 10b. MW2(t) is used to generate

[0048] The first microwave signal f MW1 To generate the first frequency modulated signal f(t), the signal generator 12 includes a first frequency mixer 13a, which mixes a first frequency modulated signal f SB1 (t) and the first oscillator signal f MW1 The first frequency modulation signal f SB1 To generate (t), the signal generator 12 includes a first signal generator 14a. MW To generate 1, the signal generator 12 has a first oscillator 15a.

[0049] The second microwave signal f MW2 To form the second frequency modulated signal f(t), the signal generator 12 includes a second frequency mixer 13b, which mixes the second frequency modulated signal f SB2 (t) and the second oscillator signal f MW2 The second frequency modulated signal f SB2 To generate (t), the signal generator 12 includes a second signal generator 14b. In addition, a second oscillator signal f MW2 To generate the signal, the signal generator 12 has a second oscillator 15b.

[0050] The first signal generator 14a generates a carrier frequency f SB1 , modulation frequency f m1 , and the modulation amplitude f d1 A first frequency modulated signal f SB1 (t) is formed to occur in the following equation: f SB1 (t)=f SB1 +f d1 cos(2πf m1 t)

[0051] Correspondingly, the second signal generator 14b generates a carrier frequency f SB2 , modulation frequency f m2 , and the modulation amplitude f d2A second frequency modulated signal f SB2 (t) is formed to occur in the equation shown below. f SB2 (t)=f SB2 +f d2 cos(2πf m2 t)

[0052] The first oscillator 15a generates a constant first oscillation frequency (simply f MW1 A first oscillator signal f MW1 Correspondingly, the second oscillator 15b generates a constant second oscillation frequency (simply f MW2 A second oscillator signal f MW2 Generate.

[0053] As described above, the first frequency mixer 13a outputs the first frequency modulated signal f SB1 (t) and the first oscillator signal f MW1 A first microwave signal f MW1 After frequency mixing in the first frequency mixer 13a, the first microwave signal f MW1 (t) has the following frequency components: f MW1 +f SB1 +f d1 cos(2πf m1 t) and f MW1 -f SB1 +f d1 cos(2πf m1 t+π)

[0054] By frequency mixing, the first microwave signal f MW1 (t) two frequency components f d1 cos(2πf m1 t) and f d1 cos(2πf m1 There is a phase difference of π or 180° between the time t and the time t+π.

[0055] The above is converted by the second frequency mixer 13b into a second frequency modulated signal f SB2(t) and the second oscillator signal f MW2 A second microwave signal f MW2 The same applies to 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), and the two frequency components have a phase difference of π or 180° with respect to each other.

[0056] First frequency modulated signal f SB1 (t) carrier frequency f SB1 and the first oscillator signal f MW1 Oscillation frequency f MW1 is the magnetic field B in the first measurement area 3a a For a given magnetic field strength of m, two frequency components with a phase offset of π are generated, as shown in FIG. S = 0 state and m S =+1 state or m S The two resonant frequencies f = -1 and f = -2 are + ,f - Correspondingly, the second frequency modulated signal f SB2 (t) carrier frequency f SB2 and a second oscillator signal f MW2 Oscillation frequency f MW2 m of NV center 4b S = 0 state and m S =+1 state or m S The two resonant frequencies f = -1 and f = -2 are + ,f - is selected so that it is corresponded in the second measurement area 3b.

[0057] In the magnetic field gradiometer 1 in Fig. 3, the magnetic field B a is measured in the first measurement area 3a. The magnetic field B at the position of the second measurement area 3b b is used to measure the ambient magnetic field. a -B bBy forming a difference between the first magnetic field Ba in the first measurement area 3a and the second magnetic field Bb in the second measurement area 3b, the noise of the ambient magnetic field can be suppressed. a ,B b Since the magnetic field strengths of the first and second frequency modulated signals f are different, typically SB1 (t),f SB2 (t) is the different carrier frequency f SB1 ,f SB2 or a first oscillator signal f MW1 and a second oscillator signal f MW2 Different oscillation frequencies f MW1 ,f MW2 It is necessary to have

[0058] As can be seen from FIG. 3, the evaluation device 11 determines from the fluorescent light detected by means of the first detector 8a or from a corresponding fluorescent signal v FL1 (t) from the first demodulated double resonance signal v LIA1 DR Correspondingly, the evaluation device 11 comprises a first demodulator 16a for forming a corresponding fluorescence signal v of the second measurement area 3b from the fluorescence light detected by means of the second detector 8b. FL2 (t) from the second demodulated double resonance signal v LIA2 DR The two demodulators 16a and 16b are formed as lock-in amplifiers and each include a frequency mixer 17a, 17b and a low-pass filter 18a, 18b. The first frequency mixer 17a receives a first microwave signal f MW1 Frequency component f of (t) d1 cos(2πf m1 t) is supplied to the second frequency mixer 17b. In response to this, the second microwave signal f MW2 Frequency component f of (t) d2 cos(2πf m2 t) is provided. The respective first or second double resonance signal vL IA1 DR , or vLIA2 DR has the distributed form known from FIG. 2b.

[0059] FIG. 5 shows the demodulated double resonance signal v LIA DR to illustrate the effect of magnetic field, temperature, and pressure changes on the demodulated double resonance signal v LIA DR is, for example, the first or second demodulated double resonance signal v in FIG. LIA1 DR , or v LIA2 DR The demodulated double resonance signal v LIA DR To understand the effect of changes in the above quantities on the respective demodulated single resonance signals v LIA SR is shown in the left and center diagrams of Figure 5. The relative modulation phase of π or 180° between the two frequency components of the microwave magnetic field results in m S =+1 state, or m S = A single resonance signal of two magnetic resonances in the -1 state v LIA SR A vertical mirroring of the

[0060] As can be seen in the lower left diagram of Figure 5, the resonance shift Δ B is the single resonance signal v LIA SR This leads to a shift in the resonance signal, resulting in a constructive addition (see the lower left point in Figure 5). On the other hand, the resonance shift Δ T (or Δ P , not shown) is a single resonance signal v LIA SR (See the lower dot in the center diagram of Figure 5.) As shown in the right diagram of Figure 5, the demodulated double resonance signal v LIA DR is the magnetic field B in the first measurement area 3a or the second measurement area 3b. a ,B b Magnetic field-induced resonance shift Δ BAs can be seen in the right diagram of Figure 5, the demodulated double resonance signal v LIA DR is Δ B Correspondingly, the demodulated double resonance signal v LIA DR Based on this, the magnetic field-dependent resonance shift Δ B can be calculated as follows:

[0061] The above discussion is based on the first demodulated double resonance signal v of the magnetic field gradiometer 1 shown in FIG. LIA1 DR

number

[0062] First magnetic field B in evaluation device 11 a To determine the resonant frequency f +,0 and f -,0 In order to determine the single resonance spectrum of the two magnetic resonances (see FIG. 2a or FIG. 4), the first demodulated double resonance signal v LIA1 DR is obtained in the above manner during calibration, and the scalar coefficient α or Δ B Then, the slope of the first frequency modulated signal f SB1 (t) carrier frequency f SB1 , and the first oscillator signal f MW1 Oscillation frequency f MW1 Select or set the carrier frequency f SB1 and oscillation frequency f MW1 , the magnetic field B aFor a magnetic field strength of m, there are two frequency components with a phase offset of π. S = 0 state and m S =+1 state, or m S The two resonant frequencies of the transition between the -1 state and the -2 state are f = +,0 and f -,0 (See also FIG. 4) to correspond to the above.

[0063] After calibration, the first magnetic field B a is continuously determined using the following formula:

number

[0064] Second magnetic field B b The first or second demodulated double resonance signal v LIA1 DR Or v LIA2 DR From each magnetic field B a ,B b To calculate , the evaluation device 11 comprises a respective calculation logic 19a, 19b formed in the form of suitable hardware and / or software.

[0065] Fig. 6 shows a magnetic field gradiometer 1 based on the same measurement principle as the magnetic field gradiometer 1 shown in Fig. 3. The magnetic field gradiometer 1 shown in Fig. 6 differs from the magnetic field gradiometer shown in Fig. 3 essentially in the configuration of a signal generator 12 used to individually adjust the output of the frequency bands corresponding to the respective magnetic resonances. The signal generator 12 generates a first microwave signal f MW1 A first frequency modulated microwave signal f having two frequency components that are out of phase with each other by π to form (t) MW1- (t) and a further first frequency modulated microwave signal f having two frequency components that are out of phase with each other by π. MW1+ (t) and a first output summer 20a for summing the second frequency modulated microwave signal f MW2A second frequency modulated microwave signal f having two frequency components that are out of phase with each other by π to form (t) MW2- (t) and a second frequency modulated microwave signal f having two frequency components that are out of phase with each other by π. MW2+ (t) and a second output adder 20b for adding (t).

[0066] The signal generator 12 generates a first frequency modulated microwave signal f MW1- To form (t), a frequency modulated signal f SB (t) and the first oscillator signal f MW1- Correspondingly, the signal generator 12 has a first frequency mixer 13a for frequency mixing a further first frequency modulated microwave signal f MW1+ To form (t), a frequency modulated signal f SB (t) and a further first oscillator signal f MW1+ and a further first frequency mixer 13a' for frequency mixing the first and second frequencies.

[0067] The signal generator 12 further generates a second frequency modulated microwave signal f MW2- To form (t), a frequency modulated signal f SB (t) and the second oscillator signal f MW1- The signal generator 12 includes a second frequency mixer 13b for frequency mixing a further second frequency modulated microwave signal f MW2+ (t), the further second frequency mixer 13b' comprising a frequency modulated signal f SB (t) and a further second oscillator signal f MW2+ It is used to mix the frequencies.

[0068] Frequency modulated signal f SB (t) is generated in the signal generator 14 of the signal generating device 12. The first oscillator signal f MW1- and a further first oscillator signal f MW1+ is generated in the first oscillator 15a or in the further first oscillator 15a′. Correspondingly, the second oscillator signal f MW2-and a further second oscillator signal f MW2+ is generated in the second oscillator 15b or in a further second oscillator 15b'.

[0069] The signal generator 12 generates a first frequency modulated microwave signal f MW1- (t) and a further first frequency modulated microwave signal f MW1+ The signal generator 12 is also configured to set the outputs of the second frequency modulated microwave signal f MW2- (t) and a further second frequency modulated microwave signal f MW2+ The outputs of (t) are configured to be set independently of each other.

[0070] For this purpose, the frequency modulated signal f SB The output of (t) is first split into four equal shares in the output splitter 21. The frequency modulated signal f SB The power contributions of (t) are individually set using programmable attenuators 22. Alternatively or additionally, suitable oscillator signals f MW1- ,f MW1+ ,f MW2- ,f MW2+ It is also possible to form four oscillators 15a, 15a', 15b, 15b' so as to generate:

[0071] FIG. 7 shows the first microwave signal f generated by the signal generator 12 of FIG. MW1 7, the frequency components of the first microwave signal f(t) are shown in a representation similar to that of FIG. MW1 (t) has two sets, each set having two frequency components that are out of phase with each other by π. The output of each frequency band, or the first frequency modulated microwave signal f MW1- (t) and a second frequency modulated microwave signal f MW1+ By being able to set the output of (t) independently, it is possible to minimize the difference in the scalar coefficient α of each single resonance spectrum, resulting in a double resonance signal vLIA1 DR The above considerations can be applied to maximize the suppression of temperature and pressure changes in the second microwave signal f MW2 It is understood that the same applies to (t).

[0072] 8 shows a magnetic field gradiometer 1 which differs from the magnetic field gradiometers shown in Figs. 3 and 6 in the configuration of the signal generator 12 and the evaluation device 11. In the magnetic field gradiometer 1 shown in Fig. 8, the signal generator 12 generates two first frequency modulated microwave signals f MW1+ (t),f MW1- (t) for the first microwave radiating unit 10a. MW1 Correspondingly, the signal generator 12 includes a first output summer 20a for forming two second frequency modulated microwave signals f MW2+ (t),f MW2- (t) to obtain a second microwave signal f MW2 and a second output summer 20b for forming a first frequency modulated microwave signal f MW1+ (t),f MW1- (t) is generated in the respective first or further first signal generator 14a, 14a'. Correspondingly, a second frequency modulated microwave signal f MW2+ (t),f MW2- (t) is generated in a respective second or further second signal generator 14b, 14b'.

[0073] The following equation expresses the first frequency modulated microwave signal f MW1+ (t),f MW1- (t) applies. f MW1± (t)=f MW1± +f d cos(2πf m1± t) where f MW1± are the respective carrier frequencies, f m1± are the respective modulation frequencies, and f ddenotes the modulation amplitude. The two first frequency modulated microwave signals f MW1+ (t),f MW1- (t) carrier frequency f MW1± are different from each other in order to excite one of the two magnetic resonances of the NV center 4a in the first measurement region 3a.

[0074] Therefore, the fluorescence signal V detected by the first detector 8a of the magnetic field gradiometer 1 fl1 (t) is f m1± The evaluation device 11 measures the fluorescence signal V detected from the first measurement area 3a. fl1 The first output distributor 23a distributes the output of the fluorescent signal V(t) detected from the second measurement area 3b to the first pair of demodulators 16a, 16a'. Correspondingly, the second output distributor 23b distributes the fluorescent signal V(t) detected from the second measurement area 3b to the first pair of demodulators 16a, 16a'. fl2 (t) to a second pair of demodulators 16b, 16b'. In each demodulator 16a, 16a', 16b, 16b', a demodulated single resonance signal v LIA1+ ,v LIA1- ,v LIA2+ ,v LIA2- is generated, which can be further analyzed by the methods described above in relation to FIG. 2b.

[0075] The first magnetic field B in the first measurement area 3a a For , the following relationship is obtained:

number

[0076] In summary, the above magnetic field gradiometer 1 can be used to measure the magnetic field B in a manner independent of temperature and pressure changes without limiting the bandwidth of the magnetic field gradiometer 1. a ,B b of, or magnetic field gradient B a -B b can be determined, thereby utilizing the full dynamic range of the magnetic field gradiometer 1. Also, as a rule, there is no need to heat or cool the respective diamond crystals 2a, 2b in order to stabilize their temperature. [Explanation of symbols]

[0077] 1. Magnetic field gradiometer 2a The first diamond crystal 2b Second diamond crystal 3a First measurement area 3b Second measurement area 4a,4b NV center 5a,5b Excitation light source 6a, 6b Excitation light 7a,7b Fluorescent light 8a First detector 8b Second detector 9a First microwave field 9b Second microwave field 10a First microwave radiating section 10b Second microwave radiating section 11 Evaluation equipment 12 Signal Generator 13a First frequency mixer 13a' further first frequency mixer 13b Second Frequency Mixer 13b' Further second frequency mixer 14a First signal generator 14b Second signal generator 15a First Oscillator 15a' Further first oscillator 16a First demodulator 16a, 16a' First pair of demodulators 16b Second Demodulator 16b,16b' Second pair of demodulators 20a First output adder 20b Second output adder 21 Output distributor 22 Attenuator 23a First output distributor 23b Second output distributor B a First magnetic field B b Second magnetic field B a -B b Magnetic field gradient f MW Carrier Frequency f MW1 First oscillator signal, oscillation frequency f MW1- First Oscillator Signal f MW1+ Further first oscillator signals f MW2 Second oscillator signal, oscillation frequency f MW1 (t) First microwave signal f MW1- (t) a first frequency modulated microwave signal f MW1+ (t) a further first frequency modulated microwave signal. f MW2 (t) Second microwave signal f MW2- (t) a second frequency modulated microwave signal f MW2+ (t) a further second frequency modulated microwave signal f SB1 ,f SB2 Carrier Frequency f SB1 (t) First frequency modulated signal f SB2 (t) Second frequency modulated signal v FL1 (t), v FL2 (t) Fluorescence signal v LIA1 DR First demodulated double resonance signal v LIA2 DR Second demodulated double resonance signal v LIA1 SR First demodulated single resonance signal v LIA2 SR Second demodulated single resonance signal Δ B Magnetic field dependent resonance deviation

Claims

1. Magnetic field gradient (B ａ -B ｂ A magnetic field gradiometer (1) for determining at least one excitation light source (5a, 5b) for emitting excitation light (6a, 6b); two spatially spaced measurement areas (3a, 3b) for measuring magnetic fields, each having colour centres in diamond, preferably NV centres (4a, 4b) which emit fluorescent light (7a, 7b) when excited by excitation light (6a, 6b); a first detector (8a) for detecting fluorescent light (7a) from the first measurement area (3a); a second detector (8b) for detecting the fluorescent light (7b) from the second measurement area (3b); a first microwave radiator (10a) for applying a first microwave field (9a) to the first measurement area (3a); a second microwave radiator (10b) for applying a second microwave field (9b) to the second measurement area (3b); Based on the fluorescent light (7a) detected from the first measurement area (3a) and the fluorescent light (7b) detected from the second measurement area (3b), the magnetic field gradient (B ａ -B ｂ an evaluation device (11) configured to determine In a magnetic field gradiometer (1) comprising: A first microwave signal (f) having at least two frequency components phase-offset from each other by π for the first microwave radiating portion (10a). ＭＷ１ (t)) and a second microwave signal (f) having at least two frequency components phase-offset from each other by π for the second microwave radiating portion (10b). ＭＷ２ (t)) and a signal generating device (12) configured to generate Magnetic field gradiometer (1).

2. The signal generating device (12) generates the first microwave signal (f ＭＷ１ The first frequency mixer (13a) mixes a first frequency modulated signal (f ＳＢ１ (t)) and the first oscillator signal (f ＭＷ１ ) and frequency mixing, The signal generating device (12) generates the second microwave signal (f ＭＷ２ The second frequency mixer (13b) mixes the second frequency modulated signal (f ＳＢ２ (t)) and a second oscillator signal (f ＭＷ２ 2. The magnetic field gradiometer (1) according to claim 1, which is configured to frequency mix a first frequency component and a second frequency component.

3. The first and second frequency modulated signals (f ＳＢ１ (t), f ＳＢ２ (t)) are different carrier frequencies (f ＳＢ１ , f ＳＢ２ ) and / or The first oscillator signal (f ＭＷ１ ) and the second oscillator signal (f ＭＷ２ ) have different oscillation frequencies, A magnetic field gradiometer (1) according to claim 2.

4. The signal generating device (12) generates the first microwave signal (f ＭＷ１ A first frequency modulated microwave signal (f) having two frequency components phase offset from each other by π to form a ＭＷ１－ (t)) and a further first frequency modulated microwave signal (f ＭＷ１＋ (t)) and a first output adder (20a) for adding The signal generating device (12) generates the second microwave signal (f ＭＷ２ A second frequency modulated microwave signal (f(t)) having two frequency components phase offset from each other by π. ＭＷ２－ (t)) and a further second frequency modulated microwave signal (f ＭＷ２＋ (t)) and a second output adder (20b) for adding A magnetic field gradiometer (1) according to claim 1.

5. The signal generator (12) generates the first frequency modulated microwave signal (f ＭＷ１－ (t)) and the further first frequency modulated microwave signal (f ＭＷ１＋ (t)) independently of one another; and The signal generator (12) generates the second frequency modulated microwave signal (f ＭＷ２－ (t)) and the further second frequency modulated microwave signal (f ＭＷ２＋ (t)) independently of one another; A magnetic field gradiometer (1) according to claim 4.

6. The signal generating device (12) The first frequency modulated microwave signal (f ＭＷ１－ To form a frequency modulated signal (f ＳＢ (t)) and the first oscillator signal (f ＭＷ１－ a first frequency mixer (13a) for frequency mixing the first and second frequencies; The further first frequency modulated microwave signal (f ＭＷ１＋ To form a frequency modulated signal (f ＳＢ (t)) and a further first oscillator signal (f ＭＷ１＋ and a further first frequency mixer (13a') for frequency mixing the first and second inputs, A magnetic field gradiometer (1) according to claim 4 or 5.

7. The signal generator (12) generates the frequency modulated signal (f ＳＢ 7. The magnetic field gradiometer (1) according to claim 6, comprising at least one programmable attenuator (22) for adjustable attenuation of the respective power contribution of (t).

8. The signal generating device (12) The first oscillator signal (f ＭＷ１－ a first oscillator (15a), preferably with adjustable output, for generating a The further first oscillator signal (f ＭＷ１＋ a further first oscillator (15a'), preferably with adjustable output, for generating A magnetic field gradiometer (1) according to claim 6 or 7, comprising:

9. The evaluation device (11) The detected fluorescent light (v ＦＬ１ (t)) to obtain a first demodulated double resonance signal (v ＬＩＡ１ ＤＲ a first demodulator (16a) for forming a The detected fluorescent light (v ＦＬ２ (t)) to obtain a second demodulated double resonance signal (v ＬＩＡ２ ＤＲ a second demodulator (16b) for forming a A magnetic field gradiometer (1) according to any one of the preceding claims, comprising:

10. 10. The magnetic field gradiometer (1) according to claim 9, wherein the two demodulators (16a, b) are configured as lock-in amplifiers.

11. The evaluation device (11) The first demodulated double resonance signal (v ＬＩＡ１ ＤＲ ) based on the first magnetic field (B ａ ) magnetic field dependent resonance deviation (Δ Ｂ ) and The second demodulated double resonance signal (v ＬＩＡ２ ＤＲ ) based on the second magnetic field (B ｂ ) magnetic field dependent resonance deviation (Δ Ｂ ) is formed to determine A magnetic field gradiometer (1) according to claim 9 or 10.

12. A magnetic field gradiometer (1) according to the preamble of claim 1, The signal generator (12) generates two first frequency modulated microwave signals (f ＭＷ１＋ (t), f ＭＷ１－ (t)) to obtain a first microwave signal (f ＭＷ１ a first output summer (20a) for forming a sum of the first and second inputs (t), The signal generator (12) generates two second frequency modulated microwave signals (f ＭＷ２＋ (t), f ＭＷ２－ (t)) for the second microwave radiating portion (10b) to obtain a second microwave signal (f ＭＷ２ a second output summer (20b) for forming The evaluation device (11) The fluorescent light (v ＦＬ１ a first output distributor (23a) for distributing the output of the demodulator (t) to a first pair of demodulators (16a, 16a'); The fluorescent light (v ＦＬ２ a second output distributor (23b) for distributing the output of the demodulator (t) to a second pair of demodulators (16b, 16b'); Characterized in that it has Magnetic field gradiometer (1).

13. The signal generating device (12) The two first frequency modulated microwave signals (f ＭＷ１＋ (t), f ＭＷ１－ (t)), and The two second frequency modulated microwave signals (f ＭＷ２＋ (t), f ＭＷ２－ (t) A magnetic field gradiometer (1) according to claim 12.

Images