Method for measuring magnetic fields and magnetic field measuring device

Abstract

The invention relates to a method for measuring magnetic fields using a magnetic field measuring unit with at least one nitrogen-vacancy center magnetometer unit, comprising: driving (400) the magnetic field measuring unit for a reference measurement (402) with pulsed operation to acquire magnetic signals by means of the magnetic field measuring unit and to obtain a reference data set (404); driving (410) the magnetic field measuring unit for a target measurement (412) with continuous operation to acquire the magnetic signals by means of the magnetic field measuring unit and to obtain a measurement data set (414); determining (420), based on the reference data set (404), of interference signals in the magnetic signals; and at least partially removing (430) the interference signals from the measurement data set.

Description

[0001] The present invention relates to a method for a magnetic field measuring device, a computing unit and a computer program for carrying out the method, and a magnetic field measuring device. Background of the invention

[0002] Optically pumped quantum sensors or those based on NV centers in diamond are particularly suitable as sensors for measuring very small magnetic field strengths. DE 10 2022 204 526.2 describes a magnetometer that uses optically pumped and optically detected magnetic resonance (ODMR). This method exploits the fact that the energy levels of certain spin states of unpaired electrons split under the influence of an external magnetic field, the so-called Zeeman effect. The splitting of the energy levels results in altered transitions during relaxation from excited states, which can then be measured, for example, by optical excitation and frequency-dependent detection of the resulting fluorescence radiation or by observing optical properties such as light absorption. The measured optical parameters can then be used to determine the magnetic field strength. Disclosure of the invention

[0003] According to the invention, a method for a magnetic field measuring device, a computing unit and a computer program for implementing the method, as well as a magnetic field measuring device with the features of the independent patent claims are proposed. Advantageous embodiments are the subject of the dependent claims and the following description.

[0004] The invention relates to magnetic field measurement using a magnetic field measuring unit with at least one nitrogen vacancy center magnetometer unit (also referred to as an NV magnetometer unit). As mentioned, such a magnetic field measuring unit is described by way of example in DE 10 2022 204 526.2.

[0005] Two different types of measurement protocols or two different ways of controlling or operating the magnetic field measuring unit are considered. In so-called continuous measurement or continuous operation (CW, continuous wave), microwave and laser radiation are irradiated at a constant intensity throughout the entire measurement. In so-called pulsed measurements or pulsed operation, however, the laser and microwave radiation are coordinated and precisely switched on and off, resulting in the suppression of decoherence of the electron spins under consideration.

[0006] While pulsed measurements can thus exhibit higher sensitivities than continuous measurements, the bandwidth, i.e., the frequency range, of the detectable magnetic signals is limited by the so-called dephasing time T2*. As a result, measurements of quasi-static magnetic field changes, in particular, do not benefit from pulsed measurement mode.

[0007] If magnetic field measurements are carried out in an inhabited environment - typical magnetic field measurements are also carried out in hospitals or doctor's offices - periodic magnetic fields, e.g. from alternating current lines, represent a significant source of noise, i.e. they generate interference signals that are superimposed on the magnetic signals actually being measured. These interference signals usually comprise mixtures (linear combinations) of monofrequency tones, which in turn have a fixed or slowly changing amplitude ratio to one another. The total level of this noise, however, depends, for example, on the distance of the magnetic field measuring unit (sensor) from a power line or other source of interference signals and is therefore variable and unknown prior to the measurement. To denoise or eliminate interference from the measured data, filters can be used after the measurement; their parameters can be optimized for the expected noise amplitudes.

[0008] Magnetic field measurements in unshielded environments have been shown to regularly exhibit mono- or oligofrequency interference in the fundamental frequency and harmonics. These can be removed using filtering techniques, such as notch filters. A notch filter is an electronic filter that can filter out frequencies within a narrow frequency range. A notch is inserted into the frequency diagram to illustrate this.

[0009] The parameters of such filters, such as noise suppression and the suppressed frequency interval, can be specified in advance or based on a measurement in that frequency range. In both cases, the filtering is not optimized for the specific measurement under consideration (i.e., the target measurement) of an unknown signal; that is, either the interference signals are not completely removed, or parts of the useful signal are also filtered out.

[0010] The invention now proposes performing a reference measurement with pulsed operation of the magnetic field measuring unit in order to detect magnetic signals using the magnetic field measuring unit and obtain a reference data set, and also performing a target measurement with continuous operation of the magnetic field measuring unit in order to detect the magnetic signals using the magnetic field measuring unit and obtain a measurement data set. The magnetic field measuring unit can be controlled accordingly for this purpose.

[0011] Based on the reference data set, interference signals in the magnetic signals are then determined, which are then at least partially removed from the measurement data set.

[0012] This enables optimized denoising of those interference signals (noise) which are known or at least assumed to be present in constant proportions in fundamental and harmonic frequencies, e.g., by the interference signal originating from a single source. Pulsed and continuous magnetic field measurements are combined in a novel way. By assuming, as a rule, that an interference signal is present in fundamental and harmonic frequencies, i.e., a specific fundamental frequency and multiples thereof, an interference frequency (then in a harmonic) can be identified in the generally higher-frequency reference frequency range of the reference measurement, and a conclusion can be drawn about a usually lower interference frequency (then in a lower harmonic and / or the fundamental frequency) in the generally lower-frequency (but wider) target frequency range of the target measurement. This can then be specifically removed, e.g.,through a filter like the one mentioned.

[0013] For this purpose, in one embodiment, one or more reference interference frequencies can be determined in the reference data set, and based thereon, one or more target interference frequencies can then be determined in the measurement data set such that the one or at least one of the plurality of reference interference frequencies corresponds to the one or at least one of the plurality of reference interference frequencies or a multiple thereof.

[0014] The particular advantage of this is that the reference measurement with pulsed operation takes significantly less time than a reference measurement with continuous operation. This allows the aforementioned advantages of pulsed magnetic field measurements with NV centers in diamond to be exploited.

[0015] It is initially irrelevant whether the reference measurement or the target measurement is taken first. If the reference measurement is taken first, the interference signals can be removed immediately after, or possibly even directly during, the reference measurement. This ensures that the corrected measurement is available as quickly as possible after it has been performed. If, however, the target measurement is taken first, it can be corrected subsequently or during post-processing once the reference measurement is available.

[0016] A computing unit according to the invention, e.g. a control and / or signal processing unit for a magnetic field measuring unit, is configured, in particular in terms of programming, to carry out a method according to the invention.

[0017] A magnetic field measuring device according to the invention comprises a magnetic field measuring unit with at least one nitrogen vacancy center magnetometer unit, as well as a computing unit according to the invention.

[0018] The implementation of a method according to the invention in the form of a computer program or computer program product with program code for carrying out all method steps is also advantageous, since this entails particularly low costs, in particular if an executing control unit is also used for other tasks and is therefore already present. Finally, a machine-readable storage medium is provided with a computer program stored thereon, as described above. Suitable storage media or data carriers for providing the computer program are, in particular, magnetic, optical and electrical memories, such as hard disks, flash memories, EEPROMs, DVDs, and others. Downloading a program via computer networks (Internet, intranet, etc.) is also possible. Such a download can be wired or cable-based or wireless (e.g. via a WLAN network, a 3G, 4G, 5G or 6G connection, etc.).

[0019] Further advantages and embodiments of the invention will become apparent from the description and the accompanying drawings.

[0020] The invention is illustrated schematically in the drawing using an embodiment and is described below with reference to the drawing. Short description of the drawings Fig. 1 shows in a schematic block view the essential components of a magnetic field measuring unit with an NV magnetometer unit, as can be used within the scope of the invention. Fig. 2 shows a schematic diagram of a magnetic field measuring device as it can be used in the context of the invention. Fig. 3 shows a frequency diagram to explain the invention in one embodiment. Fig. 4 shows schematically a method according to the invention in one embodiment. Embodiment(s) of the invention

[0021] Fig. 1 schematically shows the essential components of a magnetic field measuring unit 100 with an NV magnetometer unit according to one embodiment. Initially, a diamond 110 with nitrogen vacancies (NV) is present as the sensor medium. The optical excitation of the NV centers can be achieved by a suitable light source 120, such as a pump laser. For example, a frequency-doubled Nd:YAG laser or a semiconductor laser in the green range of approximately 510-532 nm is suitable, e.g., at 532 nm for off-resonance excitation. Alternatively, LEDs in suitable wavelength ranges can also be used. Depending on the arrangement, the light from the light source 120 can be irradiated into the diamond 110 via suitable optical elements 122, such as mirrors, beam splitters, focusing optics such as lenses, and optionally via fiber optic elements.

[0022] In addition, the excitation light can be irradiated continuously or in pulsed mode, so that, for example, time windows are kept free for interference-free fluorescence light measurement. As mentioned, both variants, continuous and pulsed operation, are used within the scope of the present invention.

[0023] Furthermore, the magnetometer can comprise a microwave source 150 capable of generating an electromagnetic field in the sensor medium over a bandwidth covering the desired resonance frequency, i.e., in the region of the NV centers of the diamond 110. A microwave resonator structure can be used to homogeneously distribute the generated microwaves across the volume of the measurement area in the diamond. The resonator structure or the microwave source 150 is preferably tuned to the frequency of the electron spin resonances. To enable vector magnetometry, an additional bias magnetic field is generated by means of a device 140. This makes the measurement intrinsically vectorial. For this purpose, various spatial directions in the crystal structure are used.A Helmholtz coil, for example, is suitable for generating such a magnetic field. A pair of coils can generate a substantially homogeneous magnetic field within a limited area. Other devices 140 can also be used, such as a simple coil, an elongated coil, permanent magnet solutions such as those in a Hallbach array, etc.

[0024] The resulting fluorescent light 112 from the diamond 110 can in turn be guided via suitable optical elements 134, such as optical filters, beam splitters, lenses, and / or fiber optic elements, to a first photodetector 130, which is sensitive at least in the range of the fluorescence wavelength. The first photodetector 130 can also be arranged directly on the diamond 110. A second photodetector 132 is arranged such that it can detect at least a portion of the excitation light from the light source 120, which can be coupled out, for example, by a beam splitter, a filter, or a partially transparent element. This detector signal 132 of the excitation light can be used as a reference signal, for example, to eliminate background signals and to emphasize the resonance signal of interest by modulating the excitation light using a lock-in amplifier.Additionally or alternatively, this reference signal can be used to account for fluctuations in the excitation light. Appropriate circuits 160, such as a preamplifier, a logarithmic amplifier, a lock-in amplifier, signal filters, or others, are thus provided to receive the signals from the first and second photodetectors and preprocess the signals appropriately for further evaluation. Finally, the preprocessed fluorescence signal can be evaluated by a signal processing unit 170, e.g., using a suitable microcontroller or processor, to obtain the desired parameters of the detected magnetic field from the signal, in particular the magnetic field strength and the direction of the magnetic field.

[0025] It is understood that such a device may also comprise additional units not shown, such as communication units or interfaces for outputting the measurement results. Such a device may also advantageously be integrated into an ASIC or FPGA.

[0026] Such a magnetic field measurement unit with an NV magnetometer unit offers a variety of advantages for this application. In addition to the aforementioned very high sensitivity, a wide measurement range (> 1 Tesla) can also be covered. The underlying Zeeman effect is linearly dependent on the existing magnetic field and, moreover, exhibits no degradation because the measurement is based on quantum mechanical states. Furthermore, NV magnetometer units offer the possibility of determining external magnetic fields vectorially based on the various orientations present in the diamond lattice.

[0027] There are also alternative ways to electrically read magnetic spin resonance in diamond. This involves detecting charge carriers that have been lifted into the diamond's conduction band by two-photon ionization of the NV centers. If such a method is used to read resonance effects, the fluorescent light detection components in the previous examples are not required and are replaced by suitable photocurrent detectors on the diamond. Apart from that, the method for magnetic field measurement can be adapted accordingly and applied in all embodiments with NV magnetometer units.

[0028] In order to be usable in an everyday environment, magnetic fields that do not originate from desired weak sources - i.e. interference signals - should be eliminated from the measurement as far as possible, e.g. magnetic signals or fields caused by alternating voltage lines.

[0029] In Fig. 2 schematically shows a magnetic field measuring device 200, as it can be used within the scope of the invention. The magnetic field measuring device 200 comprises a magnetic field measuring unit 210 with at least one nitrogen vacancy center magnetometer unit, as well as a computing unit 220. The magnetic field measuring unit 210 can be, for example, the magnetic field measuring unit 100 according to Fig. 1 or another variant. The computing unit 220 can be a control unit for the magnetic field measuring unit 100. Likewise, the computing unit 220 can, for example, perform the function of the signal processing unit 170 according to Fig. 1 include.

[0030] A magnetic field measurement can or should now be performed using the magnetic field measuring device 200 or the magnetic field measuring unit 210, for which purpose magnetic signals are detected by the magnetic field measuring unit 210. The magnetic signals then include, for example, those of a magnetic field 232 to be measured from, for example, a magnetic field source 230. However, there may now be interference sources such as an AC line 240 that generate interference signals or an interference magnetic field 242 that overlay the magnetic field 232 to be measured.

[0031] In Fig. Figure 3 is a schematic frequency diagram illustrating one embodiment of the invention. It shows the amplitude (in arbitrary units) of a magnetic field plotted against a frequency f in Hz.

[0032] The frequency diagram shown here includes magnetic signals with a certain background or noise floor 330 as well as signals with higher amplitude (peaks) at certain frequencies.

[0033] For example, the frequency range 310 Hz, in this example from 0 to 500 Hz, can now be recorded as part of a target measurement with continuous operation of the magnetic field measuring unit. However, any interference signals should be removed or calculated out.

[0034] These interference signals are often known to be present in the frequency spectrum at the fundamental frequency and integer multiples thereof (harmonics). In the example of the AC line, a fundamental frequency can be 50 Hz, for example, in Fig. 3 is labeled with the interference signal 340. The integer multiples of this can also be seen (at 100 Hz, 150 Hz, etc.), but not all are labeled.

[0035] These harmonics partially extend into the frequency range for which higher sensitivities are achieved with pulsed measurements. A frequency range that can be measured with pulsed operation is designated as 320, for example, and covers 1100 Hz to 1250 Hz. It should be noted that the background noise here is lower than with a measurement using continuous operation.

[0036] It is often also known from previous measurements that all frequency components of the interference signal are correlated. For example, if the magnetic field measuring unit moves away from the power line or other interference source, the local maxima, i.e., the peaks, of the frequency spectrum decrease with the same ratios to each other.

[0037] From a pulsed reference measurement in the 320 Hz frequency range (reference frequency range), it is now possible to infer the interference in the target measurement in the 310 Hz frequency range (target frequency range). For example, the depth and frequency width of a notch filter at 50 Hz (here, the fundamental frequency) can be determined using pulsed measurements in the 1200 Hz range (here, a harmonic or an integer multiple). Compared to the actual measurement of the quasi-static frequencies, the pulsed measurement takes only a fraction of the time. Only a slight time delay occurs.

[0038] In Fig. 4 schematically illustrates a method according to the invention in one embodiment. For this purpose, in a step 400, the magnetic field measuring unit is controlled for a reference measurement 402, specifically with a pulsed operation, in order to detect magnetic signals by means of the magnetic field measuring unit and to obtain a reference data set 404. The reference data set 404 is, for example, the magnetic signals in the frequency range 320 according to Fig. 3.

[0039] In a step 410, the magnetic field measuring unit is controlled for a target measurement 412, namely, with a continuous operation, in order to detect magnetic signals by means of the magnetic field measuring unit and to obtain a measurement data set 414. The measurement data set 414 is, for example, the magnetic signals in the frequency range 310 according to Fig. 3, i.e. the actual or desired measurement.

[0040] As mentioned, the reference measurement 402 or the target measurement can be performed first.

[0041] In a step 420, based on the reference data set 404, interference signals in the magnetic signals are determined, as is also the case with respect to Fig. 3. In one embodiment, in step 422, one or more reference interference frequencies are determined in the reference data set, e.g., the interference signals 342 at the frequency 1200 Hz according to Fig. 3.

[0042] In a step 424, based on the one or at least one of the several reference interference frequencies 432 in the measurement data set, one or more target interference frequencies are determined, e.g. the interference signals 340 at the frequency 50 Hz according to Fig.3. This is done in such a way that the one or at least one of the several reference interference frequencies corresponds to the one or at least one of the several reference interference frequencies or a multiple thereof, i.e. are fundamental and harmonic oscillations.

[0043] In a step 430, the interference signals are then at least partially removed from the measurement data set. For this purpose, for example, in step 432, based on the interference signals, a width and / or depth of a frequency filter 434, in particular a notch filter, can be determined. The frequency filter is then applied to the measurement data set in step 436. QUOTES CONTAINED IN THE DESCRIPTION

[0000] This list of documents submitted by the applicant was generated automatically and is included solely for the convenience of the reader. This list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions. Cited patent literature

[0000] DE 10 2022 204 526.2 [0002, 0004]

Claims

[1] A method for magnetic field measurement using a magnetic field measuring unit (100, 210) with at least one nitrogen vacancy center magnetometer unit, comprising: Controlling (400) the magnetic field measuring unit for a reference measurement (402) with a pulsed operation in order to detect magnetic signals by means of the magnetic field measuring unit and to obtain a reference data set (404); Controlling (410) the magnetic field measuring unit for a target measurement (412) with a continuous operation in order to detect the magnetic signals by means of the magnetic field measuring unit and to obtain a measurement data set (414); Determining (420), based on the reference data set (404), interference signals in the magnetic signals; and at least partially removing (430) the interference signals from the measurement data set. [2] The method of claim 1, wherein the reference measurement (402) covers a reference frequency range (320) of the magnetic signals, and wherein the target measurement (412) covers a target frequency range (310) of the magnetic signals, wherein the reference frequency range includes higher frequencies than the target frequency range, and in particular does not overlap with it. [3] The method of claim 1 or 2, wherein determining (420), based on the reference data set (404), interference signals in the magnetic signals comprises: Determining (422), in the reference data set, one or more reference interference frequencies (342); Determining (424), based on the one or at least one of the plurality of reference interference frequencies (342) in the measurement data set, one or more target interference frequencies (340) such that the one or at least one of the plurality of reference interference frequencies corresponds to the one or at least one of the plurality of reference interference frequencies or a multiple thereof. [4] Method according to one of the preceding claims, wherein the at least partial removal (430) of the interference signals from the measurement data set comprises: Determining (432) based on the interference signals, a width and / or a depth of a frequency filter, (434) in particular a notch filter; and Apply (436) the frequency filter to the measurement data set. [5] Method according to one of the preceding claims, wherein the reference measurement (402) is carried out before the target measurement (412). [6] Method according to one of claims 1 to 4, wherein the reference measurement (402) is carried out after the target measurement (412). [7] Computing unit (220) which is configured to carry out all method steps of a method according to one of the preceding claims. [8] Magnetic field measuring device (200) comprising a magnetic field measuring unit (100, 210) with at least one nitrogen vacancy center magnetometer unit and a computing unit (220) according to claim 7. [9] Computer program which causes a computing unit to carry out all the method steps of a method according to one of claims 1 to 6 when executed on the computing unit. [10] A machine-readable storage medium having stored thereon a computer program according to claim 9.

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