Magnetic field detection device for detecting magnetic signals emanating from a measuring location

Abstract

The invention relates to a magnetic field detection device (300) for detecting magnetic signals emanating from a measuring location (330), comprising a magnetic field measuring unit (310) with at least one nitrogen vacancy center (NV) magnetometer unit connected to a signal processing unit (340), wherein the magnetic field measuring unit (310) is arranged on a movable manipulator unit (315), wherein the magnetic field detection device (300) is configured to move the magnetic field measuring unit (310) by means of the movable manipulator unit (315) to at least two different positions relative to the measuring location (330) and to detect a magnetic field strength and field direction at each of the at least two different positions,and to determine by means of the signal processing unit (340) at least one field vector comprising an effective magnetic field strength and an effective field direction from the magnetic field strengths and field directions detected at the at least two different positions.

Description

[0001] The present invention relates to a magnetic field detection device for detecting magnetic signals emanating from a measuring location. 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 A1 describes a magnetometer that uses optically pumped and optically detected magnetic resonance (ODMR). This utilizes 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 magnetic field detection device for detecting magnetic signals emanating from a measuring location is proposed, having the features of patent claim 1. Advantageous embodiments are the subject of the subclaims and the following description.

[0004] The invention proposes arranging a magnetic field measuring unit with at least one nitrogen vacancy center (NV) magnetometer unit on a movable manipulator unit. The magnetic field strength and field direction, i.e., a magnetic field vector, are measured at different positions relative to a measurement location and an effective magnetic field vector is determined from this. The movable arrangement thus allows a multitude of different positions relative to the measurement location to be measured with only one magnetic field measuring unit, thus obtaining a three-dimensional image of the prevailing magnetic field, in particular the magnetic field emanating from an object at the measurement location. The movable arrangement enables a three-dimensional reconstruction of the magnetic field source distribution, e.g., the underlying magnetic dipoles or electric currents.The invention makes it possible to provide a magnetic field detection device which can be used for imaging, for example in the manner of a computer tomograph or magnetic resonance tomograph.

[0005] In detail, a magnetic field detection device for detecting magnetic signals emanating from a measurement location is now presented. The device comprises a magnetic field measuring unit with at least one NV magnetometer unit connected to a signal processing unit, wherein the magnetic field measuring unit is arranged on a movable manipulator unit. The magnetic field detection device is configured to move the magnetic field measuring unit by means of the movable manipulator unit to at least two different positions relative to the measurement location and to detect a magnetic field strength and field direction at each of the at least two different positions, and to determine, by means of the signal processing unit, at least one field vector comprising an effective magnetic field strength and an effective field direction from the magnetic field strengths and field directions detected at the at least two different positions.Both a wireless and a wired connection between the sensors and the signal processing unit is provided.

[0006] Diamond NV magnetometers are based on the reading of magnetic resonances from specific defect centers in diamond, particularly nitrogen vacancies (NV), which occur as impurities in the carbon lattice of diamond and can also be deliberately introduced. If the NV center is optically excited in the ground state, for example, by irradiating a pump laser beam with a suitable wavelength (in this case in the green wavelength range, e.g., at 532 nm for off-resonance excitation), the electrons are lifted from the triplet ground state to the excited triplet state and relax, emitting fluorescent light in the red wavelength range at 650 - 800 nm (637 nm = zero phonon line). Since the probability for non-spin-conserving transitions from the spin state increases with the spin quantum number m s=±1 is larger, continuous excitation pumping ensures that the NV centers are mostly in the spin state m s =0 hyperpolarized.

[0007] Between the m s = 0 and m s =±1 spin states in the ground state, there is an energy difference, which in this case is about 2.87 GHz. Therefore, if microwave radiation is irradiated into the diamond in addition to the optical excitation, a dip in the red fluorescence occurs at this resonance frequency of 2.87 GHz, since the spin-polarized electrons are deflected by the microwave field from the m s = 0 in the m s =±1 ground state and from there by the pump light into the m s =±1 excited state. From there, however, mainly non-radiative transitions and weak infrared fluorescence transitions occur via the singlet state, while fluorescence in the red region disappears.

[0008] If an external magnetic field (to be measured) is present, the so-called Zeeman effect causes the splitting of the otherwise equal-energy m s=±1 triplet levels into energetically equidistant Zeeman levels. When the fluorescence is plotted against a frequency spectrum of the microwave excitation, two dips are observed in the fluorescence spectrum, the frequency spacing of which is proportional to the magnetic field strength of the external magnetic field. The magnetic field sensitivity is primarily defined by the minimum resolvable frequency shift and can reach 1 pT / √Hz or less. Since the NV center in single-crystal diamond has four possible arrangements in the crystal lattice, the presence of an additional bias magnetic field causes the NV centers present in the crystal to react with varying degrees to the external magnetic field depending on their position within the crystal. Ideally, this results in four pairs of fluorescence minima appearing in the spectrum, from whose shape and position relative to each other, both the magnetic field strength (magnitude) and the direction of the external magnetic field can be unambiguously determined.

[0009] If vectorial magnetic field measurements are to be enabled, the magnetic field detection device also includes a device for generating the bias magnetic field in the region of the sensor medium. This can be a Helmholtz coil arrangement, with at least the sensor medium arranged within the Helmholtz coil arrangement. Other devices can also be used, such as a simple coil, an elongated coil, permanent magnet solutions such as in a Hallbach array, etc. In order to additionally determine vectorial (direction-dependent) magnetic information, a defined direction of the bias magnetic field is required.

[0010] Since measurements are taken at different positions, these positions have a different orientation to the magnetic field at the measurement location. This allows the position and strength of this magnetic field to be determined. Since a background field (especially the Earth's magnetic field) is usually essentially the same (same strength and orientation) at different positions, it can be eliminated. This eliminates the need for magnetic shielding. However, to improve measurement results, the magnetic field detection device can also be magnetically shielded from the environment.

[0011] Another advantage of NV sensors is their size, especially the size of the sensor medium. NV sensors have a very small active sensor volume. This small size also allows the sensors to be used in a geometric array. In particular, very high-resolution arrays are possible due to the very small active sensor volume.

[0012] In one embodiment, the magnetic field detection device is configured to determine the effective magnetic field strength and the effective field direction from a difference between the magnetic field strengths and field directions detected at at least two different positions. This is similar to a gradiometer connection of two magnetic field measuring units at different positions (circuitry gradiometry), but uses only one magnetic field measuring unit, whose measurement results are calculated, thus corresponding to computational gradiometry. Through gradiometry, i.e. essentially vector arithmetic of the measured object, the magnetic field gradient approximately corresponds to the field emanating from the source, while significantly stronger background fields (which are essentially the same at both positions) are eliminated (see also above). The invention is accordingly particularly suitable for the unshielded measurement of weak magnetic fields.Technical details of gradiometer solutions that can also be used in the context of the present invention are disclosed in DE 10 2022 201 690 and are intended to be included here.

[0013] In one embodiment, the at least two different positions are equally spaced from the measurement location or from an axis running through the measurement location. In other words, the magnetic field measuring unit is moved on a circular or spiral path around the measurement location or the axis. A spiral path is particularly suitable for extended objects, such as people arranged along the axis, e.g. on a couch. On the one hand, this measure is technically easy to implement and is known from other applications, such as computer tomography, and can be adopted from them. On the other hand, it is very easy to always find two different positions that are well suited to computational gradiometry.

[0014] In one embodiment, two different positions of the at least two different positions and the measurement location or a point on an axis passing through the measurement location lie on a straight line. In other words, the positions for two measurements are rotated by 180° relative to the measurement location, for example, by 90° and 270°, on a circular path whose center is the measurement location. This makes it particularly easy to eliminate background fields.

[0015] In one embodiment, the magnetic field measuring unit comprises at least two NV magnetometer units arranged in one plane. This makes it very easy to provide, for example, circuit-based gradiometry. It can also be used, for example, to determine multiple field vectors, each comprising a magnetic field strength and a field direction, from the magnetic field strengths and field directions detected at the at least two different positions.

[0016] Further advantages and embodiments of the invention will become apparent from the description and the accompanying drawings.

[0017] The invention is illustrated schematically in the drawing using exemplary embodiments 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 an exemplary magnetic field measuring unit with a plurality of NV magnetometer units for use in a magnetic field detection device according to an embodiment of the invention in a schematic view. Fig. 3 shows an exemplary magnetic field detection device according to an embodiment of the invention in a schematic view. Fig. 4 schematically shows the measurement of magnetic signals in a magnetic field detection device according to an embodiment of the invention in a schematic view. Embodiment(s) of the invention

[0018] Fig. 1 schematically shows the essential components of a magnetic field measuring unit 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.In addition, the excitation light can be emitted continuously or in pulsed form by the laser, so that, for example, time windows are kept free for interference-free fluorescence light measurement.

[0019] Furthermore, the magnetic field measuring unit 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.

[0020] 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 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.

[0021] It is understood that such a device may also include 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.

[0022] 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.

[0023] 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.

[0024] In order to be usable in an everyday environment, magnetic fields that do not originate from desired weak sources should be eliminated from the measurement as far as possible, especially the Earth's magnetic field in the range of 10 -5 Tesla (some microtesla).

[0025] The elimination of background magnetic fields can be achieved by shielding or by a gradiometer arrangement during magnetic field measurement according to exemplary embodiments. Gradiometers are generally magnetic field measuring units capable of measuring not only the field strength but also the field gradient. For this purpose, measurement information from at least two spatially different positions can be offset against each other.

[0026] In Fig. Figure 2 shows a schematic view of a section of an exemplary magnetic field measuring unit 200 with multiple NV magnetometer units for use in a magnetic field detection device according to an embodiment of the invention. Several NV magnetometer units are arranged in a plane in a regular two-dimensional array on a substrate 201, such as a printed circuit board (PCB).

[0027] Each NV magnetometer unit comprises a sensor medium 210 in the form of a diamond crystal, to which a microwave resonator 251 is assigned, which is connected via a waveguide 252 to a microwave source 250, for example in the form of an integrated circuit (IC). Further components, such as an excitation light source for radiating light into the sensor media 210 and a device for generating a bias magnetic field in the region of the sensor media 210, are not shown, but can be assigned to all sensor media 210. Furthermore, each sensor medium 210 can be assigned a (separate) detector, for example as in connection with Fig. 1 described.

[0028] Fig. 3 shows an exemplary magnetic field detection device 300 according to an embodiment of the invention in a schematic view. The illustrated magnetic field detection device 300 has a couch 320 for accommodating a patient 330, which in this case represents the measurement location. Furthermore, the magnetic field detection device 300 has a magnetic field measuring unit 310 with at least one NV magnetometer unit connected to a signal processing unit 340. The signal processing unit 340 can simultaneously serve as a control unit for the magnetic field detection device 300.

[0029] The magnetic field measuring unit 310 is arranged on a movable manipulator unit 315, which in this case is configured to move the magnetic field measuring unit 310 in the circumferential direction U around the couch 320 and in the axial direction z along the couch 320. In the example shown, the manipulator unit 315 is ring-shaped and, in terms of structure and circumferential movement, is essentially similar to a computer tomography scanner, but is axially movable, so that the magnetic field measuring unit 310 can be moved on a spiral path 350 around the couch 320 and the patient 330 as the measurement location.

[0030] The magnetic field detection device 300, e.g. via its control unit, is configured to move the magnetic field measuring unit 310 by means of the movable manipulator unit 315 to at least two different positions (e.g. P1, P1' in Fig. 4) to move relative to the couch 320 and patient 330 and to detect a magnetic field strength and field direction at each of the at least two different positions. Using the signal processing unit 340, at least one field vector comprising an effective magnetic field strength and an effective field direction can be determined from the magnetic field strengths and field directions detected at the at least two different positions.

[0031] In particular, numerous measurements can be performed on the spiral path 350. The step size of the axial movement between two measuring positions can be selected in particular as a function of a resolution or image width of the magnetic field measuring unit 310 in the z-direction, in particular such that after one orbit, the z-position has increased by a maximum of one image width. Likewise, the step size in the circumferential direction between two measuring positions can be selected as a function of a resolution or image height of the magnetic field measuring unit 310 in the circumferential direction, in particular such that after each step, the U-position has increased by a maximum of one image height. In this way, a complete three-dimensional image of the magnetic fields at the measurement location can be recorded. The magnetic field measuring unit 310 can move either alone or with other detectors (e.g., X-ray detectors for CT imaging).The entire magnetic field detection device 300 may be magnetically shielded (e.g., by wall coverings) or unshielded.

[0032] Fig. Figure 4 shows schematically the measurement of magnetic signals in a magnetic field detection device, for example according to Fig. 3, in a schematic view in a view of a plane in which the magnetic field measuring unit (e.g. 310) is moved. With reference to Fig. 3 this corresponds to a cross-sectional view perpendicular to the direction z, e.g. on the circle U.

[0033] A magnetic field 420 emanating from a current-carrying electrical conductor, such as nerves, at the measuring location 330 extends in a circle around the measuring location 330, with a field strength illustrated by 410. Background fields (e.g., the Earth's magnetic field) are designated by 440.

[0034] For example, opposite measurements at positions P1, P1' allow a reduction of the magnetic noise: If the magnetic fields from position P1 and P1' are subtracted from each other, the double amplitude of a circular signal field results, while the ambient fields 440 with the same orientation are subtracted from each other and disappear.

Claims

[1] Magnetic field detection device (300) for detecting magnetic signals emanating from a measuring location (330), comprising a magnetic field measuring unit (310) with at least one NV magnetometer unit connected to a signal processing unit (170, 340), wherein the magnetic field measuring unit (310) is arranged on a movable manipulator unit (315), wherein the magnetic field detection device (300) is configured to to move the magnetic field measuring unit (310) by means of the movable manipulator unit (315) to at least two different positions (P1, P1') relative to the measuring point (330) and to detect a magnetic field strength (410) and field direction (420) at each of the at least two different positions, and to determine an effective magnetic field strength and an effective field direction from the magnetic field strengths (410) and field directions (420) detected at the at least two different positions (P1, P1') by means of the signal processing unit (170, 340) comprising at least one field vector. [2] Magnetic field detection device (300) according to claim 1, wherein the magnetic field detection device (300) is configured to determine the effective magnetic field strength and the effective field direction from a difference of the magnetic field strengths (410) and field directions (420) detected at the at least two different positions (P1, P1'). [3] Magnetic field detection device (300) according to claim 1 or 2, wherein the at least two different positions (P1, P1') are equidistant from the measuring location (330) or from an axis passing through the measuring location. [4] Magnetic field detection device (300) according to one of the preceding claims, wherein two different positions of the at least two different positions (P1, P1') and the measuring location (330) or a point on an axis passing through the measuring location are on a straight line. [5] Magnetic field detection device (300) according to one of the preceding claims, wherein the magnetic field measuring unit (310) comprises at least two NV magnetometer units arranged in a plane (201). [6] Magnetic field detection device (300) according to one of the preceding claims, wherein the at least one NV magnetometer unit comprises as sensor medium (110; 210) a diamond crystal or a section of a diamond crystal with nitrogen vacancy centers, wherein the magnetic field detection device (300) is configured to detect the magnetic field strength and field direction by reading out a spin resonance in the sensor medium (110; 210) that depends on the magnetic field strength. [7] Magnetic field detection device (300) according to claim 6, further comprising at least one excitation light source (120) for illuminating light (124) into the sensor medium (110; 210), at least one microwave source (150) for generating a resonant field in the sensor medium and at least one photodetector (130) for detecting resonance-dependent fluorescence light (112) from the sensor medium (110; 210). [8] Magnetic field detection device (300) according to claim 7, wherein the magnetic field measuring unit (310) comprises at least two NV magnetometer units, wherein the at least two NV magnetometer units are assigned the same excitation light source (120) and / or the same microwave source (150). [9] Magnetic field detection device (300) according to one of claims 6 to 8, further comprising at least one device (140) for generating a substantially homogeneous bias magnetic field at the location of the sensor medium (110; 210) of the at least one NV magnetometer unit. [10] Magnetic field detection device (300) according to one of the preceding claims, wherein the signal processing unit (170, 340) is configured to determine several field vectors, each comprising a magnetic field strength and a field direction, from the magnetic field strengths and field directions detected at the at least two different positions.

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