Sensor unit for measuring magnetic fields

Abstract

The invention relates to a sensor unit (100) for measuring weak magnetic fields without shielding, comprising: two magnetometers (300, 400a, 400b) which are arranged at a fixed distance from one another, wherein each magnetometer has a sensor medium (101, 102, 304) and is configured to detect a magnetic field strength at a measurement location by reading a spin resonance that is dependent on the magnetic field strength in the sensor medium, wherein the sensor unit also has at least one excitation light source (120) for radiating light (124) into the sensor media (101, 102) of the magnetometers, wherein the sensor unit also has at least one signal processing unit (406) for determining a magnetic field gradient (100, 200, 300, 400) as the difference between the output signals from the two magnetometers.

Description

[0001] Description

[0002] title

[0003] Sensor unit for measuring magnetic fields

[0004] The present invention relates to a sensor unit for measuring magnetic fields based on magnetic resonance.

[0005] Background of the invention

[0006] Various sensor technologies are known for measuring very small magnetic field strengths. One of the most sensitive sensor types is SQU ID magnetometers (Superconducting Quantum Interference Devices). These are based on superconducting components and can resolve individual magnetic flux quanta or fields down to the range of a few picoteslas (pT). The disadvantage of SQU ID sensors, however, is the need to cool them below the superconducting transition temperature, which for most superconducting materials is below 63 Kelvin, i.e., below the temperature of liquid nitrogen. Furthermore, the measuring range of these sensors is limited and cannot operate in large interference fields, so measurements are only possible in specially magnetically shielded rooms.

[0007] Likewise, quantum-based magnetic sensors based on nitrogen vacancy centers are known, for example from DE 10 2018 220 234 A1 or DE 10 2018 214 617 A1, which are based on the detection of spin resonances in magnetic fields.

[0008] Disclosure of the invention According to the invention, a sensor unit for measuring magnetic fields is proposed with the features of patent claim 1. Advantageous embodiments are the subject of the subclaims and the following description.

[0009] In particular, a sensor unit is proposed which has two magnetometers which are arranged at a fixed distance from one another, wherein each magnetometer has a sensor medium and is configured to detect a magnetic field strength at a measuring location by reading out a spin resonance in the sensor medium which is dependent on the magnetic field strength, wherein the sensor unit further has at least one excitation light source for radiating light into the sensor media of the magnetometers and at least one signal processing unit for determining a magnetic field gradient as the difference between the output signals of the two magnetometers.

[0010] By using two magnetometers at a fixed distance from each other, which can then be arranged so that one magnetometer is further away from a weak magnetic field source than the second magnetometer, the magnetic field gradient approximately corresponds to the field emanating from the weak source, while eliminating much stronger background fields. This eliminates the need for magnetic shielding, making magnetic field measurement possible in everyday environments. The sensor unit is therefore particularly suitable for unshielded measurement of weak magnetic fields.

[0011] In particular, a possible embodiment proposes a sensor unit in which each magnetometer comprises a nitrogen vacancy center magnetometer, wherein the sensor medium comprises a diamond crystal or a section of a diamond crystal with nitrogen vacancy centers, and wherein the sensor unit further comprises at least one microwave source for generating a resonant field in the sensor medium and at least one photodetector for detecting resonance-dependent fluorescent light from the sensor medium. Nitrogen vacancy center magnetometers offer very high sensitivity, but are also easy to implement in a compact design, thus resulting in small sensor heads. Complex cooling, such as with SQU ID sensors, is not necessary. Furthermore, NV vacancy centers inherently allow vectorial magnetic field measurements to be performed.

[0012] The sensor medium can, for example, comprise a thin diamond plate, since a large volume is not required for fluorescence measurement. This allows for a more compact and cost-effective sensor design. Alternatively or additionally, the sensor medium of each magnetometer can also comprise a section of the same diamond crystal. This design, in addition to ensuring compactness, also ensures that the two sections used as the sensor medium have essentially the same properties and thus exhibit the same optical properties when reading the spin resonance, thus eliminating any sources of error in the gradiometer's differential calculation.

[0013] It is also possible for a diamond crystal used as the sensor medium to be designed to serve as a waveguide for guiding the light from the excitation light source to the measurement locations of the two magnetometers. This also eliminates the need for supply cables.

[0014] Preferably, the same excitation light source and / or microwave source are used for both magnetometers in all variants. This way, fluctuations and noise components from these elements are automatically eliminated by differential signal generation (common noise rejection).

[0015] Furthermore, the sensor unit can comprise a microwave resonator in the region of the sensor medium that is tuned to the spin resonance of the sensor medium. A microwave resonator can generate a homogeneous microwave field in the relevant locations of the sensor medium, while the microwave source can also be located remotely and fed into the resonators of the magnetometers via suitable connections. To enable vector magnetic field measurements, the sensor unit can also comprise a device for generating a substantially homogeneous bias magnetic field in the region of the sensor medium. Since the NV center in diamond has four possible arrangements, each of which is visible in the fluorescence spectrum as a separate frequency split, the magnetic field direction and strength can be extracted for each crystal direction, allowing the source of the magnetic field to be localized.To generate such a bias magnetic field, a Helmholtz coil arrangement can be used, for example, within which at least the sensor medium or the sensor heads of the two magnetometers are located. Helmholtz coils can generate a particularly homogeneous field.

[0016] The distance between the two magnetometers, i.e., between the measurement locations on the sensor media, can range from millimeters to centimeters and ultimately also depends on the strength of the magnetic field to be measured. For example, a distance between 1 and 30 millimeters can be selected, preferably between 5 and 20 millimeters.

[0017] Further advantages and embodiments of the invention will become apparent from the description and the accompanying drawings.

[0018] The invention is illustrated schematically in the drawing using exemplary embodiments and is described below with reference to the drawing.

[0019] Short description of the drawings

[0020] Figure 1 shows a schematic overview of a sensor unit according to an exemplary embodiment of the invention;

[0021] Figure 2 shows magnetic field strengths when measuring brain magnetic fields at different distances from the head; Figure 3 shows a sensor head that can be used as part of a sensor unit according to an exemplary embodiment; and

[0022] Figure 4 shows a sensor unit with two of the sensor heads shown in Figure 3.

[0023] Embodiments of the invention

[0024] Quantum-based or optically pumped magnetometers are particularly suitable as sensors for measuring very small magnetic field strengths. Diamond NV magnetometers are described in the following embodiments. However, it is also conceivable to integrate other magnetometers with comparable properties into the sensor unit.

[0025] The magnetometers described here as examples use optically pumped and / or optically detected magnetic resonance (ODRM). 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.

[0026] Diamond NV magnetometers are based on the readout of magnetic resonances from specific defect centers in diamond, in particular 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 normal state without an existing magnetic field, 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 with the emission of fluorescent light in the red wavelength range at 637 nm. 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.

[0027] 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.

[0028] If an external magnetic field is present, the so-called Zeeman effect causes the splitting of the otherwise equally energetic 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 linearly 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 up to 1 pTA / Hz. Since the NV center in single-crystal diamond has four possible arrangements in the crystal lattice, the presence of a directed magnetic field causes the NV centers present in the crystal to react with varying degrees of intensity to the external magnetic field depending on their position within the crystal. Ideally, this can result 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 clearly determined.The NV-center magnetometer offers a variety of advantages for this application. In addition to the aforementioned very high sensitivity, it can also cover a wide measurement range (> 1 Tesla). 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, an NV-center magnetometer offers the possibility of vectorially determining external magnetic fields based on the various orientations present in the diamond lattice.

[0029] 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 components for detecting the fluorescent light 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 center magnetometers.

[0030] To be usable in everyday environments, magnetic fields that do not originate from the desired weak source should be eliminated from the measurement as much as possible. Especially in the automotive sector, comparatively high and barely shielded magnetic fields arise in the background, which are approximately in the range of 10'. 6 up to 10'9 Tesla (Nanotesla), in addition the Earth's magnetic field is in the range of 10' 5 Tesla (a few microtesla). In contrast, biomagnetic fields, which serve as an example here, are in the range of 10' 12 Tesla (Picotesla) or even less.

[0031] According to exemplary embodiments, the elimination of background magnetic fields can be achieved by using a gradiometer arrangement for magnetic field measurement. Gradiometers are generally sensor units capable of detecting not only the field strength but also the gradient of the field. For this purpose, at least two individual magnetometers arranged at spatially different locations can be used. As an example, a sensor unit using two NV center magnetometers in a gradiometer arrangement is described below in conjunction with Figure 1.

[0032] For each magnetometer, a diamond 101, 102 with nitrogen vacancies can initially be present as the sensor medium. The optical excitation of the NV centers in the diamond is achieved by a suitable light source 120, such as a pump laser. A Nd:YAG laser in the green range at a frequency of 532 nm is suitable for exciting the respective transitions. The light 124 of the pump laser 120 can be irradiated into the diamonds via suitable optical elements 128, such as mirrors, beam splitters, lenses, and optionally via fiber optic elements 122. In this case, a single light source 120 is used for both diamonds 101, 102, from which the excitation light is guided into the respective diamond via an optical fiber. Furthermore, the excitation light can be irradiated continuously or pulsed by the laser, so that, for example, time windows are kept free for interference-free fluorescence light measurements.

[0033] In addition, a microwave source 140 can be present in the sensor unit, which is capable of generating an electromagnetic field across a bandwidth that covers the desired resonance frequency in the sensor medium, i.e., in the region of the NV centers in the diamonds 101, 102. Preferably, the same microwave source 140 can be used for both sensors or diamonds. The resulting fluorescent light 125 can in turn be guided via the fiber optics and detected by a photodetector 130 that is sensitive in the range of the fluorescence wavelength; alternatively, one or more photodetectors that receive the fluorescent light could be arranged directly on the sensor diamonds.Here, too, suitable reflective or semi-transparent mirrors, filters, beam splitters, lenses for beam shaping, coupling lenses for fiber optics or for the diamond crystals, and other optical elements 126, 128 can be used along the optical path of the fluorescent light 125. The signal from the photodetector 130 can then be further processed by various elements, such as a preamplifier 132 or a processing unit 136 for signal evaluation.

[0034] The elements of the sensor unit are shown only schematically in the figure; it is understood that, for example, the microwave source is designed and arranged to generate the RF field in the sensor diamonds. Other elements such as microwave resonators or housing components are also not explicitly shown here.

[0035] The distance d between the two diamonds or sensor heads of a gradiometer unit corresponds to the distance between the locations where simultaneous magnetic field measurements are taken. As long as the distance between the measurement locations is relatively small, it can be assumed that the strength of the additional background magnetic field B envis approximately the same size at both locations. In contrast, the weak magnetic field of interest will decrease significantly with increasing distance from the magnetic field source. By placing two sensors at different distances from the source, e.g., at different distances from the skull surface, the background field can be eliminated by calculating the difference between the recorded sensor values, and the small magnetic field of interest, or rather its gradient, can be extracted: Since the magnetic field decreases with the square of the distance, the largest change in the magnetic field is detected by the sensor diamond near the source. For this purpose, for example, two magnetometers can be arranged one above the other in an axial gradiometer configuration.

[0036] The following applies to an axial gradiometer with a first and a second magnetic field sensor:

[0037] Blsens — Bl + Benv ,

[0038] B2sens— B2 + Benv ,

[0039] Bgrad — Blsens B2sens — Bl - B2 ~ Bl where Bisens is the measured field strength at the first sensor, which is closer to the magnetic field source of interest, where Bi is the fraction of the weak magnetic field strength of interest at the location of the first sensor, where B2sens is the measured field strength at the second, more distant sensor, where B2 is the fraction of the weak magnetic field strength of interest at the location of the second sensor, and where B gra d is the resulting gradient of the magnetic field strength.

[0040] Since the small magnetic field B2 at the second sensor, which is further away from the source, is many times smaller than at the first sensor, the gradient B gra d of the total magnetic field at this point is approximately equal to the magnetic field Bi of interest at the first, closer sensor head, while the approximately uniform background fields B env be eliminated.

[0041] The distance d between the sensor heads within a gradiometer unit, in this case the distance between the two sensor diamonds, can range from mm to several cm, for example, between 0.5 cm and 2 cm. Good results have been achieved for values ​​in this range, for example, for measuring biomagnetic fields in the picotesla range. However, as long as sufficient independence of the signals from the background fields is achieved, larger or smaller distances or other gradiometer configurations, such as sensors arranged side by side, can also be used. If the distance between the individual sensor heads is too large, particularly in the presence of local background fields (electrical cables, etc.), this can lead to interference.) the background field may no longer be identical at both locations; if the distance between the sensor heads is too small, however, the difference between the field strengths of the magnetic field to be measured between the two locations may be too small. The selection of the appropriate distance between the two sensors of a gradiometer unit therefore preferably also depends on the type and orientation of the magnetic field source of interest and the expected field strength. The distance of the sensor unit from the magnetic field source can also be in the mm to cm range.

[0042] Figure 2 shows a comparison of exemplary magnetic field strengths of biomagnetic brain magnetic fields occurring at different distances from the skull surface. These very weak magnetic fields are induced by the currents flowing in the brain neurons during neuronal activity and therefore, similar to an electroencephalogram, provide clues to various mental or health conditions. The x-axis indicates the depth of the actual magnetic field source, i.e., the depth of neuronal activity in the brain that generates the magnetic field, while the logarithmic y-axis shows the maximum magnetic field strength in femtotesla (fT).

[0043] The lower curve 210 shows the magnetic field strength that occurs 3 cm from the skull surface and, depending on the depth of the brain activity, is between about 10- 13 Tesla (about 100 fT) and 5-10' 15Tesla (about 5 fT). The upper curve 220, on the other hand, shows magnetic field strengths at a distance of about 6 mm from the scalp, which range from about 10' 12 Tesla (approx. 1 pT) up to about 10' 14 Tesla (approximately 10 fT). It can be seen that across the entire range, the measurable field strengths of the brain's magnetic field drop by at least fivefold between 6 mm and 3 cm from the scalp, so that the compact sensor units according to the present invention, which, for example, do not rely on complex nitrogen cooling like SQID magnetometers, enable significantly more sensitive measurements.

[0044] Instead of two sensor diamonds, as shown in Figure 1, a single diamond with NV centers could be used, which has a certain spatial extension at least along one direction to enable a gradiometer configuration. For this purpose, the diamond is excited at at least two locations, and the resulting fluorescent light is read out by at least two suitably arranged photodetectors. For this purpose, a diamond waveguide with NV centers, for example, can also be used, allowing the excitation light to propagate along the gradiometer axis in the waveguide and reach both readout locations. Perpendicular to this, no significant extension of the crystal is required, so that, for example, thin diamond platelets are conceivable as sensor heads.

[0045] If the optimal distance between the sensor heads for a gradiometer configuration is greater than the possible or desired extension of a single diamond crystal, two diamonds with identical properties are preferable. The optical properties are influenced, for example, by the type and number of NV centers or defect density and possible mechanical stresses in the diamond crystal. To achieve the most homogeneous properties possible, two pieces can then be cut from a larger diamond crystal. Synthetically produced diamonds, which can be synthesized, for example, using chemical vapor deposition (CVD) or the high-pressure, high-temperature (HPHT) process, are particularly suitable.

[0046] Figure 3 shows an oblique view of an exemplary embodiment of a single sensor head 300 of a gradiometer unit formed from two of these sensor heads. Excitation light from an excitation light source is coupled into an optical fiber 301 via an optical fiber coupler. The fiber is held by a fiber holder 305, which can also serve to stabilize the entire system. The embodiment shown is therefore only an example.

[0047] The end of the optical fiber 301 can be bonded to a gradient refractive index (GRIN) lens 303 using an optical adhesive. This GRIN fiber composite can then, in turn, be bonded to a diamond 304 using a transparent adhesive layer. The diamond 304 has a first side 304a and a second side 304b, with the GRIN fiber composite being bonded to at least a portion of the diamond surface on the first side 304a. Optionally, an optical filter layer can be inserted between the surfaces of the GRIN lens 303 and the diamond 304, so that the emitted fluorescent light of the at least one NV center of the diamond cannot escape unhindered from the corresponding surface of the diamond 304, but is reflected back toward the second side 304b.On this second side 304b, in the figure, below the diamond 304, a photodiode system 308 is arranged, comprising a first photodiode 309 and a second photodiode 310. The photodiodes can be designed such that the first photodiode 309 can detect the emitted fluorescence but blocks the wavelength of the excitation light, while the second photodiode 310 blocks the fluorescence emitted by the at least one NV center and allows only the excitation light to pass through. This can be achieved, for example, by optical filters, e.g., in the form of coatings.

[0048] With the help of appropriate signal processing, e.g., a logarithmic circuit, a balanced measurement signal can be generated from the two measured signals of the two photodiodes 309 and 310, which already takes fluctuations in the excitation light into account. Furthermore, the sensor head 300 has a microwave resonator structure 306 to homogeneously distribute the microwaves generated by a suitable microwave source across the volume of the diamond 304. The microwaves can be introduced into the microwave resonator structure via a radio-frequency connector 307. The resonator structure 306 is tuned to the frequency of the electron spin resonances.

[0049] Figure 4 provides an overview of a gradiometer unit comprising two identical sensor heads, as shown in Figure 3. In this example, the two sensor heads 400a, 400b are mounted on a mechanical support plate at a defined distance from each other. However, a different mounting or arrangement for the sensor heads is also possible and can be selected depending on the subsequent application, the required distance between the sensor heads, or the type of gradiometer configuration. Both sensor heads 400a and 400b are preferably supplied by the same excitation light source 420, whereby the excitation light can be guided, for example, through a fiber splitter 421 to the optical fibers of both sensor heads. Likewise, both sensor heads can be supplied by the same microwave source 440, whereby a microwave splitter 441 is used accordingly.

[0050] In order to determine the location of a magnetic field source, i.e. to enable vector magnetometry, an additional bias magnetic field 150 can be generated that is identical at both measuring locations of the gradiometer (i.e. in the area of ​​the sensor heads or sensor diamonds). The field gradient is to be determined in three axes. The direction in which the magnetic field gradient is maximum indicates the location of the magnetic field source. The bias magnetic field is indicated here by way of example by a single coil 450, within which the two sensitive sensor heads 400a and 400b are located. For example, the sensor heads can be used in a Helmholtz coil arrangement, which generates a bias magnetic field that is as homogeneous as possible in the area of ​​the two sensor heads and is supplied by a suitable power source 451 (e.g. a battery).

[0051] If the same microwave source and light source are used for both sensor diamonds or sensor heads, noise components from these sources (noise from the excitation light or frequency and amplitude noise from the microwave source) will also occur at both sensor heads. These simultaneously occurring noise components are automatically eliminated by the gradiometer arrangement. This reduces the requirements for the noise characteristics of these components. For a microwave source, for example, a cost-effective standard oscillator with higher phase and amplitude noise can be used instead of a high-precision, expensive, and temperature-controlled quartz oscillator. For the bias magnetic field, the use of a single field source and current source also ensures that the same noise components can be expected at both sensors, which are again eliminated by gradient formation.

[0052] To further suppress the gradiometer's sensor noise, a modulation method can be used, for example, in which the microwave frequency or the amplitude of the microwaves is modulated. Modulating the microwave frequency with a specific modulation depth and modulation frequency generates a fluorescence modulation. Demodulating the detected fluorescence signal then yields a signal in the magnetic resonance frequency range that is linearly dependent on the magnetic field. Alternatively, or additionally, the light intensity of the excitation light can also be modulated.

[0053] The fluorescence signals from the two sensor heads in a gradiometer are then further processed in a suitable manner. To obtain the field gradient, a difference between the signals can be calculated, as already described, which can be achieved, for example, using a simple or logarithmic differential amplifier. The signal can then be fed to a post-amplifier, an AD converter, and / or frequency filters, for example. The resulting preprocessed differential signal, which represents the weak magnetic field of interest, can then be evaluated for further purposes.

[0054] In addition to or as an alternative to a gradiometer configuration, lock-in detection can be used for a magnetometer to eliminate unwanted background fields from the measurement. For this purpose, for example, the excitation light or microwave source can be modulated, and a lock-in amplifier (134 in Figure 1) can be provided in the signal processing area of ​​sensor unit 100, which essentially acts as a narrowband bandpass filter for the resonance signal of interest.

[0055] Such a highly sensitive sensor based on a gradiometric arrangement of two magnetometers can be used in a wide variety of areas.

[0056] This allows, for example, the brain magnetic fields mentioned above to be measured by placing one or more sensor units near a user's head. The sensor units can be integrated, for example, into head-worn mounts or headgear, or can be integrated into elements located near the head, such as a headrest in a vehicle or the headboard of a bed.

[0057] By measuring brain magnetic fields, various conditions of a user can be recorded, such as fatigue or stress, but it can also be used to detect emergencies or neurological diseases such as epilepsy, strokes, Parkinson's disease, and others. Furthermore, the measurement of brain magnetic fields can be used, as with brainwave measurements, to control various devices (e.g. prostheses, vehicles) by recording and evaluating the neuronal activity of an imagined command or involuntary reflexes and patterns. Other biomagnetic fields can also be measured in a similar way. For example, small electrical currents are present in muscle cells and in nerve activity throughout the body, which in turn induce a weak magnetic field. Magnetic field measurements on the body can therefore be used, for example, to monitor the activity of the heart muscle.Likewise, nerve signals can be recorded in any part of the body, even outside the brain, which can then be evaluated and implemented, for example, to control prostheses. In further embodiments, a highly sensitive magnetic field sensor as presented here can generally measure small electrical currents (e.g., in cables or on technical components) that also induce a magnetic field. Thus, such a sensor can be used, for example, to indirectly determine the current strength. In other embodiments, a sensor unit can be used to monitor electrical switchgear.

Claims

Claims 1. Sensor unit (100) for measuring magnetic fields, comprising: two magnetometers (300, 400a, 400b) arranged at a fixed distance from each other, each magnetometer having a sensor medium (101, 102, 304) and being configured to detect a magnetic field strength at a measurement location by reading out a spin resonance in the sensor medium which depends on the magnetic field strength, at least one excitation light source (120) for shining light (124) into the sensor media (101, 102) of the magnetometers, at least one signal processing unit (406) for determining a magnetic field gradient (100, 200, 300, 400) as the difference of the output signals of the two magnetometers.

2. Sensor unit according to claim 1, wherein each of the magnetometers (101, 102, 400a, 400b) comprises a nitrogen vacancy center magnetometer, wherein the sensor medium comprises a diamond crystal (101, 102) or a section of a diamond crystal with nitrogen vacancy centers, and wherein the sensor unit further comprises at least one microwave source (140) for generating a resonant field in the sensor medium and at least one photodetector (130) for detecting resonance-dependent fluorescence light (125) from the sensor medium.

3. Sensor unit according to claim 2, wherein the sensor medium comprises a thin diamond plate.

4. Sensor unit according to one of claims 2 or 3, wherein the sensor medium of the two magnetometers each comprises a section of the same diamond crystal.

5. Sensor unit according to one of claims 2 to 4, wherein the diamond crystal is designed to serve as a waveguide for directing the light from the excitation light source (120, 420) to the measuring points of the two magnetometers.

6. Sensor unit according to one of claims 2 to 5, wherein the two magnetometers are assigned the same excitation light source (120, 420) and / or the same microwave source (140, 440).

7. Sensor unit according to one of claims 2 to 6, wherein the sensor unit has a microwave resonator (306) in the area of ​​the sensor medium which is tuned to the spin resonance of the sensor medium.

8. Sensor unit according to one of the preceding claims, further comprising a device (450) for generating a substantially homogeneous bias magnetic field (150) in the area of ​​the sensor medium.

9. Sensor unit according to claim 9, wherein the device (450) for generating the bias magnetic field comprises a Helmholtz coil arrangement, wherein at least the sensor medium of the two magnetometers is arranged within the Helmholtz coil arrangement.

10. Sensor unit according to one of the preceding claims, wherein the distance (d) between the two magnetometers (101 , 102, 400a, 400b) is between 1 and 30 millimeters, preferably between 5 and 20 millimeters.

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