Patent Publication Number: US-11391793-B2

Title: Device and method for generating and controlling a magnetic field strength

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a national stage application, filed under 35 U.S.C. § 371, of International Patent Application No. PCT/EP2019/079992, filed on Nov. 1, 2019, which claims the benefit of German Patent Application No. 10 2018 127 394.0, filed Nov. 2, 2018. 
     TECHNICAL FIELD 
     The present disclosure relates to a device for generating and controlling a magnetic field strength, and to a method for generating and controlling a magnetic field strength. 
     BACKGROUND 
     Magnetic fields and associated specific magnetic field strengths are usually generated by means of coil arrangements. The problem is that, in doing so, external magnetic fields cannot be taken into account. These can only be eliminated via magnetic shielding. 
     An equally difficult object is to generate reference values of physical variables. Such reference values are very important in science and technology. This is immediately obvious with respect to time, yet reference values are also very important as base values for other physical variables, such as current strength, voltage, magnetic field, electric field and the like. 
     The reference value for time is the second, which is defined as 9,192,631,770 times the period of the radiation corresponding to the transition between the two hyperfine levels of the ground state of atoms of the cesium isotope  133 Cs. Reference value transmitters for presetting the duration of a second are known through so-called “atomic clocks.” 
     For the length, a reference value for the meter can be derived from the definition that one meter is equal to the length of the distance that light travels in a vacuum during 1/299792458th of a second. Such reference value is therefore traced back to the reference value for time. 
     Current strength is defined by the fact that 1 A is the strength of the time-constant electric current that would produce a force of 2·10 −7  Newtons per meter of conductor length in a vacuum between two parallel, infinitely long, straight conductors of a negligibly small, circular cross-section and the distance of 1 m between such conductors. From this, it is not possible to find a direct solution for providing a reference value for current strength that is both economical and practical. This also applies, for example, to voltage, magnetic field and electric field. 
     Finally, the measurement of magnetic flux densities is a basic function of sensor technology, whereby the goal is to further improve the resolution and robustness of corresponding sensors. 
     From DE 10 2015 208 151 A1 and DE 10 2017 205 099 A1, methods are known with which the fluorescence intensity of a diamond material with a nitrogen-vacancy center is evaluated in order to be able to deduce the magnetic field acting on the nitrogen-vacancy center and the electric current of a conductor causing such magnetic field. Thereby, two different excitations, specifically an optical and a microwave excitation of the nitrogen vacancy center, are used, which makes the corresponding device expensive and complex. 
     From A. Wickenbrock et al. “Microwave-free magnetometry with nitrogen-vacancy centers in diamond,” Appl. Phys. Lett. 109, 053505 (2016), the measurement of magnetic fields without the use of microwaves is known. However, the problem exists that the flux density must be precisely adjusted. 
     Within the framework of the present disclosure, the magnetic field is distinguished between magnetic field strength (magnetic intensity) and magnetic flux density. Strictly speaking, the magnetic flux density B depends on the magnetic intensity H according to the relation B=μ 0  μ r  H, where μ 0  is the vacuum permeability and μ r  is the relative permeability. Thereby, the magnetic flux density is measured, while a magnetic field with a certain field strength is generated when current flows through a conductor, in particular a coil. 
     SUMMARY 
     It is the object of the present disclosure to provide a device and a method with which a magnetic field strength can be generated and controlled. Preferably, this is to eliminate one or more of the aforementioned disadvantages. Preferentially, this is to allow a reference value of a physical variable to be generated relatively simply and economically. In particular, the device is also to be very small and used in mobile form. Furthermore, it would be advantageous if the device could also be used to measure magnetic flux densities with high resolution and, in particular, highly robustly. 
     Such object is achieved with the device as claimed and the method as claimed. 
     On the part of the inventor, it was recognized that this object can be achieved in a surprising way by the fact that the magnetic field strength is traced back to a resonance extreme value in the luminescence, which is dependent on it in a material, or in the photocurrent, as the case may be. Since this dependence is very well known and also very stable, corresponding magnetic field strengths can be controlled very reliably and reproducibly. More precisely, the magnetic field strength essentially depends on the magnetic flux density B triggering the resonance extreme value at the location of the luminescence or the photocurrent center, as the case may be, and thus advantageously exclusively depends on natural constants that do not require calibration. For example, the resonance point represents a pole of the inverted luminescence value as a function of the magnetic flux density B. Due to this point-shaped resonance point in relation to the parameter of the magnetic flux density B, the smallest deviations of the magnetic flux B lead to a very high signal sensitivity, such that the magnetic field strength can be generated very accurately, independent of other influences. From the literature (see, for example, J. Lepper, “ Der Zusammenhang von Elektronenemission und Lumineszenzerscheinungen angeregter Kristalle,”  Z. Naturforschg. 10a, 47-52 (1955), it is known that there is a close relationship between the luminescence and the photocurrent. 
     The device for generating and controlling a magnetic field strength is characterized by:
         a first material that generates a luminescence and/or a photocurrent upon physical excitation, wherein the luminescence or the photocurrent, as the case may be, has an extreme value (in the form of a resonance) with respect to a magnetic field at at least one magnetic flux density,   means for generating the physical excitation,   means for magnetic field generation, wherein the first material is exposed to the generated magnetic field, wherein the magnetic field strength is adjustable,   means for measuring the luminescence or the photocurrent, as the case may be, with a detection signal that represents the strength of the detected luminescence signal or photocurrent signal, as the case may be,   means for control, which are adapted to output a control signal for controlling the means for magnetic field generation and to control the magnetic field strength as a function of the detection signal, such that the detection signal assumes an extreme value.       

     “Means for generating the physical excitation” means any means by which the physical excitation can be generated. In doing so, it does not matter that the magnitude of the physical excitation is actually variable. It is sufficient if the physical excitation can be generated such that the resonance occurs in the luminescence or the photocurrent, as the case may be. In a preferred further development, the magnitude of the excitation, for example its frequency, can be tuned discretely or continuously over at least a certain range by the means for generating the physical excitation. 
     To determine the extreme value, there may preferentially be means for detecting the first and/or the second derivative of the detection signal as a function of the magnetic flux density. While maxima or minima, as the case may be, in the detection signal can be determined via zero points in the first derivative, extreme values in the first derivative can be controlled via zero points in the second derivative. The second variant is highly advantageous, because at that point control can take place with the highest sensitivity. “Extreme value in the detection signal” thus means not only a maximum or minimum, as the case may be, in the detection signal, but also a maximum or minimum in the first derivative of the detection signal; “extreme value in the detection signal” is thus preferentially to be read as “extreme value in the detection signal or in the first derivative of the detection signal.” 
     In an advantageous further development, means are provided for generating a physical variable as a function of the control signal. This allows reference values to be produced very accurately and reproducibly. Thereby, a strict distinction is made between “physical variable,” which refers to the reference value, and “physical excitation,” which refers to a specific physical variable that is used to generate the luminescence or the photocurrent, as the case may be, in the material. 
     In an advantageous further development, it is provided that the physical variable is a variable from the group consisting of electric current, electric voltage, magnetic field (magnetic flux density B or magnetic field strength H), electric field, time and frequency. Such reference values are needed particularly often and for different fields of application. 
     In an advantageous further development, it is provided that the means for generating the physical variable comprises at least one element selected from the following group: Current mirror, source impedance, coil, capacitor, transconductance amplifier and operational amplifier. Then, reference values for electric current, electric voltage, magnetic field, electric field, time and frequency can be generated. 
     In an advantageous further development, it is provided that furthermore there are:
         a further material that generates a luminescence and/or a photocurrent upon a further physical excitation, wherein the luminescence or the photocurrent, as the case may be, has an extreme value (in the form of a resonance) with respect to a magnetic field at at least one magnetic flux density,   further means of generating the further physical excitation,   further means for magnetic field generation, wherein the further material is exposed to the generated magnetic field, wherein the magnetic field strength is adjustable, wherein the further means for magnetic field generation is adapted to expose the further material to the same or a different magnetic field as the first material,   further means for measuring the luminescence or the photocurrent, as the case may be, of the further material with a further detection signal, which represents the strength of the detected further luminescence signal or the further photocurrent signal, as the case may be. At that point, gradients in a magnetic field can be determined very easily. Instead of a combination of this further material and the further means, any combination thereof or even the further material or any of the further means alone may be used. A plurality of further materials and/or a plurality of further means can also be used.       

     The “physical excitation” can be identical to the “further physical excitation,” such that the “means for generating the physical excitation” can also be identical to the “further means for generating the further physical excitation.” Moreover, the “means for magnetic field generation” can be identical to the “further means for magnetic field generation.” 
     In an advantageous further development, it is provided that the first material and/or the further material has a crystal structure with at least one vacancy, wherein the first material or the further material, as the case may be, is preferably diamond, silicon carbide or silicon and the vacancy is a color center, in particular a nitrogen vacancy center or a nitrogen vacancy center in conjunction with a europium vacancy center, a vanadium vacancy center or a manganese vacancy center. When using diamond, the use of ST1 and L2 centers is also conceivable. Preferably, it is a fluorescent material. Such materials have very well-known resonance points in their luminescence. At this point, reference should be made to A. Zaitsev, “Optical Properties of Diamond,” 2001, Springer, Berlin, where numerous suitable color centers in diamond are indicated. The disclosure content of such publication with respect to color centers in diamond is hereby incorporated by reference in its entirety. 
     In an advantageous further development, it is provided that the means for generating the physical excitation or the further means for generating the physical excitation, as the case may be, provide an electrical or an electromagnetic excitation, preferably an excitation in the optical range and/or in the microwave range, in particular a LASER excitation or an LED excitation or an excitation by means of ionizing radiation. This makes it very easy to excite a luminescence or a photocurrent, as the case may be, and the means can be kept very compact. By using microwave excitation, fine structures in the luminescence or photocurrent spectrum can be resolved, further improving the accuracy of the generated reference value. 
     If electrical excitation (for example with electrical contacts) is used instead of electromagnetic excitation, then LEDs can be omitted for optical excitation, for example, which makes the corresponding sensor system much more compact. In addition, it would be possible to greatly increase the spatial resolution of a corresponding sensor. More specifically, excitation could be limited to a much smaller area by electrical contacts and, unlike optical detection, signals would not be obtained from the entire material. Until now, electrical excitation had failed because the NV center in diamond was ionized; however, this can be prevented by raising the Fermi level, for example, by phosphorus doping in diamond. 
     In an advantageous further development, it is provided that electrical contacts are transparent on the first material and/or the further material. For example, so-called “transparent conducting oxides (TCO),” such as indium tin oxide (“ITO”), fluorine tin oxide, aluminum zinc oxide or antimony tin oxide, could be used. Such contacts can be used both within the framework of reading the photocurrent and for electrical excitation. 
     Exemplary methods of electrical excitations of color centers in diamond are known, for example, from DE 43 22 830 A1 and from B. Burchard “Elektronische und optoelektronische Bauelemente und Bauelemente Grundstrukturen auf Diamantbasis,” Dissertation, Hagen, 1994. In particular, the excitation of luminescence can be carried out by PIP or PIN diodes. In this regard, exemplary reference should also be made to B. Tegetmeyer et al., “Charge state modulation of nitrogen vacancy centers in diamond by applying a forward voltage across a p-i-n junction” Diamond and Related Materials, Volume 65, May 2016, pages 42-46 and B. Tegetmeyer et al., “Electroluminescence from silicon vacancy centers in diamond p-i-n diodes” Diamond and Related Materials Volume 65, May 2016, pages 42-46. Excitation via PN diodes is also conceivable. With the exemplary use of diamond and a PIP diode, when a sufficiently high voltage is applied to the PIP diode, a first doped region, for example a p+ doped region, injects charge carriers—in this case holes—into the intrinsic region. There, they are accelerated by the electric field of the applied voltage and excite the color centers by impacts, whereupon they start to glow. Excitation via injection of electrons and holes, for example in PIN diodes, is more efficient. By means of bandgap engineering or the use of other materials, direct transitions can also be created, but then other color centers must be used, since color centers always exist in a manner specific to the material. It is conceivable to couple LEDs, such as those described above, only capacitively or by means of transformers or otherwise in a manner galvanically isolated to an AC signal source. This is particularly advantageous for CMOS integration. 
     In an advantageous further development, it is provided that the means for magnetic field generation or the further means for magnetic field generation, as the case may be, comprise at least one coil, which can be excited by an electric current, wherein there is preferably a magnetic conductor, in particular in the form of a yoke with an air gap, wherein the first material and/or the further material are arranged in the air gap. This allows the magnetic field to be produced very easily and at the same time very homogeneously over the first material or the further material, as the case may be. Helmholtz coil arrangements can also be used, possibly combined with one or more yoke elements in order to achieve a concentration, wherein once again an air gap can be used. 
     In an advantageous further development, the means for measuring luminescence and/or the further means for measuring luminescence comprise a photodetector, and preferably a filter that attenuates the physical excitation. This makes it very easy to measure luminescence, wherein miniaturization is readily possible. 
     In an advantageous further development, it is provided that the means for measuring the photocurrent and/or the further means for measuring the photocurrent are formed in such a manner that a voltage is applied between two electrical contacts on the first material and the further material, as the case may be, and the photocurrent is measured in series, wherein the measurement is carried out, for example, with a lock-in amplifier or simply via the voltage drop across a known resistor. Photocurrent measurement could also be carried out using photodiodes and/or CCD elements and/or avalanche photodiodes and/or SPADs (single photon avalanche photodiodes). Another simple device for measuring the photocurrent is known from DE 10 2010 060 527 B3. When using such technology, the controllers must be clocked. They are then preferably implemented in whole or in part in switched-capacitor technology. At that point, the measurement of the photocurrent can be done particularly easily. In particular, the corresponding device can be set up in a particularly compact manner. 
     In an advantageous further development, it is provided that the means for control are formed as a continuous linear controller, preferably as a PI or PID controller. This enables particularly precise control to the extreme value of the resonance, which further improves the accuracy and stability of the reference value. 
     In an advantageous further development, it is provided that at least one high-pass filter exists for filtering the detection signal and/or the further detection signal. At that point, a high-frequency useful signal imposed on the magnetic field can be determined, by which a broadband information transmission is enabled. In addition, a magnetic background field can also be analyzed very precisely. 
     In an advantageous further development, it is provided that there is at least one low-pass filter for filtering the control signal. This low-pass filter can exist separately; however, the means for control can also provide such a low-pass filter function. This prevents high-frequency components from flowing into the control and not being present in the detection signal or only being present in a modified form. 
     In an advantageous further development, it is provided that the magnitude of the lower cut-off frequency of the high-pass filter is greater than the magnitude of the upper cut-off frequency of the low-pass filter or the means for control, as the case may be. At that point, the broadband information transmission or the analysis of the background magnetic field, as the case may be, is possible very precisely, wherein, in particular, disturbances in the control of the operating point of the magnetic flux density B acting on the material are prevented. The device can then be used for typically highly broadband reception of useful signals by means of naturally or artificially modulated electromagnetic radiation by evaluating the signal of the magnetic flux density. Naturally modulated radiation can be, for example, electromagnetic fields generated during the transport of nerve signals in organisms. The invention proposed herein, if suitably designed to detect such biologically modulated signals, can find particular application in medical technology. The device and method are therefore particularly suitable for electroencephalography. 
     In an advantageous further development, it is provided that the means for magnetic field generation have at least two, preferably three, magnetic regions that have different magnetic resistances, such that at least two, preferably three, material regions of the first material or of the further material, as the case may be, are exposed to different magnetic fields, wherein each material region is assigned its own means for measuring the luminescence or the photocurrent, as the case may be, with a respective detection signal that represents the strength of the detected luminescence signal or photocurrent signal, as the case may be, wherein the means for control are adapted to output a control signal for the control of the means for magnetic field generation and to control the magnetic field strength as a function of the detection signal in such a manner that the detection signal of a material region assumes an extreme value. This effectively prevents the means for control from “running out” of the resonance point. Preferably, the different magnetic resistances are adjusted so that the resulting deviation in the magnetic flux density B is not greater than the statistical value Sigma in the signal curve approximated as a Gaussian function at the resonance point. In particular, the magnetic resistances are to be adapted so that the resulting magnetic flux densities differ in the range of 0.5 to 3 mT, preferably 1-2 mT. At that point, a particularly stable control is achieved. 
     In an advantageous further development, it is provided that the means for magnetic field generation and/or the further means for magnetic field generation have magnetic shielding against external magnetic fields. At that point, the generated magnetic field strength is independent of external magnetic fields. 
     In an advantageous further development, it is provided that the means for generating the magnetic field are adapted to supply an external magnetic field to the material. At that point, such external magnetic fields can be analyzed particularly easily. 
     In an advantageous further development, it is provided that the means for generating the physical excitation are adapted to undertake a modulation of the physical excitation, wherein means exist for correlating the modulation of the physical excitation with the detection signal. Thus, there is a very precise and stable generation of the magnetic field strength. 
     A method for generating and controlling a magnetic field strength includes the use:
         of a first material that generates a luminescence and/or a photocurrent upon physical excitation, wherein the luminescence or the photocurrent, as the case may be, has an extreme value (in the form of a resonance) with respect to a magnetic field at at least one magnetic flux density,   of means for generating the physical excitation,   of means for magnetic field generation, wherein the first material is exposed to the generated magnetic field, wherein the magnetic field strength is adjustable, and   of means for measuring the luminescence or the photocurrent, as the case may be, with a detection signal that represents the strength of the detected luminescence signal or photocurrent signal, as the case may be,   wherein a control signal is output for controlling the means for magnetic field generation in order to control the magnetic field strength as a function of the detection signal such that the detection signal assumes an extreme value.       

     A receiver in accordance with the disclosure receives electromagnetic waves modulated with a useful signal with at least one color center, wherein the color center has a luminescence and wherein the luminescence is dependent on a magnetic flux density at the location of the color center, wherein the receiver is adapted to demodulate the useful signal via such luminescence and/or wherein the color center is an antenna for the modulated electromagnetic waves. This allows a broadband receiver to be provided, particularly for encrypted communications. 
     A color center may be used as an antenna as a sub-device for receiving and/or demodulating a modulated electromagnetic signal, in particular a modulated message or measurement signal. 
     Finally, a color center may be used as an antenna as a sub-device for receiving and/or demodulating a biologically modulated electromagnetic signal, in particular a modulated nerve signal. 
     In an advantageous further development, it is provided that the device in accordance with the invention is used. 
     The features and further advantages of the present invention will become apparent below from the description of preferred exemplary embodiments in connection with the figures. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1 a    shows the dependence of the photoluminescence of a NV center in diamond on the magnitude of the magnetic flux density. 
         FIG. 1 b    shows the dependence of the first derivative of the photoluminescence of a NV center in diamond on the magnitude of the magnetic flux density. 
         FIG. 2  illustrates the basic principle on which the present invention is based. 
         FIG. 3  shows an alternative embodiment of the basic principle according to  FIG. 2 . 
         FIG. 4  shows a device for generating and controlling a magnetic field strength according to a first preferred embodiment. 
         FIG. 5  shows a device for generating and controlling a magnetic field strength according to a second preferred embodiment. 
         FIG. 6  shows a device for generating and controlling a magnetic field strength according to a third preferred embodiment. 
         FIG. 7  shows a device for generating and controlling a magnetic field strength according to a fourth preferred embodiment. 
         FIG. 8  shows a device for generating and controlling a magnetic field strength according to a fifth preferred embodiment. 
         FIG. 9  shows a device for generating and controlling a magnetic field strength according to a sixth preferred embodiment. 
         FIG. 10  shows the planar implementation of the sixth preferred embodiment according to  FIG. 9 . 
         FIG. 11  shows a device for generating and controlling a magnetic field strength according to a seventh preferred embodiment. 
         FIG. 12  shows a device for generating and controlling a magnetic field strength according to an eighth preferred embodiment. 
         FIG. 13  shows a PI controller for use in one of the devices for generating and controlling a magnetic field strength. 
         FIG. 14  shows a controller for use in the device according to  FIG. 3 . 
         FIG. 15  shows a current mirror arrangement for use in one of the devices according to  FIGS. 4, 5 or 7 . 
     
    
    
     DETAILED DESCRIPTION 
     In the following, identical reference signs are used for identical elements. 
     Color centers are lattice defects in crystals that absorb electromagnetic radiation (for example in the UV, visible and/or IR range). This creates bands with a specific wavelength. 
     The so-called “nitrogen vacancy” (NV center) in diamond, one of over 100 known defects in diamond, which is caused by a carbon atom in the diamond lattice being substituted by a nitrogen atom, and the neighboring atom of which is missing, has been very well investigated. The type  1   b  diamond, with which such NV centers are uniformly distributed over the entire diamond structure, is known. 
     The photoluminescence of such NV center is strongly dependent on the magnetic flux of a magnetic field acting on the NV center. 
       FIGS. 1 a  and 1 b    show the photoluminescence and the first derivative of the photoluminescence of such a NV center in diamond, as a function of the magnitude of the magnetic flux density, wherein the field is aligned in the NV axis. At a flux density of |B|=102.4 mT, the photoluminescence assumes a minimum. This flux density is also called “GSLAC flux density” (GSLAC—ground state level anti crossing). The theory predicts that photoluminescence should have a clear minimum here. The minimum is extremely sharp and has a contrast of more than 4.5%, which maximizes sensitivity. Moreover, this point does not depend on environmental parameters, which is a significant advantage. (Theoretically, there are dependencies on an external E-field, the pressure and the temperature. However, since such dependencies are very weak, they are negligible). 
     In addition, at about 60 mT, the NV-NV resonance and, at about 51.2 mT, the NV-P1 resonance exist. 
     Luminescence of negatively charged NV centers occurs in a wavelength range of 600-800 nm with a zero-phonon line at 638 nm. The NV center can be excited with wavelengths less than or equal to 638 nm. By means of impressed mechanical stresses in the crystal (for example by cavities or a high pressure), it is possible to shift the necessary excitation energies, and to change the luminescence characteristics. 
       FIG. 2  shows the electrical control circuit  10  of the basic principle of the present disclosure. It can be seen that the control circuit  10  has a magnetic subcircuit  12 , which comprises, by way of example, a yoke arrangement  14  comprising, for example, two cores  16 ,  18  of preferentially ferromagnetic material (for example, an iron core), between which an air gap  20  is arranged. For example, the first core  16  is wound with a coil  22  having a certain number of windings, which is used to magnetically excite the magnetic subcircuit  12 . 
     For example, the air gap  20  contains a type  1   b  diamond  24  with a NV center  26  as the luminescent material, which is irradiated by a LASER diode  28  with electromagnetic radiation  30  of a first wavelength, which is sufficient to excite photoluminescence in the diamond  24 . The NV center  26  then emits electromagnetic radiation  32  of a second wavelength, which is different from the first wavelength. For example, the first wavelength is 532 nm and the second wavelength is in the range between 600 and 800 nm. 
     Furthermore, there is a photodetector  34  that receives the emitted radiation  32 . In order to keep the electromagnetic radiation  30  of the first wavelength out of the reception path for the electromagnetic radiation  32  of the second wavelength, an optical filter  35  is inserted into the emission path, which does not let through or at least sufficiently attenuates the electromagnetic radiation  30  of the first wavelength, and which transmits the electromagnetic radiation  32  of the second wavelength or at least sufficiently transmits it without attenuation. Other optical components, such as beam splitters and the like, can also be provided in the optical path as needed. 
     The photodetector  34  detects the second wavelength electromagnetic radiation  32  and generates a detection signal  36  as a function of such detected second wavelength electromagnetic radiation  32  from the NV center  26 . The value of the detection signal  36  is preferably proportional to the amplitude magnitude of the energy intensity of the electromagnetic radiation  32  of the second wavelength. At a minimum, however, the detection signal  36  is dependent on the energy intensity of the electromagnetic radiation  32  of the second wavelength. 
     Finally, a controller  38  is provided, which generates the control signal  40  from the detection signal  36  to control the coil  22  via the electrical conductor  42 . The magnetic excitation generated by the coil  22  thus depends on the control signal  40 . Thereby, the control of the controller  38  is selected to minimize the emission of electromagnetic radiation  32  of the second wavelength by the first NV center  26 . Thereby, the controller is, for example, a PI or PID controller of known type, wherein a PID controller is preferred, with which control is performed on the zero point of the derivative of the detection signal measured by the photodetector  34 . The derivative can be the first or second derivative of the detection signal. 
     With suitable magnetic shielding (not shown) of the diamond  24 , the control signal  40  is dependent only on the geometric parameters of the magnetic subcircuit  12 , such that the control signal  40  can be taken as the basis for a physical reference value. However, even if there is no magnetic shielding, the control signal  40  can be taken as the basis for a physical reference value, specifically if the magnitude of an external magnetic field, such as the earth&#39;s magnetic field, is known. Such an external magnetic field can be used to selectively reduce the control signal  40 , which is useful for certain applications. In either case, the control circuit  10  provides a magnetic field strength H by exciting the coil  22 . 
     For clarity, it is assumed that the luminescence L around the resonance site is approximated by the function 
             L   =         L   0     [     1   -     e     -         (     B   -     B   0       )     2       σ   2             ]     .           
Thereby, the flux density B 0  is the position of the resonance point and σ its width. At the beginning of the control, a starting value for the magnetic flux density B is to be taken, which ensures that the control proceeds in the direction of the luminescence extremum.
 
     Just below the resonance point is the sensitivity 
     
       
         
           
             
               
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     Thus, the luminescence can be estimated as a function of the magnetic flux density B just below the resonance point B 0  at a location B k  for a flux density deviation ΔB as 
     
       
         
           
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     Thus, as the level P 36  of the detection signal  36  of the photodetector  34 , assuming that it provides a value proportional to the intensity of the luminescence L, the following is obtained: 
     
       
         
           
             
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     Thereby, k is a constant of proportionality. The controller  38  then provides an electrical current proportional to the level P 36  of the detection signal  36  from the photodetector  34 . The integrating property of the controller  38 , which is preferably a PI controller, is omitted here for simplicity, and only a variant with a P controller is described. 
     The control signal R 40  is then described as: 
     
       
         
           
             
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                       Δ 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       B 
                     
                   
                   ) 
                 
                 . 
               
             
           
         
       
     
     Thereby, f is another constant of the controller, typically the gain. The sign of -f is preferably selected in such a way that the change ΔB is compensated for later, so that stability is established. The coil  22  converts the control signal  40  into an additional magnetic excitation H, which acts as an additional magnetic flux ΔB L  again on the NV center  26 . Thus, this results in 
     
       
         
           
             
               Δ 
               ⁢ 
               
                 B 
                 L 
               
             
             ∼ 
             
               g 
               * 
               f 
               * 
               k 
               * 
               
                 ( 
                 
                   
                     L 
                     0 
                   
                   - 
                   
                     2 
                     ⁢ 
                     
                       
                         
                           B 
                           k 
                         
                         - 
                         
                           B 
                           0 
                         
                       
                       
                         σ 
                         2 
                       
                     
                     ⁢ 
                     
                       
                         e 
                         
                           - 
                           
                             
                               
                                 ( 
                                 
                                   
                                     B 
                                     k 
                                   
                                   - 
                                   
                                     B 
                                     0 
                                   
                                 
                                 ) 
                               
                               2 
                             
                             
                               σ 
                               2 
                             
                           
                         
                       
                       ⁡ 
                       
                         ( 
                         
                           
                             Δ 
                             ⁢ 
                             B 
                           
                           + 
                           
                             Δ 
                             ⁢ 
                             
                               B 
                               L 
                             
                           
                         
                         ) 
                       
                     
                   
                 
                 ) 
               
             
           
         
       
     
     Resolution according to ΔB L  giving rise to: 
     
       
         
           
             
               Δ 
               ⁢ 
               
                 B 
                 L 
               
             
             ∼ 
             
               
                 
                   g 
                   * 
                   f 
                   * 
                   k 
                   * 
                   
                     ( 
                     
                       
                         L 
                         0 
                       
                       - 
                       
                         2 
                         ⁢ 
                         
                           
                             
                               B 
                               k 
                             
                             - 
                             
                               B 
                               0 
                             
                           
                           
                             σ 
                             2 
                           
                         
                         ⁢ 
                         
                           e 
                           
                             - 
                             
                               
                                 
                                   ( 
                                   
                                     
                                       B 
                                       k 
                                     
                                     - 
                                     
                                       B 
                                       0 
                                     
                                   
                                   ) 
                                 
                                 2 
                               
                               
                                 σ 
                                 2 
                               
                             
                           
                         
                       
                     
                     ) 
                   
                 
                 
                   1 
                   - 
                   
                     g 
                     * 
                     f 
                     * 
                     k 
                     * 
                     
                       ( 
                       
                         
                           L 
                           0 
                         
                         - 
                         
                           2 
                           ⁢ 
                           
                             
                               
                                 B 
                                 k 
                               
                               - 
                               
                                 B 
                                 0 
                               
                             
                             
                               σ 
                               2 
                             
                           
                           ⁢ 
                           
                             e 
                             
                               - 
                               
                                 
                                   
                                     ( 
                                     
                                       
                                         B 
                                         k 
                                       
                                       - 
                                       
                                         B 
                                         0 
                                       
                                     
                                     ) 
                                   
                                   2 
                                 
                                 
                                   σ 
                                   2 
                                 
                               
                             
                           
                         
                       
                       ) 
                     
                   
                 
               
               ⁢ 
               Δ 
               ⁢ 
               B 
             
           
         
       
     
     This can be written as: 
     
       
         
           
             
               Δ 
               ⁢ 
               
                   
               
               ⁢ 
               
                 B 
                 L 
               
             
             ∼ 
             
               
                 1 
                 
                   
                     1 
                     
                       g 
                       * 
                       f 
                       * 
                       k 
                       * 
                       
                         ( 
                         
                           
                             L 
                             0 
                           
                           - 
                           
                             2 
                             ⁢ 
                             
                               
                                 
                                   B 
                                   k 
                                 
                                 - 
                                 
                                   B 
                                   0 
                                 
                               
                               
                                 σ 
                                 2 
                               
                             
                             ⁢ 
                             
                               e 
                               
                                 - 
                                 
                                   
                                     
                                       ( 
                                       
                                         
                                           B 
                                           k 
                                         
                                         - 
                                         
                                           B 
                                           0 
                                         
                                       
                                       ) 
                                     
                                     2 
                                   
                                   
                                     σ 
                                     2 
                                   
                                 
                               
                             
                           
                         
                         ) 
                       
                     
                   
                   - 
                   1 
                 
               
               ⁢ 
               Δ 
               ⁢ 
               
                   
               
               ⁢ 
               B 
             
           
         
       
     
     Now, the gain is adjusted to be very high, preferably in the range around 10 10  or higher. At that point, if the sign of -f is selected correctly, the equation is simplified to:
 
ΔB L ˜ΔB
 
     Yet, this also means that 
     
       
         
           
             
               R 
               
                 4 
                 ⁢ 
                 0 
               
             
             ∼ 
             
               f 
               * 
               k 
               * 
               
                 
                   ( 
                   
                     
                       L 
                       0 
                     
                     - 
                     
                       2 
                       ⁢ 
                       
                         
                           
                             B 
                             k 
                           
                           - 
                           
                             B 
                             0 
                           
                         
                         
                           σ 
                           2 
                         
                       
                       ⁢ 
                       
                         e 
                         
                           - 
                           
                             
                               
                                 ( 
                                 
                                   
                                     B 
                                     k 
                                   
                                   - 
                                   
                                     B 
                                     0 
                                   
                                 
                                 ) 
                               
                               2 
                             
                             
                               σ 
                               2 
                             
                           
                         
                       
                       ⁢ 
                       Δ 
                       ⁢ 
                       
                         B 
                         L 
                       
                     
                   
                   ) 
                 
                 . 
               
             
           
         
       
     
     The control signal is thus differentially proportional to the additional flux density ΔB L  superimposed in the NV center  26 . Thus, the control signal  40  can be used as a measure for an additional superimposed magnetic flux density ΔB L . 
     By combining one NV center with different paramagnetic centers NV-EuV, NV-NV, NV-MgV, different calibration points can be generated and read out. Examples of other suitable centers are the St1 and L2 centers. The St1 center has an emission line of 581 nm and must be excited with light of higher energies than the NV center. 
     Furthermore, it is possible to create suitable spin systems in the form of suitable color centers in SiC and Si in place of the diamond  24  and the NV center in diamond  24 . 
     Through the selection of suitable materials and their defects along with the suitable selection of the parameters of the magnetic subcircuit  12 , the required magnetic flux density and thus also the required control signal  40  can thus be selected in a targeted manner. 
     Since, depending on the control algorithm, the magnetic flux density is preferably linearly dependent on the control signal  40 , such control signal thus only needs to be calibrated once with respect to the actual parameters of the magnetic subcircuit  12 , thus in particular the winding diameter, the number of windings, the conductor thickness and the yoke arrangement  14 . In the case of high-precision manufacturing techniques, such as microtechnical manufacturing processes, calibration may not even be necessary. 
       FIG. 3  shows an alternative embodiment of the control loop  50 . It can be seen that here, in turn, a luminescent material  24  is arranged in the air gap  52  of a yoke arrangement  54  and is irradiated by a LASER diode  28 . 
     In contrast to the control circuit  10  according to  FIG. 2 , the yoke arrangement  54  here is closed except for the air gap  52 , wherein, however, there are three different magnet regions  56 ,  58 ,  60  arranged parallel to each other at the air gap  52 . Each of such magnetic regions  56 ,  58 ,  60  affects different NV centers  62 ,  64 ,  66  in the diamond  22 , resulting in different electromagnetic radiations  68 ,  70 ,  72  of a second wavelength being emitted after excitation by the LASER diode  28  with an electromagnetic radiation  30  of a first wavelength. There is in turn a filter  74  for the electromagnetic radiation  30  of the first wavelength, and each electromagnetic radiation  68 ,  70 ,  72  of the second wavelength is received by a respective associated photodetector  76 ,  78 ,  80  and the corresponding detection signals  82 ,  84 ,  86  are fed as input signals to a PID controller  88 , which outputs a control signal  90  via the electrical conductor  92  to generate an electric field strength by means of the coil  94 . 
     The magnetic regions  56 ,  58 ,  60  are formed so that they each have different magnetic resistances. More specifically, for example, the magnetic regions  56 ,  58 ,  60  are formed such that the magnetic flux B in the gap  52  has a value B for the magnetic region  58 , a value B−1 mT for the magnetic region  56 , and a value B+1 mT for the magnetic region  60 . 
     This realizes a multidimensional control, which prevents controller  88  from running outside the actual zero crossing of the first derivative of the GSLAC resonance point. Due to the spaced flux densities of the different magnet regions  56 ,  58 ,  60 , the controller  88  always recognizes where it is located in the resonance point. The distance between the generated magnetic flux densities is preferably not more than the width of the resonance point, i.e. 1σ. Thus, the value of ±1 mT can also be selected differently, but it should not be greater than 1σ. Preferably, such distances are thereby selected symmetrically to a symmetry point. 
     It can be that only two such magnet regions are used, but three magnet regions  56 ,  58 ,  60  are preferred because, at that point, the second derivative of the detection signal, in this case the luminescence curve, can be formed. At that point, it is possible to control for zero crossings in this second derivative, i.e. maxima or minima, as the case may be, in the first derivative, wherein the highest control sensitivity exists. 
     In principle, such embodiment according to  FIG. 3  can be advantageously used in all devices  100 ,  200 ,  300 ,  400 ,  500 ,  600 ,  700 ,  800  described below. 
       FIG. 4  shows a first preferred embodiment of the device  100  for generating and controlling a magnetic field strength. 
     It can be seen that such device has essentially the control circuit  10  of  FIG. 2 , with the exception that, instead of the magnetic subcircuit  12  with the two cores  16 ,  18 , a magnetic circuit  102  with a continuous magnetic yoke  104  with the air gap  106  has been selected here, by which the magnetic field induced by the coil  108  is particularly homogeneous in the air gap  106 . 
     In other respects, the device  100  comprises an electric current mirror  110 , by which the electric current for the coil  108  provided by the control signal  112  is converted into an output current  114  (I 0 ). If the device  100  is sufficiently shielded against irradiation of magnetic and electromagnetic fields from the outside, the magnitude |I 0 | of this current  114  is independent of environmental parameters. Therefore, the device  100  is suitable as a high accuracy reference current source. By adjusting different mirror ratios, it is very easy to adjust different reference current values, which are then determined only by the ratios of the transistor areas within the current mirror  110 . With regard to circuit technology, reference is made here by way of example to Tietze, Schenk, “Halbleiter-Schaltungstechnik,” Springer. 
       FIG. 5  shows a second preferred embodiment of the device  200  for generating and controlling a magnetic field strength. 
     It can be seen that there is a modification in this regard with respect to the device  100  of  FIG. 4 , in that a resistor  202  is used as a source resistor in addition to the current mirror  110 . Through this source resistor  202  (Rq), the output current  114  is converted to a reference voltage  204  (V 0 ). Therefore, the device  200  can be considered a high accuracy reference voltage source with internal resistor (Rq). Through the suitable selection or adjustment, as the case may be, of the source resistor  202  and/or suitable adjustment of the ratio of the current mirror  110 , the value of the reference voltage  204  can be accurately determined. 
       FIG. 6  shows a third preferred embodiment of the device  300  for generating and controlling a magnetic field strength. 
     It can be seen that, unlike the device  100  of  FIG. 4 , no current mirror was used here, but a second coil  304  wound around a core  302  was connected in series with the first coil  108 . 
     Through the selection of the geometry of the second coil  304  and the number of its coil windings and the materials of the core  302 , the magnetic field of such second magnetic circuit  306  can be adjusted in a targeted manner. If necessary, a current divider (not shown) can be used to adjust in a targeted manner and, if necessary, adapt the excitation current of the second coil  304 . 
     Thus, such device  300  provides a highly accurate reference source for a magnetic field, and specifically for a magnetic field strength H, which is generated by the magnetic subcircuit  306 . Instead of such a magnetic subcircuit  306 , magnetic circuits can of course also be used. With respect to the construction and use of magnetic circuits, reference is made to “Küpfmüller, Theoretische Elektrotechnik Elektromagnetische Felder, Schaltungen and elektronische Bauelemente,” Springer. 
       FIG. 7  shows a fourth preferred embodiment of the device  400  for generating and controlling a magnetic field strength. 
     It can be seen that here, in contrast to the device  200  of  FIG. 4 , a capacitor  402  is connected in parallel in addition to the source resistor  202 , by which the capacitor  402  charges until its voltage is identical to the voltage V 0  across the source resistor  202 . 
     Through the selection of the geometry of the capacitor  402  and its materials, the electric field between the capacitor plates  404 ,  406  can be adjusted in a targeted manner. If necessary, a voltage divider (not shown) can be used to adjust in a targeted manner and, if necessary, adapt the excitation voltage of the capacitor  402 . 
     Such device  400  thus provides a highly accurate reference source for an electric field within the capacitor  402 . 
     On the other hand, the source resistor  202  could also be omitted, building up a voltage ramp across the capacitor  402  that can provide a reference time or a reference frequency, as the case may be. 
       FIG. 8  shows a fifth preferred embodiment of the device  500  for generating and controlling a magnetic field strength. 
     It can be seen that, in contrast to the control circuit  10 , a magnetic circuit  502  corresponding to  FIG. 4  is used here in principle, wherein, however, additional elements  504 ,  506  exist for introducing an external magnetic field. 
     The external magnetic field superimposes an external magnetic field strength on the magnetic field strength generated by the coil  108 , by which the magnetic flux density in the gap  106  is defined. The external magnetic field strength can thus be calculated from the control signal  112 . 
     Thus, such device  500  can be used to accurately determine an external magnetic field strength that is fed in. 
       FIGS. 9 and 10  show a sixth preferred embodiment of the device  600  for generating and controlling a magnetic field strength. 
     It can be seen that, here again, there is a magnetic circuit  602  formed by a yoke arrangement  604  and a coil  606 , wherein there are two air gaps  608 ,  610  in the yoke arrangement. Each of the air gaps contains a diamond  611   a,    611   b  with respective NV centers  612 ,  614 , each of which is excited by LASER diodes  616 ,  618  with electromagnetic radiation  620 ,  622  of a first wavelength. 
     Electromagnetic radiation  624 ,  626  of a second wavelength emitted from the respective diamond  611   a,    611   b  with NV centers  612 ,  614  is passed through a filter  628 ,  630 , in order to filter on electromagnetic radiation  620 ,  622  of the first wavelength. 
     Photodetectors  632 ,  634  detect respective electromagnetic radiation  624 ,  626  of the second wavelength and generate a respective detection signal  636 ,  638  as a function of such detected electromagnetic radiation  624 ,  626  of the second wavelength of the NV centers  612 ,  614 . The value of the detection signals  636 ,  638  is preferably proportional to the amplitude amount of the energy intensity of the respective electromagnetic radiation  624 ,  626  of the second wavelength. At least, however, the detection signal  636 ,  638  is dependent on the energy intensity, respectively, of the electromagnetic radiation  32  of the second wavelength. 
     Finally, on the side of the first NV center  612 , a controller  640  is provided, which generates the control signal  642  from the detection signal  636  to control the coil  606  via the electrical conductor  644 . The magnetic excitation generated by the coil  606  thus depends on the control signal  642 . The control of the controller  640  is thereby in turn selected to minimize the emission of electromagnetic radiation  624  of the second wavelength by the first NV center  612 . Thereby, the controller is, for example, a PI or PID controller of known type, wherein a PID controller is preferred, with which control is performed on the zero point of the derivative of the detection signal measured by the photodetector  632 . The derivative can be the first or second derivative of the detection signal. 
     Since the second NV center  614  is identical to the first NV center  612  and the other magnetic components, in particular the magnetic resistance in the air gap  610 , are also formed to be identical in principle, it is to be expected that the magnetic flux density B in both air gaps  608 ,  610  is identical and thus there is also a minimum for the detection signal  638  of the second NV center  614 . 
     However, due to magnetic field gradients of a background field, such as the earth&#39;s magnetic field, there is a deviation from the minimum of the detection signal  638 . Thus, from the magnitude of such deviation and the distance between the two NV centers  612 ,  614 , such gradient of the magnetic flux density of the background field can be directly inferred. Thereby, calibration only needs to be undertaken once with respect to the strength of the magnetic field deviation with respect to the detection signal  638  for the second NV center  614 . Since it is to be expected that the deviations in the magnetic field caused by the gradient are only very small over the distance between the two NV centers  612 ,  614 , a measuring amplifier  646  is also provided, which outputs a corresponding measurement signal  648 , which can be used to analyze the magnetic field gradient. Such embodiment of the device  600  can thus be used to determine a gradient in the background magnetic field with high accuracy. 
       FIG. 10  shows a planar embodiment of the device  600 . It can be seen that all elements are arranged on a planar substrate  650 . Such planar structure can be manufactured in a CMOS-compatible manner, wherein reference is made to DE 100 00 707 A1 for the generation in particular of the light sources (preferably LASER diodes)  616 ,  618 , the disclosure content of which is fully incorporated in this respect. 
     If there is magnetic shielding (not shown) of the first NV center  612 , at that point there is no influence of the magnetic background on the flux density B in the air gap  608 . As a result, only the magnetic flux density B in the air gap  610  is determined by the magnetic background, such that the detection signal  638  is a measure of the background magnetic field. Thus, such alternative embodiment of the device  600  can be used to determine a background magnetic field with high accuracy. 
       FIG. 11  shows a seventh preferred embodiment of the device  700 . 
     It can be seen that, here in turn, as in the device  100  of  FIG. 4 , there is a magnetic circuit  102  with a yoke  104 , in whose air gap  106  a diamond  24  with a NV center  26  is arranged. 
     The NV center  26  is in turn excited by a LASER diode  28  with electromagnetic radiation  30  of the first wavelength, by which the NV center  26  emits electromagnetic radiation  32  of the second wavelength. This radiation is in turn filtered  35  and fed to a photodetector  34 , which outputs the detection signal  36 . This detection signal  36  is fed to the PID controller  38 , which outputs the control signal  40  to the coil  108  via the current line  41 . 
     In addition, there is a high-pass filter  702  with a lower cut-off frequency f gu  for transmission. The controller  38  itself or an optionally provided low-pass filter  704  have an upper cut-off frequency f go  for transmission. Such upper cut-off frequency f go  is smaller than the lower cut-off frequency of the high-pass filter  702 , in order to thereby prevent interference in the control of the operating point of the magnetic flux density B, which acts on the NV center  26 . 
     Now, if an alternating electromagnetic field  706 , such as a modulated magnetic field, is irradiated and acts on the NV center  26 , the atoms or defects, as the case may be, involved will follow this alternating field  706  and the emitted fluorescence  32  will also be subject to modulation at a frequency that corresponds to the frequency of the alternating field  706 . 
     Due to the controller  38  and its low-pass characteristic or the additional low-pass filter  704 , as the case may be, there is no control on such modulation in the fluorescence if the frequency of the modulation is greater than the lower cut-off frequency f gu  of the high-pass filter  702 , such that only the basic fluorescence is controlled to a minimum by the controller  38 , but the modulation remains. The resulting high-frequency modulation of the detection signal  36  is read out by the high-pass filter  702  and output as a useful signal  708 . 
     Thus, the device  700  provides a highly sensitive antenna for the alternating magnetic field  706 . Based on the structure of the device  700  and its physical operation, it is an extremely wide band receiver, which can be used for ultra-wide band communications in particular. The high-pass filter  702  can preferably be formed as a measuring amplifier. 
     In addition to analyzing the directed modulations by an alternating electromagnetic field  706 , such device  700  can also be used to analyze the electromagnetic radiation spectrum or portions of the electromagnetic radiation spectrum at the location of the NV center  26 . 
     As is familiar to those skilled in the art, analysis of the useful signal  708  can include demodulation using, for example, mixers and filters. Here, reference is made, for example, to M. Bossert, “Nachrichtenübertragung” Vieweg+Teubner Verlag; Edition: 5., rev. ed. and add. edition 2011 (Aug. 4, 2011), A. Mertins, “Signaltheorie: Grundlagen der Signalbeschreibung, Filterbänke, Wavelets, Zeit-Frequenz-Analyse, Parameter- and Signalschätzung,” Springer Vieweg; Edition: 3., revised and ext. edition 2013 (Dec. 8, 2012) along with Lüke, “Signal Transmission: Fundamentals of Digital and Analog Message Transmission Systems,” Springer Vieweg; Edition: 12 (Feb. 19, 2015). 
       FIG. 12  shows an eighth preferred embodiment of the device  800 . 
     It can be seen that such device, which is an alternative to device  700  according to  FIG. 11 , is in principle based on the device  600  according to  FIG. 9 , wherein here, corresponding to the device  700  according to  FIG. 11 , the controller  640  has a low-pass characteristic or, optionally, there is a separate low-pass filter  802  with an upper cut-off frequency f go  for transmission, which filters the control signal  642 . In addition, the detection signal  638  from the photodetector  634  is filtered by the high-pass filter  804 , which has a lower cut-off frequency f gu  for transmission, and is fed to the sense amplifier  646 , which outputs a corresponding useful signal  806 . Furthermore, a magnetic shield  808 , for example made of a soft magnetic material, exists to prevent influences of electromagnetic radiation  706  or an electromagnetic background on the NV center  612 , as the case may be, in the air gap  608 . 
     Due to the magnetic shielding  808 , the magnetic flux density B in the air gap  608  is independent of external electromagnetic influences  706 . The first NV center  612  thus serves as a reference center for controlling the magnetic field strength H by means of the coil  606  via the controller  640 . 
     Only the second NV center  614  in the air gap  610  responds to the electromagnetic field  706 , such that its high-frequency components can be analyzed by means of the high-pass filter  804 . 
     Thus, such device  800  can provide even more accurate and stable information transmission or analysis, as the case may be, to background electromagnetic fields. 
       FIG. 13  shows an example of a PI controller  900  that can be used as a controller  38 ,  88 ,  640 ,  646 ,  702  in any of the devices  10 ,  50 ,  100 ,  200 ,  300 ,  400 ,  500 ,  600 ,  700 . 
     It can be seen that this PI controller  900  has a transconductance amplifier (OTA)  902  at its core. The OTA  902  shown here has two outputs  904 ,  906  formed by its two output current sources I 1  and I 2 . The two current sources (I 1 )  904  and (I 2 )  906  are coupled together, and each generates an output current, which is proportional to the difference in the input voltages at the positive input (+)  908  minus the negative input (−)  910  with a respective constant of proportionality. The positive input (+)  908  is connected to a reference potential  912 . The output current from the first output current source  904  (I 1 ) of the OTA  902  is fed into the node of the negative input  910  (−) of the OTA  902  via the optional fourth resistor (R 4 )  914 . The optional resistor (R 4 )  914  can also be bypassed and then be missing. The photodetector  34  feeds its voltage output signal through the terminal  36  and the resistor (R 2 )  916 . Such two currents are transformed into the input voltage for the negative input (−)  910  of the OTA  902  via the working resistor (R 3 )  918 , which is connected to the reference potential (GND)  920 . For a suitable starting point of the control, an offset current from a reference voltage (Vref)  924  is fed in via the first resistor (R 1 )  922  in this example. The capacitor (C)  926  provides a low-pass characteristic of the controller, which then behaves like a PI controller at low frequencies. Here, the return line of the magnetic field exciting coil  108  is connected to the reference potential (GND)  920 . 
     Note that the PI controller  900  does not have a current limit, which would cause the luminescence minimum to overflow. Therefore, such a limitation (not shown, but familiar to the expert) would have to be provided. 
       FIG. 14  shows a controller  88  for use in the device in  FIG. 3 . 
     It can be seen that two gradient signals  1006 ,  1008  are formed from the three measurement signals  82 ,  84 ,  86  of the three photodetectors  76 ,  78 ,  80  by means of the two subtraction circuits  1002  (resistors R 5 , R 4 , R 1 , R 11  and operational amplifier OP 1 ) and  1004  (resistors R 7 , R 8 , R 16 , R 10  and operational amplifier OP 2 ). By means of the subsequent addition stage  1010  (R 13 , R 12 , R 14 , operational amplifier OP 3 ), a sum signal is now formed, which is used to control the second current source (I 2 )  1012 , which feeds the control signal  90  for the coil  94 . The feedback network  1014  (R 17 , R 15 , C 2 , C 3 ) is used here to realize a PID characteristic. 
     Finally,  FIG. 15  shows a preferred embodiment of the current mirror arrangement  110 , for example for use in one of the devices according to  FIGS. 4, 5 or 7 . 
     It can be seen that the current mirror arrangement  110  includes a first MOS transistor (T 1 )  1102 , which is connected as a MOS diode. The first MOS transistor (T 1 )  1102  detects the current coming from the controller  38  and generates a reference signal, which is used to control the second transistor (T 2 )  1104  at its control electrode. Thereby, the second transistor (T 2 )  1104  operates as a current source for the cross current through the third transistor (T 3 )  1106 . This cross current is determined by the ratio of transistors (T 1 )  1102  to (T 2 )  1104 , as is known for current mirrors, for example, from Tietze, Schenk, “Halbleiter-Schaltungstechnik,” Springer. Transistors (T 1 )  1102  and (T 2 )  1104  are preferably N-channel transistors in this example. The third transistor (T 3 )  1106  detects the cross current generated by the second transistor (T 2 )  1104  and is in turn connected as a MOS diode. It is preferably a P-channel transistor. It generates an internal reference voltage on whose value the currents generated by the two current source transistors (T 4 )  1108  and (T 5 )  1110  depend. Thereby, in the example, the fourth transistor (T 4 )  1108  provides the current  112  for controlling the magnetic field generating coil  108 . In this example, the fifth transistor (T 5 )  1110  provides the reference current (I 0 )  114 , which can then be used elsewhere in subsequent circuits  100 ,  200 ,  400 . 
     Transistors (T 3 )  1106 , (T 4 )  1108 , and (T 5 )  1110  are preferably P-channel transistors in this example. The input current coming from the controller  38  through the transistor (T 1 )  1102  to ground (GND)  1112  has a fixed ratio to the cross current from the supply voltage (Vbat)  1114  through the third transistor (T 3 )  1106  and the second transistor (T 2 )  1104  to ground (GND)  1112 , which is determined by the ratio of the transistors (T 1 )  1102  to (T 2 )  1104 . The current from the supply voltage (Vbat)  1114  to coil  108 , through coil  108  to ground (GND)  1112  depends on the magnitude of the cross current corresponding to the ratio of transistors (T 3 )  1006  to (T 4 )  1108 , and is thus directly proportional to the input current through the transistor (T 1 )  1102 . The generated reference current (I 0 )  114  from the supply voltage (Vbat)  1114  to the subsequent circuit  100 ,  200 ,  400  and from there to ground (GND)  1112  depends on the magnitude of the cross current corresponding to the ratio of transistors (T 3 )  1106  to (T 5 )  1108 , and thus directly proportional to the input current through the transistor (T 1 )  1102 . Moreover, obviously then the current  112  to the coil  108  and the reference current (I 0 )  114  are directly proportional to each other. 
     Although the invention has always been described within the framework of  FIGS. 2 to 15  in connection with electromagnetic excitation by means of a LASER diode and the use of photodetectors for fluorescence, this is not mandatory. In fact, other excitations, in particular electrical excitations, for example by hot charge carriers in the surrounding material of a color center, and/or excitation can be used to read out, preferably electronically read out, the photocurrent. Since the luminescence is closely related to the photocurrent, the advantages described above also apply here. In addition, LEDs or other suitable light sources could be used in place of the preferred LASER diodes  28 ,  616 ,  618 . 
     Even though the above refers to a single NV center in the material, ensembles of NV centers can also be used in the material. For one NV center, one achieves a very good spatial resolution, which is somewhat reduced when using NV centers. However, the signal strength increases when NV ensembles are used. 
     From the foregoing, it is clear that the present disclosure provides a device  10 ,  50 ,  100 ,  200 ,  300 ,  400 ,  500 ,  600 ,  700 ,  800  and a method for generating and controlling a magnetic field strength H, wherein the generation is very stable and precise. Preferably, reference values of physical variable can be generated relatively simply and economically. In addition, magnetic flux densities can be measured with high resolution and, in particular, highly robustly. The device can also be used transmitting information, in particular ultra-wide band communication. The required devices  10 ,  50 ,  100 ,  200 ,  300 ,  400 ,  500 ,  600 ,  700 ,  800  can be very small, in particular miniature, and mobile. 
     Unless otherwise indicated, all features can be freely combined. Also, unless otherwise indicated, the features described in the description of the figures can be freely combined with the other features. A limitation of individual features of the exemplary embodiments to the combination with other features of the exemplary embodiments is expressly not intended. In addition, the features of the device can also be reformulated and used as method features, and the method features can be reformulated and used as features of the device. Such a reformulation is thus automatically disclosed. 
     LIST OF REFERENCE SIGNS 
       10  Electrical control circuit 
       12  Magnetic subcircuit 
       14  Yoke arrangement 
       16 ,  18  Cores of yoke arrangement  14   
       20  Air gap 
       22  Coil 
       24  First material, type  1   b  diamond 
       26  NV center in first material, diamond  24   
       28  LASER diode 
       30  Electromagnetic radiation of first wavelength of LASER diode  28   
       32  Emitted electromagnetic radiation of second wavelength of NV center  26   
       34  Photodetector 
       35  Optical filter 
       36  Detection signal of the photodetector  34   
       38  Controller 
       40  Control signal of the controller  38  for controlling the coil  22   
       42  Electrical conductor 
       50  Alternative embodiment of the control circuit 
       52  Air gap 
       54  Yoke arrangement 
       56 ,  58 ,  60  Magnet regions at air gap  52  arranged in parallel 
       62 ,  64 ,  66  NV centers 
       68 ,  70 ,  72  Electromagnetic radiation of a second wavelength 
       74  Filter 
       76 ,  78 ,  80  Photodetectors 
       82 ,  84 ,  86  Detection signals of photodetectors  76 ,  78 ,  80   
       88  Controllers 
       90  Control signal of controller  88   
       92  Electrical conductor 
       94  Coil 
       100  First preferred embodiment of the device for generating and controlling a magnetic field strength 
       102  Magnetic circuit 
       104  Continuous magnetic yoke 
       106  Air gap 
       108  Coil 
       110  Current mirror 
       112  Control signal 
       114  Output current (I 0 ) 
       200  Second preferred embodiment of the device for generating and controlling a magnetic field strength 
       202  Resistor, source resistor (Rq) 
       204  Reference voltage (V 0 ) 
       300  Third preferred embodiment of the device for generating and controlling a magnetic field strength 
       302  Core 
       304  Coil 
       306  Second magnetic circuit 
       400  Fourth preferred embodiment of the device for generating and controlling a magnetic field strength 
       402  Capacitor 
       404 ,  406  Capacitor plates 
       500  Fifth preferred embodiment of the device for generating and controlling a magnetic field strength 
       502  Magnetic circuit 
       504 ,  506  Additional elements for introducing an external magnetic field 
       600  Sixth preferred embodiment of the device for generating and controlling a magnetic field strength 
       602  Magnetic circuit 
       604  Yoke arrangement 
       606  Coil 
       608 ,  610  Air gaps of yoke arrangement 
       611   a  First material, diamond 
       611   b  Further material, diamond 
       612 ,  614  NV centers 
       616 ,  618  LASER diodes 
       620 ,  622  Electromagnetic radiation of a first wavelength 
       624 ,  626  Electromagnetic radiation of a second wavelength emitted from NV centers  612 ,  614   
       628 ,  630  Filter 
       632 ,  634  Photodetectors 
       636 ,  638  Detection signal of photodetectors  632 ,  634   
       640  Controller 
       642  Control signal 
       644  Electrical conductor 
       646  Measuring amplifier 
       648  Measurement signal 
       650  Planar substrate 
       700  Seventh preferred embodiment of the device for generating and controlling a magnetic field strength 
       702  High-pass filter with a lower cut-off frequency f gu  for transmission 
       704  Low-pass filter with an upper cut-off frequency f go  for transmission 
       706  Electromagnetic alternating field 
       708  Useful signal 
       800  Eighth preferred embodiment of the device for generating and controlling a magnetic field strength 
       802  Low-pass filter with an upper cut-off frequency f go  for transmission 
       804  High-pass filter with a lower cut-off frequency f gu  for transmission 
       806  Useful signal 
       808  Magnetic shielding 
       900  PI controller for use as a controller  38 ,  88 ,  640 ,  646 ,  702   
       902  Transconductance amplifier (OTA) 
       904 ,  906  Outputs 
       904  Current source (I 1 ) 
       906  Current source (I 2 ) 
       908  Positive input (+) 
       910  Negative input (−) 
       912  Reference potential 
       914  Optional fourth resistor (R 4 ) 
       916  Resistor (R 2 ) 
       918  Working resistor (R 3 ) 
       920  Reference potential (GND) 
       922  First resistor (R 1 ) 
       924  Reference voltage (Vref) 
       926  Capacitor (C) 
       1000  Subtraction circuit (resistors R 5 , R 4 , R 1 , R 11  and operational amplifier OP 1 ) 
       1004  Subtraction circuit (resistors R 7 , R 8 , R 16 , R 10  and operational amplifier OP 2 ) 
       1006 ,  1008  Gradient signals 
       1010  Addition stage (R 13 , R 12 , R 14 , operational amplifier OP 3 ) 
       1012  Second current source (I 2 ) 
       1014  Feedback network (R 17 , R 15 , C 2 , C 3 ) 
       1102  First MOS transistor (T 1 ) 
       1104  Second transistor (T 2 ) 
       1106  Third transistor (T 3 ) 
       1108  Current source transistor (T 4 ) 
       1110  Current source transistor (T 5 ) 
       1112  Ground (GND) 
       1114  Supply voltage (Vbat)