Magnetic field sensor and method for detecting a magnetic field

Abstract

The invention relates to a magnetic field sensor (1) with at least one diamond (2), which has at least one NV --center and comprising at least one optical resonator (3) in which the diamond (2) is arranged, as well as at least one pump laser (4) configured to illuminate the diamond with laser radiation with a wavelength of approximately 480 nm to approximately 637 nm, and at least one measuring laser (5) configured to couple laser radiation into the diamond (2), wherein at least one detector is further provided which is configured to detect the absorption of the laser radiation of the measuring laser (5) in the diamond (2), wherein the measuring laser (5) is configured to generate laser radiation with a wavelength of approximately 750 nm to approximately 1040 nm and / or from approximately 1044 nm to approximately 1500 nm, and the optical resonator (3) has a fineness of more than 200. The invention further relates to a corresponding method for detecting a magnetic field.

Description

[0001] The invention relates to a magnetic field sensor with at least one diamond containing at least one NV center and with at least one optical resonator in which the diamond is arranged, as well as with at least one pump laser configured to illuminate the diamond with laser radiation having a wavelength of approximately 480 nm to approximately 637 nm. Furthermore, the invention relates to a method for detecting a magnetic field with at least one diamond containing at least one NV center and arranged in an optical resonator, wherein the diamond is illuminated with laser radiation having a wavelength of approximately 480 nm to approximately 637 nm. Devices and methods of this type are suitable for detecting magnetic fields.

[0002] From Felix A. Hahl et al.: "Magnetic-field-dependent stimulated emission from nitrogen-vacancy centers in diamond." Sci. Adv. 8, eabn7192 (2022), it is known to use at least one diamond containing at least one nitrogen-vacancy center to determine a magnetic field. The diamond is located in an optical resonator and is irradiated with green light from a pump light source. The diamond exhibits magnetic-field-dependent photoluminescence, whereby stimulated emission can be induced by the optical resonator surrounding the diamond. Since the emission decreases with increasing magnetic field, the strength of the magnetic field can be determined by measuring the intensity of the stimulated emitted light. The maximum achievable contrast is 33% or less.

[0003] Based on the prior art, the invention is therefore based, among other things, on the object of enabling magnetic field measurement with greater sensitivity.

[0004] According to one aspect of the invention, a magnetic field sensor is proposed. The magnetic field sensor contains at least one diamond, which contains at least one NV center. The NV center can be a negatively charged NV - -center. In some embodiments of the invention, a plurality of NV centers may be present in the diamond. The diamond may be polycrystalline or monocrystalline. The diamond may be a synthetically produced HPHT diamond or a CVD diamond obtained by low-pressure synthesis. In some embodiments of the invention, the diamond is nominally undoped and contains as few impurities or defects as possible.

[0005] In some embodiments of the invention, the diamond is located in at least one optical resonator. The optical resonator is formed by at least two opposing mirrors which enclose a volume lying between the mirrors. At least one of the mirrors can be partially transparent so that light can be coupled into or out of the resonator through the mirror. The mirrors can be plane mirrors or have a concave shape. Through multiple reflection of light within the resonator, standing waves can form in the resonator, the wavelength of which is determined by the eigenmodes or the resonant frequency of the resonator. For the purposes of the present invention, the diamond is located within the optical resonator if it is at least partially located in the beam path of the resonator.

[0006] The device can further contain at least one pump laser which is configured to illuminate the diamond with laser radiation having a wavelength of approximately 480 nm to approximately 637 nm. For this purpose, the light from the pump laser can be coupled into the resonator and the diamond located therein through at least one mirror of the resonator. The beam path of the pump laser can lie approximately on the longitudinal axis of the resonator, i.e. the longitudinal axis and the pump laser can enclose an angle of approximately -5° to approximately +5°. In other embodiments of the invention, the angle between the laser beam of the pump laser and the longitudinal extent of the resonator can also be larger, for example between approximately 20° and approximately 90°. In this case, the light from the pump laser can be coupled directly into the diamond, past the resonator mirror.

[0007] Furthermore, the magnetic field sensor can contain at least one measuring laser. The measuring laser is configured to couple laser radiation into the diamond. Furthermore, the magnetic field sensor can contain a detector configured to detect the absorption of the laser radiation from the measuring laser in the diamond. The detector thus detects the light from the measuring laser after it has passed through at least a partial volume of the diamond at least once. For this purpose, the measuring laser and the detector can be arranged at different ends of the resonator so that the light is coupled into and out of the diamond by partially transparent mirrors of the resonator. In other embodiments of the invention, the beam path can also take on a different form. In particular, the light can be coupled into and / or out of the diamond from the side.In some embodiments of the invention, the light from the measuring laser can pass through the resonator and thus the diamond arranged in the beam path several times before falling on the detector. This determines the absorption of at least one resonator mode in the diamond.

[0008] In some embodiments of the invention, the measuring laser is configured to generate laser radiation with a wavelength of approximately 750 nm to approximately 1040 nm. In other embodiments of the invention, the measuring laser is configured to generate laser radiation with a wavelength of approximately 1044 nm to approximately 1500 nm. In yet other embodiments of the invention, the measuring laser can generate light with a wavelength of approximately 750 nm to approximately 1040 nm and of approximately 1044 nm to approximately 1300 nm. Unlike known magnetic field sensors, the measuring laser thus does not excite the singlet states 1 A and 1 E on.

[0009] In some embodiments of the invention, the optical resonator may have a finesse of more than 200. This minimizes optical losses during the measurement, so that the sensitivity can be increased.

[0010] Thus, unlike the prior art, the proposed magnetic field sensor is not based on the detection of photoluminescence or emitted red light, but rather on the absorption of infrared radiation, which changes with the external magnetic field. This allows for a significant increase in sensitivity, even though the infrared wavelength range used by the measuring laser cannot resonantly populate or depopulate a known energy level of the NV center. To perform a measurement, only pump light needs to be applied to the diamond. In some embodiments of the invention, this can be achieved using continuous-wave operation of the pump light source. The absorption of the infrared radiation from the measuring laser can then be detected using the at least one detector. After calibration, the absorption can be related to the acting magnetic field.The measurement setup used is simple and surprisingly shows a higher sensitivity than known sensors that evaluate luminescence radiation emanating from the diamond.

[0011] In some embodiments of the invention, the optical resonator may have a finesse of more than 300. In other embodiments of the invention, the optical resonator may have a finesse of more than 500. In yet other embodiments of the invention, the optical resonator may have a finesse of more than 1000.

[0012] In some embodiments of the invention, the optical resonator may have a finesse of less than 8000. In other embodiments of the invention, the optical resonator may have a finesse of less than 4000. In yet other embodiments of the invention, the optical resonator may have a finesse of less than 3000. As a result, the optical resonator may have total losses of about 3.1% to about 0.02%. Such an optical resonator can further increase the sensitivity of magnetic field measurements.

[0013] In some embodiments of the invention, the measuring laser can be configured to generate laser radiation with a wavelength of approximately 750 nm to approximately 1300 nm. In other embodiments of the invention, the measuring laser can be configured to generate laser radiation with a wavelength of approximately 800 nm to approximately 1500 nm. In yet other embodiments of the invention, the measuring laser can be configured to generate laser radiation with a wavelength of approximately 800 nm to approximately 1300 nm. In each case, a wavelength range from approximately 1012 nm to approximately 1072 nm or from approximately 1022 nm to approximately 1062 nm or from approximately 1035 nm to approximately 1049 nm is excluded. As a result, the measuring laser excites the singlet states 1 A and 1 E not on.

[0014] In some embodiments of the invention, the measuring laser is configured not to generate light having a wavelength of about 982 nm to about 942 nm. In other embodiments of the invention, the measuring laser is configured not to generate light having a wavelength of about 992 nm to about 932 nm. In yet other embodiments of the invention, the measuring laser is configured not to generate light having a wavelength of about 905 nm to about 919 nm.

[0015] In some embodiments of the invention, the measuring laser may be configured to generate laser radiation with a wavelength of about 750 nm to about 1000 nm. In other embodiments of the invention, the measuring laser may be configured to generate laser radiation with a wavelength of about 800 nm to about 1000 nm.

[0016] In some embodiments of the invention, the optical resonator can be manufactured by coating the diamond with at least one mirror layer. This results in a particularly simple and mechanically robust structure.

[0017] In some embodiments of the invention, the optical resonator can have an adjustable resonant frequency. In some embodiments of the invention, this can be achieved by moving a mirror along the longitudinal axis of the resonator, for example, using a micrometer screw or a piezoelectric actuator. This allows the resonator modes to be adapted to the wavelength of the measuring laser, thereby further increasing the measurement accuracy and / or sensitivity.

[0018] In some embodiments of the invention, the diamond may have a background absorption of about 0.1 cm -1or better. Due to the low absorption of electromagnetic radiation in diamond, the finesse of the optical resonator equipped with the diamond can be increased.

[0019] In some embodiments of the invention, the absorption of electromagnetic radiation in the diamond in the wavelength range from about 680 nm to about 760 nm may be less than about 0.2 cm -1 or less than about 0.1 cm -1 or less than about 0.05 cm -1 or less than about 0.01 cm -1 The absorption coefficient can be measured using UV-VIS spectroscopy. The measurement can be performed using an integrating sphere to detect the transmitted light. Reflection at the substrate surface due to the difference in refractive indices is taken into account.

[0020] In some embodiments of the invention, the absorption of electromagnetic radiation in the diamond in the wavelength range from about 680 nm to about 760 nm may be greater than about 0.001 cm -1 be.

[0021] In some embodiments of the invention, the diamond may have a concentration of NV - -centers of 0.5 ppm or higher. This further increases the signal-to-noise ratio of the measurement, allowing for greater accuracy and / or sensitivity in the measurement of magnetic fields.

[0022] In some embodiments of the invention, the diamond used may be NV - -centers in a concentration of less than about 10.0 ppm, or less than about 3.0 ppm, or less than about 2.5 ppm, or less than about 2.0 ppm. The concentration of NV - -centers may, however, be greater than about 1 ppm or greater than about 1.5 ppm.

[0023] In some embodiments of the invention, the diamond may have a volume of approximately 100 µm x 100 µm x 300 µm or larger. This may increase the volume available for measurement, thus increasing sensitivity.

[0024] In some embodiments of the invention, the magnetic field sensor may further comprise at least one RF device with which microwave radiation can be generated at least at the location of the diamond. In some embodiments of the invention, the microwave radiation may have a frequency of approximately 2.77 GHz to approximately 2.97 GHz. In some embodiments of the invention, the frequency of the microwave radiation may be changed during the measurement. This feature causes the ground state 3A Electrons are excited from the ground state with spin quantum number 0 to the excited state with spin quantum number ± 1. Since the degeneracy of the spin states +1 and -1 is lifted in the presence of a magnetic field, the strength of a magnetic field can be determined from the energetic splitting of the two energy levels.

[0025] When determining a magnetic field with the magnetic field sensor according to the invention, the sensitivity can be between about 8 pT / Hz up to about 40 pT / Hz This allows for a more precise determination of the magnetic field than with conventional methods.

[0026] In some embodiments of the invention, the method for detecting a magnetic field can be performed at room temperature. In some embodiments of the invention, the method for detecting a magnetic field can be performed at a temperature of more than 260 K or more than 280 K. This eliminates the effort associated with generating cryogenic temperatures.

[0027] The invention will be explained in more detail below with reference to figures and exemplary embodiments without limiting the general inventive concept. Fig. 1 the term scheme of an NV center in diamond. Fig. 2 shows a magnetic field sensor according to the invention according to a first embodiment. Fig. 3 shows a magnetic field sensor according to the invention according to a second embodiment. Fig. 4 shows a magnetic field sensor according to the invention according to a third embodiment. Fig. 5 shows a magnetic field sensor according to the invention according to a fourth embodiment. Fig. 6 shows a magnetic field sensor according to the invention according to a fifth embodiment. Fig. 7 shows the result of a magnetic field measurement with a proposed magnetic field sensor. Fig. Figure 8 shows a measurement of optically detected magnetic resonance at different wavelengths.

[0028] Fig. Figure 1 shows the term scheme of an NV center in diamond. The term scheme of the NV center can be influenced by a magnetic field. By measuring this influence, the magnetic field can be deduced.

[0029] The NV center consists of a nitrogen atom, which is incorporated into a diamond lattice site instead of a carbon atom, and a neighboring vacancy in the crystal lattice. From electron spin resonance measurements, it is known that the NV center has an even number of electrons with a total spin of 1. Both the ground state 3 A as well as the excited state 3 E thus form a triplet state. The electron spin of this state can assume the values ​​-1, 0, and +1. In the absence of an external magnetic field, the energy levels of the +1 and -1 states are degenerate. The presence of a magnetic field leads to the splitting of these levels, since the total spin can align parallel or antiparallel to the field vector. As can be seen from Fig. 1, the energy levels of the states with spin ±1 lie above the energy level of the state with spin 0 without an external applied magnetic field.

[0030] By supplying pump light from a pump light source 4, electrons can be transferred from the ground state 3 A into the excited state 3 E. The pump light used for this purpose can be selected from the green spectral range and, for example, have a wavelength of approximately 532 nm to approximately 550 nm or of approximately 480 nm to approximately 637 nm.

[0031] The excited state 3 E decays radiatively, emitting red light with a wavelength of about 637 nm to about 800 nm. Alternatively, the excited state decays with the spin quantum number ±1 via the singlet states 1 A and 1 E to the ground state 3 A.

[0032] Within the ground state 3A, electrons can be excited from the ground state with a spin quantum number of 0 to the excited state with a spin quantum number of ±1. For this purpose, high-frequency radiation from a radio frequency transmitter 6 can be used, which has a frequency of approximately 2.88 GHz. This frequency changes in the presence of an external magnetic field when the degeneracy of the +1 and -1 spin states is lifted. Therefore, according to the state of the art, a magnetic field can be determined by continuously applying pump light to the active medium and determining fluorescent light while changing the frequency of the electromagnetic radiation. The achievable contrast is 22% or less.

[0033] According to one aspect of the invention, it was recognized that infrared light having a wavelength of about 750 nm to about 1040 nm and / or about 1044 nm to about 1500 nm is absorbed by the diamond when 4 electrons are pumped from the ground state by supplying pump light from a pump light source. 3 A into the excited state 3 E. This absorption also depends on the spin state of the NV center, meaning that the absorption can be influenced by a magnetic field prevailing at the location of the NV center. Thus, spin-dependent induced absorption can be used for magnetic field measurement, achieving a contrast of 50% or more. Spin-dependent induced absorption thus has the potential to improve the sensitivity of a magnetic field sensor.

[0034] Fig. Figure 2 shows a magnetic field sensor 1 according to a first embodiment of the invention. The magnetic field sensor 1 contains a diamond 2. The diamond 2 can be polycrystalline or monocrystalline. The diamond 2 is preferably, but not necessarily, nominally undoped. The diamond 2 can have the shape of a cuboid and, for example, have a volume of approximately 100 x 100 µm x 300 µm or more. In some embodiments of the invention, the diamond has a high crystal quality, resulting in a background absorption of approximately 0.1 cm -1 or better at least in the green and infrared spectral range up to about 1500 nm.

[0035] Furthermore, the diamond contains NV centers, which are formed in a conventional manner by a nitrogen atom incorporated into a lattice site instead of a carbon atom. A lattice site adjacent to the nitrogen atom has a defect. The NV centers used for magnetic field measurement are negatively charged, and in some embodiments of the invention, the diamond has an NV - concentration of 0.5 ppm or more.

[0036] The diamond is located in an optical resonator 3, which is formed by a first mirror 31 and a second mirror 32. The mirrors can be flat or concave. The mirrors can be partially transparent, so that part of the incoming radiation is reflected and another part is transmitted. In the illustrated embodiment, the first mirror 31 is arranged at an incline in order to simultaneously function as a beam splitter for combining the laser beams of the pump laser 4 and measuring laser 5 described below. In some embodiments of the invention, the optical resonator 3 has a high resonator quality factor or finesse, which can be, for example, more than 200, more than 300, more than 500, or more than 1000.

[0037] During operation of the magnetic field sensor 1, pump light from a pump laser 4 is supplied to the diamond 2. The pump laser 4 is configured to generate laser radiation with a wavelength of approximately 480 nm to approximately 637 nm. In the illustrated embodiment, the pump light from the pump laser 4 is reflected by the first mirror 31 and coupled into the optical resonator 3 along the longitudinal axis.

[0038] The magnetic field sensor 1 further comprises a measuring laser 5, which generates laser radiation that is coupled in a straight line into the optical resonator 3 through the partially transparent first mirror 31. The measuring laser generates laser radiation that has a wavelength of approximately 750 nm to approximately 1040 nm and / or approximately 1044 nm to approximately 1500 nm. The laser radiation from the measuring laser 5 passes through the diamond 2 once or multiple times and then strikes a detector 7, which is arranged behind the likewise partially transparent second mirror 32. The detector 7 can detect the attenuation of the laser radiation from the measuring laser 7.

[0039] When the diamond 2 is exposed to a magnetic field generated by the magnetic field generating device 8, the absorption of the laser radiation from the measuring laser 5 increases with the strength of the magnetic field. Accordingly, the absorption of the laser radiation from the measuring laser 5 decreases as the magnetic field decreases. Thus, the magnetic field can be determined by detecting the intensity of the laser radiation from the measuring laser 5 using the detector 7.

[0040] Based on the Fig. Figures 3 to 6 explain further embodiments of the invention in more detail. Identical components of the invention are provided with the same reference numerals, so the following description is limited to the essential differences.

[0041] Fig. 3 shows a second embodiment of the invention. The resonator 3 according to the second embodiment contains a second mirror 32, which is designed as a planar, partially transparent mirror, as in the first embodiment described above. The first mirror 31, on the other hand, is designed as a coating on the first side 21 of the diamond 2. The laser radiation of the measuring laser 5 is coupled into the diamond 2 from the first side 21, penetrating the coating forming the first mirror 31. The pump light of the pump laser 4 is coupled into the diamond 2 from the opposite second side and reflected by the coating forming the first mirror 31 on the first side 21 of the diamond 2. In this way, the resonator 3 can be constructed with fewer components, thus preventing losses at additional interfaces and further increasing the finesse.

[0042] In some embodiments, the angle of diamond 2 to the longitudinal axis of the resonator can be selected such that the pump light of pump laser 4 is totally reflected at the interface formed on the first side 21 of diamond 2. In this case, a coating can be omitted, so that the first mirror 31 is formed directly by diamond 2.

[0043] Based on the Fig. A third embodiment of the invention is explained in more detail in Figure 4. The third embodiment differs from the first embodiment described above in that an RF device is also present, with which microwave radiation can be generated at the location of the diamond 2. The microwave radiation can have a frequency of approximately 2.77 GHz to approximately 2.97 GHz. The microwave radiation serves to 3A electrons from the ground state with spin quantum number 0 to the excited state with spin quantum number ± 1. Since the degeneracy of the spin states +1 and -1 is lifted in the presence of a magnetic field, the sensitivity of the measurement method can be further increased.

[0044] Based on the Fig. 5, a fourth embodiment of the magnetic field sensor 1 is explained in more detail. The fourth embodiment differs from the first embodiment in that it has a resonator 3 defined by a first mirror 31 and a second mirror 32, the first and second mirrors 31 and 32 each having a concave shape. Reflected light is thus focused by the first and second mirrors onto a focal point in the center of the optical resonator 3. This can further improve the resonator quality or finesse, so that the optical resonator has total losses of approximately 0.02% to approximately 3.1%. The diamond 2 can be arranged at the focal point of the mirrors.

[0045] While in the embodiments described above the first mirror 31 simultaneously functions as a beam splitter to combine the light of the pump laser 4 and the measuring laser 5 in one beam path, the embodiment described on the basis of the Fig. 5, a separate beam splitter 35 in the form of a partially transparent mirror or prism inclined to the beam path.

[0046] Finally, Fig. 5 a first detector 7a and a second detector 7b. The first detector 7a detects a resonator mode which is coupled out by the partially transparent second mirror 32. In contrast, the second detector 7b detects light which is reflected on the first side 21 of the diamond 2. Both detectors thus detect light which has passed through the diamond 2 and was partially absorbed in the process, so that the measurement signal from both detectors can serve as a measure of the acting magnetic field. The use of two detectors has the advantage that the measured values ​​from both detectors can be checked for plausibility in order to thereby increase the accuracy of the magnetic field measurement. It should be noted that in some embodiments the detector 7a can also be omitted, so that the second detector 7b acts as the sole detector to detect the absorption of the light from the measuring laser 5 in the diamond 2.

[0047] Based on the Fig. 6, a fifth embodiment is explained in more detail. The fifth embodiment essentially corresponds to the fourth embodiment, which is explained with reference to Fig. 5, wherein the RF device 6 already explained in the third embodiment is available to further increase the sensitivity and / or the measurement accuracy.

[0048] Fig. Figure 7 shows a measurement signal recorded with the magnetic field sensor described above. The signal intensity in detector 7 is plotted on the ordinate and time on the abscissa. No magnetic field is applied for the first 30 seconds, then diamond 2 is exposed to a magnetic field for 30 seconds. After 60 seconds, the magnetic field is switched off again.

[0049] The intensity of the infrared radiation emitted by the measuring laser 5 is shown for different wavelengths, namely 750 nm, 800 nm, 850 nm, 900 nm and 960 nm. Fig. As Figure 7 shows, the presence of a magnetic field causes a signal attenuation of at least 40% due to absorption in the diamond. At some wavelengths, attenuation of as much as 50% occurs. The resulting measurement contrast is at least a factor of 2 higher than with conventional magnetic field sensors, which detect the fluorescent light emitted by the diamond.

[0050] Fig.Figure 8 shows the optically detected magnetic resonance at different wavelengths. The intensity of the light emitted by the measuring laser 5, after it has been at least partially absorbed in the diamond 2, is plotted on the ordinate, and the frequency of an RF device 6 is plotted on the abscissa. It can be seen again that the absorption is particularly high for wavelengths above the fluorescence light.

[0051] Of course, the invention is not limited to the illustrated embodiments. The above description is therefore not to be considered restrictive, but rather illustrative. The following claims are to be understood as meaning that a stated feature is present in at least one embodiment of the invention. This does not exclude the presence of further features. Where the claims and the above description define "first" and "second" embodiments, this designation serves to distinguish between two similar embodiments without establishing a priority. QUOTES CONTAINED IN THE DESCRIPTION

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

[0000] Felix A. Hahl et al.: “Magnetic-field-dependent stimulated emission from nitrogen-vacancy centers in diamond”. Sci. Adv. 8, eabn7192 (2022

[0002]

Claims

[1] Magnetic field sensor (1) with at least one diamond (2) which has at least one NV - -Center and with at least one optical resonator (3) in which the diamond (2) is arranged, and with at least one pump laser (4) which is designed to illuminate the diamond with laser radiation having a wavelength of about 480 nm to about 637 nm, and with at least one measuring laser (5) which is designed to couple laser radiation into the diamond (2), wherein at least one detector (7) is also provided which is designed to detect the absorption of the laser radiation of the measuring laser (5) in the diamond (2), characterized by , that the measuring laser (5) is designed to generate laser radiation with a wavelength of approximately 750 nm to approximately 1040 nm and / or of approximately 1044 nm to approximately 1500 nm and the optical resonator (3) has a finesse of more than 200. [2] Magnetic field sensor according to claim 1, characterized by that the optical resonator (3) has a finesse of more than 300 or more than 500 or more than 1000 and / or that the optical resonator (3) has a finesse of less than 8000 or less than 4000 or less than 3000 and / or that the optical resonator (3) has total losses of about 3.09% to about 0.02%. [3] Magnetic field sensor according to claim 1 or 2, characterized by that the measuring laser (5) is designed to generate laser radiation with a wavelength of approximately 750 nm to approximately 1300 nm or of approximately 800 nm to approximately 1500 nm or of approximately 800 nm to approximately 1300 nm, wherein in each case a wavelength range of approximately 1012 nm to approximately 1072 nm or of approximately 1022 nm to approximately 1062 nm or of approximately 1035 nm to approximately 1049 nm is excluded. [4] Magnetic field sensor according to one of claims 1 or 2, characterized by that the measuring laser (5) is designed to generate laser radiation with a wavelength of approximately 750 nm to approximately 1000 nm or of approximately 800 nm to approximately 1000 nm. [5] Magnetic field sensor according to one of claims 1 to 4, characterized by that the optical resonator (3) is produced by coating the diamond (2) with at least one mirror layer or that the optical resonator (3) has an adjustable resonance frequency. [6] Magnetic field sensor according to one of claims 1 to 5, characterized by that the diamond (2) has a background absorption of about 0.1 cm -1 or better and / or that the diamond (2) has a NV - -concentration of 0.5 ppm or higher and / or that the diamond (2) has a volume of approximately 100µm x 100µm x 300µm or more. [7] Magnetic field sensor according to one of claims 1 to 6, further comprising an RF device (6) with which microwave radiation can be generated at the location of the diamond (2). [8] Method for detecting a magnetic field with at least one diamond (2) which has at least one NV - -centre and which is arranged in an optical resonator (3), wherein the diamond (2) is illuminated with laser radiation having a wavelength of about 480 nm to about 637 nm, and the absorption of the diamond (2) is measured at a different wavelength, characterized by that the absorption of the diamond (2) is measured at a wavelength of about 750 nm to about 1040 nm and / or from about 1044 nm to about 1500 nm, wherein the optical resonator (3) has a finesse of more than 200. [9] Method according to claim 8, characterized bythat the absorption of the diamond (2) is measured at a wavelength of about 750 nm to about 1300 nm or from about 800 nm to about 1500 nm or from about 800 nm to about 1300 nm, wherein in each case a wavelength range of about 1012 nm to about 1072 nm or from about 1022 nm to about 1062 nm or from about 1035 nm to about 1049 nm is excluded. [10] Method according to claim 9, characterized by that the absorption of the diamond (2) is measured at a wavelength of about 750 nm to about 1000 nm or from about 800 nm to about 1000 nm. [11] Method according to one of claims 8 to 10, characterized by that the diamond (2) is exposed to microwave radiation with a frequency of about 2.77 GHz to about 2.97 GHz. [12] Method according to one of claims 8 to 11, characterized by that it has a sensitivity of about 8 pT / sqrt(Hz) to about 40 pT / sqrt(Hz). [13] Method according to one of claims 8 to 12, characterized bythat the resonance frequency of the optical resonator (3) is changed cyclically.

Images