Method and apparatus for ODMR measurement of a magnetic field using linearly polarized light without a reference magnetic field

Abstract

Methods for measuring a magnetic field by means of a measuring body (2) which is made of a material with a plurality of spin-1 defects with C 3v -symmetry is formed, whereby for a large number of data points simultaneously a) the measuring body (2) is illuminated with light along an optical axis (6), b) the measuring body (2) is exposed to microwave radiation of a frequency v, and c) an intensity I of light emitted by the measuring body (2) is measured, wherein the light used in a) is linearly polarized at a polarization angle α, or in c) the light is measured after passing through a linear polarization filter (7) set at a polarization angle α, wherein the data points differ in the polarization angle α of the light and / or in the frequency v of the microwave radiation, and wherein the direction and / or magnitude of the magnetic field can be determined from the data points. B → are determined, wherein the light used in step a) is linearly polarized and tilted relative to the optical axis (6).

Description

[0001] The invention relates to a method and a device for measuring a magnetic field, in particular a direction and / or an magnitude of the magnetic field.

[0002] Many applications require knowledge of the direction and / or magnitude of a magnetic field, for example in navigation, soil exploration, materials science, life sciences, precision measurement and testing.

[0003] It is known that magnetic fields can be measured optically. For this purpose, crystals with defects with spin-1 and C are used. 3v-Symmetry is used, particularly in diamond with nitrogen vacancy centers (NV centers). This technique is also known as vector magnetometry. It allows the orientation of a magnetic field relative to an NV center to be determined. However, in crystals with a multitude of NV centers, it cannot be assumed that all NV centers are oriented identically. Therefore, it is known in the art to use a single NV center to measure a magnetic field. Such a measurement is, however, inaccurate. Approaches are thus known in the art to measure a magnetic field with a multitude of NV centers. However, this requires a reference magnetic field of known orientation. Such a reference field not only adds complexity to the measurement but can also affect samples that are sensitive to magnetic fields. This is the case, for example, with ferromagnetic materials.

[0004] Examples of the determination of magnetic fields using nitrogen vacancy centers are E. van Oort and M. Glasbeek, “Fluorescence detected level-anticrossing and spin coherence of a localized triplet state in diamond,” Chem. Phys., Vol. 152, No. 3, pp. 365–373, May 1991, doi: 10.1016 / 0301-0104(91)85011-5 and BJ Maertz, AP Wijnheijmer, GD Fuchs, ME Nowakowski and DD Awschalom, “Vector magnetic field microscopy using nitrogen vacancy centers in diamond,” Appl Phys Lett, Vol. 96, p. 4, 2010.

[0005] Independently of the determination of magnetic fields, it is known from PR Dolan, X. Li, J. Storteboom and M. Gu, “Complete determination of the orientation of NV centers with radially polarized beams,” Opt. Express, Vol. 22, No. 4, p. 4379, Feb. 2014, doi: 10.1364 / OE.22.004379 and from TPM Alegre, C. Santori, G. Medeiros-Ribeiro and RG Beausoleil, “Polarization-selective excitation of nitrogen vacancy centers in diamond,” Phys. Rev. B, Vol. 76, p. 5, 2007, doi: 10.1103 / PhysRevB.76.165205 to optically determine the directions of crystal axes.

[0006] Methods for determining a magnetic field are also known from EP 242 139 A1, as well as from Münzhuber, F., et al. Polarization-Assisted Vector Magnetometry with no Bias Field Using an Ensemble of Nitrogen-Vacancy Centers in Diamond. Physical Review Applied, 2020, Vol. 14, No. 1, p. 014055, and from Alegre, Thiago P. Mayer, et al. Polarization-selective excitation of nitrogen vacancy centers in diamond. Physical Review B, 2007, Vol. 76, No. 16, p. 165205.

[0007] The object of the invention is therefore to measure a magnetic field with high accuracy and low effort in a gentle manner. The magnetic field should be able to be determined as unambiguously as possible.

[0008] These tasks are solved by the method and the apparatus according to the independent claims. Further advantageous embodiments are specified in the dependent claims. The features described in the claims and in the description can be combined with one another in any technologically meaningful way.

[0009] According to the invention, a method for measuring a magnetic field is provided. B→ This is demonstrated using a measuring body. The measuring body is made of a material with a multitude of spin-1 defects with C 3v -Symmetry is formed, whereby for a large number of data points simultaneously a) a measuring body is illuminated with light along an optical axis, b) the measuring body is exposed to microwave radiation of a frequency v, and c) an intensity I of light emitted by the measuring body is measured, wherein the light used in a) is linearly polarized at a polarization angle α, or in c) the light is measured after passing through a linear polarization filter set at a polarization angle α, wherein the data points differ in the polarization angle α of the light and / or in the frequency v of the microwave radiation, and wherein the direction and / or magnitude of the magnetic field can be determined from the data points. B→ be determined.

[0010] In a preferred embodiment of the method, the material of the measuring body is diamond with a plurality of negatively charged nitrogen vacancy centers.

[0011] As an alternative to diamond, silicon carbide can be used, for example. Other materials are also conceivable for the measuring element. It is sufficient that the material has a large number of spin-1 defects with C 3vexhibits -symmetry. Negatively charged nitrogen vacancy centers are one example. Another example is divacancy centers. All other defects that exhibit spin-1 and C can also be used. 3v -have symmetry.

[0012] The case of diamond with a multitude of negatively charged nitrogen vacancy centers is favored for practical reasons. Therefore, this case will be discussed below as an example. For other materials and other spin-1 defects with C 3v- The following applies accordingly to symmetry.

[0013] This method can be used to measure a magnetic field. This can be done in any context, for example, in navigation, soil exploration, materials science, life sciences, precision measurement and testing. The method can determine the direction and / or magnitude of the magnetic field. The "AND" case is preferred. The direction and magnitude of the magnetic field refer to the magnetic flux density. B→, which can be specified vectorially. The result of the procedure can therefore be, for example, components B. x , B y and B z of the magnetic field in a Cartesian coordinate system. If a magnetic field is specified with these components, it is vectorially unique. Accordingly, both the direction and the magnitude of the magnetic field can be considered determined as soon as B x , B y and B zare known. Alternatively to Cartesian coordinates, any other coordinate system can be used, for example spherical coordinates.

[0014] The magnetic field is measured using a diamond measuring element. The measuring element can have any shape. Preferably, the measuring element has at least one flat surface on which it can be illuminated by light from a light source. Preferably, the measuring element also has at least one flat surface on which the light emitted by the measuring element can be measured. These two flat surfaces can coincide or be located on different sides of the measuring element, particularly on opposite sides.

[0015] The diamond exhibits a large number of nitrogen vacancy centers. A nitrogen vacancy center (NV) is a substitution defect center with C 3vNitrogen vacancy symmetry in diamond. In this symmetry, a position in the diamond lattice intended for a carbon atom is unoccupied, while an adjacent position for a carbon atom is occupied by a nitrogen atom. Nitrogen vacancy centers can exist in a negative, neutral, or positive charge state. Negatively charged nitrogen vacancy centers (NV-) can be used to measure magnetic fields via optically detected magnetic resonance. This is described, for example, in E. van Oort and M. Glasbeek, “Fluorescence detected level-anticrossing and spin coherence of a localized triplet state in diamond,” Chem. Phys., Vol. 152, No. 3, pp. 365–373, May 1991, doi: 10.1016 / 0301-0104(91)85011–5.

[0016] The measuring body is preferably illuminated with light such that the defects along the crystal axes in the direction

[111] ,

[111] ,

[111] , or

[111] can be optically distinguished. This is possible in various ways by illuminating the measuring body with light along an optical axis. In a preferred embodiment, the optical axis forms an angle of less than 20° with a crystal axis of the measuring body, particularly less than 10°. Ideally, the optical axis is aligned parallel to a crystal axis of the measuring body. This is particularly preferred. However, the advantages of this embodiment described above can also be achieved, at least partially, if the optical axis is not exactly parallel to a crystal axis. Therefore, in this embodiment, it is sufficient if the optical axis forms an angle of less than 20° with a crystal axis of the measuring body, particularly less than 10°.The values ​​20° and 10° have proven to be useful.

[0017] Preferably, the optical axis is aligned parallel to the

[111] direction of the measuring body or an equivalent direction, in particular to the

[111] direction,

[111] direction, or

[111] direction of the measuring body. This applies particularly to diamond as the material of the measuring body. However, a usable measurement is also possible, in particular, if the optical axis is aligned parallel to the direction [9,9,10], for example. The optical axis can also be aligned parallel to other directions, for example

[110] and all equivalent directions.

[0018] Furthermore, measurement is even possible if the optical axis is arbitrarily oriented to the crystal axes or if the orientation of the crystal axes is initially unknown.

[0019] It is therefore preferred that the optical axis encloses an angle of less than 20° with a crystal axis of the measuring body, wherein the crystal axis is particularly preferably aligned parallel to the

[111] direction or an equivalent direction or parallel to the

[110] direction or an equivalent direction.

[0020] The measurement is simplified if the optical axis is at a known angle to one of the crystal axes. Therefore, the preferred case, where the optical axis is aligned parallel to the direction

[111] , is discussed below.

[0021] Each nitrogen vacancy center is aligned along one of the crystal axes of the diamond. These crystal axes can be designated according to the directions

[111] ,

[111] ,

[111] , and

[111] . The orientation of a nitrogen vacancy center is defined as the axis between the corresponding unoccupied position of the diamond lattice and the adjacent position occupied by a nitrogen atom. With a large number of nitrogen vacancy centers in a diamond measuring body, it can generally be assumed that these orientations are distributed approximately evenly along the four crystal axes. Therefore, a magnetic field cannot, in principle, be measured unambiguously using nitrogen vacancy centers. This is because certain different directions of the magnetic field can lead to the same measurement results and thus cannot be distinguished.The described method can reduce the corresponding uncertainty or even allow the magnetic field to be uniquely determined. This is facilitated by the fact that the optical axis is preferably aligned parallel to the

[111] direction,

[111] direction,

[111] direction, or

[111] direction of the diamond of the measuring body. This is possible because diamond has a natural fracture edge perpendicular to these directions. Accordingly, so-called

[111] -diamond is known, the surfaces of which are perpendicular to the crystal axes. Preferably, the measuring body is therefore made of

[111] -diamond.

[0022] The method comprises the acquisition and evaluation of measured values. Naturally, the evaluation only takes place after the measured values ​​have been acquired. However, new measured values ​​can be acquired even while the evaluation of previously acquired values ​​is not yet complete. Preferably, the method is carried out continuously such that measured values ​​are acquired continuously or periodically and evaluated as soon as they are available. In this respect, the magnetic field can be measured almost in real time using this method.

[0023] The acquisition of measured values ​​is carried out in steps a) to c), each of which is performed simultaneously for a large number of data points. These steps essentially describe a measurement of the type used in optically detected magnetic resonance (ODMR).

[0024] In step a), the measuring body is illuminated with light along the optical axis. The light preferably originates from a light source, for example, a laser. The light excites charge carriers in the nitrogen vacancy centers of the diamond of the measuring body. The light preferably has a corresponding wavelength. This can be achieved practically by using a light source with an emission spectrum that covers the relevant wavelengths.

[0025] Nitrogen vacancy centers have a ground state with three spin-1 sublevels. Measuring a magnetic field in these centers is possible via the two sublevels with spin m. s = -1 and m s = +1 possible. These two sublevels split in a magnetic field due to the Zeeman effect. From sublevel m s Charge carriers can be moved to sublevels with spin m by applying microwave radiation. s = -1 and m s= +1. Accordingly, in step b), the measuring body is exposed to microwave radiation of a frequency v.

[0026] In step c), the intensity I of light emitted by the measuring body is measured. Specifically, the light measured is that which arises when charge carriers excited by the incident light return to their ground state. This is based on the effect of fluorescence.

[0027] For the transitions m s = 0 → -1 and m s Different energies are required for transitions from 0 to +1, which, due to the Zeeman effect, also depend on the magnetic field being measured. A magnetic field can therefore be measured by scanning a frequency range with microwave radiation. For the transitions m s = 0 → -1 and m sAs the coefficient of inertia increases from 0 to +1, the second derivative of the measured intensity I exhibits a peak. This is an absorption peak. The spacing v between the frequencies corresponding to the peaks is a measure of the projection of the magnetic field onto the crystal axis along which the respective nitrogen vacancy center is aligned. This fact can be used in the described method to measure the direction and / or magnitude of the magnetic field. This is done in the manner of vector magnetometry, which is known per se. However, since a large number of nitrogen vacancy centers are used in the described method, the magnetic field cannot be uniquely determined using vector magnetometry. This uncertainty arises because, in general, the nitrogen vacancy centers are not all aligned parallel to each other. Rather, a portion of the nitrogen vacancy centers is aligned along each of the four crystal axes of the diamond.Therefore, when the second derivative of the measured intensity is plotted against the microwave frequency v, there is not just one pair of peaks, but generally four pairs of peaks, one per crystal axis.

[0028] If the orientation of the crystal axes is known, the magnitude and direction of the magnetic field can be determined with a high degree of certainty from the four pairs of peaks. Since the optical axis is aligned parallel to one of the crystal axes, the direction of at least one of the crystal axes is known. Furthermore, the described method can also be used to determine the direction of the remaining crystal axes.

[0029] Linearly polarized light is used for this purpose. In a first embodiment, the measuring body is exposed to linearly polarized light in step a). This can be achieved by inserting a linear polarization filter between the light source and the measuring body, or by using a light source that emits linearly polarized light. In a second embodiment, a linear polarization filter is inserted between the measuring body and the sensor. In this case, light of any polarization can interact with the measuring body. However, only the light that can pass through the linear polarization filter is detected. The first and second embodiments therefore lead to the same result.

[0030] Polarization allows the orientation of crystal axes to be determined because fluorescence intensity depends on polarization. Independent of determining a magnetic field, it is generally known to determine the direction of crystal axes by polarization, for example from PR Dolan, X. Li, J. Storteboom and M. Gu, “Complete determination of the orientation of NV centers with radially polarized beams,” Opt. Express, Vol. 22, No. 4, p. 4379, Feb. 2014, doi: 10.1364 / OE.22.004379 and from TPM Alegre, C. Santori, G. Medeiros-Ribeiro and RG Beausoleil, “Polarization-selective excitation of nitrogen vacancy centers in diamond,” Phys. Rev. B, Volume 76, p. 5, 2007, doi: 10.1103 / PhysRevB.76.165205. For example, using the methods described therein, the directions of the crystal axes can be determined in the described procedure. No separate measurement is required for this.Rather, the measurements that were already recorded are used for this purpose.

[0031] Are the projections of the magnetic field P i on the crystal axes with direction e→i For at least three crystal axes known, i.e. for i = 1,2,3, the following holds: (P1P2P3)=(e1Te2Te3T)(BxByBz):=Λ(BxByBz).

[0032] Here, Λ is a matrix. By inverting this matrix, the components of the magnetic field B can be determined. x , B y and B z will be received: (BxByBz)=Λ−1(P1P2P3)

[0033] In another preferred embodiment of the method, the direction and / or magnitude of the magnetic field are derived from the data points. B→ determined by α) for several crystal axes of the measuring body, a respective orientation is determined, β) a respective projection of the magnetic field B→ is determined on the crystal axes treated in step α), γ) from the projections determined in step β) the direction and / or magnitude of the magnetic field B→ be determined.

[0034] In step α), an orientation is determined for several crystal axes of the measuring body. This is possible because the following applies to the measured intensity I: I∝|d1⋅E|2+|d2⋅E|2=|dE|2{[sin(β)sin(α−ϕ)]2+[cos(θ)sin(β)cos(α−ϕ)+sin(θ)cos(β)]2}.

[0035] Here, E represents the electric field strength of the light illuminating the measuring body, d1 and d2 the dipoles of the defect, ϕ and θ the orientation of the crystal axis in question in spherical coordinates, α the polarization angle of the light illuminating the measuring body, and β the angle at which this light is linearly polarized with respect to the optical axis. In an embodiment not belonging to the invention, this light is linearly polarized perpendicular to the optical axis. In this case, β = 90°. Then the previously given equation simplifies to: I∝|dE|2[sin2(α−ϕ)+cos2(θ)cos2(α−ϕ)].

[0036] Using the two given equations, a respective orientation can be determined for the crystal axes of the measuring body, regardless of how the measuring body is oriented and regardless of which β was chosen.

[0037] In step α), an orientation is preferably determined for several crystal axes that are not aligned parallel to the optical axis. This applies particularly in the case where a crystal axis is aligned parallel to the optical axis, so that its orientation is already known. However, even in this case, it is also possible to use a crystal axis that is aligned parallel to the optical axis.

[0038] In step α), an orientation is preferably determined for three crystal axes of the diamond of the measuring body, which are preferably not aligned parallel to the optical axis. If, for example, the

[111] direction is aligned parallel to the optical axis, the

[111] direction, the

[111] direction and the

[111] direction are preferably determined.

[0039] To determine the direction of the crystal axes, the microwave frequencies v are determined for which the second derivative of the measured intensity is maximal. These are the absorption peaks described above. The amplitude of the absorption peaks depends on the polarization angle α. In an embodiment not included in the invention, in which the measuring body is illuminated with light that is linearly polarized perpendicular to the optical axis, i.e., β = 90°, the direction of the crystal axis corresponding to the respective absorption peak can be calculated in spherical coordinates Φ and θ as follows: A(α=Φ)=Amax cos(θ)=Amin / Amax

[0040] In this case, A maxThe maximum value of the amplitude, which depends on the polarization angle α. The amplitude is maximal when α = Φ. The polarization angle α and the angle Φ are defined with the same zero point. Thus, the angle Φ can be easily determined by finding the point of maximum in the plot of the amplitude A against the polarization angle α. The point of maximum is α = Φ. The angle θ can be determined from A using the given equation. max and A min to be determined. A is... min The minimum value of the amplitude when plotted against the polarization angle α. From this plot, A max and A min can be read.

[0041] Each crystal axis is associated with two peaks of the second derivative of intensity with respect to the polarization angle. Consequently, the described determination can be performed twice for each crystal axis. Subsequently, an average value can be calculated from the respective values ​​obtained for Φ and θ to determine Φ and θ with particular accuracy. Alternatively, only one peak per crystal axis can be evaluated.

[0042] Since the measuring body is exposed to light which is linearly polarized and tilted relative to the optical axis, i.e., β ≠ 90°, the directions of the crystal axes can be calculated from the general formula for intensity given above.

[0043] In another preferred embodiment of the method, in step α) pairs of resonance frequencies v are determined for a plurality of values ​​for the polarization angle α. 1,2 and associated amplitudes A 1,2determined, from which the respective orientation of the crystal axes is determined.

[0044] The resonance frequencies v 1,2 and associated amplitudes A 1,2 These form the peaks described above. Each crystal axis corresponds to a pair of resonant frequencies v. 1,2 with associated amplitudes A 1,2 assigned.

[0045] In another preferred embodiment of the method, each pair of resonance frequencies v 1,2 each is assigned to one of the crystal axes of the measuring body.

[0046] In an embodiment not belonging to the invention, the light used in step a) could be linearly polarized perpendicular to the optical axis.

[0047] This configuration would be the simplest. Light linearly polarized perpendicular to the optical axis can be obtained using a linear polarizing filter. This allows the magnetic field to be determined almost unambiguously. Only the following uncertainty remains: B(ϕ,θ)=−B(ϕ+π,π−θ) B(ϕ,π−θ)=−B(ϕ+π,θ).

[0048] However, the light used in step a) is linearly polarized and tilted relative to the optical axis.

[0049] According to the invention, the symmetry is broken insofar as the uncertainty described for the previous embodiment no longer exists. Only an uncertainty between B and -B remains.

[0050] Obliquely polarized light can be obtained by passing linearly polarized light through a lens, with marginal rays being asymmetrically blocked.

[0051] In another preferred embodiment of the method, the magnetic field is measured without a reference magnetic field.

[0052] Another aspect of the invention is a device for measuring a magnetic field. B→ presented. The facility includes: - a measuring body made of a material with a multitude of spin-1 defects with C 3v -symmetry is formed, - a light source which is designed to illuminate the measuring body along an optical axis, - a microwave source which is designed to bombard the measuring body with microwave radiation of a frequency v, - a sensor configured to measure an intensity I of light emitted by the measuring body, wherein the light source is configured to emit linearly polarized light at a polarization angle α, or the device further comprises a linear polarization element located between the light source and the sensor, which only transmits light that is linearly polarized at a polarization angle α, wherein the light is linearly polarized at an angle tilted relative to the optical axis. and wherein the device further comprises - a control device which is configured to determine the direction and / or magnitude of the magnetic field from a large number of data points B→ to determine, wherein each of the data points has a value for the intensity I measured by the sensor for a respective value of a polarization angle α of the light and a respective value of the frequency v of the microwave radiation.

[0053] The described advantages and features of the method are applicable and transferable to the equipment, and vice versa. The method is preferably carried out using the equipment. The equipment is preferably configured for operation according to the method.

[0054] The linear polarization element is positioned between the light source and the sensor. This covers the two cases where the linear polarization element is positioned between the light source and the measuring body, and where the linear polarization element is positioned between the measuring body and the sensor.

[0055] The linear polarization element can, for example, be a linear polarization filter. Preferably, however, the linear polarization element is formed by a combination of a linear polarization filter and a λ / 2 plate. This is particularly true when the linear polarization element is arranged between the measuring body and the sensor.

[0056] The invention is explained in more detail below with reference to the figures. The figures show particularly preferred embodiments, to which the invention is not limited. The figures and the size relationships shown therein are only schematic. They show: Fig. 1: Energy levels of a nitrogen vacancy center in diamond, Fig. 2a: a first embodiment of a device according to the invention for measuring a magnetic field B→, Fig. 2b: a second embodiment of a device according to the invention for measuring a magnetic field B→, Fig. 3a: a more detailed description of the facility from Fig. 2a, Fig. 3b: a more detailed description of the facility from Fig. 2b, Fig. 4: an example of the practical implementation of the facility Fig. 2a and Fig. 3a, Fig. 5: a presentation of measurement results as obtained with the device from Fig. 2a, Fig. 3a and Fig. 4 or from Fig. 2b and Fig. 3b can be obtained, Fig. 6: a representation of a first possibility for the linear polarization of light, Fig. 6': a representation of a second possibility for the linear polarization of light, Fig. 7: An illustration of the realization of the possibility of linear polarization of light from Fig. 6, Fig. 7': an illustration of the realization of the possibility of linear polarization of light from Fig. 6', Fig. 8: Simulated values ​​for measurement results obtained during operation of the facility Fig. 2a, Fig. 3a and Fig. 4 or from Fig. 2b and Fig. 3b with polarized light according to Fig. 6 and Fig. 7 are expected, Fig. 8': Simulated values ​​for measurement results obtained during operation of the facility Fig. 2a, Fig. 3a and Fig. 4 or from Fig. 2b and Fig. 3b with polarized light according to Fig. 6' and Fig. 7' are expected, Fig. 9: a result of a method according to the invention for determining a magnetic field.

[0057] Fig. Figure 1 illustrates the energy levels of a nitrogen vacancy center in diamond. There is a ground state |g〉, an excited state |e〉, and a singlet state |s〉. Additional vibrational states are indicated. The ground state and the excited state each possess a spin-1 subsystem. The energy levels m S = 0 and m S The sublevels m are split by the zero-field splitting D = 2.87 GHz and a transition can be excited by a Rabi frequency Ω. S Levels ±1 are nearly degenerate with a zero-field splitting E « D. When an external magnetic field B is applied, the sublevels m split. S = -1 and m S = +1 due to the Zeeman effect. The state |e,0〉 decays by radiation to |g,0〉. The states |e,±1〉 decay predominantly non-radiatively via |s〉 to |g,0〉.

[0058] To measure a magnetic field, the transitions between |g,±1〉 and |g,0〉 are excited by microwave radiation. Consequently, the Zeeman splitting can be used to determine the direction and / or magnitude of the magnetic field.

[0059] The spin state can be optically read out by optically pumping the transition |g〉 → |e〉 (zero phonon line at 637 nm). For m S A value of ±1 results in low fluorescence; for m S = 0 indicates high fluorescence.

[0060] Fig. Figure 2a shows a first embodiment of a device 1 for measuring a magnetic field. The device 1 has a measuring body 2, which is made of diamond with a plurality of negatively charged nitrogen vacancy centers 5. The illustration is simplified insofar as only one nitrogen vacancy center 5 is shown. It can be seen that one position of the diamond lattice is occupied by a nitrogen atom N and that the adjacent position is unoccupied. This is indicated by a V for "vacant".

[0061] Also shown is an optical axis 6, which is aligned parallel to the nitrogen vacancy center 5. This is described as... n→ The indicated direction of the nitrogen vacancy center 5 is therefore parallel to the optical axis 6. Consequently, the optical axis 6 is aligned parallel to the

[111] direction,

[111] direction,

[111] direction or

[111] direction of the diamond of the measuring body 2.

[0062] The measuring body 2 is arranged between a light source 9 and a sensor 11. The light source 9 is configured to illuminate the measuring body 2 with light along the optical axis 6. The light source 9 provides the measuring body 2 with linearly polarized light. The polarization direction is determined by p→ The sensor 11 is designed to measure the intensity I of light emitted by the measuring body 2. To enable measurements at different polarization angles α, the polarization is rotated. This is indicated by a curved arrow.

[0063] Fig. Figure 2b shows a second embodiment of a device 1 for measuring a magnetic field. This differs from the embodiment according to Figure 2b. Fig. 2a simply by the fact that the light is only linearly polarized behind the measuring element. Both embodiments lead to the same result.

[0064] Fig. Figure 3a shows a more detailed representation of facility 1. Fig. 2a. The device 1 comprises, in addition to the measuring body 2, the light source 9 and the sensor 11, a microwave source 10. This is configured to subject the measuring body 2 to microwave radiation of a frequency v.

[0065] The device 1 further comprises a linear polarization filter 7, which is arranged between the light source 9 and the sensor 11. In addition, the device 1 comprises a control unit 12, which is configured to determine a direction and / or magnitude of the magnetic field from a multitude of data points. Each of the data points has a value for the intensity I measured by the sensor 11 for a respective value of a polarization angle α of the light and a respective value of the frequency v of the microwave radiation.

[0066] The device 1 also includes a λ / 2 plate 13, which is arranged between the linear polarization filter 7 and the measuring body 2. The linear polarization of the light is rotated by means of the λ / 2 plate 13. For this purpose, the λ / 2 plate 13 is connected to the control unit 12. This means that a drive (not shown) is provided on the λ / 2 plate 13, with which the λ / 2 plate 13 can be rotated according to a signal from the control unit 12.

[0067] Furthermore, the device 1 has a bandpass filter 14 between the measuring body 2 and the sensor 11. This can be used to increase the measurement accuracy by, for example, filtering out ambient light and the light from the light source 9.

[0068] Fig. Figure 3b shows a more detailed representation of facility 1. Fig. 2b. In contrast to Fig. 3a Here, another linear polarization filter 7 is arranged between the bandpass filter 14 and the sensor 11. This is connected to the control unit 12 in place of the λ / 2 plate 13 and can be rotated by a drive (not shown).

[0069] In the embodiments according to Fig. 3a and Fig. 3b. All elements of the device 1 are arranged in a line along the optical axis 6. Accordingly, a first side 3 of the measuring body 2 is illuminated by the light from the light source 9, while the light emitted by the measuring body 2 is measured on a second side 4 of the measuring body. However, such a setup is not mandatory. In particular, mirrors can be used to redirect the beam path. It is also possible to measure the light emitted by the measuring body 2 on the same side of the measuring body 2 that is illuminated by the light from the light source 9.

[0070] A corresponding example shows Fig. 4. Several mirrors 17 are provided therein. The light emitted by the light source 9 passes through a beam splitter 23, which is also passed through the light emitted by the measuring body 2. Measurement can be taken by reflection through the beam splitter 23. The measuring body 2 rests on a microwave antenna 22, which is connected to the microwave source 10. A lens 18 is arranged above the measuring body 2. To direct the light from the light source 9 onto the measuring body 2, the lens 18 can be moved in the z-direction by a z-movement element 21, and the microwave antenna 22, along with the measuring body 2 and the microwave antenna 10, can be moved in the x- and y-directions by an x,y-movement element 20. The sensor 11 is formed by a spectrometer 19.

[0071] Fig. Figure 5 shows a representation of measurement results as obtained with the device 1 from Fig. 2a, Fig. 3a and Fig. 4 or from Fig. 2b and Fig. 3b can be obtained. The plot shows the second derivative of the intensity I of the light emitted by the measuring body 2, expressed as d. 2 I / dv 2 in arbitrary units (au), against the frequency v of the microwave radiation emitted by the microwave source 10. A curve is fitted to the measurement results. Eight peaks can be identified. Two of the peaks each form a pair v1, v2. The four pairs correspond to the four crystal axes

[111] ,

[111] ,

[111] and

[111] .

[0072] The results in Fig. The 5 data points refer to a specific polarization angle α. A complete set of data points includes data such as in Fig. Figure 5 shows for a variety of values ​​for the polarization angle α.

[0073] Fig. Figure 6 represents a first possibility for the linear polarization of light. Here, the light is linearly polarized perpendicular to the optical axis 6. Accordingly, there is an angle β = 90° between the optical axis 6 and the electric field. E→ The figure shown reflects the polarization of the light. However, according to the invention, light is used which is linearly polarized and tilted relative to the optical axis 6.

[0074] Fig. Figure 6' represents a second possibility for the linear polarization of light. In this case, the light is linearly polarized at an angle to the optical axis 6, as is the case according to the invention. The angle β is less than 90° in this case.

[0075] Both the possibility of Fig. 6 as well as the possibility to Fig. 6' can be used both when setting up 1 from Fig. 2a, Fig. 3a and Fig. 4 as well as at the setup 1 from Fig. 2b and Fig. 3b can be used. However, according to the invention, only the possibility after Fig. 6' used.

[0076] Fig. Figure 7 is an illustration of the possibilities for linear polarization of light. Fig. 6. Figure 6 shows how linearly polarized light strikes a lens 15. The polarization of the marginal rays is indicated by arrows. Behind the lens 15, the marginal rays are oblique. Accordingly, the polarization of the individual marginal rays can also be considered oblique. However, due to symmetry, the oblique components cancel each other out, so that the overall polarization is parallel to the polarization in front of the lens 15.

[0077] Fig. Figure 7' is an illustration of the possibilities for linear polarization of light. Fig. 6'. Unlike Fig. A shield 16 is provided here, which shields the lower marginal ray. Consequently, the oblique components of the polarization behind the lens 15 no longer exactly cancel each other out. The overall polarization behind the lens 15 is therefore tilted.

[0078] Fig. Figure 8 shows simulated values ​​for measurement results obtained during the operation of device 1. Fig. 2a, Fig. 3a and Fig. 4 or the facility 1 from Fig. 2b and Fig. 3b are expected when linearly polarized light according to Fig. 6 and Fig. Figure 7 is used. Shown is the intensity I versus the polarization angle α for four crystal axes at a specific microwave frequency v. The crystal axes are given in spherical coordinates Φ and θ. For each crystal axis, the values ​​are shown for the microwave frequency v at which the intensity is maximum. This is, as shown in Figure 7, the intensity I versus the polarization angle α for a specific microwave frequency v. Fig. 5 can be seen, for each crystal axis at two microwave frequencies v. Theoretically, however, the values ​​in are used for both of these values. Fig. Curves shown in 8a are expected. Fig. Figure 8a therefore shows the curves only once. In practice, both peaks can be considered and the analysis proceeded using an average of the results.

[0079] This can be recognized by Fig. 8, that the intensity at the crystal axis with Φ = 0° = θ is constant at the maximum value 1 for all polarization angles α. This can be explained by the fact that this is the

[111] crystal axis. The three other crystal axes differ from the

[111] crystal axis by an angle θ = 109.5° and have an individual angle Φ. It can be seen that the results in Fig. 8. These three crystal axes are identical except for a phase shift. Therefore, the direction of the magnetic field can only be uniquely determined by knowing the angle Φ. This angle can be determined using the polarization of the light.

[0080] Fig. 8' shows simulated values ​​for measurement results obtained during operation of device 1. Fig. 2a, Fig. 3a and Fig. 4 or the facility 1 from Fig. 2b and Fig. 3b will be when linearly polarized light according to Fig. 6' and Fig. 7' is used. Due to the oblique polarization of the light, the symmetry is broken in such a way that the expected measurement results for the crystal axes differ from each other not merely by a phase shift.

[0081] Fig.Figure 9 shows a result of a method for determining a magnetic field. A Cartesian coordinate system is shown, in which four crystal axes 8 of the diamond are drawn. The upward-pointing crystal axis 8 corresponds to the

[111] direction, parallel to which the light in the considered example falls from the light source 9 onto the measuring body 2. The magnetic field B→ is represented as a vector. Reference symbol list 1. Facility 2 measuring bodies 3 first page 4 second page 5 Nitrogen Vacancy Center 6 Optical axis 7 linear polarization filters 8 Crystal axis 9 Light source 10 microwave source 11 Sensor 12 Control unit 13 λ / 2 plate 14 bandpass filters 15 lenses 16 Shielding 17 mirrors 18 lens 19 spectrometers 20 x,y motion element 21 z-movement element 22 Microwave antenna 23 beam splitters

Claims

[1] Method for measuring a magnetic field by means of a measuring body (2) which is made of a material with a plurality of spin-1 defects with C 3v -symmetry is formed, whereby for a large number of data points simultaneously a) the measuring body (2) is illuminated with light along an optical axis (6), b) the measuring body (2) is exposed to microwave radiation of a frequency v, and c) an intensity I of light emitted by the measuring body (2) is measured, wherein the light used in a) is linearly polarized at a polarization angle α, or in c) the light is measured after passing through a linear polarization filter (7) set at a polarization angle α, wherein the data points differ in the polarization angle α of the light and / or in the frequency v of the microwave radiation, and wherein the direction and / or magnitude of the magnetic field can be determined from the data points. B→ are determined, wherein the light used in step a) is linearly polarized and tilted relative to the optical axis (6). [2] Method according to claim 1, wherein the material of the measuring body (2) is diamond with a plurality of nitrogen vacancy centers (5) with a negative charge. [3] Method according to one of the preceding claims, wherein the direction and / or magnitude of the magnetic field can be determined from the data points B→ be determined by α) for several crystal axes (8) of the measuring body (2) a respective orientation is determined, β) a respective projection of the magnetic field B→ on the crystal axes (8) treated in step α), γ) from the projections determined in step β) the direction and / or magnitude of the magnetic field B→ be determined. [4] Method according to claim 3, wherein in step α) pairs of resonance frequencies v are selected for a plurality of values ​​for the polarization angle α. 1,2 and associated amplitudes A 1,2 are determined from which the respective orientation of the crystal axes (8) is determined. [5] Method according to claim 4, wherein each pair of resonance frequencies v 1,2 each is assigned to one of the crystal axes (8) of the measuring body (2). [6] Method according to claim one of the preceding claims, wherein the measurement of the magnetic field is carried out without a reference magnetic field. [7] Device (1) for measuring a magnetic field B→, comprehensive - a measuring body (2) which is made of a material with a multitude of spin-1 defects with C 3v -symmetry is formed, - a light source (9) which is designed to illuminate the measuring body (2) along an optical axis (6), - a microwave source (10) which is configured to subject the measuring body (2) to microwave radiation of a frequency v, - a sensor (11) which is configured to measure an intensity I of light emitted by the measuring body (2), wherein the light source (9) is configured to emit linearly polarized light at a polarization angle α or the device (1) further comprises a linear polarization element (7) which is arranged between the light source (9) and the sensor (11) and which only transmits light which is linearly polarized at a polarization angle α, wherein the light is linearly polarized at an angle relative to the optical axis (6), and wherein the device (1) further comprises - a control device (12) which is configured to determine a direction and / or magnitude of the magnetic field from a multitude of data points B→ to determine, wherein each of the data points has a value for the intensity I measured with the sensor (11) for a respective value of the polarization angle α of the light and a respective value of the frequency v of the microwave radiation.

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