Magnetic field sensor and method for calibrating the temperature of a magnetic field sensor

Abstract

The invention relates to a magnetic field sensor (10), comprising: an excitation light source (6) for emitting excitation light (4), a measurement region (7) which has colour centres (3) in a crystal (1), preferably NV centres in a diamond, which emit fluorescent light (5) when excited with the excitation light (4), a detector (8) for detecting the fluorescent light (5) from the measurement region (7), a microwave emitter (11) for applying a microwave field (9) to the measurement region (7), a signal generator (14) which is designed to generate a microwave signal (15) for the microwave emitter (11) and has two frequency components (fsb-(t), fsb+,π(t)) phase-shifted in relation to one another by π, in order to simultaneously excite magnetic field resonances at two different resonant frequencies (f-, f+) in the crystal (1) in order to determine a magnetic field (B), and an evaluation device (12) which has a demodulator (13) for forming a demodulated magnetic resonance signal from the detected fluorescent light (5) of the measurement region (7), wherein the evaluation device (12) is designed to determine a detuning (∆B) of the resonant frequencies (f-, f+) due to a change in the magnetic field (B), and to generate a frequency shift (∆f) of the microwave signal (15) according to the magnetic-field-related detuning (∆B) of the resonant frequencies (f-, f+). The magnetic field sensor (10) is designed to determine, in a temperature calibration step, a temperature-related detuning (∆T) of the resonant frequencies (f-, f+) and to take same into account in the frequency shift (∆f) of the microwave signal (15) in order to determine the magnetic field (B). The invention further relates to an associated method for calibrating the temperature of a magnetic field sensor (10).

Description

[0001] Magnetic field sensor and method for temperature calibration of a

[0002] Maqnet field sensor

[0003] The present invention relates to a magnetic field sensor comprising: an excitation light source for emitting excitation light, a measuring area having color centers in a crystal, preferably NV centers in a diamond, which emit fluorescence light when excited with the excitation light, a detector for detecting the fluorescence light from the measuring area, a microwave emitter for impinging a microwave field on the measuring area, a signal generator configured to produce a microwave signal for the microwave emitter having two frequency components phase-shifted by TT in order to simultaneously excite magnetic field resonances at two different resonance frequencies in the crystal for determining a magnetic field, and an evaluation device comprising a demodulator for generating a demodulated magnetic resonance signal from the detected fluorescence light of the measuring area.wherein the evaluation device is configured to determine a detuning of the resonance frequencies due to a change in the magnetic field and to generate a frequency shift of the microwave signal depending on the magnetic field-induced detuning of the resonance frequencies. The invention also relates to a method for temperature calibration of a magnetic field sensor, which comprises a measuring area having color centers in a crystal, preferably NV centers in a diamond, which emit fluorescent light when excited with the excitation light. The method comprises: applying a microwave field to the measuring area by means of a microwave emitter to which a microwave signal is supplied, which has two frequency components phase-shifted by TT from each other, in order to simultaneously excite magnetic field resonances at two different resonance frequencies in the crystal in order to determine a magnetic field.Forming a demodulated magnetic resonance signal from the detected fluorescence light of the measurement range, determining a detuning of the resonance frequencies due to a change in the magnetic field, and generating a frequency shift of the microwave signal depending on the magnetic field-induced detuning of the resonance frequencies.

[0004] Quantum sensors, such as magnetic field sensors, exploit the extreme sensitivity of quantum systems to their environment and therefore enable very precise measurements of physical quantities, such as magnetic fields. Crystals doped with color centers are typically used as sensor elements in quantum sensors. In particular, a diamond crystal doped with color centers, usually nitrogen vacancy centers (hereinafter referred to as nitrogen vacancy, NV, centers), can be used as a sensor element. NV centers in diamond exhibit a characteristic electronic structure that changes when certain measured quantities, such as an external magnetic field, temperature, pressure, or electric field, are altered.The color centers are excited by irradiation with excitation light in the optical range and typically by irradiation with a microwave field with varying frequency in the microwave range, and the fluorescence light induced in the color centers is detected by a detector and demodulated and analyzed in an evaluation unit in order to determine a physical quantity, for example the magnetic field.

[0005] German patent DE 10 2022 113 754 A1 describes an NV magnetometer and a method for determining an external magnetic field using the NV magnetometer with automated resonance control. In this method, a double-resonance signal with a dispersive waveform around the resonance frequencies of two simultaneously excited magnetic resonances is recorded. A detuning of the resonance frequencies is detected via the demodulated signal when microwave radiation is applied resonantly due to changes in the external magnetic field using an evaluation unit. The frequency bands of the applied microwave radiation are controlled by a controller as a function of the observed detuning, such that the frequency bands remain within the range of the resonance frequencies of the NV centers present at this detuning, even after the detuning has occurred. Using the control described therein, the microwave frequency of the microwave signal can be determined.The frequency bands are adjusted to the detuning of the resonant frequencies, thus tracking the magnetic field-sensitive frequency range of the magnetic field change. In this operating mode of the magnetic field sensor, also known as tracking, even larger magnetic field changes can be detected, and the dynamic range of the magnetic field sensor can be increased from the usual few pT to several mT (or more).

[0006] The two frequency bands used to excite the two magnetic field resonances in the magnetic field sensor have two frequency components that are phase-shifted by ΔT. When the temperature changes, the two magnetic field resonances, and thus their resonance frequencies, are shifted in the same direction. Due to this phase shift, a temperature change causes the two demodulated signals of the magnetic field resonances to undergo changes of equal magnitude but opposite sign. Since the two signals are superimposed or added by the detector, these changes do not alter the demodulated signal. Therefore, the demodulated magnetic resonance signal of the magnetic field sensor is insensitive to temperature changes, and the frequency of the microwave signal is not adjusted in response to temperature changes.

[0007] In the article “Robust high dynamic-range vector magnetometry with nitrogen vacancy centers in diamond”, H. Clevenson et al., Appl. Phys. Lett. 112, 252406 (2018), a vector magnetometer is described that uses a frequency-locked loop or lock-in detection to simultaneously excite and track the Zeeman-split resonance pairs of nitrogen vacancy centers in diamond. This is intended to decouple the influence of temperature from the magnetic field across the entire measurement bandwidth.

[0008] In W02022020943A1, a vector magnetometer and a method for distinguishing spin population transfer in different crystal defect orientations are described. It is proposed to determine the relative temperature difference of zero-field splitting and the three components of the magnetic field strength vector based on the frequency of four resonance lines. The resonance lines are determined for at least three different crystal defect orientations. The determination of the relative temperature difference and the magnetic field vector is carried out using the Rabi frequencies. These frequencies differ for the respective crystal defect orientations when the pulse parameters, in particular the pulse durations or pulse amplitudes, of the incident microwave radiation are varied.

[0009] WO 2014 / 051886A1 describes a sensor formed from a diamond material and exhibiting one or more spin defects, which may be NV centers. These NV centers allow temperature measurement by detecting variations in their axial zero-field splitting parameter. Temperature detection, such as with a temperature sensor, can be used to calibrate the effects of temperature on the magnetic field measurement. Alternatively, it is possible to use both the m s =+1 as well as the m s =-1 resonances to provide a feedback mechanism to calibrate out the temperature-related effects. The system can also be configured to use the m s = ±1 resonance transitions to perform magnetic field measurements that are not affected by temperature.

[0010] As described above, temperature changes lead to a shift or detuning of the resonant frequencies of the magnetic field resonances. In the magnetic field sensor described above, where the microwave signal contains the two frequency components offset by TT, these shifts are eliminated during the determination of the magnetic field and therefore cannot be measured. This results in an unnoticed offset between the true resonant frequency and the resonant frequency set or tracked during the tracking process, i.e., during the frequency shift of the microwave signal. This offset can cause the linear range of the resonance to be exceeded, and the magnetic field sensor may no longer function correctly. Object of the invention

[0011] The invention is based on the objective of providing a magnetic field sensor and a method for temperature calibration of a magnetic field sensor that prevent temperature-related offset in the frequency shift of the microwave signal as completely as possible.

[0012] Subject matter of the invention

[0013] This task is solved by a magnetic field sensor of the type mentioned above, which is designed to determine a temperature-related change in the resonance frequencies in a temperature calibration step and to take this into account when shifting the frequency of the microwave signal.

[0014] The magnetic field sensor according to the invention is designed to determine the temperature-related change in the resonance frequencies in a temperature calibration step and to take this into account during tracking. In this way, the occurrence of a temperature-related offset in the frequency shift of the microwave signal can be prevented. In particular, this prevents the linear range of the magnetic field resonances from being left, which would otherwise lead to the magnetic field sensor malfunctioning, especially during large temperature changes.

[0015] In the magnetic field sensor according to the invention, the determination of the magnetic field-induced detuning of the resonance frequencies due to a change in the magnetic field is carried out analogously to DE 10 2022 113 754 A1 cited above, which is incorporated in its entirety by reference into this application. This method utilizes the fact that the demodulator is a lock-in amplifier. In this case, the demodulated magnetic resonance signal exhibits a dispersive waveform and a linear region around the respective resonance frequency. The slope of the linear region can be determined as a scalar factor from the demodulated magnetic resonance signal. From this slope, the detuning of the resonance frequencies due to a change in the magnetic field can be determined, since the two resonance frequencies are shifted in opposite directions during magnetic field-induced detuning.As described above, the temperature-induced detuning of the resonance frequencies cannot be determined in this way, since the two resonance frequencies are shifted in the same direction during temperature-induced detuning. Therefore, the determination of the temperature-induced detuning in the temperature calibration step must be carried out using a different method.

[0016] In one embodiment, the magnetic field sensor is configured to determine the magnetic field based on the magnetic field-induced detuning of the resonant frequencies and to perform the temperature calibration step simultaneously. It is advantageous if the determination or measurement of the magnetic field for the temperature calibration step does not have to be interrupted, but rather if recalibration of the resonant frequencies can be performed practically in real time.

[0017] The temperature calibration step can be performed in different ways.

[0018] In one embodiment, the magnetic field sensor additionally comprises a further signal generator configured to generate at least one further microwave signal in order to excite at least two further magnetic resonances at at least two further resonance frequencies in the crystal, wherein the evaluation device is configured to determine a temperature-related change of the further resonance frequencies in the temperature calibration step.

[0019] In this embodiment, a further signal generator is used to excite additional magnetic field resonances and further resonance frequencies, from which the temperature-induced detuning of those resonance frequencies used to determine the magnetic field can be determined. For this purpose, it is necessary that a correlation exists between the temperature-induced detuning of the further resonance frequencies and the temperature-induced detuning of the resonance frequencies. Ideally, the temperature-induced detuning of the further resonance frequencies corresponds to the temperature-induced detuning of the resonance frequencies. In a further development, the signal generator is configured to excite the magnetic resonances at NV centers with a first NV axis, and the further signal generator is configured to excite the further magnetic resonances at NV centers with a second NV axis, different from the first.

[0020] In this case, the magnetic resonances and other magnetic resonances are typically the same transitions between two spin states, usually between the m s = 0 and the ms = -1 or the m s The spin state of the NV centers is 0 and ms = +1. As described above, the sum of the resonance frequencies of these two magnetic resonances depends on the temperature, and this dependence is not affected by the orientation of the NV axes of the NV centers. Therefore, by determining the other resonance frequencies, the temperature dependence or frequency shift of the resonance frequencies can be determined and taken into account when calculating the frequency shift of the microwave signal.

[0021] For the simultaneous determination of the magnetic field-induced detuning of the resonant frequencies and the temperature-induced detuning of the other resonant frequencies, the evaluation unit of the magnetic field sensor can have two demodulators or one demodulator that can generate two demodulated signals from one input signal in parallel. One detector is sufficient for the simultaneous determination of both detunings with the magnetic field sensor.

[0022] In a further embodiment, the additional signal generator is configured to excite the two additional magnetic resonances either sequentially or simultaneously, wherein the additional microwave signal, when the additional magnetic resonances are excited simultaneously, has two frequency components without a phase shift. In both cases, i.e., both with sequential excitation and with simultaneous excitation without a phase shift, a uniform temperature-induced detuning of the additional resonance frequencies occurs, which can be determined in the demodulated magnetic resonance signal and taken into account when frequency-shifting the microwave signal. With simultaneous excitation of the magnetic field resonances, the temperature-induced detuning can be determined analogously to the magnetic field-induced detuning of the resonance frequencies due to the absence of a phase shift.

[0023] In another embodiment, the signal generator is configured to produce a microwave signal during the temperature calibration step that has two frequency components without phase shift, in order to simultaneously excite the magnetic field resonances at the two different resonance frequencies in the crystal. In this embodiment, the phase shift of the two frequency components of the microwave signal is briefly set from 180° (or 180°) to 0° for the temperature calibration step. This makes it possible to determine or measure the temperature-induced detuning of the resonance frequencies and to take this into account when shifting the frequency of the microwave signal.

[0024] The magnetic field sensor or signal generator can alternatively be configured to excite the two magnetic resonances sequentially during the temperature calibration step. In this case, the temperature-dependent detuning of the resonance frequencies can also be determined and taken into account when shifting the frequency of the microwave signal.

[0025] In another alternative embodiment, the magnetic field sensor is configured to determine the temperature-induced detuning of the two resonant frequencies during the temperature calibration step by measuring the two resonant frequencies at at least two different times. It is generally necessary to measure the two resonant frequencies before determining the magnetic field, as they depend on the temperature and ambient pressure. In this case, this initial measurement can already be part of the temperature calibration step, and the subsequent measurement of the two resonant frequencies at at least one later time point constitutes another part of the temperature calibration step. If the temperature of the crystal has changed between the two time points, the two resonant frequencies will also have shifted. Adding the two resonant frequencies, or...If the mean value of the resonant frequencies is determined at a given time, the influence of temperature on the measurement can be determined and eliminated by taking into account the temperature-induced detuning of the resonant frequencies through an adjustment of the microwave frequency or frequency shift. To determine the resonant frequencies at a given time, a sweep or tuning of the microwave signal frequency across the entire spectrum or across the relevant portion of the spectrum can be performed.

[0026] In another embodiment, the magnetic field sensor includes at least one temperature sensor for measuring the temperature in the vicinity of the crystal and is configured to perform the temperature calibration step depending on the temperature measured by the temperature sensor. In this case, the temperature calibration step is triggered by the temperature sensor, for example, when the measured temperature falls below or exceeds a threshold value relative to a reference temperature. Alternatively, it is possible to perform the temperature calibration step at predetermined time intervals or, if necessary, continuously. The latter is possible, for example, if an additional signal generator is provided for the temperature calibration step (so).

[0027] Another aspect of the invention relates to a method of the type mentioned at the outset, further comprising: determining a temperature-induced detuning of the resonance frequencies in a temperature calibration step and taking the temperature-induced detuning of the resonance frequencies into account when shifting the frequency of the microwave signal. In this aspect of the invention, the temperature calibration step is performed to calibrate the magnetic field sensor, and the temperature-induced change in the resonance frequencies is taken into account when shifting the frequency of the microwave signal.

[0028] As described above, the temperature calibration step can be performed at regular intervals or triggered by an additional temperature sensor. This prevents the magnetic field sensor from leaving the linear range of its resonances and malfunctioning. In one variation of the method, the magnetic field is determined based on the detuning of the resonance frequencies, and the temperature calibration step is performed simultaneously. This variation allows for simultaneous determination of the magnetic field and monitoring of the temperature-induced change in the resonance frequencies; that is, it is not necessary to interrupt the magnetic field determination to perform the temperature calibration step.

[0029] The temperature calibration step can be carried out in different ways, i.e., with different variants of the procedure, which correspond to the embodiments described above in connection with the magnetic field sensor.

[0030] For example, in one variant at least one further microwave signal can be generated to excite at least two further magnetic resonances at at least two further resonance frequencies in the crystal, and a temperature-related detuning of the further resonance frequencies can be determined in the temperature calibration step to determine the temperature-related detuning of the resonance frequencies.

[0031] The microwave signal can be used to excite the magnetic resonances at NV centers with a first NV axis, and the additional microwave signal can be used to excite the further magnetic resonances at NV centers with a second NV axis, different from the first.

[0032] The two other magnetic resonances can be excited either sequentially or simultaneously, whereby the additional microwave signal has two frequency components without phase shift when the other magnetic resonances are excited simultaneously.

[0033] Alternatively, a microwave signal with two frequency components without phase shift can be generated in the temperature calibration step to simultaneously excite the magnetic field resonances at the two resonance frequencies in the crystal.

[0034] Alternatively, it is possible to determine the temperature-induced detuning of the two resonance frequencies in the temperature calibration step by determining the two resonance frequencies at at least two different times.

[0035] Further advantages of the invention will become apparent from the description and the drawing. Likewise, the features mentioned above and those listed below can be used individually or in any combination. The embodiments shown and described are not to be understood as an exhaustive list, but rather serve as examples illustrating the invention.

[0036] They show:

[0037] Fig. 1 is a schematic representation of a unit cell of a diamond crystal with NV centers defining two NV axes,

[0038] Fig. 2a, b schematic representations of the electron states of one of the NV centers of Fig. 1 ,

[0039] Fig. 3 shows a schematic representation of a magnetic field sensor which has a diamond crystal with NV centers,

[0040] Fig. 4 shows a schematic representation of an ODMR spectrum recorded with the magnetic field sensor,

[0041] Fig. 5a, b schematic representations of a magnetic resonance excited by a frequency-modulated microwave signal, and of a magnetic resonance signal demodulated by a lock-in amplifier, and Fig. 6a-c schematic representations of demodulated magnetic resonance signals and their dependence on changes in the magnetic field and temperature.

[0042] In the following description of the drawings, identical reference symbols are used for identical or functionally equivalent components.

[0043] Fig. 1 shows a unit cell 2 of a diamond crystal 1, which has nitrogen vacancy (NV) centers 3. Each NV center 3 consists of a nitrogen atom N and a vacancy V. An imaginary line connecting the nitrogen atom N and the vacancy V defines an NV axis of the respective NV center 3. The NV centers 3 are present in the diamond crystal 1 in four different orientations, i.e., there are four differently oriented NV axes, of which a first NV axis NV1 and a second NV axis NV2 are shown as examples in Fig. 1.

[0044] Figs. 2a, b show the simplified state spectrum of one of the NV centers 3 from Fig. 1, its excitation with the aid of excitation light 4, and the emission of fluorescence light 5 generated during excitation. The ground state 3 A and the excited state 3 E of the NV centers 3 split into three fine-structure states each with the quantum numbers ms = 0, ms = -1 and m s = +1. If no external magnetic field B is present, the states m s = -1 and m s = + 1 degenerates, as shown in Fig. 2a, b. Between these states and the energetically lower state m s At = 0, even without external influences, a splitting D(T) of approximately 2.87 GHz exists, which is also called zero-field splitting (ZFS) and is caused by the spin-spin interaction of the electrons of the NV centers 3.

[0045] The zero-field splitting D(T) depends on the temperature, crystal properties, and pressure. A temperature change dT leads to a change in the zero-field splitting D(T) according to: dD / dT = -74.2 kHz / K.

[0046] The degeneracy of states m s = +1 and m sThe difference of -1 is canceled due to the Zeeman effect in the presence of an external magnetic field B. The shift in the transition frequencies f+, f. between the two no longer degenerate

[0047] states m s = +1 and m s = - 1 is d / — ±YNVB P , where YNV denotes the gyromagnetic ratio (here: 28 GHz / T) and B P denotes the portion of the external magnetic field that lies parallel to the respective NV axis NV1, NV2, ..., i.e., the projection of the magnetic field B onto the respective NV axis NV1, NV2, ... . The transition or resonance frequencies f+, f. can therefore be written as follows: f+ = D(T~) ± YNV^P-

[0048] For determining an external magnetic field B, a magnetic field sensor 10 shown in Fig. 3 has an excitation light source 6 for emitting the excitation light 4 shown in Fig. 2a, b. The excitation light source 6 is configured to emit the excitation light 4 in the green wavelength range. The excitation light 4 excites the electrons from the ground state. 3 A into the energetically higher excited state 3 E is excited. This transition is spin-conserving. If the electrons are in the fine-structure state with quantum number m at the time of excitation... s = 0, therefore they will also be the state with m s = 0 in the excited state 3 E is occupied, as shown in Fig. 2a. From this state, the decay to the ground state proceeds. 3 A directly emitting red fluorescent light 5 at wavelengths between approximately 600 nm and 900 nm.

[0049] The diamond crystal 1 in the magnetic field sensor 10 shown in Fig. 3 has a measuring area 7 with the NV centers 3. The fluorescence light 5 emanating from the NV centers 3 of the diamond crystal 1 in the measuring area 7 is detected by a detector 8 in the form of a photodiode. The detector 8 can have a balanced evaluation circuit in which the noise of the excitation light 4 is eliminated from the detected signal. The measuring area 7 is supplied with a microwave field 9 by means of a microwave emitter 11. The microwave field 9 serves to excite the electrons from the state m s = 0 into the states m s = -1 and m s = +1 to excite, as shown in Fig. 2b. This is possible if the microwave field 9 has a frequency that matches the respective resonance or transition frequency f, f+. After interaction with the green excitation light 4, these electrons will have the corresponding spin state ms = -1 or m s = +1 in the excited state 3 E is occupied. From these spin states, the decay to the ground state proceeds. 3 A via an intermediate level 1 A, as shown in Fig. 2b. No fluorescence light 5 in the wavelength range between 600 nm and 900 nm is emitted via this decay pathway.

[0050] Fig. 4 shows an ODMR spectrum of intensity I of the fluorescence light 5 detected by detector 8 as a function of the microwave frequency fMw of the incident microwave field 9. As can be seen in Fig. 4, the decay proceeds via the intermediate level 1A leads to a dip in intensity I at detector 8 in the form of the photodiode, which is dependent on the microwave frequency fiww. Due to the hyperfine structure splitting, three closely spaced resonances result. In the example shown, the magnetic field B is aligned along the

[0111] direction of the unit cell 2 of the diamond crystal 1 of Fig. 1, i.e., parallel to the first NV axis NV1 of the diamond crystal 1.

[0051] As can also be seen in Fig. 3, the magnetic field sensor 10 has an evaluation unit 12 in the form of suitable hardware and / or software, which includes a demodulator 13. In the example shown, the demodulator 13 is a lock-in amplifier used to suppress low-frequency noise. When using the demodulator 13 as a lock-in amplifier, the microwave field 9 is typically modulated with a frequency-modulated microwave signal f. MW(t) excited, which has the following form:

[0052] / MM = fuw + fd C °s(27T / j n t), where f MW the carrier frequency, f d the modulation amplitude and f m denotes the modulation frequency. The intensity I of the fluorescence light 5 is accordingly represented by a modulation amplitude V. m and with the modulation frequency f m modulated, as shown in Fig. 5a. The fluorescence response is in the modulation amplitude V. m and in the phase of the fluorescence signal with the modulation frequency f m encoded and can be decoded using the demodulator 13 in the form of the lock-in amplifier.

[0053] Fig. 5b shows the signal VLIA generated from the detected fluorescence light by means of a demodulator 13 as a function of the frequency f. As can be seen in Fig. 5b, the demodulated signal VLIA has a dispersive shape and is dispersed around the respective resonance frequency f. ± linear. The slope α of the linear region can be determined from the demodulated signal VLIA. If the resonant frequency changes due to magnetic field, temperature, and / or pressure changes, the resonance deviation (detuning) α from the resonant frequency f can be determined. ± The evaluation is generally based on the demodulated signal VLIA. The following applies: A = VLIA / a. The curve shown in Fig. 5b is used for both magnetic field resonances or for both transitions of m. s = 0 to m s = -1 and of m s = 0 to m s = +1 obtained, which are addressed with the microwave field 9, whose carrier frequencies are centered around the resonance frequencies f., f+.

[0054] Fig. 6a shows the associated magnetic resonance signal VLIA in the case that a signal generator 14 of the magnetic field sensor 10 generates the frequency-modulated microwave signal described above. with an oscillator signal with oscillation frequency fosz, resulting in a frequency-mixed microwave signal 15 with two frequency components f that are phase-shifted by TT. S b-(t), fsb+,ir(t) is generated such that the magnetic resonance signal VLIA of the two magnetic field resonances exhibits a mirrored frequency response at the respective resonance frequencies f., f+. The resonance frequencies are given by: f = fosz - fiviw and f+ = fosz + fMw. For the following considerations, it is assumed that the two magnetic field resonances have the same linewidth and the same amplitude.

[0055] To determine the magnetic field B, the respective frequencies f, f+ are first determined, corresponding to the zero crossing of the respective magnetic field resonance when the magnetic field B = 0 is zero. The microwave signal 15 is then activated at these frequencies. If the frequency of the microwave signal 15 coincides with the zero crossing of the resonance or the resonance frequency f_, f+, the amplitude of the demodulated signal is...

[0056] Magnetic resonance signal VLIA zero, as shown in Fig. 6a.

[0057] A change in the magnetic field B from zero to a value B1 greater than zero leads to an increase in the distance between the two resonant frequencies f., f+. If the microwave signal 15 still exhibits the two frequencies f 0S z - fiviw and f oszIf the magnetic field changes, a magnetic field-induced detuning AB of the resonance frequencies f, f+ is shown in Fig. 6b, which is due to the change in the magnetic field B. The magnetic resonance signal VLIA changes from zero to the value VLIA(B1) at each resonance frequency f_, f+. Since the changes in the magnetic resonance signal VLIA have the same sign for both magnetic field resonances and are detected by the same photodiode or detector 8, they add up, and the resulting magnetic resonance signal has an amplitude of 2 VLIA(B1). Using the proportionality factor a described above, the magnetic field-induced detuning AB = VLIA / a can be calculated from the magnetic resonance signal VLIA. By taking the difference between the two detuned resonance frequencies f_ - AB and f+ + AB, the magnetic field B can be determined. P to be determined:

[0058] 2yß p = / + - / _ + 2V LIA / a

[0059] At a constant frequency of the microwave signal 15, the linear region around the respective resonance frequency f forms ±The magnetic field-sensitive dynamic range of the magnetic field sensor 10 is limited to the linewidth of the respective magnetic field resonance of the magnetic resonance signal VLIA. To increase the dynamic range of the magnetic field sensor 10, a frequency shift Af of the microwave signal 15 by the amount AB can be applied so that the frequencies of the microwave signal 15 coincide with the detuned resonance frequencies f - AB, f + AB. To implement this frequency shift Af of the microwave signal 15, a controller can be used as described in DE 10 2022 113 754 A1 cited above. By tracking or by the frequency shift Af of the microwave signal 15, the dynamic range of the magnetic field sensor 10 can be significantly increased. Fig. 6c shows the effect of changing the temperature T from a first temperature value T1 to a second temperature value T2 on the magnetic resonance signal VLIA. As shown in Fig.As can be seen in Figure 6c, a change in temperature T leads to a temperature-induced detuning AT of the resonance frequencies f_, f+. This temperature-induced detuning AT has the same sign for both resonance frequencies f_, f+. Due to the phase shift of the two magnetic resonance signals, they exhibit values ​​VLIA-(T2) and VLIA+(T2) at the two resonance frequencies f_, f+, respectively, with the same magnitude but opposite sign. Therefore, adding the two values ​​at detector 8 does not result in any change to the magnetic resonance signal VLIA; that is, the measurement of the magnetic field B is insensitive to temperature changes.

[0060] For this reason, however, the two resonant frequencies f., f+ are not tracked when the temperature T changes; that is, there is no frequency shift Af of the microwave signal 15 when the resonant frequencies f., f+ are detuned by temperature. The temperature change nevertheless leads to a shift of the resonant frequencies f., f+. If the temperature change between temperature T1 and temperature T2 is too large, this can result in the signal leaving the linear range around the resonant frequencies f., f+, and the magnetic field sensor 10 no longer functioning correctly.

[0061] To avoid this, the magnetic field sensor 10 is designed to determine the temperature-induced detuning AT of the resonance frequencies f, f+ in a temperature calibration step and to take this into account when calculating the frequency shift Af of the microwave signal 15. The temperature calibration step can be implemented in various ways using the magnetic field sensor 10.

[0062] For example, the signal generator 14 can be configured to generate a microwave signal 15 in the temperature calibration step, which contains two frequency components f S b-(t), f Sb+(t) without phase shift, in order to simultaneously excite the magnetic field resonances at the two resonance frequencies f., f+. In this way, the two values ​​VLIA-(T2) and VLIA+(T2) described above in connection with Fig. 6c are added, and the temperature-induced detuning AT of the resonance frequencies f_, f+ can be determined analogously to the magnetic field-induced detuning AB of the resonance frequencies f., f+. The frequency shift Af of the microwave signal 15 can then be adjusted analogously to the magnetic field-induced detuning AB of the resonance frequencies f_, f+, so that the frequencies of the microwave signal 15 again coincide with the - temperature-induced detuned - resonance frequencies f., f+.

[0063] For the temperature calibration step, the magnetic field sensor 10 is switched in this case from a measurement mode for measuring the magnetic field B to a temperature calibration mode by the signal generator 14 adjusting the phase shift for the two frequency components fS b-(t), f S b+,ir(t) briefly switches from TT (or from 180°) to 0° for the duration of the temperature calibration step. Therefore, no measurement of the magnetic field B can be performed during the temperature calibration step.

[0064] Another way to implement the temperature calibration step is to determine the temperature-induced detuning AT of the resonance frequencies f., f+ by measuring both resonance frequencies f., f+ at two different times. The first time point could be, for example, at the beginning of the measurement of the magnetic field B, and the second time point could be a later time point during the measurement of the magnetic field B. If the center frequency or the sum of the two resonance frequencies f. + f+ has changed between the two times, the temperature T has also changed between the two times, which can be taken into account in the frequency shift Af of the microwave signal 15.

[0065] It is understood that the two resonance frequencies f. and f+ can be determined at more than two different times. In the temperature calibration step, a time point during the measurement is compared with a preceding time point during or at the beginning of the measurement of the magnetic field B in order to determine the temperature-induced detuning AT of the resonance frequencies f. and f+. Determining the two resonance frequencies f. and f+ typically requires tuning the microwave signal 15 across the entire spectrum or a relevant portion thereof. Therefore, the temperature calibration step cannot be performed concurrently with the measurement of the magnetic field B.

[0066] The magnetic field sensor 10 shown in Fig. 3, on the other hand, is configured to perform the temperature calibration step concurrently with the determination of the magnetic field B. To achieve this, the magnetic field sensor 10 has a further signal generator 14' which is configured to generate a further microwave signal 15' in order to excite two further magnetic resonances at two further resonance frequencies f.', f+' in the crystal 1. The evaluation unit 12 of the magnetic field sensor 10 is configured to determine a temperature-related change AT' of the further resonance frequencies f.', f+' in order to determine the temperature-related change AT of the resonance frequencies f., f+ in the temperature calibration step.

[0067] In the magnetic field sensor 10 of Fig. 3, the signal generator 14 is designed to excite the magnetic resonances at NV centers 3 with a first NV axis NV1.

[0068] The further signal generator 14' is configured to excite the further magnetic resonances at NV centers 3 with a second NV axis, NV2, which is different from the first. The further magnetic field resonances, which also involve the transition from m s = 0 in the m s = - 1 or in the m s Since the spin state is +1, further resonance frequencies f.', f+' are observed, which are associated with another microwave signal 15' with a different oscillation frequency f. osz ' and are excited at a different carrier frequency fMw than is the case with the magnetic resonances described above. The temperature-induced detuning AT' of the further magnetic resonances corresponds to the temperature-induced detuning AT of the magnetic resonances described above, which are used to determine the magnetic field B.

[0069] In the example shown in Fig. 3, the additional signal generator 14' is configured to excite the two additional magnetic resonances simultaneously. For this purpose, the additional microwave signal 15' has two frequency components fsb-(t)' and fsb+(t)' without phase shift. The determination of the temperature-induced detuning AT of the additional magnetic resonances can be carried out as described above. Alternatively, it is possible for the additional signal generator 14' to excite the two magnetic resonances sequentially. In this case as well, the temperature-induced detuning AT of the additional magnetic resonances can be determined in the manner described above.

[0070] In the example shown, the demodulator 13 has two modulation / demodulation channels to simultaneously determine the magnetic field-induced detuning AB of the resonance frequencies f, f+ and the temperature-induced detuning AT of the further resonance frequencies f_', f+', as indicated in Fig. 3. The frequency shift Af of the microwave signal 15 for determining the magnetic field B takes into account the magnetic field-induced detuning AB and the temperature-induced detuning AT additively, i.e., Af = Afβ + Afr.

[0071] The temperature calibration step described above can be performed at predetermined time intervals or continuously. The magnetic field sensor 10 shown in Fig. 3 has a temperature sensor 16 for measuring a temperature T in the vicinity of or within the crystal 1. The magnetic field sensor 10 is configured to perform the temperature calibration step depending on the temperature T measured by the temperature sensor 16. For example, the temperature calibration step can be performed whenever the measured temperature T deviates from a reference temperature by a predetermined amount. In this case, the measured temperature T after the temperature calibration step can be defined as the new reference temperature.

[0072] The temperature calibration step described above allows the magnetic field sensor 10 to be used under everyday conditions where the temperature T in the vicinity of the crystal 1 is subject to strong fluctuations. Furthermore, the temperature calibration can be performed within the magnetic field sensor 10 itself; that is, no additional measuring or calibration device is required for this purpose.

Claims

Patent claims 1. Magnetic field sensor (10), comprising: an excitation light source (6) for emitting excitation light (4), a measuring area (7) having color centers in a crystal (1), preferably NV centers (3) in a diamond, which emit fluorescence light (5) when excited with the excitation light (4), a detector (8) for detecting the fluorescence light (5) from the measuring area (7), a microwave emitter (11) for impinging a microwave field (9) on the measuring area (7), a signal generator (14) configured to generate a microwave signal (15) for the microwave emitter (11) having two frequency components (fsb-(t), fsb+,ir(t)) phase-shifted by TT to simultaneously determine magnetic field resonances at two different resonance frequencies (f., f+) in the crystal (1), and an evaluation device (12) comprising a demodulator (13) for generating a demodulated magnetic resonance signal (VLIA) from the detected fluorescence light (5) of the measuring area (7), wherein the evaluation device (12) is configured to determine a detuning (AB) of the resonance frequencies (f., f+) due to a change in the magnetic field (B) and to generate a frequency shift (Af) of the microwave signal (15) as a function of the magnetic field-induced detuning (AB) of the resonance frequencies (f., f+), characterized in that the magnetic field sensor (10) is configured to determine a temperature-induced detuning (AT) of the resonance frequencies (f., f+) in a temperature calibration step and to take it into account in the frequency shift (Af) of the microwave signal (15) for determining the magnetic field (B).

2. Magnetic field sensor according to claim 1, which is configured to determine, on the basis of the magnetic field-induced detuning (AB) of the resonance frequencies (f., f+), the to determine the magnetic field (B) and to perform the temperature calibration step in parallel with the magnetic field (B).

3. Magnetic field sensor according to claim 1 or 2, further comprising: a further signal generator (14') configured to generate at least one further microwave signal (15') to excite at least two further magnetic resonances at at least two further resonance frequencies (f, f+') in the crystal (1 ), wherein the evaluation device (12) is configured to determine the temperature-induced detuning (AT) of the resonance frequencies (f_, f+) in the temperature calibration step. 1 , f+') to determine.

4. Magnetic field sensor according to claim 3, wherein the signal generator (14) is configured to excite the magnetic resonances at NV centers (3) with a first NV axis (NV1) and wherein the further signal generator (14') is configured to excite the further magnetic resonances at NV centers (3) with a second NV axis (NV2) different from the first.

5. Magnetic field sensor according to claim 3 or 4, wherein the further signal generator (14') is configured to excite the two further magnetic resonances either successively or simultaneously, wherein the further microwave signal (15') has two frequency components (fsb-(t)', f when the further magnetic resonances are excited simultaneously S b+(t)') without phase shift.

6. Magnetic field sensor according to claim 1, wherein the signal generator (14) is configured to generate a microwave signal (15) in the temperature calibration step, which comprises two frequency components (f Sb-(t), fsb+(t)) without phase shift, in order to simultaneously excite the magnetic field resonances at the two resonance frequencies (f., f+) in the crystal (1 ). . Magnetic field sensor according to claim 1, which is configured in the temperature calibration step to account for the temperature-induced detuning (AT) of the two To determine the resonance frequencies (f_, f+) by determining the two resonance frequencies (f., f+) at at least two different times.

8. Magnetic field sensor according to one of the preceding claims, further comprising: at least one temperature sensor (16) for measuring a temperature (T) in an environment of the crystal (1), wherein the magnetic field sensor (10) is configured to perform the temperature calibration step depending on the temperature (T) measured by means of the temperature sensor (16).

9. Method for temperature calibration of a magnetic field sensor (10), wherein the magnetic field sensor (10) comprises a measuring area (7) having color centers (3) in a crystal (1), preferably NV centers in a diamond, which emit fluorescence light (5) when excited with the excitation light (4), the method comprising: The measuring area (7) is subjected to a microwave field (9) by means of a microwave emitter (11) to which a microwave signal (15) is supplied, which contains two frequency components (f) that are phase-shifted by TT from each other. S b-(t), f S b+,ir(t)) to simultaneously excite magnetic field resonances at two different resonance frequencies (f., f+) in the crystal (1) to determine a magnetic field (B), forming a demodulated magnetic resonance signal (VLIA) from the detected fluorescence light (5) of the measuring range (7), Determining a detuning (AB) of the resonance frequencies (f., f+) due to a change in the magnetic field (B), as well as Generating a frequency shift (Af) of the microwave signal (15) depending on the magnetic field-induced detuning (AB) of the resonance frequencies (f., f+), characterized by Determining a temperature-induced detuning (AT) of the resonance frequencies (f., f+) in a temperature calibration step and taking into account the temperature-induced detuning (AT) of the resonance frequencies (f., f+) in the frequency shift (Af) of the microwave signal (15) to determine the magnetic field (B).

10. Method according to claim 9, wherein the magnetic field (B) is determined based on the detuning (AB) of the resonance frequencies (f., f+) and the temperature calibration step is carried out in parallel with the detuning.

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