Method for calibrating a quantum sensor, and quantum sensor

Abstract

The invention relates to a method for calibrating a quantum sensor, in particular a magnetic field sensor (1), which has a crystal (2) with color centers (2a), in particular a diamond crystal with NV centers. During calibration, the quantum sensor is operated in a calibration mode which comprises the following steps: a) automatically setting a plurality of values of at least one operating parameter (PA, PM, MD, fMW, …) which influences the sensitivity (ηB) of the quantum sensor, b) automatically determining a measure of the sensitivity (ηB) of the quantum sensor for the plurality of values of the at least one operating parameter (PA, PM, MD, fMW, …), and c) automatically selecting a value (PA,OPT, PM,OPT, …) of the at least one operating parameter (PA, PM, …) that is optimal for the sensitivity (ηB) of the quantum sensor. The invention also relates to a quantum sensor, in particular a magnetic field sensor (1), which has a calibration device (12) designed to carry out the method.

Description

[0001] Method for calibrating a quantum sensor and quantum sensor

[0002] The present invention relates to a method for calibrating a quantum sensor, in particular a magnetic field sensor, comprising a crystal with color centers, in particular a diamond crystal with NV centers. The invention also relates to a quantum sensor comprising: a crystal with color centers, in particular a diamond crystal with NV centers, an excitation light source for irradiating excitation light into the crystal, a microwave generator for coupling a microwave field into the crystal, a detector for detecting fluorescent light emitted upon excitation of the color centers, and an evaluation device for determining at least one measured variable, in particular a magnetic field, based on the detected fluorescent light. The evaluation device comprises a demodulator, preferably in the form of a lock-in amplifier.

[0003] Quantum sensors play a central role in many technical applications and can be used to determine various physical quantities. Crystals doped with color centers are typically used as sensor elements for 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 the sensor element. NV centers in diamond exhibit a characteristic electronic structure that changes when certain quantities, such as an external magnetic field, temperature, pressure, or electric field, change.

[0004] DE102022201185A1 describes a pressure sensor comprising at least one sensitive component in the form of a deflectable diamond membrane having at least a partial volume doped with color centers, preferably NV centers. The color centers are excited by irradiation with excitation light in the optical range and by irradiation with a microwave field of varying frequency in the microwave range. The fluorescent light thereby induced in the color centers is detected and analyzed by a detector.

[0005] The sensitivity of such a quantum sensor depends on various parameters. It is not easy to determine the optimal operating parameters for the most sensitive operation of a quantum sensor, especially a magnetic field sensor, through theoretical estimation or simulation, as this requires certain assumptions and simplifications. Furthermore, the optimal operating parameters can vary between different devices of the same type of quantum sensor. Manual calibration of a quantum sensor is time-consuming and has only limited validity, both in terms of time and depending on the quantum sensor's location.

[0006] The article "Sensitivity Optimization for NV-Diamond Magnetometry," by John F. Barry et al., Review of Modern Physics 92, no. 1, 2020, 015004, describes that quantum sensors in the form of magnetometers based on NV centers in diamond exhibit a sensitivity that is orders of magnitude far from the theoretical limits. The article describes various approaches for optimizing the sensitivity of magnetometers.

[0007] In the article "Fiber-coupled Diamond Magnetometry with an Unshielded 30 pT / / ~Hz Sensitivity," SM Graham et al., Physical Review Applied 19, no. 4, 044042, an NV-center magnetometer with a sensitivity on the order of approximately 30 pT / Hz for a frequency range of 10 to 500 Hz is described. This high sensitivity can be achieved, among other things, by optimizing the operating parameters of a microwave source and a lock-in amplifier.

[0008] CN115128518A describes a high-sensitivity differential magnetic sensing system based on NV centers and frequency-agility microwave modulation technology. The sensing system includes a magnetic signal calibration module that includes a magnetic coil for applying a bias magnetic field. The sensing system varies the strength of the magnetic field to obtain multiple data sets and differentially processes the output data to suppress thermal noise and improve magnetic signal sensitivity.

[0009] CN112068046B describes a magnetic field measuring device and a method for magnetic field measurement based on NV color centers. In this method, a calibration signal with a known field strength and direction is applied to the measuring device, and a fluorescence signal detected by a detection module is calibrated using the applied calibration signal.

[0010] Object of the invention

[0011] The invention is based on the object of providing a method for calibrating a quantum sensor, in particular a magnetic field sensor, which enables the operation of the quantum sensor with the highest possible sensitivity.

[0012] Subject of the invention

[0013] This object is achieved by a method of the type mentioned above, in which the quantum sensor is operated during calibration in a calibration mode comprising the following steps: a) automated specification of a plurality of values ​​of at least one operating parameter influencing the sensitivity of the quantum sensor, b) automated determination of a measure of the sensitivity of the quantum sensor for the plurality of values ​​of the at least one operating parameter, and c) automated selection of an optimal value of the at least one operating parameter for the sensitivity of the quantum sensor. The selection of the (at least one) optimal value is carried out based on the measure of the sensitivity of the quantum sensor determined in step b).

[0014] In the method according to the invention, the quantum sensor is operated in a calibration mode in which optimal values ​​of operating parameters are automatically determined, at which the sensitivity of the quantum sensor is as high as possible or maximum. Following the calibration mode, the quantum sensor can be operated in a measuring mode with the optimized value(s) of the at least one operating parameter selected in the calibration mode. The calibration of the quantum sensor in the calibration mode can be fully automated, i.e., manual calibration of the quantum sensor can be omitted.

[0015] The operating parameters, for which the majority of values ​​are set or specified, can generally be varied essentially continuously. Of the possible values ​​of the respective operating parameter, a plurality of values ​​are set one after the other. These values ​​lie within a specified value or variation range of the operating parameter. The values ​​can be set within the specified value range with a specified step size, but this is not mandatory. For each of the values, a measurement time is specified, during which the quantum sensor is operated with the set value of the operating parameter. The measurement time is selected so that the sensitivity of the quantum sensor can be determined for the respectively set value. The number of values ​​of the operating parameter that are set is usually five or more, ten or more, or one hundred or more.However, the number of values ​​that are set may also be smaller.

[0016] With this method, the specified number of values ​​can be set and the sensitivity measure determined for only one operating parameter (one-dimensional scan). However, it is also possible for the variation to occur multidimensionally, i.e., for several operating parameters together. Each set value or each combination of set values ​​of the operating parameters is assigned the value for the sensitivity measure and saved. At the end of the calibration process, the value of the operating parameter or the combination of values ​​of the operating parameter at which the sensitivity of the quantum sensor is maximum or optimal is selected.

[0017] It is understood that in addition to the operating parameters adjusted during the process, there are also parameters that influence the sensitivity of the quantum sensor, but which cannot be adjusted or can only be adjusted to a limited extent during operation. These parameters include, for example, the properties of the (diamond) crystal, such as the concentration of color centers, relaxation times, orientation, strain, impurities, etc., the collection efficiency of the fluorescent light, the noise in the current, the bias magnetic field (homogeneity, noise), the homogeneity of the microwave field, the pump laser beam shape or caustic, and the pump laser wavelength. These parameters should be optimized with regard to sensitivity during the design of the quantum sensor.

[0018] In one variant, in step a), a plurality of values ​​of a first operating parameter and a plurality of values ​​of a second operating parameter are set, and in step c), an optimal value of the first operating parameter and an optimal value of the second operating parameter are selected. In this variant, a two-dimensional variation of the values ​​of the first and second operating parameters takes place, i.e., a two-dimensional scan of the operating parameters. This is particularly advantageous if the first operating parameter and the second operating parameter are interrelated, as is the case, for example, with the (laser) power of the excitation light source and the microwave power of the microwave generator, which the quantum sensor has. See the article by C. Zhang et al., “Diamond magnetometry and gradiometry towards subpicotesla DC field measurement,” Phys. Rev. Applied, vol. 15, no. 6, p. 064075, Jun. 2021, for example Fig. 3(a).It is understood that in step a) a plurality of values ​​of a third, fourth, ... operating parameter can also be set, from which an optimal value is selected in step c).

[0019] In a further development of this variant, in step a), the majority of values ​​of the first operating parameter and the majority of values ​​of the second operating parameter are set according to a fractional factorial design. The variation of two or more operating parameters or factors can in principle be carried out using a complete factor plan, in which all combinations of values ​​of the first operating parameter and values ​​of the second operating parameter are set iteratively or one after the other. However, this is very time-consuming even for the joint variation of two operating parameters. An alternative to varying the operating parameters using a complete factor plan is varying or setting the operating parameters in a partial factor plan, in which only a certain fraction of the factor combinations contained in a complete factor plan are set.Factors are examined at two factor levels, resulting in a mathematical notation 2. k-p where k is the number of factors and p is the fraction of factor combinations tested. Using a fractional factor plan when jointly varying the operating parameters can accelerate 2-dimensional and multidimensional scans of the operating parameters.

[0020] In an alternative variant, steps a) to c) are carried out for a first operating parameter and steps a) to c) are subsequently carried out for a second operating parameter under specification of the selected optimal value of the first operating parameter. It is understood that steps a) to c) can subsequently be carried out for a third operating parameter under specification or setting of the selected optimal value of the second operating parameter, etc. In the variant described here, a one-dimensional scan of the first operating parameter is first carried out and subsequently - with the optimized value for the first operating parameter - a one-dimensional scan of the second operating parameter is carried out. In this way, the duration of the calibration operation can be reduced compared to the time required for a two- or multi-dimensional scan of the operating parameters.

[0021] In a further variant, steps a) to c) are carried out iteratively, i.e. repeated once or more than once, for at least two operating parameters. In particular, if steps a) to c) are carried out for a first operating parameter and steps a) to c) are subsequently carried out for a second operating parameter while setting the selected optimized value of the first operating parameter, steps a) to c) can then be carried out again for the first operating parameter while setting the selected optimized value of the second operating parameter, and so on, i.e. the two operating parameters can be alternately iteratively optimized. In one variant, the operating parameter or one of the operating parameters is an excitation light operating parameter of excitation light irradiated into the crystal to excite the color centers. As described above, the crystal orThe color centers of the crystal are stimulated with excitation light, the properties of which have a significant influence on the sensitivity of the quantum sensor.

[0022] In a further development, the excitation light operating parameter is a power of the excitation light or a pulse parameter of the excitation light during pulsed operation of the quantum sensor. The sensitivity of the quantum sensor is typically strongly influenced by the (laser) power of the excitation light or by the (laser) pump current of the excitation laser source. Varying the laser power during calibration operation may require adjusting the temperature control of the light source, i.e., an upstream calibration procedure for a thermoelectric cooler (TEC), for example, is carried out depending on the power of the excitation light. This is because, in both calibration operation and measurement operation of the quantum sensor, care must be taken to ensure that the excitation light source for exciting the color centers reaches a low-noise operating point. This can be achieved, for example, by readjusting the temperature or the pump current of the excitation light source.For example, an AC-coupled signal from a photodiode (e.g. a monitor photodiode in a housing of the excitation light source) or the demodulated signal of the quantum sensor itself can be used as a control signal.

[0023] The light source parameter can also be a pulse parameter of the excitation light if the quantum sensor is operated in a pulsed manner, for example the pulse duration or the pulse shape of a respective (laser) pulse, but also the temporal sequence of the different pulses.

[0024] In another variant, the operating parameter or one of the operating parameters is a microwave operating parameter of a microwave field coupled into the crystal. The properties of the microwave field coupled into the crystal or into the color centers of the crystal also have a significant influence on the sensitivity of the quantum sensor. In a further development of this variant, the microwave operating parameter is selected from the group comprising: microwave field power, microwave field modulation depth, and pulse parameters of the microwave field during pulsed operation of the quantum sensor. The microwave field power is frequency-dependent due to the frequency response of the structure generating the microwave field, for example, in the form of a microwave antenna or a microwave coupler.For measurement / calibration of the structure generating the microwave field, such as a microwave antenna, a resonance curve for the microwave antenna can be recorded by varying the bias field (in the case of electromagnets) or by applying an external bias field in a test bench. This table can then be used to set the optimal microwave field power, eliminating the need to recalibrate the microwave field power for every (e.g., temperature-dependent) shift in the (ODMR) spectrum.

[0025] The modulation depth MD of the microwave field can also be set as an operating parameter, which is defined by where A denotes the amplitude of the microwave field. Alternatively to the amplitude, the frequency or phase of the microwave field can be varied; in this case, A denotes the frequency or phase of the microwave field, respectively.

[0026] Alternatively or additionally, the modulation frequency of the microwave field can be adjusted or varied. When determining the optimal value for the modulation frequency of the microwave field, particular attention should be paid to the fact that at certain frequencies, ambient interference signals impair the sensitivity of the quantum sensor in these frequency bands (e.g., 50 Hz / 60 Hz and 16 Hz signals from the mains voltage or the railway supply). It may therefore be advisable to restrict the range of values ​​within which the microwave frequency is adjusted or varied by excluding these frequency bands and their multiples from the range of variation.

[0027] The microwave operating parameter can also be a pulse parameter of the microwave field if the quantum sensor is operated in pulsed mode. For example, it can be a pulse duration or a pulse shape of the microwave pulses, or the temporal sequence of the microwave pulses.

[0028] In another variant, the operating parameter or one of the operating parameters is a demodulator operating parameter, in particular an amplifier operating parameter of a lock-in amplifier, an evaluation device of the quantum sensor. For the evaluation, the fluorescence signal detected by a detector is demodulated with a demodulator, for example with the help of a lock-in amplifier. In calibration mode, in this variant, the sensitivity of the quantum sensor is determined based on the demodulated signal, so that the amplifier operating parameters of the lock-in amplifier influence the sensitivity or the measure of the sensitivity. The amplifier operating parameter(s) can be, for example, the low-pass bandwidth of the lock-in amplifier, the low-pass order or the spectral filter response of the lock-in amplifier, or the chunk width, if the lock-in amplifier is a software implementation.

[0029] In a further variant, the operating parameter or one of the operating parameters is a discrete, in particular binary, operating parameter of the quantum sensor. A discrete, e.g., binary, operating parameter of the quantum sensor can fundamentally only assume a limited number of values. The discrete operating parameter cannot therefore be varied continuously, as is the case, for example, with the power of the excitation light or the power of the microwave field. The discrete operating parameter can, for example, be the form of modulation of the microwave field, e.g., whether the amplitude or the frequency of the microwave field is modulated. Depending on the design of the quantum sensor, it can be the orientation of the offset magnetic field. Another discrete or binary operating parameter is the type of resonance drive of the quantum sensor, i.e., whether it is a single-resonance drive or a double-resonance drive.In single-resonance drive, the microwave field excites only the transition between the spin-O level and a spin-±1 level of an NV center, whereas in double-resonance drive, the transitions to both spin± levels are excited. Whether the hyperfine structure (HFS) driving (which is at 2.16 MHz for NV centers) is switched on or off also represents a discrete or binary operating parameter. HFS drive is an operating mode in which, in addition to a central microwave frequency, a frequency above and below the central frequency is applied. The frequency differences between these frequencies correspond to the hyperfine splitting.

[0030] In a further variant, the first operating parameter is an excitation light operating parameter, in particular a power of the excitation light, and the second operating parameter is a microwave operating parameter, in particular a power of the microwave field, or vice versa. The power of the excitation light and the power of the microwave field are operating parameters that have a significant influence on the sensitivity of the quantum sensor. These two operating parameters can be varied, as described above, by a two-dimensional scan or by two consecutive one-dimensional scans within a predetermined value range. In the latter case, the optimal value of the operating parameter of the first one-dimensional scan is set for performing the second one-dimensional scan. This can be repeated iteratively.

[0031] It is typically advantageous to optimize the operating parameters in order of their influence on the sensitivity of the quantum sensor. This means that those operating parameters that have the greatest influence on the sensitivity of the quantum sensor should be optimized first. In calibration mode, the power of the excitation light and the power of the microwave source can be optimized as the first and second operating parameters. Subsequently, other operating parameters can be optimized using the optimal values ​​of these two operating parameters, for example, the modulation depth and the modulation frequency of the microwave field. If the quantum sensor is operated in a pulsed mode, the pulse parameters, for example, the pulse duration, the pulse shape, and the sequence of the excitation light pulses or the laser pulses and the microwave pulses, can also be optimized.The discrete operating parameters and the amplifier operating parameters described above can also be optimized in one or more subsequent optimization steps.

[0032] In a further variant, after the calibration operation, the measure of the sensitivity of the quantum sensor at the selected optimal value of the at least one operating parameter is determined again and compared with the measure of the sensitivity of the quantum sensor determined in the calibration operation, wherein the comparison is preferably used to decide on whether to carry out a new calibration operation and / or to detect changes in the structural condition, in particular degradation, of the quantum sensor. After the successful calibration operation, the measuring operation is started. At predetermined time intervals during the measuring operation, the measure of the sensitivity of the quantum sensor at the selected optimal value(s) can be determined again. Based on the comparison, a decision can be made as to whether to carry out a new calibration operation.Recalibration can be performed if the quantum sensor's sensitivity deteriorates by a specified amount during the re-determination compared to the quantum sensor's sensitivity determined during calibration. During recalibration, the value ranges for varying the operating parameters can typically be selected to be significantly smaller than in calibration mode, and optimization can be achieved toward a local optimum. It should also be noted that certain components of the quantum sensor exhibit long-term fluctuations, which may also require recalibration.

[0033] By comparing the sensitivities or based on the data on how much the optimal operating point of the quantum sensor varies over time, it may also be possible to make statements about the condition of the quantum sensor's components, e.g., degradation of the laser or the laser system. For example, the drift of the optimal operating point or operating condition of the laser system of the excitation light source, e.g., in the form of the optimal laser pump current, can be monitored. In this way, for example, early detection or diagnosis regarding the service life of the quantum sensor and a predictive maintenance function can be implemented for a potentially necessary replacement of the laser system of the quantum sensor and, if necessary, for other components of the quantum sensor. In the event of a drift of the optimal laser pump current, readjustment can also be carried out if necessary, provided the excitation light passes through the quantum sensor in free-beam propagation.

[0034] In another variant, the quantum sensor's sensitivity is determined via the ODMR (optically detectable magnetic resonance) spectrum based on the fluorescence contrast (resonance depth) and resonance linewidth parameters. The magnetic resonance is recorded via the variation of the fluorescence light intensity at varying incident microwave frequencies. The fluorescence intensity can be measured by demodulation of the lock-in amplifier signal.

[0035] The following describes the determination of the sensitivity of a quantum sensor using a magnetic field sensor as an example. It should be understood that the determination of the sensitivity of a quantum sensor designed to measure a different measurand can be performed in a similar manner.

[0036] The sensitivity r)B of a magnetic field sensor (in T [Hz) can be described by the following formula: where o is the standard deviation of the noise level of the measured signal S, t is the measurement time in seconds and dS / dB is a proportionality factor or the change of the signal S depending on the magnetic field B.

[0037] In the ODMR spectrum of the detected fluorescence light of a magnetometer containing a diamond crystal with NV centers, the shot-noise-limited sensitivity r|B is given by: FWHM

[0038] PB = P - TH

[0039] YNV c det ' vn where P denotes a numerical proportionality factor that depends on the mathematical description of the resonance curve (for a Lorentz curve: P = 4 / (3 * 3) = 0.77), FWHM the spectral linewidth (FWHM) of the resonance curve, C det the signal contrast, R the detected fluorescence photon rate and y NV the gyromagnetic ratio of the NV centers (y w «28 GHz / T).

[0040] The line width FWMH and the contrast c det The baseline and the baseline can be determined in a known manner by fitting (Lorentz) resonances in the ODMR spectrum. They also depend on the optical saturation parameter s and the Rabi frequency OR. The detected photon rate R can be deduced from the baseline.

[0041] Alternatively or additionally, the sensitivity PB of the magnetic field sensor can be determined using a signal demodulated from the spectral intensity profile of the fluorescent light using a demodulator, such as a lock-in amplifier. In this case, the sensitivity can be determined according to the following formula: where VNSD denotes the noise spectral density and a the slope of the demodulated signal VLIA of the lock-in amplifier at a zero crossing slope (ZCS), and y is the gyromagnetic ratio of the NV centers. The noise spectral density VNSD is the standard deviation of the demodulated signal divided by the square root of the noise equivalent power bandwidth of the low-pass filter in the lock-in amplifier; see also the article "https: / / www.zhinst.com / europe / en / blogs / noise-spectral-density-measured-lock-amplifiers." The zero-crossing slope a of the demodulated signal is typically determined by a linear fit. To measure the magnetic field, the microwave can be tuned to a resonant frequency or to a zero crossing in the demodulated spectrum, and the resulting signal can be measured.In a further development of this variant, the sensitivity of the quantum sensor is measured by the spectral noise density of the demodulated signal and / or the zero-crossing slope of the demodulated signal. As can be seen from the formula for the sensitivity r|B of the demodulated signal, the sensitivity depends on the spectral noise density VNSD and the zero-crossing slope. Depending on the operating parameter being varied, only one of these two quantities can be determined for quick measurement or calibration.

[0042] In a further development, a measure of the sensitivity of the quantum sensor is determined by a contrast and / or a width of at least one resonance in the spectral intensity profile of the fluorescent light and / or a height of the baseline of the spectral intensity profile of the fluorescent light. These are examples of how the measure of sensitivity can be determined based on the spectral intensity profile of the fluorescent light.

[0043] A second aspect of the invention relates to a quantum sensor, in particular a magnetic field sensor, of the type mentioned at the outset, which has a calibration device designed to carry out the method described above.

[0044] The calibration device can be implemented in the form of suitable hardware and / or software and enables automated, possibly repeated, calibration operations. The quantum sensor with the automated self-calibration described here simplifies production. The calibration operation of the quantum sensor can be initiated automatically or manually by an operator, i.e., calibration can be performed at the push of a button without any specialist knowledge, as the entire calibration process runs automatically. Furthermore, wider value ranges for the operating parameters can be selected, if necessary, than with manual calibration. Continuous monitoring and recalibration, if necessary, ensure that the quantum sensor is permanently operated in optimal operating condition, as no manual "readjustment" is required.

[0045] The quantum sensor can be a magnetic field sensor designed to determine a magnetic field in a measuring volume in which the crystal with the color centers is arranged. However, it is also possible for the magnetic field sensor to be designed as a gradiometer, in which the magnetic field is measured in two or more different measuring volumes by evaluating fluorescent light from the two or more measuring volumes in order to determine a magnetic field gradient. A gradiometer typically uses two or more independent measuring heads, which generally have the components described above. With a gradiometer, it is advantageous if the method described above is carried out independently for each of the two or more measuring heads, i.e. if the calibration operation described above is carried out independently for both measuring heads.Some calibration steps, such as the calibration of the modulation frequency of one or more microwave fields, can be carried out for all measuring heads simultaneously if necessary.

[0046] If vector magnetometry is to be performed using the magnetic field sensor, it is typically necessary to perform the calibration process for all four NV center orientations, partially simultaneously. To determine the optimal (laser) power, all four orientations must be considered simultaneously, while the microwave operating parameters can be partially adjusted individually for each NV center orientation if necessary.

[0047] Further advantages of the invention will become apparent from the description and the drawings. Likewise, the above-mentioned and further listed features can be used individually or in combination. The embodiments shown and described are not intended to be exhaustive, but rather serve as examples for describing the invention.

[0048] They show:

[0049] Fig. 1 is a schematic representation of a quantum sensor in the form of a magnetic field sensor, Fig. 2 shows the dependence of the intensity of fluorescent light generated by NV centers of a diamond crystal on the frequency of a microwave field, and

[0050] Fig. 3 shows a signal demodulated by a lock-in amplifier as a function of the frequency of the microwave field.

[0051] In the following description of the drawings, identical reference symbols are used for identical or functionally identical components.

[0052] Fig. 1 shows a magnetic field sensor 1 (magnetometer) comprising a crystal 2, which is a diamond crystal doped with color centers 2a in the form of NV centers, indicated by dots in Fig. 1. The magnetic field sensor 1 comprises a microwave generator 5 for coupling a microwave field 5a into the crystal 2. For this purpose, the microwave generator 5 comprises a signal generator 3, which generates an alternating electric field with a power PM, which is fed to a microwave coupler 4.

[0053] To determine the strength of a magnetic field B, in addition to the coupling of the microwave field 5a, the crystal 2 is irradiated with excitation light 6 from an excitation light source 7, which generates the excitation light 6 with a power PA. The excitation light 6 has wavelengths in the range between 500 nm and 600 nm. When irradiated, the color centers 2a emit fluorescent light 8, which is detected by a detector 9 in the form of a photodiode. An evaluation device 10, which comprises a lock-in amplifier 11, serves to determine the strength of the magnetic field B in the crystal 2. The magnetic field B is determined in a manner known per se, see, for example, the article "Sensitivity Optimization for NV-Diamond Magnetometry" by John F. Barry et al. cited at the beginning, which is incorporated into this application in its entirety by reference.

[0054] In order to determine the optimal operating parameters for the most sensitive operation of the magnetic field sensor 1, the magnetic field sensor 1 is operated in a calibration mode prior to the actual measurement operation, in which several steps of a calibration process are performed. To carry out the calibration operation, the evaluation device 10 has a calibration device 12, which is typically suitable software and / or hardware or a computer program product designed to carry out the method steps of the calibration process.

[0055] The calibration method comprises the following steps: a) Automated setting of a plurality of values ​​of at least one operating parameter influencing the sensitivity r|B of the magnetic field sensor 1, b) Automated determination of a measure of the sensitivity r|B of the quantum sensor for the plurality of values ​​of the at least one operating parameter, and c) Automated selection of an optimal value of the at least one operating parameter for the sensitivity r|B of the quantum sensor. The (at least one) optimal value is selected based on the sensitivity measure determined in step b).

[0056] During the calibration process, optimal values ​​of operating parameters are automatically determined, at which the sensitivity of the magnetic field sensor 1 is as high or as maximum as possible. Following the calibration operation, the magnetic field sensor can be operated in a measuring mode with the optimized value(s) of the at least one operating parameter selected in the calibration mode. The calibration of the magnetic field sensor 1 in the calibration mode can be fully automated, meaning manual calibration is no longer necessary.

[0057] The operating parameters for which the plurality of values ​​are set can generally be varied essentially continuously. Of the possible values ​​of the respective operating parameter, a plurality of values ​​are set one after the other, which lie within a predetermined value range of the operating parameter. The values ​​can be set within the predetermined value range with a predetermined step size, but this is not mandatory. The method can perform a two- or multi-dimensional parameter scan, in which in step a) a plurality of values ​​of a first operating parameter and a plurality of values ​​of a second operating parameter are set, and in which in step c) an optimal value of the first operating parameter and an optimal value of the second operating parameter are selected.At the optimal values ​​of the two (or more) operating parameters, the sensitivity of the magnetic field sensor 1 is maximum. To accelerate the calibration process, in step a), the majority of values ​​of the first operating parameter and the majority of values ​​of the second operating parameter can be adjusted according to a partial factor plan.

[0058] Alternatively, in the method, steps a) to c) can be performed for a first operating parameter, and steps a) to c) can subsequently be performed for a second operating parameter by setting the selected optimized value of the first operating parameter. In this case, a first dimension or a first operating parameter is first optimized, and the optimal parameter value of the first dimension is recorded. Starting from this optimized operating point, the parameter value of the second dimension or the second operating parameter is optimized.

[0059] The or one of the operating parameters can, for example, be an excitation light operating parameter of the excitation light 6 radiated into the crystal 2 to excite the color centers 2a, for example the power PA of the excitation light 6 or a pulse parameter of the excitation light 6 during pulsed operation of the magnetic field sensor 1. The operating parameter or one of the operating parameters can also be a microwave operating parameter PM, MD, fMw, ... of the microwave field 5a coupled into the crystal 2, for example the power PM of the microwave field 5a, a modulation depth MD of the microwave field 5a, a modulation frequency fw of the microwave field 5a or pulse parameters of the microwave field 5a during pulsed operation of the magnetic field sensor 1.

[0060] The operating parameter or one of the operating parameters can also be a demodulator operating parameter, more precisely an amplifier operating parameter of the lock-in amplifier 11 of the evaluation device 10 of the magnetic field sensor 1, for example the low-pass bandwidth of the lock-in amplifier, the low-pass order or the spectral filter response of the lock-in amplifier or the chunk width.

[0061] The operating parameter or one of the operating parameters can also be a discrete operating parameter of the magnetic field sensor 1, for example, the form of modulation of the microwave field, e.g., whether the amplitude or frequency of the microwave field 5a is modulated. Depending on the design of the magnetic field sensor 1, it can also be the orientation of the offset magnetic field. Another discrete or binary operating parameter is the type of resonance drive or whether the hyperfine structure (HFS) driving is enabled or disabled.

[0062] The optimization of the magnetic field sensor 1 can be carried out, for example, in the manner described below:

[0063] First, the power PA of the excitation light 6 is optimized as the first operating parameter, and an optimized value PA, OPT of the power PA of the excitation light 6 is determined. Subsequently, the power PM of the microwave field 5a is optimized as the second operating parameter, i.e., an optimal value PM.OPT of the power PM of the microwave field 5a is determined. It is also possible for the power PA of the excitation light 6 and the power PM of the microwave field 5a to be optimized in reverse order. The power PA of the excitation light 6 and the power PM of the microwave field 5a can also be optimized jointly in the form of a 2-dimensional parameter scan, if necessary using a partial factor plan. Likewise, both parameters can be optimized alternately iteratively.

[0064] In a subsequent step, the operating parameter of the modulation depth MD of the microwave field 5a is first optimized, before the modulation frequency fMw of the microwave field 5a is optimized in a further step. In the case of pulsed operation of the magnetic field sensor 1, the duration, shape, and sequence of the pulses of the excitation light 6 and the microwave field 5a can be optimized. Subsequently, those operating parameters that have a lesser influence on the sensitivity r|B of the magnetic field sensor 1 can be optimized, for example, the discrete operating parameters and the amplifier parameters of the lock-in amplifier 11.

[0065] The automated determination of the measure of the sensitivity r|B of the magnetic field sensor 1 in step b) of the calibration procedure described above is described in detail below.

[0066] Fig. 2 schematically shows the dependence of the intensity I of the fluorescent light 8 of the magnetic field sensor 1 detected by the detector 9 on the frequency fw of the irradiated microwave field 5a. Due to a magnetic resonance, the intensity I of the fluorescent light 8 has a minimum M compared to a background value I B From the frequency fmin of the minimum M, the magnetic field B at the location of crystal 2 can be determined. The essential parameters include the fluorescence contrast Cdet and the half-width FWHM of the magnetic resonance as well as the background value I B The photon-shot noise limited sensitivity B, or the smallest theoretically measurable magnetic field Bmin, is given by the formula where T is the integration time. The photon-shot noise-limited sensitivity r)B can be determined from the frequency-dependent intensity I of the fluorescent light 8 shown in Fig. 2 by fitting it with a Lorentz resonance curve.

[0067] Fig. 3 shows a demodulated signal VLIA generated from the detected fluorescent light 8 by means of a demodulator in the form of a lock-in amplifier 11 as a function of the frequency fimv. As can be seen in Fig. 3, the demodulated signal VLIA has a dispersive form and is linear around the respective resonance frequency fmin. From the demodulated signal LIA, the zero-crossing slope a of the linear range can be determined by a linear fit. The sensitivity r)B of the magnetic field sensor 1 can be determined from the demodulated signal VLIA according to the following formula: where VNSD denotes the noise spectral density (NSD) and YNV denotes the gyromagnetic ratio of the NV centers. The noise spectral density (VNSD) is the standard deviation of the demodulated signal VLIA divided by the square root of the noise equivalent power bandwidth of the low-pass filter in the lock-in amplifier 11.

[0068] The measure of the sensitivity r)B can be determined in the manner described in connection with Fig. 2 or in connection with Fig. 3. It is also possible to use the mean value of the two measured variables determined in Fig. 2 and Fig. 3 as a measure of the sensitivity r)B. In order to realize the fastest possible calibration operation, only the spectral noise density VNSD of the demodulated signal VLIA and / or the zero-crossing slope a of the demodulated signal VLIA can be determined as a measure of the sensitivity r)B of the magnetic field sensor 1 - especially in certain parameter ranges.

[0069] After calibration has been completed, the magnetic field sensor 1 is operated in measuring mode with the optimal values ​​PA, OPT, PM.OPT, .... After calibration, the sensitivity r)B of the magnetic field sensor 1 can be determined again at specific intervals at the selected optimal value PA, OPT, PM.OPT, ... of at least one operating parameter PA, PM, ... and compared with the measure for the sensitivity r)B determined in calibration mode. If it turns out that the sensitivity r)B of the magnetic field sensor 1 has deteriorated by a predetermined value, recalibration can be carried out, i.e. the magnetic field sensor 1 can be operated in calibration mode again. During recalibration, the value ranges for the variation of the operating parameters can be selected to be significantly smaller and optimization can be carried out to a local optimum. The data can also be used to determine how strongly the optimal operating point varies over time.Statements about the condition of.

[0070] System components, such as the excitation light source 7, such as a drift of the optimal laser operating point, can be addressed. This allows for early detection of any necessary replacement of the excitation light source 7 of the magnetic field sensor 1 or of other components of the magnetic field sensor 1.

[0071] It is understood that the method described above for calibrating the magnetic field sensor 1 can also be performed with quantum sensors that are used to determine other measured variables than the strength of the magnetic field B, for example, with quantum sensors designed to determine temperature, pressure, etc. Instead of a diamond crystal with NV centers, the quantum sensor can also have another crystal with color centers.

Claims

Patent claims 1 . Method for calibrating a quantum sensor, in particular a magnetic field sensor (1), which has a crystal (2) with color centers (2a), in particular a diamond crystal with NV centers, characterized in that the quantum sensor is operated during calibration in a calibration mode, which comprises the following steps: a) Automated setting of a plurality of values ​​of at least one operating parameter (PA, PM, MD, fMw, ■■■ ) influencing the sensitivity (C|B) of the quantum sensor, b) Automated determination of a measure for the sensitivity (C|B) of the quantum sensor for the plurality of values ​​of the at least one operating parameter (PA, PM, MD, fw, ...), and c) Automated selection of an optimal value (PA, OPT, PM, OPT, ...) of the at least one operating parameter (PA, PM, MD, fMw, ... ).

2. Method according to claim 1, wherein in step a) a plurality of values ​​of a first operating parameter (PA) and a plurality of values ​​of a second operating parameter (PM) are set and wherein in step c) an optimal value (PA, OPT) of the first operating parameter (PA) and an optimal value (PM.OPT) of the second operating parameter (PM) are selected.

3. The method according to claim 2, wherein in step a) the plurality of values ​​of the first operating parameter (PA) and the plurality of values ​​of the second operating parameter (PM) are set according to a partial factor plan.

4. The method according to claim 1, wherein steps a) to c) are carried out for a first operating parameter (PA) and wherein steps a) to c) are subsequently carried out by setting the selected optimized value of the first operating parameter (PA) for a second operating parameter (PM).

5. Method according to one of the preceding claims, in which steps a) to c) are carried out iteratively for at least two operating parameters (PA, PM, MD, fw, ...).

6. Method according to one of the preceding claims, in which the operating parameter or one of the operating parameters is an excitation light operating parameter (PA) of excitation light (6) irradiated into the crystal (2) to excite the color centers (2a).

7. The method according to claim 6, wherein the excitation light operating parameter is a power (PA) of the excitation light (6) or a pulse parameter of the excitation light (6) during pulsed operation of the quantum sensor.

8. Method according to one of the preceding claims, in which the operating parameter or one of the operating parameters is a microwave operating parameter (PM, MD, fw, .. ) of a microwave field (5a) coupled into the crystal (2).

9. The method according to claim 8, wherein the microwave operating parameter is selected from the group comprising: power (PM) of the microwave field (5a), modulation depth (MD) of the microwave field (5a), modulation frequency (fMw) of the microwave field (5a), modulated size (frequency, phase or amplitude) of the microwave field (5a) and pulse parameters of the microwave field (5a) during pulsed operation of the quantum sensor.

10. Method according to one of the preceding claims, wherein the operating parameter or one of the operating parameters is a demodulator operating parameter, in particular an amplifier operating parameter of a lock-in amplifier (11), of an evaluation device (10) of the quantum sensor.

11. Method according to one of the preceding claims, wherein the operating parameter or one of the operating parameters is a discrete, in particular binary, operating parameter of the quantum sensor.

12. Method according to one of claims 2 to 11, wherein the first operating parameter is an excitation light operating parameter, in particular a power (PA) of the excitation light (6), and wherein the second operating parameter is a microwave operating parameter, in particular a power (PM) of the microwave field (5a), or vice versa.

13. Method according to one of the preceding claims, in which, after the calibration operation, the measure of the sensitivity (C|B) of the quantum sensor at the selected optimal value (PA, OPT, PM.OPT, ... ) of the at least one operating parameter (PA, PM, ... ) is determined again and compared with the measure of the sensitivity (T|B) of the quantum sensor determined in the calibration operation, wherein the comparison is preferably used to decide on carrying out a new calibration operation and / or to detect changes in the structural condition, in particular degradation, of the quantum sensor.

14. Method according to one of the preceding claims, in which the measure of the sensitivity (T|B) of the quantum sensor is determined on the basis of a spectral intensity profile (l(fMw)) of fluorescent light (8) generated upon excitation of the color centers (2a) and / or on the basis of a demodulated signal (VLIA) generated from the spectral intensity profile (l(fw)) of the fluorescent light (8) by a demodulator, in particular by a lock-in amplifier (11).

15. The method according to claim 14, wherein a spectral noise density of the demodulated signal (VLIA) and / or a zero-crossing slope (a) of the demodulated signal (VLIA) is determined as a measure of the sensitivity (QB) of the quantum sensor (1).

16. The method according to claim 14, wherein a contrast (Cdet) and / or a width (FWHM) of at least one resonance in the spectral intensity profile (l(fw)) of the fluorescent light (8) and / or a height of the baseline of the spectral intensity profile (l(fw)) of the fluorescent light (8) is determined as a measure of the sensitivity (QB) of the quantum sensor (1).

7. Quantum sensor (1), in particular a magnetic field sensor, comprising: a crystal (2) with color centers (2a), in particular a diamond crystal with NV centers, an excitation light source (7) for irradiating excitation light (6) into the crystal (2), a microwave generator (5) for coupling a microwave field (5a) into the crystal (2), a detector (9) for detecting fluorescent light (8) emitted upon excitation of the color centers (2a), and an evaluation device (10) for determining at least one measured variable, in particular a magnetic field (B), based on the detected fluorescent light (8), wherein the evaluation device (10) has a demodulator, preferably in the form of a lock-in amplifier (11), characterized by a calibration device (12) which is designed to carry out the method according to one of the preceding claims.

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