Method and device for magnetic field determination using at least one NV center

Abstract

The invention relates to a method for determining a magnetic field by means of at least one NV center with features of claim 1 and a device (22) for determining a magnetic field by means of at least one NV center with features of the independent claim.

Description

The invention relates to a method for determining the magnetic field by means of at least one NV center having features of claim 1 and to a device for determining the magnetic field by means of at least one NV center having features of the subordinate claim.To determine a magnetic field, an ODMR (optically detected magnetic resonance) spectrum can be recorded in an NV magnetometer. For this purpose, an emission of the excited triplet-triplet transition is usually measured. The contrast of the ODMR spectrum is limited here. In a measurement method in which a variable is modulated and a demodulated signal is used for measuring the magnetic field B, a slope of the signal S (i.e. the derivative dS / dB) is proportional to the ODMR contrast. The signal to be measured, and thereby the determination of the magnetic field, are thereby limited.It is therefore the object of the present invention to provide a method and a device for magnetic field determination by means of at least one NV center, wherein the contrast and thus the sensitivity in the magnetic field determination can be improved.The above object is achieved by a method for determining the magnetic field by means of at least one NV center having the features of claim 1. The method comprises the steps of:providing the NV center in a magnetic field to be determined.exciting the NV center into a triplet upper state and a singlet upper state.generating a first signal by measuring an emission of the triplet-triplet transition of the NV center.generating a second signal by measuring an emission of the singlet-singlet transition of the NV center or absorption exciting a singlet-singlet transition of the NV center.evaluating the first and second signals to determine the magnetic field.The first and / or second signal can each be generated from a current signal generated by means of a photodiode, which is converted into a voltage signal by means of a transimpendance amplifier (see below), for example.This allows the triplet-triplet and singlet-singlet transitions to be read out simultaneously. By measuring the triplet-triplet and singlet-singlet transition, the contrast of an acquired ODMR spectrum can be increased and thus the sensitivity of the magnetic field determination can be improved.In the present case, an NV (nitrogen-vacuum) center means an NV -- center (negatively charged nitrogen-vacuum center). In particular, the NV center is not meant to be the NV0 center in the present case.For magnetic field determination, the NV center can be irradiated with light in the green-orange spectral range. Depending on the initial state, the NV center can thereby be excited into a state which decays with greater or lesser probability with emission (emission) of fluorescent light with wavelengths between approximately 600 nm (nanometers) and 900 nm and decays with lesser or greater probability over a plurality of intermediate states and with emission (emission) of light with a wavelength of 1042 nm.In this case, it is possible to manipulate or set, by means of a microwave radiated in resonant fashion, for example, in which initial state the NV center is located. The resonant frequency (of the microwave) is dependent on a projection of a flux density of the external magnetic field onto a respective NV axis and the temperature. The resonant frequency can therefore be used to determine the component of the magnetic field parallel to the NV axis. In this case, it is possible to detect a break-in of the fluorescent radiation (or emission of the triplet-triplet transition) for resonant excitation with the microwave. The fluorescent radiation can be monitored by means of a photodiode.The states of the NV center are shifted in energy under the influence of an external magnetic field due to the Zeeman effect, resulting in a change in the resonant frequency of the microwave. The hyperfine structure results in three resonances which are located close to one another. The dependence between the resonant frequency of the microwave and the external magnetic field B is given by where v +1- v -1 is the distance of the resonances for the transitions from spin-0 to the spin ±1 level of the ground state and γ e is the gyromagnetic ratio of the electron.The NV center may be in eight different orientations in the diamond, the orientation being defined by an imaginary connecting axis between the nitrogen atom (N) and the defect (V). Two directions are in each case opposite and indistinguishable in the method described, for which reason four orientations are referred to below. The NV center is sensitive to the projection of the external magnetic field onto this axis, i.e. to the portion of the external magnetic field that is parallel to the NV axis. Thus, each NV center orientation experiences a different magnetic field strength for an arbitrarily oriented external magnetic field. This can be used to operate vector magnetometers and determine the vector of the external magnetic field.Without an external magnetic field, the different transitions of all four NV center orientations (or NV orientations) near the IFS (zero field splitting) are superimposed at room temperature at approximately 2.87 GHz (gigahertz). However, stresses in the crystal and temperature gradients can lead to a slight displacement of the states of the different orientations relative to one another. This slight shift results in the resulting line width of the superimposed transition being widened and the sensitivity of the NV magnetometer thereby being reduced. Even if there is only one NV center, in the IFS both resonances (of spin ±1) overlap. If these now shift slightly, the contrast is very low. In addition, the direction of the magnetic field is then unknown.In order to solve this problem, an additional, homogeneous, static offset magnetic field, which is defined in its field strength, can be generated in a defined direction, e.g. along an NV orientation. This can cancel the degeneracy of the various NV orientations. This leads to a shift of the transitions of an NV orientation in the ODMR spectrum. The linewidth of the junctions, which is directly related to the sensitivity of the NV magnetometer, can be described by the following formula:Where v is the microwave frequency, Γ p the laser power dependent pump rate, Γ 1 the spin lattice relaxation rate, Γ 2 the spin spin relaxation rate, and Ω is the rabi frequency (proportional to the microwave magnetic field).The shot noise-limited sensitivity η B can be calculated by means of the following formula:where P is a prefactor (0.77), Δv is the linewidth, γ e is the gyromagnetic ratio of the electron (about 28 GHz / T), C det is the detected contrast, and R is the rate of the detected photons. A lower sensitivity is better. Given a line width, a high contrast and a high rate of detected photons are thus advantageous.For magnetic field measurement (or determination), the fluorescence (emission) of the triplet-triplet transition (wavelength range about 600 nm to 900 nm) can be measured. When measuring an ODMR spectrum, the rates for the radiative transitions from the 3 E to the 3 A 2- states are only slightly different. The rate for the nonradiative transitions from the 3 E states to the 1 A 1- state for spin ±1 is about five times as high as for spin 0. This results in the measured contrast. As a result, in the case of a resonant microwave (i.e. the spin ±1 states are occupied), the fluorescence in the red region is reduced by a specific value which is set in relation to the fluorescence at spin=0 and is called "contrast C". A short pass filter that blocks the emission of the singlet-singlet transition can increase the contrast.Alternatively or additionally, differences in singlet-singlet transitions may be measured. This can be done in the visible range as described above. This can likewise be done by measuring an emission of the radiative transition from 1 A 1 to 1 E (wavelength: about 1042 nm) or by measuring an absorption (at the same wavelength) with excitation of the transition from 1 E to 1 A 1. During the absorption, the transmission of a laser beam (sample laser beam) at the corresponding wavelength can be measured. The absorption can be determined from at least one intensity dip of the measured transmission. Both the measurement of the emission and the measurement of the transmission (or of the absorption) offer a higher contrast than the measurement of the emission of the triplet-triplet transition in the visible range. This is caused, inter alia, by rates which are greater by a factor of 5 for the transition from the upper triplet state to the upper singlet state (1 A 1). In addition, there is no emission of the NV0 center here.The measurement of the absorption or transmission of a laser beam (sample laser beam) can generate better signals in principle, since the lifetime of the 1 A 1- state is short compared to the 1 E state and nonradiative transitions also exist here, which significantly reduces the quantum yield. In addition, an optical path can be constructed or provided with very high collection efficiency. In contrast, the emission radiates in all directions.An optical resonator can be used for measuring the emission. This can increase the rate of transition of the optical transition (spontaneous emission) at the expense of the nonradiative transitions (Purcell effect).Laser threshold magnetometry (laser threshold magnetometry) can be used to measure the transmission (or absorption), it being possible for the diamond to function as a variable absorber.According to a development of the method, if, in the step of generating a second signal by measuring an emission of the singlet-singlet transition of the NV center or absorption with excitation of a singlet-singlet transition of the NV center, the emission is measured, the NV center can be excited into the upper triplet state by means of a first laser beam (pump laser beam). A transition to the upper singlet state subsequently takes place with a certain probability.Since here the emission of the singlet-singlet transition can be measured directly, a laser suffices, which simplifies the method.According to a development of the method, in the step of evaluating the first and the second signal in order to determine the magnetic field, the first and the second signal can be subtracted from one another. This can be implemented by means of a differential amplifier (subtracting operational amplifier). In this case, the first signal can be subtracted from the second signal. It is likewise conceivable that the second signal can be subtracted from the first signal.This makes it possible to further improve the contrast and thus the sensitivity using simple means.According to a development of the method, if, in the step of generating a second signal, the absorption is measured by measuring an emission of the singlet-singlet transition of the NV center or absorption with excitation of a singlet-singlet transition of the NV center, the NV center can be excited (from the lower) into the upper triplet state by means of a first laser beam and (from the lower) into the upper singlet state by means of a second laser beam (sample laser beam). The absorption measurement can be carried out in particular by measuring the transmission of the sample laser beam.In this way, a better sensitivity can be achieved, in particular due to the longer lifetime of the 1 E state (compared to the 1 A 1- state).According to a development of the method, in the step of evaluating the first and the second signal in order to determine the magnetic field, the first and the second signal can be added to one another. This can be implemented by means of an addition amplifier.This makes it possible to further improve the contrast and thus the sensitivity using simple means.According to a development of the method, the first signal and / or the second signal can be generated in each case by means of a transimpendance amplifier.As a result, the first and / or second signal can be generated with simple means. In particular, the first and / or the second signal can each be generated from a current signal of a (measuring) photodiode. In other words, a current signal (current intensity I) of a photodiode can be converted into the first and / or the second signal by means of a transimpendance amplifier. The first and / or the second signal can each be configured as a voltage signal (current voltage U).According to a development of the method, the method can comprise the step:adjusting a noise of the first and / or the second signal. This can be implemented by adjusting at least one resistor of at least one transimpendance amplifier, the addition amplifier and / or the differential amplifier. By adjusting (or selecting) the respective resistors, the respective intensity of the first and / or of the second signal can be varied or set as desired. Accordingly, the respective noise can also be varied or set. In this case, the noise of the first signal and / or the noise of the second signal can be adapted in such a way that the noise of the first signal and the noise of the second signal are approximately of equal magnitude.In this way, optimal noise suppression of the two signals can be implemented using simple means. For noise suppression, the first signal, the second signal and / or the signal evaluated from the first and the second signal can be "balanced" with a reference signal (for example with a gradiometer or a part of the first laser beam) according to the principle of balanced detection.It is likewise conceivable that at least one (optical) resonator can be used for the respective wavelength in order to increase a local intensity.The above object is achieved by a device for determining the magnetic field having the features of the subordinate claim. The device can be designed in particular as an NV magnetometer.The apparatus comprises at least one diamond having at least one NV center. The apparatus comprises a first laser device for generating a first laser beam (pump laser beam) for exciting the NV center. The diamond may be present as a nanostructure. In other words, the diamond may have a size in the nanometer range. With a very small diamond or with a very small diamond structure, a very high Purcell enhancement is possible.The device comprises a first photodiode for measuring an emission of a triplet-triplet transition of the NV center and a first transimpendance amplifier coupled to the first photodiode for generating a first signal. The first transimpendance amplifier converts, in particular, a current signal generated by the first photodiode into the first signal, which can be designed as a voltage signal.The device comprises a second photodiode for measuring a singlet-singlet transition of the NV center or absorption under excitation of a singlet-singlet transition of the NV center (or the corresponding transmission), and a second transimpendance amplifier coupled to the second photodiode for generating a second signal. The second photodiode can be designed as an InGaAs (Indiumgalliumarsenid) photodiode. In particular, the second photodiode is not a silicon-based photodiode, since the photon energy is only just above the band gap of the silicon-based photodiode and the "response function" of the silicon-based photodiode typically drops sharply in the range around 1042 nm. The second transimpendance amplifier converts, in particular, a current signal generated by the second photodiode into the second signal, which can be designed as a voltage signal.The apparatus comprises an evaluation device for evaluating the first and the second signal. For this purpose, the evaluation device is coupled to the first and the second transimpendance amplifier.This allows the triplet-triplet and singlet-singlet transitions to be read out simultaneously. By measuring the triplet-triplet and singlet-singlet transition, the contrast of a recorded ODMR spectrum and thus the sensitivity of the magnetic field determination can be improved.According to a further development of the apparatus, the apparatus can comprise a second laser device for generating a second laser beam (sample laser beam) for exciting the NV center. In this case, the second laser beam can have a wavelength of 1042 nm. In particular, the second laser beam can be configured to meet well the zero-phonon line of the singlet-singlet transition, which is 1042 nm.As a result, the absorption (or the transmission) of the sample laser and thus the occupancy of the lower singlet state can be determined or measured with simple means.According to a development of the apparatus, the apparatus can comprise at least one separating device for separating the emission of the triplet-triplet transition and the emission of the singlet-singlet transition of the NV center or of the transmitted sample laser beam. The separating device can be designed as a dichroic filter.Thus, the beam path of the emission of the triplet-triplet transition and the beam path of the emission or of the singlet-singlet transition or of the sample laser beam can be separated from one another with simple means. The separate beam paths can thus be detected or detected by means of a (separate) photodiode in each case.According to a further development of the device, the evaluation device can be designed as a differential amplifier or as an addition amplifier.As a result, the first and the second signal can be evaluated, in particular added or subtracted from one another, using simple means.The apparatus can comprise at least one microwave source for generating a microwave radiation, in particular resonant microwave radiation.According to a development of the device, the device can be configured to carry out the method according to the above explanations.With regard to the advantages which can be achieved thereby, reference is made to the relevant explanations relating to the method. For a further embodiment of the device, the measures described in connection with the method and / or the measures explained below can be used.Further features, details and advantages of the invention are evident from the wording of the claims and from the following description of exemplary embodiments on the basis of the drawings. The following are shown: FIG. 1 shows a flow chart of a method for determining the magnetic field by means of at least one NV center, and FIG. 2 shows a schematic illustration of a device for determining the magnetic field by means of at least one NV center.FIG. 1 shows a flow chart of a method for determining the magnetic field by means of at least one NV center. The method comprises the steps of:10: providing the NV center in a magnetic field to be determined.12: exciting the NV center into a triplet upper state and a singlet upper state.14: generating a first signal by measuring an emission of the triplet-triplet transition of the NV center.16: generating a second signal by measuring an emission of the singlet-singlet transition of the NV center or absorption exciting a singlet-singlet transition of the NV center.18: evaluating the first and second signals to determine the magnetic field.When the emission is measured in generating the second signal, the NV center is excited to the upper triplet state and the upper singlet state by a first laser beam (pump laser beam). In other words, the same laser beam or the same laser beam source is used to excite into the upper triplet state and the upper singlet state.In particular, if the emission is measured when generating the second signal, the evaluation of the two signals can be implemented by subtracting the two signals from one another. This can be implemented by means of a differential amplifier.When the absorption or transmission is measured in generating the second signal, the NV center is excited into the upper triplet state by means of a first laser beam (pump laser) and into the upper singlet state by means of a second laser beam (sample laser beam). In this case, the wavelength of the second laser beam (for example, can be determined. 1042 nm) with the singlet-singlet transition. The absorption can be determined, for example, by a transmission measurement of the second laser beam. The absorption or the transmission can be determined in the form of a dip in the intensity or an intensity minimum (or intensity dip).In particular, if the absorption is measured by means of a transmission measurement when generating the second signal, the evaluation of the two signals can be implemented by adding the two signals to one another. This can be implemented by means of an addition amplifier.The first signal and / or the second signal can each be generated by means of a transimpendance amplifier. Thus, for example, a current signal generated by a respective photodiode can be converted by means of a transimpendance amplifier into a respective signal embodied as a voltage signal.The method comprises the step:20: adjusting a noise of the first and / or the second signal.This can be implemented by adjusting at least one resistor of the transimpendance amplifier, the addition amplifier and / or the differential amplifier. In this case, the noise of the first signal and / or the noise of the second signal can be adapted such that the noise of the first signal and the noise of the second signal are of equal intensity. In other words, noise adaptation or noise suppression is possible by varying (or by a corresponding selection) the respective resistor or the respective resistors.FIG. 2 shows a schematic illustration of a device 22 for determining the magnetic field by means of at least one NV center. The device can be designed as an NV magnetometer.The device 22 includes at least one diamond 24 having at least one NV center. The apparatus 22 comprises a first laser device 26 for generating a first laser beam 28 (pump laser beam) for exciting the NV center. The device 22 comprises a first photodiode 30 for measuring an emission 32 of a triplet-triplet transition of the NV center. The device 22 further comprises a first transimpendance amplifier 34 coupled to the first photodiode 30 for generating a first signal.The device 22 comprises a second photodiode 36 for measuring an emission 38 or absorption under excitation of a singlet-singlet transition of the NV center. The second photodiode 36 can be formed as an InGaAs photodiode. The device 22 further comprises a second transimpendance amplifier 40 coupled to the second photodiode for generating a second signal.The device 22 comprises an evaluation device 42 for evaluating the first and the second signal. For this purpose, the evaluation device 42 is respectively coupled to the first and the second transimpendance amplifiers 34, 40.The evaluation device 42 can be designed as a differential amplifier or as an addition amplifier.The apparatus 22 comprises a second laser device 44 for generating a second laser beam 46 (sample laser beam) for exciting the NV center. The second laser beam 46 may have a wavelength of 1042 nm.Depending on the structure of the apparatus 22 or the respective beam paths, the beam paths of the first laser device 26 and the second laser device 44 or the light signals to be measured can overlap one another. In this case, the beam paths can be (spectrally) separated, in particular for the respective measurement. A separating device 48 can be used for this purpose.In the present case, the apparatus 22 has the separating device 48 for separating the emission 32 of the triplet-triplet transition and the emission 38 of the singlet-singlet transition of the NV center or of the transmitted sample laser beam. The separating device 48 can be designed as a dichroic filter.Alternatively, the beam path of the second laser device 44 or the second laser beam 46 can be arranged in such a way that a geometrically separate beam path (separate from the first laser device 26 or the first laser beam 28) is formed (both beam paths on the diamond or the NV center are oriented orthogonally to one another, for example). A (spectral) separation of the two beam paths is then not necessary, but is nevertheless advantageous due to the emission characteristic of the emission.It is conceivable to improve the contrast of the measurement of the emission of the triplet-triplet transition (measurement at 600 nm to 900 nm) by a (short-pass) filter. For the measurement of the absorption of the singlet-singlet transition (or transmission measurement of the second laser beam), a filter that blocks the emission at 600 nm to 900 nm can also be used. Likewise, in both cases, a filter can improve the contrast that blocks light of the pump wavelength.The device 22 can be configured to carry out the method as described above. The device 22 can be configured in particular for carrying out the method illustrated in FIG. 1.

Claims

A method for magnetic field determination by means of at least one NV center comprising the steps of: - (10) providing the NV center in a magnetic field to be determined; - (12) exciting the NV center into an upper triplet state and an upper singlet state; - (14) generating a first signal by measuring an emission of the triplet-triplet transition of the NV center; - (16) generating a second signal by measuring an emission of the singlet-singlet transition of the NV center or absorption by exciting a singlet-singlet transition of the NV center; - (18) evaluating the first and the second signal in order to determine the magnetic field.The method according to claim 1, characterized in that when, in the step (16) of generating a second signal by measuring an emission of the singlet-singlet transition of the NV center or absorption exciting a singlet-singlet transition of the NV center, the emission is measured, the NV center is excited to the upper triplet state and the upper singlet state by a first laser beam.Method according to Claim 2, characterized in that, in the step (18) of evaluating the first and the second signal in order to determine the magnetic field, the first and the second signal are subtracted from one another, in particular by means of a differential amplifier.The method according to claim 1, characterized in that when, in the step (16) of generating a second signal by measuring an emission of the singlet-singlet transition of the NV center or absorption exciting a singlet-singlet transition of the NV center, the absorption is measured, the NV center is excited to the upper triplet state by means of a first laser beam and to the upper singlet state by means of a second laser beam, the transmission of which is measured.Method according to Claim 4, characterized in that, in the step (18) of evaluating the first and the second signal in order to determine the magnetic field, the first and the second signal are added to one another, in particular by means of an addition amplifier.Method according to Claim 3 or 5, characterized in that the first signal and / or the second signal are each generated by means of a transimpendance amplifier.Method according to one of claims 3, 5 or 6, characterized in that the method comprises the step of: - (20) adapting a noise of the first and / or of the second signal, in particular by means of adapting at least one resistance of the transimpendance amplifier, of the addition amplifier and / or of the differential amplifier, in particular wherein the noise of the first signal and / or the noise of the second signal are adapted such that the noise of the first signal and the noise of the second signal are of equal magnitude.Device (22) for magnetic field determination, in particular an NV magnetometer, comprising: - at least one diamond (24) with at least one NV center, - a first laser device (26) for generating a first laser beam (28) for exciting the NV center, - a first photodiode (30) for measuring an emission (32) of a triplet-triplet transition of the NV center, - a first transimpendance amplifier (34) coupled to the first photodiode (30) for generating a first signal, - a second photodiode (36), in particular an InGaAs photodiode, measuring an emission (38) of a singlet-singlet transition of the NV center or an absorption of a sample laser beam with excitation of a singlet-singlet transition of the NV center, - a second transimpendance amplifier (40) coupled to the second photodiode (36) for generating a second signal, - an evaluation device (42) coupled to the first and second transimpendance amplifiers (34, 40) for evaluating the first and second signals.Device (22) according to claim 8, characterised in that the device (22) comprises a second laser device (44) for generating a second laser beam (46), in particular with a wavelength of 1042 nm, for exciting the NV centre.Device (22) according to claim 8 or 9, characterised in that the device (22) comprises at least one separating device (48), in particular a dichroic filter, for separating the emission (32) of the triplet-triplet transition and the emission (38) or transmission of the singlet-singlet transition of the NV centre.Device (22) according to one of Claims 8 to 10, characterized in that the evaluation device (42) is designed as a differential amplifier or an addition amplifier.Device (22) according to one of Claims 8 to 11, characterized in that the device (22) is designed to carry out the method according to one of Claims 1 to 7.

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