Patent Grant
12339332
Embodiments of the present invention relate to a method for determining an external magnetic field with a nitrogen-vacancy (NV) magnetometer with automated resonance control, as well as to the type of NV magnetometer and to the use of one or more NV magnetometers of the type for gradiometric measurements.
The nitrogen-vacancy center is one of over 100 known defects in the diamond lattice. These defects are impurities in the pure carbon lattice of diamond. The nitrogen-vacancy center is also referred to as the NV center. Quantum sensors such as NV magnetometers utilize the extreme sensitivity of quantum systems with respect to their environment and allow for significantly improved sensors to be created. By capturing an additional electron, NV centers form a microscopic compass via the spin of the electron. By precisely manipulating and measuring the state of this NV spin, even the smallest changes in the surrounding magnetic field can be measured. This technique is called NV magnetometry and is based on the interaction between the NV center and the surrounding magnetic field. The presence of a magnetic field manifests itself in a change in the energies of the spin states of the NV center. The effect of the magnetic field can easily be recognized from the change in the microwave excitation spectrum of the NV center.
A decisive advantage of NV magnetometry stems from the well-understood behavior of the NV center spin transition frequencies, which provide clear information about the strength and direction of the magnetic field. This ability to provide self-calibrated, quantitative data combined with very high reproducibility (every NV center is the same) are key features of NV magnetometers. A major advantage of NV magnetometers is their high sensitivity to small signals. The sensitivity of an NV magnetometer to static magnetic fields depends in particular on the density of the NV centers in the diamond chip, on the quantum coherence of the NV centers and on the efficiency of the optical readout. Changes in the magnetic field, temperature and/or pressure shift the magnetic resonances of the NV center ground state, which are used for the magnetic field measurement of the NV magnetometer. This leads to a limited dynamic range of the NV magnetometer and a poorer signal-to-noise ratio (SNR).
The article by H. Clevenson et al. in Appl. Phys. Lett 112, 252406 (2018) discloses a robust, scale-factor-free vector magnetometer that uses a closed-loop frequency-locking scheme to simultaneously track Zeeman-split resonance pairs of nitrogen-vacancy centers (NV centers) in diamond. Nevertheless, it would be desirable to further increase the dynamic range and bandwidth of the NV magnetometer with regard to the magnetic field changes that can be detected, as well as to improve the SNR of the measurements and enable an extension to gradiometric magnetic field measurements while simultaneously suppressing temperature and pressure fluctuations.
Embodiments of the present invention provide a method for determining an external magnetic field with a nitrogen-vacancy (NV) magnetometer with automated resonance control. The NV magnetometer includes a sensor material with NV centers with respective spin states ms=0, ±1 and resonance frequencies f± for transitions between the spin states ms=0↔±1. A difference f+−f− of the resonance frequencies f± depends on the external magnetic field B. The method includes recording single resonance signals VSR-LIA for determining the resonance frequencies f± by using frequency-modulated microwave radiation emitted onto the sensor material in frequency bands based on a microwave frequency fMW and a carrier frequency fSB. The microwave frequency fMW and the carrier frequency fSB are selected such that the frequency bands address two magnetic resonances in succession. The method further includes recording a double resonance signal VDR-LIA with a dispersive form and a linear range around the resonance frequencies f±. The microwave frequency fMW and the carrier frequency fSB are selected such that fMW±fSB=f± applies so that the two magnetic resonances are excited simultaneously. The method further includes determining a slope of the linear range as a scalar factor α from the double resonance signal VDR-LIA, observing a detuning ΔB=VDR-LIA/α of the resonance frequencies f± via the double resonance signal VDR-LIA with resonant emission of the microwave radiation due to external magnetic field changes by using an evaluation unit, controlling the frequency bands fMW±fSB with a controller including a control algorithm based on the observed detuning ΔB so that the frequency bands fMW±fSB remain in the range of the resonance frequencies f± of the NV centers present at the detuning ΔB even after the detuning ΔB has occurred, and determining the external magnetic field B from a distance 2*fSB of the frequency bands and the detuning ΔB by using the evaluation unit.
Subject matter of the present disclosure will be described in even greater detail below based on the exemplary figures. All features described and/or illustrated herein can be used alone or combined in different combinations. The features and advantages of various embodiments will become apparent by reading the following detailed description with reference to the attached drawings, which illustrate the following:
Embodiments of the invention provide an NV magnetometer that enables an increased dynamic range and a higher bandwidth with respect to the detectable magnetic field changes as well as a better SNR in the measurements.
According to some embodiments, a method for determining an external magnetic field with an NV magnetometer with automated resonance control, wherein the NV magnetometer comprises a sensor material with NV centers with respective spin states ms=0, ±1 and resonance frequencies f± for transitions between the spin states ms=0↔±1, wherein a difference f+−f− of the resonance frequencies f± depends on the external magnetic field B, comprising the following steps:
The term “sensor material” refers to the material that has the NV centers. This is diamond, for example.
For a more detailed explanation of the spin states ms=0, ±1 and the resonance frequencies f± for transitions between the spin states ms=0↔±1, please refer to
The term “magnetic resonance” refers to the excitation of the energetically higher spin states in the NV centers and thus the absorption of microwave radiation at the resonance frequencies f±. During the transition to the ground state, the corresponding radiation is emitted again. The magnetic resonance can be measured in absorption spectra or emission spectra, for example.
In this regard, the carrier frequency is a fixed frequency which, depending on the modulation method, is changed in its amplitude, phase position or frequency in the rhythm of the modulation frequency. For example, in frequency modulation, the frequency changes. The carrier frequency is added again when the signal is demodulated.
The so-called “scalar factor α” corresponds to the slope of the demodulated signal VDR-LIA in the linear range around the resonance frequency, where it has a zero crossing. Depending on the influence of the external magnetic field, the zero crossing of the demodulated signal VDR-LIA can be shifted in its frequency, as the external magnetic field shifts the actual resonance frequency compared to the resonance frequency without the influence of an external magnetic field. This shift is referred to as resonance detuning ΔB. By repeatedly controlling the operating point of the NV magnetometer at the point of the steepest slope of the demodulated signal (largest value of α) using the control algorithm, an improved signal-to-noise ratio of the measurement signal is obtained.
In this regard, the control algorithm can be installed and executed as a software program on a controller, wherein the controller is connected to the evaluation unit and the microwave transmitter for emitting the microwave radiation in order to excite the magnetic resonance via data or control lines so that the microwave frequency fMW can be controlled according to the control algorithm depending on the detuning ΔB observed with the evaluation unit. In this regard, the evaluation unit can be designed as a processor connected to a storage medium so that the corresponding evaluation program(s) can be executed and the evaluated data can be stored.
In the NV magnetometer according to embodiments of the invention, the intelligent control algorithm, including the hardware configuration necessary for this purpose, ensures that the frequency of the microwave excitation required for sensitive sensor operation remains within the magnetic resonance range.
With the method according to embodiments of the invention, an NV magnetometer can be operated with an increased dynamic range, a higher bandwidth with respect to the detectable magnetic field changes and a better SNR in the measurements. The above-stated advantages also enable gradiometric measurements with one or more such NV magnetometers.
In one embodiment of the method or the NV magnetometer, the controlling comprises the following steps:
By continuously adjusting the operating point of the NV magnetometer, the dynamic range of the NV magnetometer is significantly increased. After magnetic field or temperature changes, which can cause the resonance range to be exceeded, a time-consuming search for the resonance in a broad frequency spectrum is therefore no longer necessary. The bandwidth of the NV magnetometer is also increased accordingly.
In a preferred embodiment of the method or the NV magnetometer, the controller is an integral software-based controller controlling according to the rule ΔfMW[n]=K*(e[0]+e[1]+ . . . +e[n] with K as the proportionality factor of the controller with i=0, 1, . . . , n as iteration steps. Here, the proportionality factor K is preferably equal to 1/α.
In one embodiment of the method or the NV magnetometer, the controller has a control period Tc, after which the microwave frequency fMW is again controlled with the controller, wherein the control period Tc is between 1 μs and 10 s, preferably between 0.05 s and 1 s, more preferably 0.5 s. A short control period Tc enables the tracking of rapidly changing magnetic field signals, but can also limit the bandwidth of the NV magnetometer, since setting the frequency bands incurs a dead time. In addition, a long control period Tc worsens the signal-to-noise ratio SNR, as the optimum operating point (resonance point/zero crossing) cannot be adequately maintained. A control period Tc=0.5 s is therefore preferred.
In a preferred embodiment of the method or the NV magnetometer, a signal generator generates a signal with the microwave frequency fMW and a further signal generator generates a frequency-modulated signal with the carrier frequency fSB, a modulation frequency fm and a modulation amplitude fd with fSB(t)=fSB+fd*cos(2πfmt) with t as the time with a respective power.
In a preferred embodiment of the method or the NV magnetometer, the two signals are frequency-mixed by means of a frequency mixer and the microwave radiation emitted onto the sensor material comprises the two frequency bands fMW+fSB(t) and fMW−fSB(t), wherein fSB(t) is phase-shifted by π at fMW−fSB(t). Preferably, the microwave frequency fMW and carrier frequency fSB are selected such in this regard that the two frequency bands are each resonant with the resonance frequencies f± in order to excite both magnetic resonances simultaneously.
In a preferred embodiment of the method or the NV magnetometer, both frequency bands are emitted with individually adjusted powers. By being able to adjust the power for both frequency bands separately, the difference in the scalar factors of the single resonance spectra can be minimized and thus the suppression of temperature and pressure changes in the double resonance signal can be maximized. Here, depending on the embodiment of the hardware configuration of the NV magnetometer, the powers can be individually adjusted by means of a programmable attenuator or by adjusting the power of the signal generators.
Embodiments of the invention further relate to an NV magnetometer for determining an external magnetic field B, designed to carry out a method for automated resonance control according to any one of the preceding claims, comprising
In the NV magnetometer according to embodiments of the invention, the intelligent control algorithm, including the hardware configuration necessary for this purpose, ensures that the frequency of the microwave excitation required for sensitive sensor operation remains within the magnetic resonance range.
The NV magnetometer according to embodiments of the invention enables an increased dynamic range, a higher bandwidth with respect to the detectable magnetic field changes as well as a better SNR in the measurements. The above-stated advantages also enable gradiometric measurements with one or more such NV magnetometers.
In this regard, due to its special properties, the sensor material is preferably diamond.
In one embodiment, the microwave transmitter comprises a signal generator for generating a signal with the microwave frequency fMW and a further signal generator for generating a frequency-modulated signal with the carrier frequency fSB, a modulation frequency fm and a modulation amplitude fd with fSB(t)=fSB+fd*cos(2πfmt) with t as the time with a respective power.
In a further embodiment, the signal generators are designed to adjust the power of the signals or the signal generators have an attenuator connected upstream for individual power adjustment.
Embodiments of the invention further relate to the use of one or more NV magnetometers according to embodiments of the invention for gradiometric measurements. Gradiometry is the measurement of a component of a gradient field, for example the gradient of a magnetic field. In this context, a vector component is registered simultaneously with two sensors (in this case two NV magnetometers) that are positioned a fixed distance apart. Gradiometry is used in geomagnetics. The gradient measurement requires precise and fast measurements, which are made possible by the NV magnetometer according to embodiments of the invention and the corresponding methods according to embodiments of the invention.
It should be expressly noted that expressions using solely “at least” have been avoided wherever possible in order to improve readability. Rather, an indefinite article (“one”, “two”, etc.) is normally to be understood as “at least one, at least two, etc.,” unless it is clear from the context that “exactly” the specified number is meant.
At this point, it should also be mentioned that in the context of the present patent application, the term “in particular” is always to be understood as introducing an optional, preferred feature. The expression is therefore not to be understood as “specifically” and not as “namely”.
It is to be understood that features of the solutions described above or in the claims can also be combined in deviation from the claim references, if applicable, in order to be able to realize the advantages and effects that can be achieved in the present case, either individually or cumulatively.
In addition, further features, effects and advantages of the embodiments of the present invention are explained with reference to the attached drawing and the following description. Components which at least substantially correspond in terms of their function in the individual figures are designated with the same reference symbols here, wherein the components need not be numbered and explained in all figures.
Temperature and pressure fluctuations can be separated from magnetic field changes if both resonance frequencies f± are known by subtracting the two resonance frequencies from one another and the difference is only proportional to the external magnetic field B:
γ is the gyromagnetic ratio of the NV center in the sensor material 2 here.
The signals are then frequency-mixed in a power combiner 92 so that the signal emitted onto the sensor material 2, for example a diamond, by means of an antenna 10 contains the following frequency portions:
As can be seen, the frequency mixing results in a phase difference of π or 180° between the two frequency portions. The system parameters fMW and fSB are selected such that the frequency portions are each resonant with the transitions ms=0↔±1 (see
The respective frequency bands are emitted onto the sensor material 2 via respective antennas 10 and the resulting magnetic resonance (fluorescence FL) is measured via a respective optical photodetector 8 and converted into a double resonance signal VDR-LIA by means of a respective lock-in amplifier 3 with a frequency mixer 31 and low-pass filter 32. In order to generate the two separate frequency bands, the circuit according to
The following measurement protocol can be derived from this:
The controller 5 then shifts the frequency bands in the opposite direction by ΔfMW (by adjusting the system parameters fMW and fSB), see
Here, K is the proportionality factor of the controller and i=0, 1, . . . , n the iteration step. In this regard, the proportionality factor K should be selected as follows in order to eliminate the deviation (e=0): K=1/α. The following preferably applies to the control period Tc: a short control period enables the tracking of rapidly changing magnetic field signals, but can also limit the bandwidth of the NV magnetometer 1, since setting the frequency bands incurs a dead time. In addition, a long control period worsens the signal-to-noise ratio, as the optimum operating point (resonance point/zero crossing) cannot be adequately maintained. For example, a control period of TC=0.5 s can be used. For the output signal of the control system 2γB=f+−f−+2VDR-LIA/α, f± denotes the position of the frequency bands after the last operation of the controller 5.
At this point, it should be explicitly noted that features of the solutions described above or in the claims and/or figures can also be combined, if applicable, in order to be able to implement or achieve the features, effects and advantages explained in a correspondingly cumulative manner.
It should be understood that the exemplary embodiment described above is merely an initial embodiment of the present invention. In this respect, the embodiment of the invention is not limited to this exemplary embodiment.
While subject matter of the present disclosure has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive. Any statement made herein characterizing the invention is also to be considered illustrative or exemplary and not restrictive as the invention is defined by the claims. It will be understood that changes and modifications may be made, by those of ordinary skill in the art, within the scope of the following claims, which may include any combination of features from different embodiments described above.
The terms used in the claims should be construed to have the broadest reasonable interpretation consistent with the foregoing description. For example, the use of the article “a” or “the” in introducing an element should not be interpreted as being exclusive of a plurality of elements. Likewise, the recitation of “or” should be interpreted as being inclusive, such that the recitation of “A or B” is not exclusive of “A and B,” unless it is clear from the context or the foregoing description that only one of A and B is intended. Further, the recitation of “at least one of A, B and C” should be interpreted as one or more of a group of elements consisting of A, B and C, and should not be interpreted as requiring at least one of each of the listed elements A, B and C, regardless of whether A, B and C are related as categories or otherwise. Moreover, the recitation of “A, B and/or C” or “at least one of A, B or C” should be interpreted as including any singular entity from the listed elements, e.g., A, any subset from the listed elements, e.g., A and B, or the entire list of elements A, B and C.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 2022 113 754.6 | May 2022 | DE | national |
This application is a continuation of International Application No. PCT/EP2023/063409 (WO 2023/232491 A1), filed on May 17, 2023, and claims benefit to German Patent Application No. DE 10 2022 113 754.6, filed on May 31, 2022. The aforementioned applications are hereby incorporated by reference herein.
| Number | Name | Date | Kind |
|---|---|---|---|
| 9551763 | Hahn | Jan 2017 | B1 |
| 10088336 | Fisk | Oct 2018 | B2 |
| 10712408 | Pham et al. | Jul 2020 | B2 |
| 20100308813 | Lukin | Dec 2010 | A1 |
| 20170038411 | Yacobi | Feb 2017 | A1 |
| 20170212046 | Cammerata | Jul 2017 | A1 |
| 20170212183 | Egan | Jul 2017 | A1 |
| Entry |
|---|
| Fescenko Ilja et al, Diamond magnetometer enhanced by ferrite flux concentrators, Physical Review Research, vol. 2, No. 2, Jun. 24, 2020, DOI: 10.1103/PhysRevResearch.2.023394. |
| Hannah Clevenson et al, Robust High-Dynamic-Range Vector Magnetometry via Nitrogen-Vacancy Centers in Diamond, arxiv.org, Feb. 27, 2018, United States, DOI: 10.1063/1.5034216. |
| Number | Date | Country | |
|---|---|---|---|
| 20250093432 A1 | Mar 2025 | US |
| Number | Date | Country | |
|---|---|---|---|
| Parent | PCT/EP2023/063409 | May 2023 | WO |
| Child | 18961530 | US |
