Abstract
A sensor unit for detecting a magnetic field is disclosed. The sensor unit includes (i) a light source for generating excitation light, (ii) at least one first sensor for determining a measurement signal of an object, and (iii) a second sensor for determining a background magnetic field. The first sensor is designed as a diamond-based NV magnetometer and includes a highly sensitive diamond having at least one negatively charged NV center that has a fluorescent effect and thus emits fluorescence.
Claims
1. A sensor unit for detecting a magnetic field, comprising: a light source configured to generate excitation light, at least one first sensor configured to determine a measurement signal of an object, and a second sensor configured to determine a background magnetic field, wherein the first sensor is designed as a diamond-based NV magnetometer and comprises a highly sensitive diamond having at least one negatively charged NV center that has a fluorescent effect and thus emits fluorescence.
2. The sensor unit of claim 1, wherein the first sensor is configured to be arranged in close proximity to an object to be measured.
3. The sensor unit of claim 1, wherein the second sensor is a gas vapor cell magnetometer or a superconducting quantum interference device (SQUID) magnetometer.
4. The sensor unit according to claim 1, wherein: the sensor unit comprises an optical fiber connected to the light source, and the optical fiber is configured to excite the at least one NV center by way of the excitation light of the light source.
5. The sensor unit according to claim 4, wherein the second sensor is fastened to the optical fiber such that a defined distance of the second sensor to the first sensor results.
6. The sensor unit according to claim 1, wherein: the sensor unit comprises an evaluation unit, and the evaluation unit comprises at least one signal processing unit and control unit for determining a first measurement signal based on the first sensor and a second measurement signal based on the second sensor.
7. The sensor unit according to claim 1, wherein the sensor unit comprises a photodetector configured to receive the emitted fluorescence.
8. The sensor unit according to claim 7, wherein the sensor unit comprises a lens configured to separate the excitation light and the emitted fluorescence so that only the emitted fluorescence falls on the photodetector.
9. A method for detecting a magnetic field with a sensor unit according to claim 1, comprising a first sensor and a second sensor.
10. The method according to claim 9, comprising: arranging the first sensor in close proximity to an object to be measured, measuring a background magnetic field using the second sensor, determining the background magnetic field at the location of the first sensor, and calibrating a measurement signal of the first sensor.
Description
[0041] The figure shows a purely schematic representation:
[0042] FIG. 1: nitrogen defects (NV centers) in a diamond;
[0043] FIG. 2: an absorption and emission spectrum of the NV center;
[0044] FIG. 3: an optically detected magnetic resonance of a single NV center;
[0045] FIG. 4: the Zeeman effect within the energy diagram of the negatively charged NV center;
[0046] FIG. 5: pulsed excitation;
[0047] FIG. 6: the construction of a sensor unit according to the invention with respect to an object;
[0048] FIG. 7: the construction of the sensor unit of FIG. 6 in greater detail, and
[0049] FIG. 8: a method diagram of a method according to the invention.
[0050] FIG. 1 shows a crystal lattice on the left side, in this case a diamond, wherein the crystal lattice as a whole is designated with reference numeral 10. The crystal lattice 10 comprises a number of carbon atoms 12 and an NV center 14, which in turn comprises a nitrogen atom 16 and a defect or vacancy 18. The nitrogen defect 18 is aligned along one of the four possible binding directions in the diamond crystal.
[0051] On the right-hand side, the energy level scheme 30 of the negatively charged NV center 14 is shown. A ground state .sup.3A.sub.2 32 is a spin-triplet with a total spin s=1. The states 34 with magnetic spin quantum number m.sub.s=+?1 are energetically shifted from state 36 with m.sub.s=0. A state .sup.3E 38 and an intermediate state 40 are further shown. Bracket 42 illustrates a microwave frequency of 2.87 GHz, which corresponds to a splitting energy or zero field splitting D.sub.gs. The zero field splitting is an intrinsic variable that is independent of the radiated MW field or frequency. It is about 2.87 GHz, and in particular is temperature dependent. The following relationship applies to determining the resonance frequency:
v?D.sub.gs+?*?T?y.sub.NV*B.sub.0; [0052] wherein ?T is the deviation from room temperature, ? is the temperature shift of the zero-field splitting with ? at about ?74.2 kilohertz/kelvin, y.sub.NV is the gyromagnetic ratio of the NV center, and B.sub.0 is the field strength of an external magnetic field.
[0053] FIG. 2 shows in a graph 50 the absorption and emission spectrum of the NV center shown in FIG. 1. In graph 50, the wavelength [nm] is plotted at an abscissa 52 and the absorption coefficient [cm.sup.?1] is plotted at a first abscissa 54 and the fluorescence is plotted at a second abscissa 56. A first curve 60 shows the absorbent spectrum, a second curve 62 shows the emission spectrum. A first arrow 70 denotes NV.sup.0 ZPL, a second arrow 72 denotes NV absorption, a third arrow 74 denotes NV fluorescence. Furthermore, NV-ZPL 76 is registered at 637 nm.
[0054] FIG. 3 shows in a graph 100 the optically detectable magnetic resonance (ODMR) of a single NV center for different background magnetic fields. In the graph 100, the microwave frequency is plotted on an abscissa 102, the magnetic field B is plotted on a first ordinate 104, and the fluorescence is plotted on a second ordinate 106.
[0055] A first curve 110 shows the resonance for B=0, a second curve 112 shows the resonance at B=2.8 mT with the negative peaks ?.sub.1 114 and ?.sub.2 116, a third curve 120 shows the resonance for B=5.8 mT, and a fourth curve 122 shows the resonance for 8.3 mT.
[0056] FIG. 4 shows the Zeeman effect in the ground state 150 of the NV center. Furthermore, the excited state 152 and the intermediate state 154 are registered. A first arrow 160 shows a transition with a high probability or transition rate, a dashed arrow 162 shows a transition with low probability or transition rate. In a box 170, a transition 172 without a magnetic field and a transition 174 with a magnetic field are shown.
[0057] FIG. 5 shows the pulsed excitation based on its temporal progression plotted at a time axis 250. Here, the laser excitation is shown at the top 252 and the microwave excitation at the bottom 254. Note that the sequence of a laser pulse and a microwave pulse are repeated periodically. The laser pulse is used to initialize the electron spin of the NV defects (second portion of the pulse 260) and to read the electron spin after manipulation (first portion of the laser pulse 262). The microwave pulse 270 is used to manipulate the electron spin, as a function of the magnetic field, on which the measurement principle is based.
[0058] FIG. 6 shows a schematic illustration of a sensor unit 400 comprising a first sensor 401 and a second sensor 402. While the first sensor 401 is configured as a diamond-based NV magnetometer, the second sensor 402 is a gas vapor cell magnetometer or a SQUID magnetometer. There is a defined distance 405 between the two. The first sensor 401 may be brought in close proximity to an object 300 to be examined.
[0059] FIG. 7 shows in greater detail the sensor unit 400 comprising a first sensor 401 and a second sensor 402 of FIG. 6. Furthermore, in FIG. 7, the light source 403 can be seen, which serves to generate light, in other words, excitation light 407. Via an optical fiber 406, further preferably a fiber coupler 406a, the light is supplied to the first sensor (401), namely the diamond 404. The excitation light 407 passes through a lens 409, namely a dichroic mirror 410, which is arranged to allow the excitation light 407 to pass through unhindered.
[0060] Furthermore, FIG. 7 shows a source 411 for generating microwaves necessary for a corresponding split of the energy levels of the NV centers. The fluorescence 408 triggered by electron transitions is passed through the same optical fiber 406, however does not pass through the lens 409 but is redirected therefrom such that only fluorescence 408 meets the photodetector 412. In other words, the lens 409 ensures that the emitted fluorescence 408 can strike the photodetector 412 separately from the excitation light 407. The optical fiber 406 used for the first sensor 401 can be used to fasten the second sensor 402.
[0061] FIG. 8 shows a flowchart of the method 500 according to the invention. In a first step, the first sensor 401 is arranged in close proximity to an object 300 to be measured 501. A background magnetic field 502 is measured using the second sensor 402 and the background magnetic field is determined at the location of the first sensor 401, 503. In this way, the measurement signal of the first sensor 401 may be calibrated 504 by subtracting the background magnetic field at the location of the first sensor 401 from the measurement signal of the first sensor 401.
