Magnetic field sensor

Abstract

To provide a magnetic field sensor in which excitation of a quantum spin is spatially uniform using no microwave.SOLUTION: A magnetic field sensor includes a substrate having a spin-operable lattice defect and capable of generating a surface acoustic wave, and a plurality of comb-shaped electrode groups disposed on the substrate so as to face each other. In this magnetic field sensor, a voltage is applied to the comb-shaped electrode groups to generate a plurality of surface acoustic waves while shifting a phase and a direction, thereby selectively exciting a spin state and specify an excited energy level.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The technology disclosed in this specification relates to a magnetic field sensor.

Background Art

[0002] Non-Patent Document 1 discloses a technique related to quantum spin magnetic field measurement performed from ODMR (Optical Detection Magnetic Resonance) by applying microwaves to quantum spins.

Prior Art Documents

Non-Patent Documents

[0003]

Non-Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] When applying microwaves to quantum spins, an antenna is used to apply microwaves to the quantum spins to excite the quantum spins. In this case, if the microwaves are not applied uniformly to the element, the excitation of the quantum spins becomes spatially non-uniform, making it difficult to confirm the magnetic field distribution by ODMR. Therefore, in magnetic field measurement with the application of microwaves, it is necessary to apply microwaves uniformly to the element. However, it is difficult to create an antenna that can apply microwaves uniformly to the element. Therefore, there is a need for a magnetic field sensor that can excite quantum spins without using microwaves. This specification aims to provide a magnetic field sensor in which the excitation of quantum spins is spatially uniform without using microwaves.

Means for Solving the Problems

[0005] The magnetic field sensor disclosed in this specification has lattice defects capable of spin manipulation and a substrate capable of generating surface acoustic waves, and has a plurality of comb-shaped electrode groups arranged opposite to each other on the substrate. In this magnetic field sensor, a voltage is applied to the comb-shaped electrode group to selectively excite the spin state by generating a plurality of surface acoustic waves while shifting the phase and direction, and the excited energy level is specified.

[0006] The above magnetic field sensor generates surface acoustic waves using a comb-shaped electrode group, excites the spin state, and specifies the excited energy level. Since the energy of the surface acoustic wave decreases due to the motion of the quantum spin, the energy spectrum of the quantum spin state can be estimated by measuring the energy decrease, and magnetic field measurement can be performed. That is, the above magnetic field sensor can perform magnetic field measurement without using microwaves.

Brief Description of the Drawings

[0007]

Figure 1

Figure 2

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Figure 6

Modes for Carrying Out the Invention

[0008] Referring to FIGS. 1 and 2, the magnetic field sensor 10 will be described. As shown in FIG. 1, the magnetic field sensor 10 has a substrate 8 and four comb-shaped electrode groups 4 (4a, 4b, 4c, 4d) disposed on the substrate 8. The comb-shaped electrode groups 4 are formed of a metal mainly composed of Au. The comb-shaped electrode groups 4a and 4c are disposed opposite to each other in the vertical direction (y direction), and the comb-shaped electrode groups 4b and 4d are disposed opposite to each other in the horizontal direction (x direction). That is, the comb-shaped electrode groups 4a and 4c are disposed opposite to each other in a direction (y direction) orthogonal to the direction (x direction) in which the comb-shaped electrode groups 4b and 4d are disposed opposite to each other. A gap of approximately 100 μm is provided between the comb-shaped electrode groups 4a and 4c. Similarly, a gap of approximately 100 μm is provided between the comb-shaped electrode groups 4b and 4d. Note that, as the material of the comb-shaped electrode groups 4, a metal mainly composed of Al or Ti can be used instead of the metal mainly composed of Au. Further, the comb-shaped electrode groups 4 can be manufactured using a lithography technique after forming an electrode material on the substrate 8.

[0009] Each comb-shaped electrode group 4 is connected to a vector network analyzer 2 via wiring 6 (6a, 6b, 6c, 6d). Specifically, each comb-shaped electrode group 4 includes a positive electrode connected to the vector network analyzer 2 via the wiring 6 and a grounded negative electrode. The positive and negative electrodes are comb-shaped, and the negative electrode penetrates between the positive electrodes, and the positive electrode penetrates between the negative electrodes. Note that a gap of approximately 5 μm is provided between the positive and negative electrodes. The vector network analyzer 2 can apply a voltage to each comb-shaped electrode group 4 individually (selectively). For example, the vector network analyzer 2 can apply a voltage to the comb-shaped electrode groups 4a and 4b with a phase shift.

[0010] As shown in FIG. 2, the substrate 8 has a first part 14 mainly composed of diamond NV centers and a second part 12 mainly composed of ZnO. ZnO is an example of a substance having a piezoelectric effect. The second part 12 is provided on the first part 14, and a comb-shaped electrode group 4 is arranged on the second part 12. The diamond NV center is a lattice defect in which a C atom in the diamond is replaced by an N atom, creating a vacancy next to it. Therefore, the diamond NV center is a system that can maintain a quantum state even at room temperature and atmospheric pressure, and is a quantum spin material.

[0011] Instead of the diamond NV center, the first part 14 can also be mainly composed of SiC, GaN, etc. Also, instead of ZnO, the second part 12 can be mainly composed of AlN, etc. Note that the same material can also be used for the first part 14 and the second part 12. Specifically, as the materials for the first part 14 and the second part 12, materials mainly composed of SiC, GaN, hBN, ZnO, AlN, etc. can also be used. In this case, the substrate 8 can be regarded as being formed of a single layer (one kind of material). In any of the above cases, it can be said that the substrate 8 is formed of a quantum spin material.

[0012] In the magnetic field sensor 10, a voltage is applied to the comb-shaped electrode group 4 to generate a surface acoustic wave, and the magnetic field is measured by measuring the decrease in the surface acoustic wave associated with the movement of the quantum spin of the substrate 8 with a vector network analyzer 2. As described above, a gap of approximately 100 μm is provided between the comb-shaped electrode groups 4a and 4c, and between the comb-shaped electrode groups 4b and 4d. Therefore, a relatively uniform surface acoustic wave is generated on the substrate 8.

[0013] When performing magnetic field measurement using the magnetic field sensor 10, for example, surface acoustic waves are applied to the quantum spin system with a phase shift of ±π / 2 from the x direction and the y direction (see FIG. 1). The surface acoustic wave deforms the lattice of the substrate 8, and the magnetic anisotropy also shifts from the z axis due to the surface acoustic wave. As a result, the Hamiltonian becomes D(n·S) 2It can be described by , and the magnetic anisotropy n precesses around the z-axis. Note that the right-handed and left-handed precessions correspond to a phase difference of ±π / 2. The Hamiltonian of the time-dependent magnetic anisotropy is similar to that of a spin system to which a rotating magnetic field is applied. Therefore, if a surface acoustic wave with a frequency corresponding to spin splitting is applied, Rabi oscillations (repeated transitions between two energy states) can be observed.

[0014] Figure 3 shows the change in the surface acoustic wave 22 when a voltage is applied to the comb-shaped electrode group 4. As shown in Figure 3, when a voltage is applied to the comb-shaped electrode group 4, the surface acoustic wave 22 is generated. At the resonance frequency, since the quantum spin 20 moves violently, the Gilbert damping increases and the magnitude of the surface acoustic wave 22 decreases like the surface acoustic wave 24. The magnitude of the surface acoustic wave 24 can be estimated by the vector network analyzer 2 (see also Figure 1). By measuring the decrease from the surface acoustic wave 22 to the surface acoustic wave 24 with the vector network analyzer 2, the energy spectrum of the quantum spin state can be estimated.

[0015] Note that as shown in Figure 4, since the circularly polarized surface acoustic wave has a finite angular momentum, it can selectively excite |1〉 and |-1〉 (corresponding to the phase difference of ±π / 2 of the surface acoustic wave). For example, the right-handed surface acoustic wave 30 excites |-1〉, and the left-handed surface acoustic wave 32 excites |1〉. Therefore, the magnetic field sensor 10 can distinguish the spin state without applying a known external magnetic field and functions as a magnetic field sensor without applying a known external magnetic field. From the above, the magnetic field sensor 10 enables magnetic field measurement by a quantum spin without using optical measurement by the circularly polarized surface acoustic wave and the vector network analyzer 2.

[0016] The following shows the analytical calculation when a surface acoustic wave is applied while shifting the phase in the vertical direction. Hereinafter, the case where a surface acoustic wave with a wave number q and a frequency ω is applied in the x - direction and the y - direction will be described. In this case, the elastic body exists at z < 0, and z > 0 is a vacuum. In this case, the displacement of the elastic body is expressed as in the following Equation 1. In Equation 1 below, A1 and A2 represent the amplitudes of the surface acoustic waves in the x - direction and the y - direction, respectively, and φ represents the phase shift between the surface acoustic waves in the x - direction and the y - direction.

Number

[0017] Also, the reciprocals of the penetration lengths of the longitudinal wave and the transverse wave are expressed as in the following Equation 2. In Equation 2 below, υl and υt represent the velocities of the bulk longitudinal wave and transverse wave elastic waves.

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[0018] When the above - mentioned surface acoustic wave is applied to an element having a quantum spin, the Hamiltonian of the quantum spin is represented by the following Equation 3.

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[0019] The magnetic anisotropy axis n originally points in the z - direction, but when an elastic wave is applied, it is displaced as in the following Equation 4.

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[0020] Specifically, φ is represented by the following Equation 5.

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[0021] Hereinafter, assuming A1 = A2 and appropriately assuming z and φ, let φx, φy, and φz be as in the following Equation 6, respectively.

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[0022] When D = 1, the Hamiltonian of spin 1 is shown by the following Equation 7.

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[0023] Since the Hamiltonian of the above Equation 7 has time-dependence, consider the unitary transformation shown in the following Equation 8 to eliminate the time-dependence.

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[0024] Using the above Equation 8, the Hamiltonian and the wave function are respectively the following Equation 9.

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[0025] By considering the unitary transformation, the time-dependence of the Schrödinger equation disappears, and the following Equation 10 is obtained.

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[0026] As described above, the Hamiltonian after the transformation is shown by the following Equation 11.

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[0027] The energy eigenvalue can be obtained by solving the following Equation 12 for ε.

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[0028] Solving the above Equation 12 with εi = λi + 2 / 3 gives the following Equation 13.

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[0029] The eigenstate in the rotating frame is shown by the following Equation 14, and the Schrödinger equation of the quantum spin by the circularly polarized surface acoustic wave can be solved.

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[0030] Next, consider the state transition when |0〉 is selected as the initial condition in the laboratory frame. The state |0t〉 after time t in the laboratory frame is shown by the following Equation 15.

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[0031] The probability that the state of the above Equation 15 is |1〉 after time t is shown by the following Equation 16.

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[0032] The above Equation 16 causes Rabi oscillation when ω = D. Also, when ω = D, |〈−1|0t〉| 2 is zero. In the case of reverse polarization (near ω = -D), |〈−1|0t〉| 2 causes Rabi oscillation, and |〈1|0t〉| 2 is zero.

[0033] Here, referring to FIGS. 5 and 6, a prior art magnetic field sensor will be described. In the prior art, a diamond NV center is used as the substrate 8. In the prior art magnetic field sensor, with a known external magnetic field 32 applied to the substrate 8, quantum spins are excited by microwaves 30. Also, in the prior art magnetic field sensor, a laser beam 34 is applied to the substrate 8, and the magnitude of the magnetic field is measured based on the presence or absence of luminescence (fluorescent emission) 36. A quantum spin system such as a diamond NV center has spin 1, and the Hamiltonian of the hard-axis magnetic anisotropy is given by D(S z ) 2 . D is positive, and the spin state |0〉 has an energy eigenvalue of 0, and the states |±1〉 have energy eigenvalues of D and are degenerate.

[0034] The conventional magnetic field sensor operates on the principle of ODMR, and linearly polarized microwaves are applied for magnetic resonance. In the conventional magnetic field sensor, for example, when there is no external magnetic field, since the spin states |±1〉 are degenerate, the microwaves will excite both states. Therefore, the strength of the magnetic field can be detected using ODMR, but the direction of the magnetic field is unknown. Thus, in the conventional magnetic field sensor, it is necessary to apply a known external magnetic field to lift the degeneracy of |±1〉.

[0035] As shown in FIG. 6, when the NV center is not excited by microwaves, in the diamond NV center 3 A2 is the ground state and is described by the Hamiltonian of spins with hard-axis magnetic anisotropy. When a visible light laser (532 nm) is applied to the ground state |0〉, the state is excited as shown by the arrow 40. Then, after excitation, the state 3 E to the ground state of 3When relaxing to A2, it emits red light 36. Also, when the frequency of the microwave matches the energy of the excited state |±1〉, when a laser is applied to the diamond NV center, upon relaxation from the excited state, it returns to the ground state through a non-emissive process as shown by arrow 42. Since ODMR can observe the emission / non-emission process (|0〉 and |±1〉), the energy levels of |±1〉 Zeeman split by the magnetic field can also be estimated from the microwave frequency. As described above, since |1〉 and |-1〉 are excited without distinction by microwaves, the magnitude of the magnetic field can be measured by ODMR, but the direction cannot be measured. Therefore, in order to distinguish between |1〉 and |-1〉, it is necessary to apply a known magnetic field in advance to lift the degeneracy.

[0036] As described above, in the prior art, it is necessary to apply a known external magnetic field to the quantum spin in advance to lift the spin degeneracy. Therefore, in the prior art, it is necessary to incorporate a magnet, a coil, etc. into the device. As a result, in the prior art, the size of the magnetic field sensor becomes large and the cost also increases. As described above, the magnetic field sensor 10 can selectively excite the degenerate quantum state (distinguish the spin state) by using circularly polarized surface acoustic waves without applying a known external magnetic field. Therefore, in the magnetic field sensor 10, it is not necessary to incorporate a magnet, a coil, etc. into the device, and miniaturization and cost reduction of the sensor can be achieved. Also, since the magnetic field sensor 10 does not need to incorporate a magnet, a coil, etc. into the device, problems such as the magnetic field interfering with the measurement target can also be prevented.

[0037] Note that the magnetic field sensor 10 may measure the magnitude of the magnetic field by ODMR, but the magnitude of the magnetic field can be measured by measuring the decrease in surface acoustic waves with a vector network analyzer. That is, the magnetic field sensor 10 also has the advantage that ODMR can be omitted compared to the prior art.

[0038] In the prior art, an optical system facility for irradiating laser light is required to observe the excitation of quantum spins. Since the magnetic field sensor 10 can also omit the optical system facility, it also has the advantage of being easy to handle (user-friendly) as a sensor.

[0039] Hereinafter, the features of the technology disclosed in this specification will be sorted out. Note that the technical elements described below are each independent technical elements, which exhibit technical usefulness alone or in various combinations, and are not limited to the combinations described in the claims at the time of filing.

[0040] (Feature 1) A substrate having lattice defects capable of spin operation and capable of generating surface acoustic waves, A plurality of comb-shaped electrode groups arranged opposite to each other on the substrate, and By applying a voltage to the comb-shaped electrode group and generating a plurality of surface acoustic waves while shifting the phase and direction, selectively exciting the spin state, A magnetic field sensor for specifying the excited energy level.

[0041] (Feature 2) The magnetic field sensor according to Feature 1, wherein the comb-shaped electrode groups are arranged in orthogonal directions.

[0042] (Feature 3) The magnetic field sensor according to Feature 1 or 2, wherein the substrate has a first part mainly composed of diamond NV centers and a second part provided on the first part and mainly composed of a substance having a piezoelectric effect.

[0043] (Feature 4) The magnetic field sensor according to claim 1 or 2, wherein the substrate is a material mainly composed of SiC, GaN, h-BN, ZnO or AlN.

[0044] As described above, specific examples of the present invention have been described in detail, but these are merely examples and do not limit the scope of the claims. The technology described in the claims includes various modifications and changes to the specific examples exemplified above. In addition, the technical elements described in this specification or the drawings exhibit technical utility alone or in various combinations, and are not limited to the combinations described in the claims at the time of filing. Further, the technology exemplified in this specification or the drawings can achieve a plurality of purposes simultaneously, and achieving one of those purposes itself has technical utility.

Explanation of Reference Numerals

[0045] 4: Comb-shaped electrode group 8: Substrate 10: Magnetic field sensor

Claims

1. A substrate having lattice defects capable of spin operation and capable of generating surface acoustic waves, and a plurality of comb-shaped electrode groups arranged opposite to each other on the substrate, wherein a voltage is applied to the comb-shaped electrode groups to generate surface acoustic waves from a plurality of directions, and the phases of the surface acoustic waves generated in different directions are shifted to selectively excite the spin state, and a magnetic field sensor for specifying the excited energy level.

2. The magnetic field sensor according to claim 1, wherein the comb-shaped electrode groups are arranged in orthogonal directions.

3. The magnetic field sensor according to claim 1 or 2, wherein the substrate has a first part mainly composed of a diamond NV center and a second part provided on the first part and mainly composed of a material having a piezoelectric effect.

4. The magnetic field sensor according to claim 1 or 2, wherein the substrate is a material mainly composed of SiC, GaN, h-BN, ZnO or AlN.

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