A method and device for measuring microwave-free electromagnetic field based on NV color centers

Abstract

This invention discloses a microwave-free electromagnetic field measurement method and apparatus based on NV color centers, relating to the field of quantum precision measurement. The apparatus includes a magnetic field generating structure, a laser excitation structure, a color center selection structure, and a fluorescence detection structure. The magnetic field generating structure applies a magnetic field of known direction and magnitude to diamond; the laser excitation structure generates a laser and irradiates the diamond containing NV color centers; the color center selection structure uses a triaxial displacement stage to focus the objective lens at the micrometer level, selecting the diamond NV color center corresponding to the magnetic field magnitude and direction; the fluorescence detection structure collects the fluorescence emitted by the NV color centers and converts it into an electrical signal for measurement. This invention achieves target electromagnetic field detection by applying a magnetic field of magnitude 1024G along the NV axis within an angle of 0.1°, utilizing the sensitivity of the transverse component of the electromagnetic field under a strong magnetic field. This invention is a microwave-free electromagnetic field measurement method and apparatus based on NV color centers, applicable to the measurement of magnetic and electric fields in all-optical devices without microwave components.

Description

Technical Field

[0001] This invention belongs to the field of quantum precision measurement, specifically relating to a microwave-free electromagnetic field measurement method and device based on NV color centers. Background Technology

[0002] Electromagnetic field detection technology plays a crucial role in aerospace, biomedicine, geological exploration, and integrated circuit chip testing. Diamond nitrogen-vacancy centers (NV centers), due to their unique quantum spin properties, have become an important tool in quantum magnetic measurement. Sensors fabricated using this principle possess characteristics such as high spatial resolution and high sensitivity. When irradiated with a 532nm laser, they emit 637nm fluorescence, with the ground state spin shifting from the ms=±1 state to the ms=0 state. The ms=0 state emits more fluorescence photons, allowing population resolution through fluorescence intensity. Under the influence of microwaves of corresponding energy, resonant transitions between energy levels occur, a principle known as photodetector magnetic resonance.

[0003] Commonly used diamond NV centers at room temperature require the combined action of optical, magnetic, and microwave fields. While they can be used to measure physical quantities such as magnetic fields, electric fields, temperature, and stress, microwave magnetic fields are unsuitable for certain scenarios or specific samples. Furthermore, microwave magnetic fields introduce power and frequency noise, leading to measurement errors and reduced sensitivity. Microwave-free sensing techniques, based on the photoluminescence changes of NV centers under optical pumping conditions near ground-state anti-crossing (GSLAC), require an external strong magnetic field of 1024 G. Schemes using Helmholtz coils and electromagnets require a large current, generating significant heat and affecting the ambient temperature field. These devices are also bulky and power-consuming, and the inductance in the coils prevents timely generation and removal of the strong magnetic field during switching. Permanent magnets can achieve stable magnetic field application, but it is difficult to linearly change the magnitude of the magnetic field by altering the magnet's position. Summary of the Invention

[0004] To address the aforementioned issues, this invention discloses a microwave-free electromagnetic field measurement method and apparatus based on NV color centers. By adjusting the position of a permanent magnet to apply a constant strong magnetic field, and generating an adjustable magnetic field through a current-carrying conductor, the combination of a permanent magnet and a current-generated magnetic field eliminates the need for a complex Helmholtz coil and cooling. In the anti-crossing region of the ground state energy level, rapid changes in the bias magnetic field can be achieved in a short time. By controlling the on / off state of the current, a magnetic field pulse can be constructed, thereby solving the problem of miniaturization of the overall device and improving the sensitivity of electromagnetic field measurement based on NV color centers.

[0005] To achieve the above objectives, the technical solution of the present invention is as follows:

[0006] A microwave-free electromagnetic field measurement device based on NV color centers includes: a magnetic field generating structure, a laser excitation structure, a color center selection structure, and a fluorescence detection structure;

[0007] The magnetic field generating structure is used to generate a bias magnetic field, which includes a constant strong magnetic field and an adjustable small magnetic field; the direction of the constant strong magnetic field is parallel to the direction of the NV axis, and it is used to provide a bias magnetic field of 1024G for the diamond ensemble sample; the adjustable small magnetic field is used to provide an adjustable small magnetic field from -10G to 10G for the diamond ensemble sample.

[0008] Optionally, the magnetic field generating structure includes a permanent magnet, an adjustable current source, a current-carrying wire, an arbitrary signal generator, a Z-axis lifting platform, an angular displacement platform, and an XYZ three-axis displacement platform;

[0009] The permanent magnet is a rare earth neodymium boron magnet, which is fixed on a mechanical platform consisting of a Z-axis lifting platform, an angle displacement platform, and an XYZ three-axis displacement platform;

[0010] The Z-axis lifting platform is used to adjust the dynamic range of the magnetic field applied by the permanent magnet;

[0011] The angular displacement stage is used to align the permanent magnet's magnetic field and the direction of the NV axis;

[0012] The XYZ three-dimensional displacement stage is used to adjust the distance between the permanent magnet and the NV color center at the μm level. The displacement stage has a stroke of ±1.25cm, enabling more precise setting of the magnetic field size and alignment.

[0013] The energized conductor is connected to an adjustable current source, and the switching of the current controls the switching of the adjustable magnetic field. The magnitude and direction of the current correspond to the magnitude and direction of the magnetic field.

[0014] The arbitrary signal generator is used to provide a specified sequence of voltage pulses to lasers, photodetectors, and energized wires.

[0015] The laser generating structure includes a laser, a focusing lens, an acousto-optic modulator, a dichroic mirror, and a reflecting mirror, used to focus a 532nm green pump laser onto diamond.

[0016] The laser is used to provide a 532nm green pump laser;

[0017] The focusing lens is used to focus the laser emitted by the laser into the acousto-optic modulator;

[0018] The acousto-optic modulator is used to control and modulate the laser intensity;

[0019] The dichroic mirror is used to reflect the output 532nm green laser and collect the red fluorescence through the objective lens;

[0020] The reflector is used to reflect the laser onto the diamond NV system sample;

[0021] The color center selection structure includes a diamond NV ensemble sample, a triaxial displacement stage, and a high numerical aperture objective lens. By adjusting different positions of the objective lens, NV color centers of different concentrations and under different magnetic fields are selected for laser excitation and fluorescence collection.

[0022] The diamond NV ensemble sample is <100> Diamond has four axial NV color centers, each forming an angle of 54.7° with the normal vector of the diamond facet.

[0023] The high numerical aperture objective lens is used to focus the laser onto the NV color center and collect the red fluorescence emitted by the color center and return it to the fluorescence optical path;

[0024] The triaxial displacement stage is used to adjust the position of the objective lens and select color centers with different magnetic fields and concentrations.

[0025] The fluorescence detection structure includes a defocusing lens, a high-pass filter, and a photodetector, used to collect the red fluorescence emitted by the NV color center of diamond. The photodetector is placed at the focal point of the optical path and converts the collected fluorescence into an electrical signal for processing.

[0026] The defocusing lens is used to determine the focal point of the fluorescence light path;

[0027] The high-pass filter allows fluorescence in the 650nm-800nm ​​range to pass through while filtering out green light passing through the dichroic mirror.

[0028] This invention also provides a microwave-free electromagnetic field measurement method based on NV color centers, employing the microwave-free electromagnetic field measurement device based on NV color centers as described above, and including the following steps:

[0029] S1. Alignment of the permanent magnet along the NV color center axis under a weak magnetic field is achieved through the Z-axis lifting platform and the angle displacement platform.

[0030] S2. Determine the position of the permanent magnet on the XYZ displacement stage and apply a constant strong magnetic field of magnitude 1024G along the NV axis;

[0031] S3. By using an adjustable small magnetic field, the fluorescence intensity can be swept with respect to the magnetic field to determine the magnitude of the constant strong magnetic field and the magnitude of the longitudinal magnetic field being measured.

[0032] S4. Ground state spin polarization with a bias magnetic field magnitude far away from 1024G is achieved by switching the adjustable small magnetic field and the laser.

[0033] S5. Turn off the laser and the adjustable small magnetic field for a duration of t. The ground state spin spontaneously transitions between ms = 0 and ms = -1 at 1024 G in the form of Rabi oscillations. The Rabi oscillation frequency is positively correlated with the transverse component of the electromagnetic field; Ω∝γ e B ⊥ γ e For gyromagnetic ratio, B ⊥ Let Ω represent the magnitude of the transverse magnetic field to be measured, Ω be the Rabi frequency, and ∝ represent proportionality to Ω.

[0034] S6. By applying a laser pulse to read out the spin, the fluorescence distribution and the magnitude of the transverse magnetic field are calculated. The magnitude of the transverse magnetic field is expressed as the difference between the transverse magnetic fields calculated by the Rabi frequency when there is / is no measured magnetic field in the two measurements.

[0035] The above-described technical solution of the present invention has the following advantages:

[0036] This invention has a reasonable structure and ingenious design. By adjusting the position of the permanent magnet to apply a constant strong magnetic field, the conductor generates an adjustable magnetic field. The combination of the permanent magnet and the magnetic field generated by the current eliminates the need for a complex Helmholtz coil and cooling. It can also achieve rapid changes in the bias magnetic field in a short time in the anti-crossing region of the ground state energy level. By controlling the on and off of the current to construct magnetic field pulses, it solves the problem of miniaturization of the overall device and improves the sensitivity of electromagnetic field measurement of NV color centers. Attached Figure Description

[0037] Figure 1 This is a schematic diagram of the microwave-free electromagnetic field measurement device based on NV color centers in this invention.

[0038] List of identifiers in attached diagrams:

[0039] 1. Z-axis lifting stage; 2. Angular displacement stage; 3. XYZ three-axis displacement stage; 4. Permanent magnet; 5. Diamond NV ensemble sample; 6. Wire; 7. Objective lens; 8. Three-axis displacement stage; 9. Reflector; 10. Defocusing lens; 11. Dichroic mirror; 12. Long-pass filter; 13. Photodetector; 14. Acousto-optic modulator; 15. Focusing lens; 16. Pinhole filter; 17. Laser; 18. Adjustable current source; 19. Arbitrary waveform generator.

[0040] Figure 2 This is a schematic diagram of the microwave-free electromagnetic field measurement method based on NV color centers in this invention. Detailed Implementation

[0041] The present invention will be further illustrated below with reference to the accompanying drawings and specific embodiments. It should be understood that the following specific embodiments are for illustrative purposes only and are not intended to limit the scope of the invention.

[0042] Microwave-free sensing technology, based on the photoluminescence changes of NV color centers under optical pumping conditions near the ground-state anti-crossing (GSLAC), requires an external strong magnetic field of 1024 G. Without considering the hyperfine splitting of spin nuclear polarization, the ground-state spin exhibits a triplet state, with the energy level splitting between ms=0 and ms=±1 at D=2870 MHz. After applying the magnetic field, ms=±1 undergoes degeneracy due to Zeeman splitting, and the energy of the ms=±1 level changes proportionally with the magnetic field strength about 2870 MHz. The energy difference between the ms=0 and ms=±1 levels is Δ=D±γB, where γ is the gyromagnetic ratio. When B=1024 G, the energies between ms=-1 and ms=0 are the same; this region is called the ground-state anti-crossing region.

[0043] The microwave-free electromagnetic field measurement device and method based on NV color centers described in this invention utilizes the magnetic field frequency sweep of an adjustable magnetic field to measure the magnitude of the longitudinal component of the electromagnetic field under test, and utilizes the spontaneous resonant transition of the ground state energy level anti-cross region to construct a combined pulse of magnetic field and laser to measure the transverse component of the electromagnetic field under test, thus realizing the three-dimensional vector detection of the electromagnetic field.

[0044] As shown in the figure, the microwave-free electromagnetic field measurement device based on NV color centers of the present invention includes a magnetic field generating structure, a laser excitation structure, a color center selection structure, and a fluorescence detection structure. Specifically, wherein:

[0045] The magnetic field generating structure includes a permanent magnet 4, an adjustable current source 18, a wire 6, an arbitrary signal generator 19, a Z-axis lifting platform 1, an angle displacement platform 2, and an XYZ three-axis displacement platform 3. The permanent magnet 4 is fixed on a mechanical platform composed of the Z-axis lifting platform 1, the angle displacement platform 2, and the XYZ three-axis displacement platform 3. The Z-axis lifting platform 1 is used to adjust the dynamic range (vertical direction) of the magnetic field applied by the permanent magnet. The angle displacement platform 2 is used to adjust the angle between the permanent magnet and the diamond normal vector. The permanent magnet faces an angle of 54.7° with the diamond surface normal vector (because the position of the color center is not determined and the magnetic field direction is not necessarily uniform, the angle between the magnetic field and NV is not strictly aligned, requiring further operation). The XYZ three-dimensional displacement platform 3 is used for micrometer-level adjustment of the permanent magnet position to achieve the application of a constant strong magnetic field of magnitude 1024G within a 0.1° angle along the NV axis. The adjustable current source 18 is connected to the wire 6, and the current is adjusted to control the magnitude of the adjustable magnetic field. The arbitrary signal generator 19 provides a specified voltage sequence pulse to the adjustable current source 18 to control the switching of the adjustable magnetic field.

[0046] The laser excitation structure includes a laser 17, a pinhole filter 16, a focusing lens 15, an acousto-optic modulator 14, a dichroic mirror 11, a defocusing lens group 10, and a reflector 9. The laser 17 provides a 532nm green pump laser, which enters the acousto-optic modulator 14 after passing through the pinhole filter 16 and the focusing lens 15. The acousto-optic modulator 14 acts as an optical switch to control the laser intensity and output the laser beam. After two reflections by the dichroic mirror 11, the defocusing lens 10, and the reflector 9, the laser beam is output as a parallel laser beam.

[0047] The color center selection structure includes a diamond NV ensemble sample 5, a triaxial stage 8, and a high numerical aperture objective lens 7. The objective lens 7 is fixed on the triaxial stage 8. Adjusting the triaxial stage 8 changes the position of the objective lens 7 to focus the lens on different positions of the diamond NV ensemble sample 5.

[0048] The fluorescence detection structure includes a high-pass filter 12 and a photodetector 13. The red fluorescence collected by the objective lens 7 is reflected twice by the mirror 9, the defocusing lens 10 and the dichroic mirror 11 into the fluorescence optical path. The high-pass filter 12 allows fluorescence in the 650nm-800nm ​​range to pass through and filters out the green light passing through the dichroic mirror. The photodetector 13 is placed at the focal point of the silver light path and converts the collected fluorescence into an electrical signal for processing.

[0049] The arbitrary signal generator 19 provides a specified sequence of pulses to the laser 17, the photodetector 13 and the adjustable current source 18 to control the spin polarization, manipulation and readout.

[0050] A microwave-free electromagnetic field measurement method based on NV color centers, the measurement method includes:

[0051] (1) Fix the permanent magnet 4 with the known magnetic field direction on the mechanical platform, adjust the angle displacement stage 2 to 54.7°, adjust the Z-axis lifting stage 1 along the Z direction, gradually reduce the distance between the permanent magnet 4 and the diamond 5, and achieve alignment of the permanent magnet along the NV color center axis under the weak magnetic field.

[0052] (2) Change the Z-axis of the XYZ displacement stage 3. When the fluorescence intensity suddenly extinguishes, determine the position of the permanent magnet in the Z-axis direction. Change the X and Y axes of the XYZ displacement stage. The fluorescence intensity emitted by the diamond is the strongest. Determine the position of the permanent magnet in the X and Y axis directions. Apply a constant strong magnetic field of magnitude 1024G along the NV axis using the permanent magnet.

[0053] (3) Adjust the position of the permanent magnet 4 along the Z-axis to make the fluorescence suddenly extinguish. Change the direction and magnitude of the current in the current-carrying wire 6 to achieve a frequency sweep of the fluorescence intensity with respect to the magnitude of the magnetic field, and determine the magnitude of the constant strong magnetic field. When the magnetic field to be measured exists, determine the magnitude of the longitudinal magnetic field to be measured based on the magnitude of the current intensity applied when the fluorescence brightness is darkest.

[0054] (4) Use the arbitrary waveform generator 19 to control the adjustable current source 18 and the laser 17 to turn on, fix the current output of the adjustable current source 18 to remain unchanged and the laser output power to be stable, and make the bias magnetic field of the NV color center far away from 1024G in the conductive and energized state, wait for several microseconds, and under the action of the laser, the ground state spin polarization to ms=0 state.

[0055] (5) With the laser and tunable current source turned off for a duration of t, the spin state spontaneously transitions between ms = 0 and ms = -1 at 1024 G in the form of Rabi oscillations. The Rabi oscillation frequency is positively correlated with the transverse component of the electromagnetic field; Ω ∝ γ e B ⊥ γ e For gyromagnetic ratio, B ⊥ Let Ω represent the magnitude of the transverse magnetic field to be measured, Ω be the Rabi frequency, and ∝ represent proportionality to Ω.

[0056] (6) The arbitrary waveform generator 19 controls the photodetector 13 and the laser 17 to turn on, and reads out the spin state by applying a laser pulse, and calculates the spin fluorescence distribution degree and the magnitude of the transverse magnetic field. The magnitude of the transverse magnetic field to be measured is expressed as the difference between the transverse magnetic field calculated by the Rabi frequency when the measured magnetic field is present / absent in two measurements.

[0057] The magnitude and direction of the adjustable magnetic field generated by the energized conductor along the NV axis are known. The laser can be focused onto the color centers subjected to different magnetic fields by adjusting the three-axis displacement stage 8.

[0058] It should be noted that the above content merely illustrates the technical concept of the present invention and should not be construed as limiting the scope of protection of the present invention. For those skilled in the art, various improvements and modifications can be made without departing from the principle of the present invention, and all such improvements and modifications fall within the scope of protection of the claims of the present invention.

Claims

1. A microwave-free electromagnetic field measurement method based on NV color centers, characterized in that, The method includes: A microwave-free electromagnetic field measurement device based on NV color centers includes: a magnetic field generating structure, a laser generating structure, a color center selection structure, and a fluorescence detection structure; The magnetic field generating structure includes a permanent magnet, an adjustable current source, a current-carrying wire, an arbitrary signal generator, a Z-axis lifting platform, an angular displacement platform, and a three-axis displacement platform. The magnetic field generating structure is used to generate a bias magnetic field, which includes a constant strong magnetic field and an adjustable small magnetic field. The laser generating structure includes a laser, a focusing lens, an acousto-optic modulator, a dichroic mirror, and a reflecting mirror. The laser generating structure is used to focus a 532nm green pump laser onto a diamond. The color center selection structure includes a diamond NV ensemble sample, a triaxial displacement stage, and a high numerical aperture objective lens. The fluorescence detection structure includes a defocusing lens, a high-pass filter, and a photodetector; (1) Initially determine the direction of the magnetic field of the permanent magnet, and adjust the Z-axis lifting platform and the angle displacement platform to achieve alignment along a certain axis of the NV color center under the condition of weak magnetic field. (2) Adjust the Z-axis of the triaxial displacement stage to increase the magnetic field strength. When the fluorescence intensity suddenly extinguishes, determine the position of the permanent magnet in the Z-axis direction. Adjust the X and Y directions of the triaxial displacement stage to make the fluorescence intensity emitted by the diamond stimulated to be the strongest, and determine the position of the permanent magnet in the X and Y directions to determine the direction of the magnetic field. Apply a constant strong magnetic field of magnitude 1024G along the NV axis using the permanent magnet. (3) Apply an adjustable small magnetic field, change the magnitude and direction of the current in the conductor to achieve a frequency sweep of fluorescence intensity with respect to the magnetic field. When the fluorescence is darkest without current, adjust the constant strong magnetic field to 1024G. (4) Fix the magnitude of the small magnetic field so that the magnitude of the bias magnetic field on the NV color center is far away from 1024G. Turn on the laser pulse, and the electron transitions from the ground state to the excited state. Wait for a few microseconds, and the spin polarization of the ground state reaches ms = 0. (5) When the laser and the tunable magnetic field are turned off for a duration of t, the ground state spin spontaneously transitions from ms = 0 to ms = -1 at 1024G in the form of Rabi oscillations. The oscillation frequency is positively correlated with the transverse component of the electromagnetic field. Ω∝γ e B ⊥ γ e With a gyromagnetic ratio of 2.8 MHz / G, B ⊥ The value represents the magnitude of the transverse magnetic field to be measured, Ω is the Rabi frequency; ∝ represents proportional to; (6) The spin state is read out by loading a laser pulse, and the fluorescence distribution degree and the magnitude of the transverse magnetic field are further calculated; (7) The fluorescence suddenly extinguishes when the position of the permanent magnet is adjusted along the Z-axis. The direction and magnitude of the current in the conductor are changed to achieve a frequency sweep of the fluorescence intensity with respect to the magnitude of the magnetic field. When the magnetic field to be measured exists, the magnitude of the longitudinal magnetic field component is calculated based on the magnitude of the current intensity applied when the fluorescence brightness is darkest.

2. The microwave-free electromagnetic field measurement method based on NV color centers according to claim 1, characterized in that: The magnetic field has a slight deviation along the NV axis, and there is a transverse component other than the measured magnetic field. The magnitude of the measured transverse magnetic field is expressed as the difference between the transverse magnetic fields calculated by the Rabi frequency when the measured magnetic field is present or absent in the two measurements.

3. The microwave-free electromagnetic field measurement method based on NV color centers according to claim 1, characterized in that, The constant strong magnetic field is parallel to the NV axis and is used to provide a 1024G bias magnetic field for the diamond ensemble sample; the adjustable small magnetic field is used to provide an adjustable small magnetic field from -10G to 10G for the diamond ensemble sample. The color center selection structure selects NV color centers of different concentrations and under different magnetic fields for laser excitation and fluorescence collection by adjusting the different positions of the objective lens; the fluorescence detection structure is used to collect the red fluorescence emitted by the diamond NV color centers, and the photodetector is placed at the focal point of the optical path to convert the collected fluorescence into an electrical signal for processing.

4. The microwave-free electromagnetic field measurement method based on NV color centers according to claim 1, characterized in that: The permanent magnet is a rare-earth neodymium boron magnet, fixed on a mechanical platform consisting of a Z-axis lifting platform, an angular displacement platform, and a three-axis displacement platform. The Z-axis lifting platform is used to adjust the dynamic range of the magnetic field applied by the permanent magnet. The angular displacement platform is used to align the magnetic field of the permanent magnet with the direction of the NV axis. The three-axis displacement platform is used to adjust the distance between the permanent magnet and the NV color center at the μm level, with a displacement travel of ±1.25 cm, achieving more precise magnetic field magnitude setting and direction alignment. The conductor is connected to an adjustable current source, and the on / off state of the current controls the switching of the adjustable magnetic field. The magnitude and direction of the current correspond to the magnitude and direction of the magnetic field. The arbitrary signal generator is used to provide a specified voltage sequence pulse to the laser, photodetector, and conductor.

5. The microwave-free electromagnetic field measurement method based on NV color centers according to claim 1, characterized in that: The laser is used to provide a 532nm green pump laser; the focusing lens is used to focus the laser emitted by the laser into the acousto-optic modulator; the acousto-optic modulator is used to control and modulate the laser intensity; the dichroic mirror is used to reflect the output 532nm green laser and collect the red fluorescence through the objective lens; the reflecting mirror is used to reflect the laser onto the diamond NV system sample.

6. The microwave-free electromagnetic field measurement method based on NV color centers according to claim 1, characterized in that: The diamond NV ensemble sample is <100> The diamond has four axial NV color centers, each forming a 54.7° angle with the normal vector of the diamond facet. The high numerical aperture objective lens is used to focus the laser onto the NV color centers and collect the red fluorescence emitted by the color centers and return it to the fluorescence optical path. The triaxial displacement stage is used to adjust the position of the objective lens and select color centers with different magnetic fields and concentrations.

7. The microwave-free electromagnetic field measurement method based on NV color centers according to claim 1, characterized in that: The defocusing lens is used to determine the focal point of the fluorescence optical path; the high-pass filter allows fluorescence in the 650nm-800nm ​​range to pass through while filtering out green light passing through the dichroic mirror; the photodetector is placed at the focal point of the fluorescence optical path and converts the optical signal into an electrical signal for processing.

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