Magnetic field gradient measurement method and device based on atomic coherent state control

Abstract

The invention relates to a magnetic field gradient measurement method and device based on atomic coherent state control, which are characterized in that an alkali metal atom hyperfine energy level structure is modulated through microwave pulse, a carrier signal is converted into a polarization orthogonal optical sideband through parametric frequency conversion, and a differential magnetic field of a diatomic air chamber is encoded in the frequency of the optical sideband; after a polarization beam splitter is adopted to separate a carrier wave from an optical sideband, an optical sideband beat frequency signal of the diatomic gas chamber is extracted; and through combination of microwave, light and atom three-resonance modulation, significant suppression of phase noise and common-mode noise is realized. Magnetic field gradient information is directly extracted through microwave excitation optical sideband interference, magnetometer calibration is not needed, and the method has the advantages of being high in robustness, ultrahigh in sensitivity, resistant to environmental interference, capable of being designed in an integrated mode and the like.

Description

Technical Field

[0001] This invention relates to the field of magnetic field gradient detection technology in quantum sensing and precision measurement, specifically to a magnetic field gradient measurement method and device based on atomic coherent state manipulation, applicable to fields such as biomagnetic sensing, geophysical exploration, non-destructive testing, and biomedical imaging. Background Technology

[0002] Magnetic field gradient measurement technology has significant applications in biomedical imaging, geological exploration, and national defense. Traditional magnetic gradiometers primarily rely on a dual magnetometer differential architecture, using two spatially separated sensors to independently measure magnetic field strength and subtract them to calculate the gradient. However, this method requires periodic calibration of the dual magnetometers to eliminate device differences and environmental drift, and common-mode noise (such as geomagnetic field fluctuations and electromagnetic interference) is difficult to suppress effectively in differential operations, resulting in a significantly limited signal-to-noise ratio. Superconducting quantum interference devices (SQUIDs) form a built-in gradiometer by connecting two sets of magnetic flux pickup coils with opposite polarities, but these magnetic gradiometers are bulky, power-consuming, and require ultra-low temperatures, making portable integration difficult.

[0003] Gradient measurement technology based on alkali metal atomic magnetometers has attracted much attention due to its ultra-high sensitivity at the femtosecond level. Magnetic field detection is achieved by laser-polarizing atomic spin states and utilizing the linear relationship between Larmor precession frequency and magnetic field. However, existing atomic magnetic gradiometers still use a dual-magnetometer architecture, requiring post-processing and subtraction of the outputs from two independent sensors, leading to system delay and error accumulation.

[0004] Existing technologies face bottlenecks in robustness and integrated design under high-noise environments, necessitating a calibration-free, common-mode noise-resistant, and chip-integrated magnetic field gradient measurement scheme to overcome the sensitivity limits and application scenario limitations of current technologies. This invention proposes a magnetic field gradient measurement mechanism based on microwave sideband interferometry. By exciting polarization-orthogonal optical sidebands through microwave modulation of atomic hyperfine levels, the magnetic field gradient difference between two detection points is encoded as a sideband frequency difference. Combined with pulse timing control and all-optical differential readout, efficient common-mode suppression is achieved outside the gradient meter bandwidth. Summary of the Invention

[0005] The technical problem solved by this invention is the limitation of traditional magnetic gradiometers by the dual magnetometer architecture and insufficient common-mode suppression. It proposes a magnetic field gradient measurement method and device based on atomic coherent state manipulation. By using microwave-excited optical sideband interference to manipulate atomic coherent states to measure magnetic field gradients, it is beneficial to achieve calibration-free, integrable, and highly sensitive gradient detection.

[0006] The technical solution of the present invention is as follows:

[0007] A method for measuring magnetic field gradients based on atomic coherent state manipulation, characterized by comprising the following steps:

[0008] Step 1: The microwave signal emitted by the microwave module is used to simultaneously act on the alkali metal atoms in the first atomic gas chamber and the second atomic gas chamber in the magnetic field gradient measuring device. The first atomic gas chamber and the second atomic gas chamber are located in the same non-magnetic heating oven with a spatial separation distance.

[0009] Step 2: The atoms in the first atomic gas cell are optically polarized by the first pump, and the atoms in the second atomic gas cell are optically polarized by the second pump. The ground-state atoms undergo population inversion through pump optical polarization and return to the ground state through spontaneous emission. Then, the polarized ground-state atoms are excited by microwave pulses to coherently superimpose the ground-state hyperfine level atoms.

[0010] Step 3: The probe light passing through the first and second atomic gas chambers interacts with atoms in a coherent superposition state to generate optical sidebands;

[0011] Step 4: Obtain the magnetic field gradient through optical sideband extraction and gradient inversion.

[0012] In step 1, the microwave module includes a microwave antenna. The microwave signal is a 6.8GHz π / 2 pulse signal. The spatial separation distance is adapted to the full wavelength spacing of microwaves at the 6.8GHz frequency.

[0013] In step 2, both the first pump light and the second pump light are 795nm circularly polarized pump lights.

[0014] In step 3, the probe light is a 780nm linearly polarized probe light.

[0015] Step 4 includes the following expression:

[0016] △f=△f1-△f2=△v BG +3γ|ΔB|,

[0017] Where Δf is the beat frequency signal extracted from the optical sideband, Δf1 is the frequency difference between the first sideband light generated by the first atomic gas cell and the probe light, Δf2 is the frequency difference between the second sideband light generated by the second atomic gas cell and the probe light, and Δv BG γ is the pressure frequency shift of the ground-state buffer gas, γ is the gyromagnetic ratio, and ΔB is the magnetic field gradient.

[0018] A magnetic field gradient measurement device based on atomic coherent state manipulation, used to perform the aforementioned magnetic field gradient measurement method based on atomic coherent state manipulation, is characterized by comprising a pump laser, wherein the pump laser is connected to the input side of a first polarization beam splitter via a first half-wave plate; the transmission side of the first polarization beam splitter is connected to the input side of a beam splitting prism via an acousto-optic modulator, an aperture, a first beam expander lens group, and a quarter-wave plate in sequence; the reflection side of the beam splitting prism is connected to a first atomic gas cell; and the transmission side of the beam splitting prism is connected to a second atomic gas cell via a first reflecting mirror. The first atomic gas chamber and the second atomic gas chamber are both located inside a non-magnetic heating oven. The non-magnetic heating oven is located inside a triaxial magnetic field coil. The triaxial magnetic field coil includes an x-axis Lee-Whiting shimming coil, a y-axis saddle-shaped shimming coil, and a z-axis saddle-shaped shimming coil. The triaxial magnetic field coil is located inside a magnetic shielding barrel. The microwave antenna is located between the magnetic shielding barrel and the triaxial magnetic field coil, and between the first pump light and the second pump light.

[0019] The device includes a detection laser, a second half-wave plate, a second polarizing beam splitter, a second beam expander lens group, a Glan Taylor prism, a first atomic gas cell, a second atomic gas cell, and a third polarizing beam splitter connected in sequence to a photodetector. The third polarizing beam splitter is connected to a Fabry-Perot interferometer in sequence through a third mirror and a fourth mirror. The second polarizing beam splitter is connected to a second wavelength meter in sequence through a fifth mirror and a second fiber coupler.

[0020] Both the first atomic gas chamber and the second atomic gas chamber are square gas chambers.

[0021] The technical advantages of this invention are as follows: This invention provides a magnetic field gradient measurement method and device based on atomic coherent state manipulation. Based on optical sideband interferometry and atomic coherent state manipulation technology, it achieves an innovative breakthrough in the field of magnetic field gradient detection. Through a dual-atom gas cell differential architecture, the system directly encodes the magnetic field gradient information as the frequency difference of optical sidebands, eliminating the complex calibration process required by traditional dual magnetometers, significantly simplifying operation and reducing error sources. Pulse timing control technology effectively suppresses relaxation noise and common-mode interference, enabling precise capture of extremely weak magnetic field gradient changes. The miniaturizable integration solution significantly reduces device size and power consumption, providing possibilities for mobile applications such as unmanned aerial vehicles and wearable devices.

[0022] Compared with existing technologies, this invention addresses several key differences. Traditional magnetic field gradient detection techniques generally rely on a dual-sensor differential architecture, requiring repeated calibration to eliminate device variations. This cumbersome operation is susceptible to environmental drift, and common-mode noise suppression is limited, making it difficult to meet high-precision requirements. While superconducting quantum interference devices (SQIs) offer high sensitivity, they are dependent on cryogenic environments and are bulky. Existing atomic magnetometer solutions partially address the sensitivity issue but still require post-processing of dual-sensor data, leading to system delays and error accumulation. In contrast, this invention directly encodes gradient information into optical signals via microwave sideband interference, achieving calibration-free measurement. This fundamentally eliminates human intervention errors, reduces common-mode interference, and enhances sensitivity while ensuring system robustness. This technological innovation significantly surpasses existing solutions in terms of accuracy, efficiency, and applicability, opening up broader application prospects for precision magnetic gradient detection. Attached Figure Description

[0023] Figure 1 This is a schematic diagram of the structure of a magnetic field gradient measurement device based on atomic coherent state manipulation, which implements the present invention.

[0024] Figure 2 This is a schematic diagram of optical sideband generation and frequency distribution.

[0025] Figure 3 This is a diagram of the energy levels of rubidium-87 atoms.

[0026] Figure 4 This is a schematic diagram of energy level transitions.

[0027] Figure 5 This is a schematic flowchart illustrating the implementation of a magnetic field gradient measurement method based on atomic coherent state manipulation according to the present invention. Figure 5 The process includes: Step 1, setting a microwave module in the magnetic field gradient measurement device so that the microwave signal emitted by the microwave module simultaneously acts on alkali metal atoms in the first and second atomic gas chambers, which are located in the same non-magnetic heating oven with a spatial separation distance; Step 2, using a first pump light to polarize the atoms in the first atomic gas chamber and a second pump light to polarize the atoms in the second atomic gas chamber. The ground-state atoms undergo population inversion through pump light polarization and return to the ground state through spontaneous emission. Then, the polarized ground-state atoms are excited by microwave pulses to coherently superimpose the hyperfine energy level atoms; Step 3, the probe light penetrating the first and second atomic gas chambers interacts with the atoms in the coherent superposition state to generate optical sidebands; Step 4, obtaining the magnetic field gradient through optical sideband extraction and gradient inversion.

[0028] The following are the annotations for the attached diagrams: 1-Pump laser, 2-Half-wave plate, 3-Polarizing beam splitter, 4-Acousto-optic modulator, 5-Aperture, 6-Beam expander lens group, 7-Quarter-wave plate, 8-Beam splitter prism, 9-Reflector, 10-Fiber optic coupler, 11-Wavemeter, 12-Detection laser, 13-Glan Taylor prism, 14-Magnetic shielding barrel, 15-Lee-Whiting shimming coil, 16-Saddle-shaped shimming coil, 17-Non-magnetic heating oven, 18-Square gas cell, 19-Microwave antenna, 20-Photodetector, 21-Fabry-Perot interferometer, F-Ground hyperfine level, mF-Zeman level, f1-First sideband frequency, f2-Second sideband frequency, f p - Probe light frequency, S- Ground state, S 1 / 2 - P-excited state with quantum number 1 / 2, P 3 / 2 - Excited state with quantum number 3 / 2, P 1 / 2 - The excited ground state with a quantum number of 1 / 2, δ c -Pump optical frequency detuning, δ w -Probe light frequency detuning, δ p -Sideband light frequency detuning, w c -Pump optical frequency, w p -Probe optical frequency, w w - Microwave frequency, |1>~|4>- First energy level to fourth energy level. Detailed Implementation

[0029] The following is in conjunction with the attached diagram ( Figures 1-5 The invention will be described in the following sections and examples.

[0030] Figure 1 This is a schematic diagram of the structure of a magnetic field gradient measurement device based on atomic coherent state manipulation, which implements the present invention. Figure 2 This is a schematic diagram of optical sideband generation and frequency distribution. Figure 3 This is a diagram of the energy levels of rubidium-87 atoms. Figure 4 This is a schematic diagram of energy level transitions. Figure 5 This is a schematic flowchart illustrating the implementation of a magnetic field gradient measurement method based on atomic coherent state manipulation according to the present invention. (Reference) Figures 1 to 5As shown, a magnetic field gradient measurement method based on atomic coherent state manipulation includes the following steps: Step 1, using microwave signals emitted by a microwave module to simultaneously act on alkali metal atoms in the first and second atomic gas chambers of a magnetic field gradient measurement device, wherein the first and second atomic gas chambers are located in the same non-magnetic heating oven with spatial separation; Step 2, using a first pump photopolarization to polarize the atoms in the first atomic gas chamber and a second pump photopolarization to polarize the atoms in the second atomic gas chamber, the ground-state atoms undergo population inversion through pump photopolarization and return to the ground state through spontaneous emission, and then the polarized ground-state atoms are excited by microwave pulses to coherently superimpose the hyperfine energy level atoms of the ground state; Step 3, the probe light penetrating the first and second atomic gas chambers interacts with the atoms in the coherent superposition state to generate optical sidebands; Step 4, the magnetic field gradient is obtained through optical sideband extraction and gradient inversion.

[0031] In step 1, the microwave module includes a microwave antenna. The microwave signal is a 6.8GHz π / 2 pulse signal. The spatial separation distance is adapted to the full wavelength spacing of microwaves at the 6.8GHz frequency.

[0032] In step 2, both the first and second pump beams are 795nm circularly polarized pump beams. In step 3, the probe beam is a 780nm linearly polarized probe beam.

[0033] Step 4 includes the following expression:

[0034] △f=△f1-△f2=△v BG +3γ|ΔB|,

[0035] Where Δf is the beat frequency signal extracted from the optical sideband, Δf1 is the frequency difference between the first sideband light generated by the first atomic gas cell and the probe light, Δf2 is the frequency difference between the second sideband light generated by the second atomic gas cell and the probe light, and Δv BG γ is the pressure frequency shift of the ground-state buffer gas, γ is the gyromagnetic ratio, and ΔB is the magnetic field gradient.

[0036] A magnetic field gradient measurement device based on atomic coherent state manipulation, used to perform the aforementioned magnetic field gradient measurement method based on atomic coherent state manipulation, includes a pump laser 1. The pump laser 1 is connected to the input side of a first polarization beamsplitter (i.e., polarization beamsplitter 3) via a first half-wave plate (i.e., half-wave plate 2). The transmission side of the first polarization beamsplitter is connected to the input side of a beam-splitting prism 8 via an acousto-optic modulator 4, an aperture 5, a first beam-expanding lens group (i.e., beam-expanding lens group 6), and a quarter-wave plate 7. The reflection side of the beam-splitting prism 8 is connected to a first atomic gas cell (i.e., square gas cell 18). The transmission side of the beam-splitting prism 8 is connected to a second atomic gas cell (i.e., square gas cell 18) via a first reflecting mirror (i.e., reflecting mirror 9). The reflecting side of the polarization beam splitter (i.e., polarization beam splitter 3) is connected to the first wavelength meter (i.e., wavelength meter 11) in sequence through the second reflecting mirror (i.e., reflecting mirror 9) and the first fiber coupler (i.e., fiber coupler 10). The first atomic gas cell and the second atomic gas cell are both located inside the non-magnetic heating oven 17. The non-magnetic heating oven 17 is located inside the triaxial magnetic field coil. The triaxial magnetic field coil includes an x-axis Lee-Whiting shimming coil 15, a y-axis saddle-shaped shimming coil (i.e., saddle-shaped shimming coil 16), and a z-axis saddle-shaped shimming coil (i.e., saddle-shaped shimming coil 16). The triaxial magnetic field coil is located inside the magnetic shielding barrel 14. The microwave antenna 19 is located between the magnetic shielding barrel 14 and the triaxial magnetic field coil, and between the first pump light and the second pump light.

[0037] The system comprises a detection laser 12, a second half-wave plate (i.e., half-wave plate 2), a second polarization beam splitter (i.e., polarization beam splitter 3), a second beam expander lens group (i.e., beam expander lens group 6), a Glan Taylor prism 13, a first atomic gas cell (i.e., square gas cell 18), a second atomic gas cell (i.e., square gas cell 18), and a third polarization beam splitter (i.e., polarization beam splitter 3), which are connected to a photodetector 20. The third polarization beam splitter is connected to a Fabry-Perot interferometer 21 via a third mirror (i.e., mirror 9) and a fourth mirror (i.e., mirror 9). The second polarization beam splitter is connected to a second wavelength meter (i.e., wavelength meter 11) via a fifth mirror (i.e., mirror 9) and a second fiber optic coupler (i.e., fiber optic coupler 10). Both the first and second atomic gas cells are square gas cells 18.

[0038] A method and apparatus for measuring magnetic field gradients based on atomic coherent state manipulation are disclosed. This method modulates the hyperfine level structure of alkali metal atoms using microwave pulses, converts the carrier signal into an optical sideband orthogonal to its polarization using parametric frequency conversion, and encodes the differential magnetic field of the diatomic gas cell into the optical sideband frequency. A polarization beamsplitter separates the carrier and optical sidebands, extracting the beat frequency signal of the optical sidebands from the diatomic gas cell. By combining microwave, light, and atomic triple resonance modulation, significant suppression of phase noise and common-mode noise is achieved. This invention utilizes microwave-excited optical sideband interference to directly extract magnetic field gradient information without requiring magnetometer calibration, and features high robustness, ultra-high sensitivity, resistance to environmental interference, and integrable design capabilities.

[0039] A method for measuring magnetic field gradients based on atomic coherent state manipulation includes the following steps:

[0040] Step 1, pumping photopolarized atoms, characterized in that the ground-state atoms of the two spatially separated alkali metal atom gas cells undergo population inversion through pumping photopolarization and return to the ground state through spontaneous emission;

[0041] Step 2, preparing a coherent superposition state of ground-state atoms, characterized in that the polarized ground-state atoms are excited by microwave pulses to coherently superimpose the hyperfine energy level atoms of the ground-state;

[0042] Step 3: The probe light interacts with coherent atoms to generate optical sidebands, characterized in that the probe light generates optical sidebands orthogonal to its polarization through parametric frequency conversion; the frequency of the probe light is f. p The optical sideband frequencies generated in the two spatially separated atomic gas cells are f1 and f2, respectively.

[0043] Step 4, optical sideband extraction and gradient inversion, characterized in that the optical sidebands with the same polarization but different frequencies generated by the dual air chambers are separated from the probe light by a polarization beam splitter; the two optical sidebands interfere at the photodetector and detect the beat frequency signal Δf = f1 - f2, and the beat frequency signal Δf can be directly inverted to obtain the magnetic field gradient ΔB.

[0044] The frequency of the 795nm circularly polarized pump light in step 1 is w. c The fine energy level D1 line of the alkali metal Rb-87 atomic ensemble is used to polarize the atoms, transferring most of them to the ground state |F=2, mF=2>; the microwave in step 2 is of frequency W. w A π / 2 pulse of 2π × 6.8 GHz is used to adjust the atomic population between the hyperfine levels of the ground state so that they are in a coherent superposition state; the frequency of the 780 nm linearly polarized probe light in step 3 is w. p =2πf p It is used to detect the fine energy level D2 line of the Rb-87 atomic ensemble and generate optical sidebands due to ground state coherence.

[0045] The energy level transition of the pump light is The energy level transition of the probe light is as follows: The energy level transition of the sideband light is Where: F represents the ground state hyperfine level, F′ represents the excited state hyperfine level, mF represents the Zeeman level, S represents the ground state, and P represents the excited state; the microwave, light, and atomic triple resonance phenomenon is transmitted through δ c =δ w =δ p =0 is achieved, where: δ c δ w δ p The frequency detuning is respectively for pump light, probe light, and sideband light.

[0046] The two spatially separated atomic chambers contain Rb-87 alkali metal atoms and a small amount of N2 buffer gas. The frequency difference between the sideband light and the probe light generated by the first atomic chamber is Δf1 = f1 - f p =Δv HFS +Δv BG +3γ|B1|, where: Δv HFS =6.8GHz is the ultrafine energy level The frequency difference between them, Δv BG =584Hz / Torr represents the pressure frequency shift of the ground-state buffer gas, γ = 2π × 6.99Hz / nT is the gyromagnetic ratio, and |B1| is the magnitude of the absolute magnetic field in the first atomic chamber; the magnetic field gradient can be obtained through the sideband light frequency difference Δf = Δf1 - Δf2 = Δv generated by the two chambers. BG +3γ|ΔB| inversion.

[0047] An apparatus for implementing the above method includes: a diatomic gas chamber unit, a pump light source, a probe light source, a microwave module, and a signal processing unit, wherein: the two rubidium-87 atomic gas chambers contain nitrogen gas of 30 Torr as a buffer gas, and the distance between the two gas chambers is 44 mm, which is the full wavelength distance of microwaves at a frequency of 6.8 GHz; the diatomic gas chambers are placed in a dual-chamber oven without magnetic heating, and the dual-chamber oven is placed in a triaxial magnetic field coil; the x-axis magnetic field coil is a Lee-Whiting shimming coil for generating a uniform magnetic field, and the y / z-axis magnetic field coils are two saddle-shaped shimming coils, the magnitude of the magnetic field gradient can be changed by adjusting the voltage of the z-axis coils at the two gas chambers; the triaxial magnetic field coils are placed in a permalloy magnetic shielding barrel.

[0048] The pump light source section includes, in sequence, a pump laser for generating the pump light source, a half-wave plate and a polarization beam splitter for polarization beam splitting, an acousto-optic modulator as a pump optical switch, an aperture, a beam expander, a quarter-wave plate for converting to circularly polarized light, and a beam splitter prism with a splitting ratio of 50:50 for generating two beams of identically polarized light. The pump light source generates a 795nm laser for rubidium atom polarization. The reflected light from the polarization beam splitter is coupled to a wavelength meter via optical fiber for monitoring the wavelength.

[0049] The probe light source section includes, in sequence, a detection laser for generating the probe light source, a half-wave plate and a polarizing beam splitter for polarization beam splitting, a beam expander lens, a Glan Taylor prism for generating linearly polarized light, a polarizing beam splitter for separating the probe light from the sideband light, a photodetector for detecting the signal, and a Fabry-Perot interferometer for testing the spectral distribution.

[0050] The microwave module is a microwave antenna that outputs a 6.8 GHz π / 2 pulse through a microwave signal source; the signal processing unit includes a lock-in amplifier for demodulating the magnetic field gradient component in the beat frequency signal and a DAQ data acquisition system.

[0051] A method for measuring magnetic field gradients based on atomic coherent state manipulation includes the following steps:

[0052] Step 1: Pump optically polarized atoms;

[0053] Step 2: Prepare coherent superposition states of ground-state atoms;

[0054] Step 3: The probe light interacts with coherent atoms to generate optical sidebands;

[0055] Step 4: Optical sideband extraction and gradient inversion.

[0056] Figure 1 This is a schematic diagram of the overall structure of a magnetic field gradient measurement method based on atomic coherent state manipulation according to the present invention, including: a diatomic gas chamber unit 18, a pump light source 1, a probe light source 12, a microwave module 19, and a signal processing unit. The diatomic square gas chamber 18 contains rubidium-87 atoms and 30 Torr nitrogen gas; the diatomic gas chamber 18 is placed in a dual-chamber oven 17 without magnetic electric heating, and the dual-chamber oven 17 is placed in a triaxial magnetic field coil; the x-axis magnetic field coil is a Lee-Whiting shimming coil 15 for generating a uniform magnetic field, and the y / z-axis magnetic field coils are two saddle-shaped shimming coils 14; the triaxial magnetic field coil is placed in a permalloy magnetic shielding barrel 14.

[0057] The pump source section of the testing device includes, in sequence, a pump laser 1, a half-wave plate 2, a polarizing beam splitter 3, an acousto-optic modulator 4, an aperture 5, a beam expander lens 6, a quarter-wave plate 7, a beam splitter prism 8, a reflector 9, an optical fiber coupler 10, and a wavelength meter 11. The beam splitter prism 8 splits two identically polarized beams with a 50:50 splitting ratio. The probe source section of the testing device includes, in sequence, a detection laser 12, a half-wave plate 2, a polarizing beam splitter 3, a beam expander lens 6, a GlanTeller prism 13, a photodetector 20, and a Fabry-Perot interferometer 21. The Fabry-Perot interferometer 21 uses equal-inclination piezoelectric interference to control the cavity length and scan the spectrum.

[0058] Figure 2 This diagram illustrates the experimental principle and frequency distribution at each stage of a magnetic field gradient measurement method based on atomic coherent state manipulation, as described in this invention. The two spatially separated atomic gas chambers are 44 mm apart, and the probe light frequency is f. p The light frequency of the probe light in the first atomic gas cell 18 is converted into the first sideband light frequency f1, and the light frequency of the probe light in the second atomic gas cell 18 is converted into the second sideband light frequency f2. The polarization beam splitter 3 ensures that only the sideband light with the same polarization is retained in the optical path. The optical sideband interference beat frequency signal generated by the two gas cells is realized at the photodetector 20 and used to invert the magnetic field gradient.

[0059] like Figure 3 and Figure 4 As shown, a schematic diagram of energy level transitions during the implementation of a magnetic field gradient measurement method based on atomic coherent state manipulation according to the present invention, and the energy level structure of rubidium-87 atoms are drawn. The rubidium-87 atom has an electron spin angular momentum S = 1 / 2, a ground-state s-shell electron orbital angular momentum L = 0, an excited-state p-shell electron orbital angular momentum L = 1, and a nuclear spin angular momentum I = 1 / 2. The ground-state energy level angular momentum is J = S + L = 1 / 2, and the excited-state energy level angular momentum is J = L + S = 3 / 2 or J = L S = 1 / 2. The ground-state hyperfine energy level angular momentum is F = I + J = 1 or 2. The Zeeman level is the atomic angular momentum projection along the quantization axis generated by the interaction between the atom and the external magnetic field, mF = {-1, 0 + 1} or mF = {-2, -1, 0 + 1, +2}. The distances between the ground-state hyperfine energy levels are... The frequency difference is Δv HFS =6.8GHz. For example... Figure 4 As shown, take Figure 3 Medium |F=1, mF=1>=|1>, |F=2, mF=2>=|2>, 5 2 P 1 / 2 |F′=2,mF=2>=|3>、5 2 P 3 / 2|F′=2,mF=2>=|4>,where: F represents the ground state hyperfine level, F′ represents the excited state hyperfine level, mF represents the Zeeman level, S represents the ground state, and P represents the excited state; the pump light frequency w c Leap indicates Energy level transition, probe light frequency w p Leap indicates Energy level transition, microwave frequency w w Leap indicates Energy level transition, the sideband light generation represents the interval The energy level transition frequency is f1, and the frequency difference between the sideband light and the probe light is Δf = f1 - f p =Δv HFS +Δv BG +3γ|B|, where: Δv HFS =6.8GHz is the ultrafine energy level The frequency difference between them, Δv BG =584Hz / Torr represents the pressure frequency shift of the ground-state buffer gas, γ = 2π × 6.99Hz / nT is the gyromagnetic ratio, and |B| is the magnitude of the absolute magnetic field within the gas chamber. The frequency difference includes the ground-state hyperfine level frequency difference Δv HFS Pressure frequency shift Δv of ground-state buffer gas BG And 3γ|B|, the three times Larmor precession frequency can be obtained through Figure 3 The Zeeman level structure is obtained by mF2 + mF1 = 3, and the nonlinear Zeeman effect in this process is ignored. The microwave, light, and atomic triple resonance phenomenon is obtained through δ c =δ w =δ p =0 is achieved, where: δ c δ w δ p The frequency detuning is respectively for pump light, probe light, and sideband light.

[0060] like Figure 5 As shown, the implementation steps of the magnetic field gradient measurement method based on atomic coherent state manipulation of the present invention include: actively applying a magnetic field gradient in a square gas cell with two spatially separated spaces through a triaxial coil; pumping optically polarized rubidium atoms to make most atoms be in the ground state |F=2, mF=2>; exciting the atoms to coherent population with microwave pulses; generating an optical sideband orthogonal to the polarization of the probe light by the interaction between the probe light and the coherent atoms; separating and extracting the sideband light through a polarization beam splitter and measuring the beat frequency signal; and calculating the inverted magnetic field gradient through the hyperfine energy level and the Larmor precession frequency.

[0061] Contents not described in detail in this specification are prior art known to those skilled in the art. It is hereby indicated that the above description is intended to help those skilled in the art understand this invention, but does not limit the scope of protection of this invention. Any equivalent substitutions, modifications, improvements, and / or simplifications of the above descriptions that do not depart from the essential content of this invention fall within the scope of protection of this invention.

Claims

1. A method for measuring magnetic field gradient based on atomic coherent state manipulation, characterized in that, The method comprises the following steps: Step 1: the microwave signal emitted by a microwave module acts on alkali atoms in a first atomic cell and a second atomic cell in a magnetic field gradient measuring device, and the first atomic cell and the second atomic cell are located in a same non-magnetic heating oven with a spatial separation interval; Step 2: a first pumping light polarizes atoms in the first atomic cell, and a second pumping light polarizes atoms in the second atomic cell, the ground state atoms are subjected to population inversion through polarization by the pumping light, and then return to the ground state through spontaneous emission, and then the polarized ground state atoms excite the coherent superposition of ground state hyperfine level atoms through a microwave pulse; Step 3: probe light penetrating through the first atomic cell and the second atomic cell interacts with the atoms in the coherent superposition state to generate optical sidebands; Step 4: the magnetic field gradient is obtained through inversion of the optical sidebands.

2. The method of magnetic field gradient measurement based on atomic coherent state manipulation according to claim 1, characterized in that, The microwave module in step 1 comprises a microwave antenna, the microwave signal is a 6.8 GHz π / 2 pulse signal, and the spatial separation interval is adapted to the full wavelength interval of the microwave at a frequency of 6.8 GHz.

3. The method of magnetic field gradient measurement based on atomic coherent state manipulation according to claim 1, characterized in that, The first pumping light and the second pumping light in step 2 are both 795 nm circularly polarized pumping light.

4. The method of magnetic field gradient measurement based on atomic coherent state manipulation according to claim 1, characterized in that, The probe light in step 3 is 780 nm linearly polarized probe light.

5. The method of magnetic field gradient measurement based on atomic coherent state manipulation according to claim 1, wherein, Step 4 comprises the following expression: Δf = Δf1 - Δf2 = Δv BG + 3γ|ΔB|, where Δf is the beat frequency signal extracted from the optical sideband, Δf1 is the frequency difference between the first sideband light generated by the first atomic cell and the probe light, Δf2 is the frequency difference between the second sideband light generated by the second atomic cell and the probe light, Δv BG is the pressure shift of the ground state buffer gas, γ is the gyromagnetic ratio, and ΔB is the magnetic field gradient.

6. An atomic coherent state manipulation based magnetic field gradient measuring apparatus for performing the atomic coherent state manipulation based magnetic field gradient measuring method according to any one of claims 1 to 5, characterized by The pumping laser is connected to the input side of a first polarization beam splitter through a first half-wave plate, the transmission side of the first polarization beam splitter is connected to the input side of a beam splitting prism through an acousto-optic modulator, an aperture, a first expansion lens group and a quarter-wave plate in sequence, the reflection side of the beam splitting prism is connected to the first atomic cell, the transmission side of the beam splitting prism is connected to the second atomic cell through a first mirror, the reflection side of the first polarization beam splitter is connected to a first wavemeter through a second mirror and a first fiber coupler in sequence, the first atomic cell and the second atomic cell are located in a non-magnetic heating oven, the non-magnetic heating oven is located in a three-axis magnetic field coil, the three-axis magnetic field coil comprises an x-axis Lee-Whiting shimming coil, a y-axis saddle shimming coil and a z-axis saddle shimming coil, the three-axis magnetic field coil is located in a magnetic shielding barrel, a microwave antenna is located between the magnetic shielding barrel and the three-axis magnetic field coil and between the first pumping light and the second pumping light.

7. The atomic coherence state manipulation based magnetic field gradient measurement device of claim 6, wherein, The detection laser is connected to the photodetector through a detection laser, a second half-wave plate, a second polarization beam splitter, a second expansion lens group, a Glan-Taylor prism, the first atomic cell, the second atomic cell and a third polarization beam splitter in sequence, the third polarization beam splitter is connected to a Fabry-Perot interferometer through a third mirror and a fourth mirror in sequence, and the second polarization beam splitter is connected to a second wavemeter through a fifth mirror and a second fiber coupler in sequence.

8. The atomic coherence state manipulation based magnetic field gradient measurement device of claim 6, wherein, The first atomic cell and the second atomic cell are both square cells.

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