A three-axis vector near zero and extremely weak magnetic field absolute measurement device and method

Abstract

A kind of three-axis vector near zero extremely weak magnetic field absolute measuring device and method, adopt helium-3 nuclear spin precession as magnetic field scale, by along z axis and x axis respectively ±B0, ±2B0 and ±B1, ±2B1 Eight state bias sequence is applied, in combination with SERF (Spin-Exchange-Relaxation-Free, no spin exchange relaxation) atomic magnetometer linear reading to the projection of helium-3 atom macroscopic magnetic moment in y direction, the absolute measurement of residual vector magnetic field Bx, By, Bz three axes is realized, and the value is transferred to the SERF magnetometer to be calibrated, to determine its scale factor and zero offset.The present application does not need to know coil constant in advance, measurement precision is high, suitable for extremely weak magnetic field measurement and atomic magnetometer calibration.

Description

Technical Field

[0001] This invention relates to the field of quantum sensing and precision measurement technology, specifically to a three-axis vector near-zero extremely weak magnetic field absolute measurement device and method. Background Technology

[0002] Absolute measurements of extremely weak magnetic fields are of great significance in fundamental physics research, geophysical exploration, biomagnetic imaging, inertial navigation, and quantum metrology standards. In magnetic field measurements on the sub-nanotescalar (sub-nT) to femtotescalar (fT) scale, spin-exchange-relaxation-free (SERF) atomic magnetometers have become the mainstream technique due to their ultra-high sensitivity. However, SERF atomic magnetometers are essentially relative measuring instruments; the proportional relationship (calibration coefficient) and zero-point bias (zero bias) between their output signal and the magnetic field can drift with operating conditions such as temperature, light intensity, and atomic density. Therefore, in-situ calibration using an independent magnetic field standard is necessary to achieve traceability and absolute measurement. The helium-3 (³He) nuclear spin has a relatively long transverse relaxation time T² and a definite gyromagnetic ratio γ. He The precession frequency of helium-3 nuclei satisfies the Larmor relation with the magnetic field strength, thus serving as a magnetic field scale associated with fundamental physical constants. However, in near-zero field environments below sub-nanotesla, the precession frequency of helium-3 nuclei spins is only on the order of millihertz, making it difficult to achieve high-resolution triaxial vector magnetic field reconstruction directly through frequency measurement. Furthermore, SERF atomic magnetometers must operate under near-zero field conditions to maintain linear sensitivity, while applying a large bias field along their sensitive axis disrupts the SERF's operating mechanism. How to achieve high-precision triaxial vector magnetic field absolute measurement using helium-3 nuclei spin precession while maintaining near-zero operating conditions for the SERF is a pressing technical challenge in the field of extremely weak magnetic field metrology. However, current technology lacks a device and method that can organically combine a helium-3 nuclei spin precession scale with the high-sensitivity readout of the SERF, enabling self-calibrated absolute measurement of the triaxial vector magnetic field without prior calibration of coil constants. Summary of the Invention

[0003] The purpose of this invention is to overcome the aforementioned shortcomings of the prior art and provide a three-axis vector near-zero extremely weak magnetic field absolute measurement device and method. This invention separates the magnetic field scale function of the helium-3 nucleus spin precession from the high-sensitivity reading function of the SERF atomic magnetometer. While maintaining the near-zero field condition of the SERF linear sensitive axis (along the y-axis magnetic field direction), an eight-state controllable bias magnetic field sequence is applied along the z-axis and x-axis respectively. The Bx and Bz components and the applied bias magnetic field value are decoupled from the helium-3 nucleus spin precession frequency through self-calibration. Then, the By component is determined from the decoupled Bx and Bz components, the applied bias magnetic field value, and the read helium-3 nucleus spin precession frequency, thereby achieving absolute measurement of the three-axis vector magnetic field and transmitting the value to an external SERF magnetometer to be calibrated.

[0004] The technical solution of the present invention is as follows:

[0005] A three-axis vector near-zero extremely weak magnetic field absolute measurement device is characterized by comprising two gas chambers, one being a helium-3 atom gas chamber (1) and the other being an alkali metal atom gas chamber (12) in a SERF atomic magnetometer. Both gas chambers are located within the same three-axis magnetic field coil (23), which is located within a permalloy magnetic shield (24). The negative z-axis side of the helium-3 atom gas chamber is connected to a helium-3 atom pump laser via a beam expander (4), a helium atom beam reflector (5), and a helium atom beam circular polarization generator (6). 7) The outer wall of the helium-3 atom gas chamber is attached with a radio frequency discharge electrode (2), the radio frequency discharge electrode is connected to a radio frequency drive power supply (3), and the triaxial magnetic field coil is connected to a pulse timing controller (21) through a low noise current source (22). Using the spin precession of the helium-3 nucleus as the magnetic field scale, by applying a total of eight-state bias sequence of ±B0, ±2B0 and ±B1, ±2B1 along the z-axis and x-axis respectively, combined with the linear reading of the projection of the macroscopic magnetic moment of the helium-3 atom in the y-direction by the SERF atomic magnetometer, the absolute measurement of the triaxial vector residual magnetic field (Bx, By, Bz) is realized.

[0006] A SERF magnetometer (25) to be calibrated is also provided inside the triaxial magnetic field coil. It is used to receive the known vector magnetic field reconstructed by the triaxial magnetic field coil. The calibration coefficient K and zero bias δ ​​of the SERF magnetometer to be calibrated are determined by the reading result of the SERF magnetometer to be calibrated and the known vector magnetic field.

[0007] The alkali metal atom gas chamber is located inside the ceramic oven (11). The negative y-axis side of the alkali metal atom gas chamber is connected to the alkali metal atom beam reflector (9) through the alkali metal atom circular polarization generator (10). The alkali metal atom beam reflector is connected to the alkali metal atom pump laser (8). The positive z-axis side of the alkali metal atom gas chamber is connected to the alkali metal atom detection laser (13) through the polarization beam splitter (14). The negative z-axis side of the alkali metal atom gas chamber is connected to the computer host (20) in sequence through the half-wave plate (15), the polarization differential polarization beam splitter (16), the photodetector (17), the differential circuit (18), and the transimpedance amplifier (19).

[0008] Including the following formulas:

[0009]

[0010] in It is a bias magnetic field. It is the z-axis component of the vector residual magnetic field. It is a bias magnetic field. It is the x-axis component of the vector residual magnetic field. It is the y-axis component of the vector residual magnetic field. It is the nuclear spin gyromagnetic ratio of a helium-3 atom, Ω {z,+2} The helium-3 nuclear spin precession angular frequency corresponding to +2B0 is applied along the z-axis, Ω {z,-2} The helium-3 nuclear spin precession angular frequency corresponding to -2B0 is applied along the z-axis, Ω {z,+1} The helium-3 nuclear spin precession angular frequency corresponding to +B0 is applied along the z-axis, Ω {z,-1} The helium-3 nuclear spin precession angular frequency corresponding to -B0 is applied along the z-axis, Ω {x,+1} The helium-3 nuclear spin precession angular frequency corresponding to +B1 is applied to the x-axis, Ω {x,-1} The helium-3 nuclear spin precession angular frequency corresponding to -B1 is applied along the x-axis, Ω {x,+2} The helium-3 nuclear spin precession angular frequency corresponding to +2B1 is applied along the x-axis, Ω {x,-2} It is the spin precession angular frequency of the helium-3 nucleus corresponding to -2B1 applied along the x-axis.

[0011] Including the following expressions:

[0012] , Where V + The output voltage, V, of the SERF magnetometer to be calibrated is given under the first known vector magnetic field state. - The output voltage of the SERF magnetometer to be calibrated is given under the known second vector magnetic field state. The baseline polarizability of SERF under the first known vector magnetic field state. It is the baseline polarizability of SERF under the known second vector magnetic field state.

[0013] A method for absolute measurement of near-zero weak magnetic fields using a three-axis vector, characterized by employing the aforementioned absolute measurement device for near-zero weak magnetic fields using a three-axis vector.

[0014] Includes the following steps:

[0015] Step 1: Place the helium-3 atom gas chamber (1) and the alkali metal atom gas chamber (12) at the center of the triaxial magnetic field coil (23) inside the permalloy magnetic shield (24). Use non-magnetic electric heating technology to heat the ceramic oven (11) and the alkali metal atom gas chamber (12) therein to 150-200°C, so that the number density of alkali metal atoms reaches 10¹³-10¹⁰. 14 1 / cm³; turn on the alkali metal atom pump laser (8) and the alkali metal atom detection laser (13) to make the alkali metal atoms work in the SERF state;

[0016] Step 2: Turn on the helium-3 atom pump laser (7) to output circularly polarized pump light with a wavelength of 1083nm. At the same time, through the radio frequency drive power supply (3) and the radio frequency discharge electrode (2), maintain the metastable state discharge of helium-3 in the helium-3 atom gas chamber (1) to perform metastable exchange light pump polarization on the helium-3 nuclear spin.

[0017] Step 3: The π / 2 tilting pulse is applied by the RF drive power supply (3) synchronously triggered by the pulse timing controller (21) to tilt the helium-3 nucleus spin to the transverse plane. Then the tilting pulse is turned off. The helium-3 nucleus spin undergoes free induction decay precession at angular frequency Ω in the residual vector magnetic field. The precession signal is read by the SERF detection link composed of alkali metal atom gas cell (12), polarization beam splitter (14), half-wave plate (15), polarization differential polarization beam splitter (16), photodetector (17), differential circuit (18) and transimpedance amplifier (19).

[0018] Step 4: The low-noise current source (22) is controlled by the pulse timing controller (21) to drive the triaxial magnetic field coil (23) to apply four bias magnetic fields of +B0, -B0, +2B0, and -2B0 in the z-axis direction in sequence, and the corresponding helium-3 nucleus spin precession angular frequency Ω is obtained in each bias state. {z,+1} Ω {z,-1} Ω {z,+2} Ω {z,-2} ;

[0019] Step 5: Apply four bias states (+B1, -B1, +2B1, -2B1) sequentially along the x-axis, and obtain the corresponding helium-3 nuclear spin precession angular frequency Ω under each bias state. {x,+1} Ω {x,-1} Ω{x,+2} Ω {x,-2} ;

[0020] Step 6: In the computer host (20), calculate the bias magnetic field amplitudes B0 and B1 and the residual magnetic field components Bz and Bx from the eight precession angular frequencies according to the self-calibration decoupling formula. {z,+1} Combine the decoupled Bz and Bx with the formula:

[0021]

[0022] The residual magnetic field component By is determined, thus obtaining the complete three-axis vector residual magnetic field (Bx, By, Bz), where γ He It is the nuclear spin gyromagnetic ratio of a helium-3 atom;

[0023] Step 7: The known vector magnetic field reconstructed in Step 6 is used as a standard value and transmitted to the SERF magnetometer (25) to be calibrated placed next to it. The output voltage of the SERF magnetometer (25) to be calibrated is read under two or more known magnetic field conditions, and its calibration coefficient K and zero bias δ ​​are determined by weighted regression.

[0024] The technical advantages of this invention are as follows: This invention provides a three-axis vector near-zero extremely weak magnetic field absolute measurement device and method. It uses the spin precession of helium-3 nuclei as a magnetic field scale. By applying an eight-state bias sequence (±B0, ±2B0, ±B1, ±2B1) along the z-axis and x-axis respectively, and combining this with a SERF (Spin-Exchange-Relaxation-Free) atomic magnetometer to linearly read the projection of the macroscopic magnetic moment of helium-3 atoms in the y-direction, absolute measurement of the residual vector magnetic field along the three axes (Bx, By, Bz) is achieved. The measured values ​​are then transferred to the SERF magnetometer to be calibrated to determine its calibration coefficient and zero bias. This invention does not require prior knowledge of the coil constant, has high measurement accuracy, and is suitable for extremely weak magnetic field measurement and atomic magnetometer calibration.

[0025] The advantages of this invention compared to the prior art are:

[0026] (1) Conventional helium-3 magnetometers have a precession frequency of only millihertz to subhertz in near-zero field, which is insufficient for direct frequency measurement resolution. This invention applies an eight-state bias sequence of ±B0, ±2B0 and ±B1, ±2B1 along the z-axis and x-axis to upconvert the low-frequency precession signal to be measured to a higher frequency range corresponding to a magnetic field of 100 pT. At the same time, it utilizes the high sensitivity of the SERF atomic magnetometer to read the FID signal (SERF, Spin-Exchange-Relaxation-Free; FID, free induction decay), thus taking into account both the absolute scale characteristics of the helium-3 gyromagnetic ratio and the ultra-high sensitivity advantage of SERF.

[0027] (2) Conventional vector magnetic field calibration methods require the precise determination of the coil constant of the triaxial coil in advance, and the coil constant is affected by temperature, assembly error and magnetic shielding environment. This invention utilizes the self-calibration decoupling relationship, and the bias field amplitude and residual field component can be solved simultaneously from the precession angular frequency of the eight bias states. It does not require prior knowledge of the coil constant, and fundamentally eliminates the influence of coil constant calibration error on the measurement results.

[0028] (3) Conventional SERF magnetometer calibration usually approximates the zero bias as the average value of the positive and negative magnetic field outputs, ignoring the residual field response and the SERF nonlinear denominator. Based on the explicit reconstruction of the Bx, By, and Bz triaxial residual fields, this invention accurately determines the By component and the magnetometer zero bias δ, and further suppresses random noise through multi-state weighted regression, which significantly improves the calibration accuracy.

[0029] (4) The present invention keeps the SERF linear sensitive axis (By direction) always in near-zero field conditions, while applying bias fields only on the z-axis and x-axis. This maintains the SERF working mechanism and realizes the complete reconstruction of the three-axis vector magnetic field, thus solving the contradiction between near-zero field absolute measurement and SERF working conditions. Attached Figure Description

[0030] Figure 1 This is a schematic diagram of the structure of a three-axis vector near-zero extremely weak magnetic field absolute measurement device of the present invention.

[0031] The following are the annotations in the attached figures: 1-Helium-3 atom gas chamber; 2-RF discharge electrode; 3-RF drive power supply; 4-beam expander; 5-Helium atom beam reflector; 6-Helium atom beam circular polarization generator; 7-Helium-3 atom pump laser; 8-Alkali metal atom pump laser; 9-Alkali metal atom beam reflector; 10-Alkali metal atom circular polarization generator; 11-Ceramic oven; 12-Alkali metal atom gas chamber; 13-Alkali metal atom detection laser; 14-Polarization beam splitter prism; 15-Half-wave plate; 16-Polarization beam splitter prism for polarization difference; 17-Photodetector; 18-Differential circuit; 19-Transimpedance amplifier; 20-Computer host; 21-Pulse timing controller; 22-Low noise current source; 23-Triaxial magnetic field coil; 24-Permalloy magnetic shield; 25-SERF magnetometer to be calibrated; xyz-Cartesian coordinate system three axes (i.e., x-axis, y-axis, and z-axis). Detailed Implementation

[0032] The following is in conjunction with the attached diagram ( Figure 1 The present invention will be described in conjunction with the examples.

[0033] Figure 1 This is a schematic diagram of the structure of a three-axis vector near-zero extremely weak magnetic field absolute measuring device according to the present invention. (Reference) Figure 1 As shown, a three-axis vector near-zero extremely weak magnetic field absolute measurement device is characterized by comprising two gas chambers, one being a helium-3 atom gas chamber (1) and the other being an alkali metal atom gas chamber (12) in a SERF atomic magnetometer. Both gas chambers are located within the same three-axis magnetic field coil (23), which is located within a permalloy magnetic shield (24). The negative z-axis side of the helium-3 atom gas chamber is connected to a helium-3 atom pump laser via a beam expander (4), a helium atom beam reflector (5), and a helium atom beam circular polarization generator (6) in sequence. The device (7) has an RF discharge electrode (2) attached to the outer wall of the helium-3 atom gas chamber. The RF discharge electrode is connected to the RF drive power supply (3). The triaxial magnetic field coil is connected to the pulse timing controller (21) through a low-noise current source (22). Using the spin precession of the helium-3 nucleus as the magnetic field scale, the absolute measurement of the triaxial vector residual magnetic field (Bx, By, Bz) is achieved by applying an eight-state bias sequence of ±B0, ±2B0 and ±B1, ±2B1 along the z-axis and x-axis respectively, combined with the linear reading of the projection of the macroscopic magnetic moment of the helium-3 atom in the y-direction by the SERF atomic magnetometer.

[0034] A SERF magnetometer (25) to be calibrated is also provided inside the triaxial magnetic field coil. It is used to receive the known vector magnetic field reconstructed by the triaxial magnetic field coil. The calibration coefficient K and zero bias δ ​​of the SERF magnetometer to be calibrated are determined by the reading result of the SERF magnetometer to be calibrated and the known vector magnetic field.

[0035] The alkali metal atom gas chamber is located inside the ceramic oven (11). The negative y-axis side of the alkali metal atom gas chamber is connected to the alkali metal atom beam reflector (9) through the alkali metal atom circular polarization generator (10). The alkali metal atom beam reflector is connected to the alkali metal atom pump laser (8). The positive z-axis side of the alkali metal atom gas chamber is connected to the alkali metal atom detection laser (13) through the polarization beam splitter (14). The negative z-axis side of the alkali metal atom gas chamber is connected to the computer host (20) in sequence through the half-wave plate (15), the polarization differential polarization beam splitter (16), the photodetector (17), the differential circuit (18), and the transimpedance amplifier (19).

[0036] Including the following formulas:

[0037]

[0038] in It is a bias magnetic field. It is the z-axis component of the vector residual magnetic field. It is a bias magnetic field. It is the x-axis component of the vector residual magnetic field. It is the y-axis component of the vector residual magnetic field. It is the nuclear spin gyromagnetic ratio of a helium-3 atom, Ω {z,+2} The helium-3 nuclear spin precession angular frequency corresponding to +2B0 is applied along the z-axis, Ω {z,-2} The helium-3 nuclear spin precession angular frequency corresponding to -2B0 is applied along the z-axis, Ω {z,+1} The helium-3 nuclear spin precession angular frequency corresponding to +B0 is applied along the z-axis, Ω {z,-1} The helium-3 nuclear spin precession angular frequency corresponding to -B0 is applied along the z-axis, Ω {x,+1} The helium-3 nuclear spin precession angular frequency corresponding to +B1 is applied to the x-axis, Ω {x,-1} The helium-3 nuclear spin precession angular frequency corresponding to -B1 is applied along the x-axis, Ω {x,+2} The helium-3 nuclear spin precession angular frequency corresponding to +2B1 is applied along the x-axis, Ω {x,-2} It is the spin precession angular frequency of the helium-3 nucleus corresponding to -2B1 applied along the x-axis.

[0039] Including the following expressions:

[0040] ,

[0041] Where V + The output voltage, V, of the SERF magnetometer to be calibrated is given under the first known vector magnetic field state. - The output voltage of the SERF magnetometer to be calibrated is given under the known second vector magnetic field state. The baseline polarizability of SERF under the first known vector magnetic field state. It is the baseline polarizability of SERF under the known second vector magnetic field state.

[0042] This invention discloses a three-axis vector near-zero extremely weak magnetic field absolute measurement device and method, belonging to the field of quantum sensing and precision measurement technology. It solves the problem of accurately measuring three-axis vector absolute magnetic fields near zero magnetic fields. The device includes a helium-3 atom gas chamber, radio frequency discharge electrodes, a radio frequency drive power supply, a beam expander, a helium atom beam reflector, a helium atom beam circular polarization generator, a helium-3 atom pump laser, an alkali metal atom pump laser, an alkali metal atom beam reflector, an alkali metal atom circular polarization generator, a ceramic oven, an alkali metal atom gas chamber, an alkali metal atom detection laser, a polarization beam splitter, a half-wave plate, a polarization beam splitter for polarization difference, a photodetector, a differential circuit, a transimpedance amplifier, a computer host, a pulse timing controller, a low-noise current source, a three-axis magnetic field coil, and a permalloy magnetic shield. This invention uses the spin precession of helium-3 nuclei as a magnetic field scale. By applying an eight-state bias sequence (±B0, ±2B0, ±B1, ±2B1) along the z-axis and x-axis respectively, and combining this with a SERF (Spin-Exchange-Relaxation-Free) atomic magnetometer to linearly read the projection of the macroscopic magnetic moment of helium-3 atoms in the y-direction, absolute measurement of the residual vector magnetic field along the three axes (Bx, By, Bz) is achieved. The measured values ​​are then transferred to the SERF magnetometer to be calibrated to determine its calibration coefficient and zero bias. This invention requires no prior knowledge of the coil constant, offers high measurement accuracy, and is suitable for measurement of extremely weak magnetic fields and calibration of atomic magnetometers.

[0043] A triaxial vector near-zero extremely weak magnetic field absolute measurement device includes: a helium-3 atom gas chamber (1), a radio frequency discharge electrode (2), a radio frequency drive power supply (3), a beam expander (4), a helium atom beam reflector (5), a helium atom beam circular polarization generator (6), a helium-3 atom pump laser (7), an alkali metal atom pump laser (8), an alkali metal atom beam reflector (9), an alkali metal atom circular polarization generator (10), a ceramic oven (11), an alkali metal atom gas chamber (12), an alkali metal atom detection laser (13), a polarization beam splitter (14), a half-wave plate (15), a polarization beam splitter for polarization difference (16), a photodetector (17), a differential circuit (18), a transimpedance amplifier (19), a computer host (20), a pulse timing controller (21), a low-noise current source (22), a triaxial magnetic field coil (23), and a permalloy magnetic shield (24).

[0044] The permalloy magnetic shield (24) is internally equipped with the triaxial magnetic field coil (23), which in turn is internally equipped with the helium-3 atom gas chamber (1), the ceramic oven (11), and the alkali metal atom gas chamber (12). The radio frequency drive power supply (3) is electrically connected to the radio frequency discharge electrode (2), which is attached to the outer wall of the helium-3 atom gas chamber (1). The pump light output from the helium-3 atom pump laser (7) passes sequentially through the helium atom beam circle. The polarization generator (6), beam expander (4), and helium atom beam reflector (5) are incident on the helium-3 atom gas chamber (1); the pump light output from the alkali metal atom pump laser (8) passes sequentially through the alkali metal atom circular polarization generator (10) and the alkali metal atom beam reflector (9) and is incident on the alkali metal atom gas chamber (12); the detection light output from the alkali metal atom detection laser (13) passes sequentially through the polarization beam splitter (14) and the half-wave plate (15) and is incident on the alkali metal atom gas chamber (12). The detection light passing through the alkali metal atomic gas cell (12) is incident on the polarization differential beam splitter (16). The two output terminals of the polarization differential beam splitter (16) are optically coupled to the two photodetectors (17), respectively. The output terminals of the two photodetectors (17) are electrically connected to the differential circuit (18), and the output terminal of the differential circuit (18) is electrically connected to the transimpedance amplifier (19). The output of the transimpedance amplifier (19) is... The output end is electrically connected to the computer host (20); the computer host (20) is controlled and connected to the radio frequency drive power supply (3), helium-3 atom pump laser (7), alkali metal atom pump laser (8), alkali metal atom detection laser (13) and low noise current source (22) respectively through the pulse timing controller (21); the low noise current source (22) is electrically connected to the triaxial magnetic field coil (23); the alkali metal atom gas chamber (12) is placed inside the ceramic oven (11).

[0045] The helium-3 atom gas chamber (1) is a glass gas chamber filled with elemental helium-3 gas; the radio frequency discharge electrode (2) consists of two symmetrical metal foil electrodes attached to the outer wall of the helium-3 atom gas chamber (1), which are subjected to a radio frequency signal with a frequency in the megahertz range and a power in the watt range by a radio frequency driving power supply (3) to maintain the metastable population of helium-3 atoms; the helium-3 atom pump laser (7) outputs a linearly polarized laser with a wavelength of 1083 nm, corresponding to the C8 transition spectral line of helium-3 atom 2³S1→2³P0, which is converted into σ by the helium atom beam circular polarization generator (6). +The circularly polarized light is expanded by the beam expander (4) and then reflected by the helium atom beam reflector (5) and incident into the helium-3 atom gas chamber (1), realizing the metastable exchange light pumping of the helium-3 nuclear spin.

[0046] The alkali metal atom gas chamber (12) is a sealed glass gas chamber filled with alkali metal atoms, quenching gas, and buffer gas. The alkali metal is potassium (K) or rubidium-87 (K). 87 Rb), with an alkali metal atomic number density of 10¹³ to 10¹⁰. 14 The ceramic oven (11) uses non-magnetic boron nitride ceramic material and heats the alkali metal atom gas chamber (12) to 150-200°C by hot air or AC heating. The alkali metal atom pump laser (8) outputs a laser with a wavelength corresponding to the D1 line of the alkali metal, which is converted into a circularly polarized laser by the alkali metal atom circular polarization generator (10) and then incident on the alkali metal atom gas chamber (12) through the alkali metal atom beam reflector (9) to realize the spin polarization of alkali metal atoms. The alkali metal atom detection laser (13) outputs a linearly polarized detection light with a wavelength corresponding to the D1 line of the alkali metal, which is incident on the alkali metal atom gas chamber (12) through the polarization beam splitter (14) and the half-wave plate (15). The polarization beam splitter (14) and the half-wave plate (15) work together to adjust the polarization direction of the detection light.

[0047] The polarization differential beam splitter (16) splits the detection light passing through the alkali metal atom gas cell (12) into two beams with orthogonal polarization directions. These beams are received by two photodetectors (17) and converted into photocurrent signals. The differential circuit (18) performs differential operations on the two photocurrent signals and outputs a differential current signal to the transimpedance amplifier (19). The transimpedance amplifier (19) converts the differential current signal into a voltage signal and sends it to the computer host (20) for data acquisition and processing. This polarization differential detection structure is used to suppress laser common-mode intensity noise and improve the transverse polarizability P of alkali metal atoms. x Detection sensitivity.

[0048] The triaxial magnetic field coil (23) consists of three sets of mutually orthogonal circular or square Helmholtz coil pairs, corresponding to the x, y, and z axes respectively, and is driven by a low-noise current source (22). The computer host (20) outputs a synchronous trigger signal through a pulse timing controller (21) to control the low-noise current source (22) to output eight bias states. These eight bias states drive the triaxial magnetic field coil (23) to generate four bias magnetic fields in the z-axis direction: +B0, -B0, +2B0, and -2B0, and apply four bias magnetic fields in the x-axis direction: +B1, -B1, +2B1, and -2B1. The amplitudes of B0 and B1 are in the tens of nT range. The permalloy magnetic shield (24) is a multi-layer permalloy structure used to attenuate the external environmental magnetic field to below the nanotesla level.

[0049] The computer host (20) has a built-in data acquisition card and signal processing software, which is used to acquire the free induction decay (FID) signal output by the transimpedance amplifier (19), extract the spin precession angular frequency Ω of the helium-3 nucleus through digital fitting, and calculate the three components of the residual vector magnetic field Bx, By, and Bz according to the self-calibration decoupling formula; the pulse timing controller (21) is a multi-channel digital delay pulse generator with a time jitter of less than 10ns between channels, which is used to perform unified timing synchronization of the optical pump pulse of the helium-3 atom pump laser (7), the π / 2 tilting pulse of the radio frequency drive power supply (3), and the bias magnetic field switching of the low noise current source (22).

[0050] It also includes a SERF magnetometer (25) to be calibrated. The SERF magnetometer (25) to be calibrated is placed inside a permalloy magnetic shield (24) adjacent to the helium-3 atom gas chamber (1) and the alkali metal atom gas chamber (12). It is used to receive the known vector magnetic field reconstructed by the triaxial magnetic field coil (23). By comparing its output voltage with the known magnetic field value, the calibration coefficient K and zero bias δ ​​of the SERF magnetometer (25) to be calibrated are determined.

[0051] A method for absolute measurement of near-zero weak magnetic fields using a three-axis vector vector includes the following steps:

[0052] Step (1): Place the helium-3 atom gas chamber (1) and the alkali metal atom gas chamber (12) at the center of the triaxial magnetic field coil (23) inside the permalloy magnetic shield (24), and heat the ceramic oven (11) and the alkali metal atom gas chamber (12) therein to 150-200°C using non-magnetic electric heating technology, so that the alkali metal atom number density reaches 10¹³-10¹⁰. 14 1 / cm³; turn on the alkali metal atom pump laser (8) and the alkali metal atom detection laser (13) to make the alkali metal atoms work in the SERF state;

[0053] Step (2): Turn on the helium-3 atom pump laser (7) to output circularly polarized pump light with a wavelength of 1083nm. At the same time, through the radio frequency drive power supply (3) and the radio frequency discharge electrode (2), maintain the metastable discharge of helium-3 in the helium-3 atom gas chamber (1) to perform metastable exchange light pump polarization on the helium-3 nuclear spin.

[0054] Step (3): The π / 2 tilting pulse is applied by the RF drive power supply (3) synchronously triggered by the pulse timing controller (21) to tilt the helium-3 nucleus spin to the transverse plane. Then the tilting pulse is turned off. The helium-3 nucleus spin undergoes free induction decay precession at angular frequency Ω in the residual vector magnetic field. The precession signal is read by the SERF detection link composed of alkali metal atom gas cell (12), polarization beam splitter (14), half-wave plate (15), polarization differential polarization beam splitter (16), photodetector (17), differential circuit (18) and transimpedance amplifier (19).

[0055] Step (4): The low-noise current source (22) is controlled by the pulse timing controller (21) to drive the triaxial magnetic field coil (23) and apply four bias magnetic fields of +B0, -B0, +2B0 and -2B0 in the z-axis direction in sequence. The corresponding helium-3 nucleus spin precession angular frequency Ω is obtained under each bias state. {z,+1} Ω {z,-1} Ω {z,+2} Ω {z,-2} ;

[0056] Step (5): Apply four bias states, +B1, -B1, +2B1, and -2B1, sequentially along the x-axis, and obtain the corresponding helium-3 nuclear spin precession angular frequency Ω under each bias state. {x,+1} Ω {x,-1} Ω {x,+2} Ω {x,-2} ;

[0057] Step (6): In the computer host (20), the bias magnetic field amplitudes B0 and B1 and the residual magnetic field components Bz and Bx are calculated from the eight precession angular frequencies according to the self-calibration decoupling formula. {z,+1} Combine the decoupled Bz and Bx with the formula:

[0058]

[0059] The residual magnetic field component By is determined, thus obtaining the complete three-axis vector residual magnetic field (Bx, By, Bz), where γ He It is the nuclear spin gyromagnetic ratio of a helium-3 atom;

[0060] Step (7): The known vector magnetic field reconstructed in step (6) is used as a standard value and passed to the SERF magnetometer (25) to be calibrated placed next to it. The output voltage of the SERF magnetometer (25) to be calibrated is read under two or more known magnetic field conditions, and its calibration coefficient K and zero bias δ ​​are determined by weighted regression.

[0061] See Figure 1This embodiment provides a triaxial vector near-zero extremely weak magnetic field absolute measurement device, including a helium-3 atom gas chamber (1), a radio frequency discharge electrode (2), a radio frequency drive power supply (3), a beam expander (4), a helium atom beam reflector (5), a helium atom beam circular polarization generator (6), a helium-3 atom pump laser (7), an alkali metal atom pump laser (8), an alkali metal atom beam reflector (9), an alkali metal atom circular polarization generator (10), a ceramic oven (11), an alkali metal atom gas chamber (12), an alkali metal atom detection laser (13), a polarization beam splitter (14), a half-wave plate (15), a polarization beam splitter for polarization difference (16), a photodetector (17), a differential circuit (18), a transimpedance amplifier (19), a computer host (20), a pulse timing controller (21), a low-noise current source (22), a triaxial magnetic field coil (23), a permalloy magnetic shield (24), and a SERF magnetometer to be calibrated (25).

[0062] In this embodiment, the helium-3 atom gas chamber (1) is a spherical borosilicate glass gas chamber with a radius of R=35mm, filled with helium-3 gas at a pressure of several mbar. The radio frequency discharge electrode (2) consists of two copper foil electrodes symmetrically attached to the outer wall of the gas chamber. A radio frequency signal with a frequency in the tens of MHz range and a power in the watt range is applied by the radio frequency driving power supply (3) to maintain the 2³S1 metastable population of helium-3 atoms through weak radio frequency discharge. The helium-3 atom pump laser (7) is a fiber laser that outputs a linearly polarized laser with a wavelength of 1083nm, corresponding to the C8 transition spectral line of helium-3 atom 2³S1→2³P0. The laser is converted into σ by the helium atom beam circular polarization generator (6) (composed of a linear polarizer and a quarter-wave plate). + The circularly polarized light is reflected by the helium atom beam reflector (5), and then expanded by the beam expander (4) into a parallel collimated beam with a diameter of about 40 mm before being incident on the helium-3 atom gas cell (1) along the z-axis, thereby realizing the metastable exchange light pump polarization of the helium-3 nuclear spin.

[0063] In this embodiment, the alkali metal atom gas chamber (12) is a cubic borosilicate glass gas chamber with a side length of about 4 mm, filled with alkali metal atoms (potassium K is used in this embodiment, but rubidium-87 can also be used in other embodiments). 87 Rb) and approximately 2200 Torr nitrogen are used as quenching and buffering gases, and the alkali metal atomic number density is 10¹³ to 10¹⁰ at the operating temperature. 14The ceramic oven (11) is made of non-magnetic boron nitride ceramic material. The alkali metal atom gas chamber (12) is heated to 180°C by non-magnetic AC resistance heating. The outer layer of the oven is covered with heat insulation material to reduce the influence of heat radiation on other components. The alkali metal atom pump laser (8) outputs a laser with a wavelength corresponding to the potassium D1 line (about 770nm) or the rubidium D1 line (about 795nm). The laser is converted into circularly polarized light by the alkali metal atom circular polarization generator (10), and then reflected by the alkali metal atom beam reflector (9) and incident on the alkali metal atom gas chamber (12). The alkali metal atoms are optically pumped and polarized in a direction orthogonal to the detection light. The alkali metal atom detection laser (13) outputs a linearly polarized detection light with a wavelength corresponding to the same alkali metal D1 line. The detection light frequency is detuned by 65GHz compared to the D1 line. After being polarized by the polarization beam splitter (14) and the polarization direction is adjusted by the half-wave plate (15), the light is incident on the alkali metal atom gas chamber (12).

[0064] In this embodiment, the polarization beam splitter (16) used for polarization differential is a Wollaston prism or a polarization beam splitter cube, which splits the detection light passing through the alkali metal atom gas cell (12) into two beams, s-polarized and p-polarized, according to the orthogonal polarization direction. These beams are received and converted into photocurrent signals by two photodetectors (17) with matched performance parameters (using low-noise, large-area photodiodes). The differential circuit (18) uses a low-noise instrumentation amplifier structure to perform differential operations on the two photocurrent signals, effectively suppressing laser common-mode intensity noise. The differential current signal is then passed through a transimpedance amplifier (19) (with a gain of approximately 10). 6 ~10 7 The voltage (V / A) is converted into a voltage signal and sent to the high-precision data acquisition card built into the computer host (20) for digital acquisition.

[0065] In this embodiment, the triaxial magnetic field coil (23) consists of three pairs of mutually orthogonal circular Helmholtz coils. The coil frame is made of non-magnetic material (such as G10 fiberglass or acrylic). Each pair of coils is independently driven by a low-noise current source (22) (current noise below 1nA / √Hz). The permalloy magnetic shield (24) is a five-layer permalloy cylindrical nested structure with an openable end cap along the axial direction. Its shielding coefficient is better than 10 at 1Hz. 5 It can attenuate the external environmental magnetic field (approximately 50 μT of geomagnetic field and environmental electromagnetic interference) to below the nanotesla level. The SERF magnetometer (25) to be calibrated is placed inside the magnetic shielding adjacent to the helium-3 atom gas chamber (1) and the alkali metal atom gas chamber (12), and the center distance between the two is determined according to the requirements of dipole field uniformity.

[0066] In this embodiment, the pulse timing controller (21) is a multi-channel digital delay pulse generator with no less than 8 channels, a time jitter between channels of less than 10ns, and an output level of TTL level. The computer host (20) programs and configures the pulse timing controller (21) through a USB or Ethernet interface. The pulse timing controller (21) outputs synchronous trigger signals to control the π / 2 tilting pulse of the RF drive power supply (3), the light source seed switch of the helium-3 atom pump laser (7), and the bias magnetic field switching of the low-noise current source (22).

[0067] The measurement method steps of this embodiment are described in detail below:

[0068] After the system is powered on and thermally balanced, step (1) is performed: turn on the heating power of the ceramic oven (11) to heat the alkali metal atom gas chamber (12) to about 180°C, so that the potassium atom number density reaches about 10¹³ atoms / cm³; turn on the alkali metal atom pump laser (8) and adjust the pump light power to about 2mW, so that the spin polarization of the alkali metal atom reaches more than 0.5; turn on the alkali metal atom detection laser (13), adjust the detection light wavelength to lock to 65GHz detuned to the potassium D1 line, and the detection light power to about 2mW, so that the alkali metal atom works in the SERF state.

[0069] After completing step (1), proceed to step (2): turn on the helium-3 atom pump laser (7), outputting a laser with a wavelength of 1083nm and a power of approximately 2W, which is then converted into σ by the helium atom beam circular polarization generator (6). + Circularly polarized light is expanded and incident into the helium-3 atom gas chamber (1); at the same time, the radio frequency driving power supply (3) is turned on, and a radio frequency signal with a frequency of 25 MHz and a power of about 2 W is applied through the radio frequency discharge electrode (2) to maintain a weak discharge. The optical pumping lasts for about 5 minutes, so that the spin polarization p of the helium-3 nucleus reaches 0.1 to 0.5.

[0070] After completing step (2), proceed to step (3): synchronously trigger the RF drive power supply (3) to output a π / 2 tilting pulse (the pulse width is calibrated according to the nuclear spin Rabi frequency, so that the longitudinal magnetization vector is deflected by 90° to the transverse plane). After the tilting pulse ends, the helium-3 nuclear spin in the residual vector magnetic field moves at an angular frequency Ω=γ. He • B undergoes free induction decay precession. The rotating dipole magnetic field generated by the helium-3 polarized atoms in the alkali metal atom gas chamber (12) drives the transverse polarizability Px of the alkali metal atoms to oscillate at the same angular frequency Ω. The polarization differential detection link converts the Px oscillation into a voltage signal and sends it to the computer host (20). The computer host (20) performs digital quadrature demodulation and exponential decay sine fitting on the acquired FID signal to extract the precession angular frequency Ω.

[0071] After completing step (3), proceed to step (4): control the low-noise current source (22) to drive the triaxial magnetic field coil (23) through the pulse timing controller (21), and apply four bias magnetic fields of +B0, -B0, +2B0, and -2B0 in sequence in the z-axis direction, with B0 being 30nT; obtain the precession angular frequency Ω in each bias state. {z,+1} Ω {z,-1} Ω {z,+2} Ω {z,-2} .

[0072] After completing step (4), proceed to step (5): switch the bias magnetic field to the x-axis direction and apply four bias magnetic fields in sequence: +B1, -B1, +2B1, and -2B1, with B1 also set to 30 nT; obtain the precession angular frequency Ω in each bias state. {x,+1} Ω {x,-1} Ω {x,+2} Ω {x,-2} .

[0073] After completing step (5), proceed to step (6): In the computer host (20), calculate the z-axis bias field amplitude B0 and the residual magnetic field z-component Bz based on the z-axis four-state precession angular frequency; calculate the x-axis bias field amplitude B1 and the residual magnetic field x-component Bx based on the x-axis four-state precession angular frequency. Then, calculate the x-axis bias field amplitude B1 and the residual magnetic field x-component Bx based on B0 and Ω. {z,+1} Solve for the residual magnetic field y-component By using the decoupled Bz and Bx. Turn off all bias magnetic fields and read the SERF baseline polarizability in a zero-bias state. The obtained Bx and Bz are used to iteratively solve the residual magnetic field y component By as a check and comparison. The total relaxation rate Γ = Rop + Rrel is determined by the optical pump rate Rop and the transverse relaxation rate Rrel. Rop can be estimated by the pump light intensity and absorption cross section, and Rrel can be obtained by measuring the SERF response bandwidth. At this point, all three components (Bx, By, Bz) of the residual vector magnetic field are determined. During the solution process, the computer host (20) also uses the internal consistency diagnostic conditions to check the measured data. If the conditions are not met, the data of the corresponding bias state is re-acquired.

[0074] After completing step (6), proceed to step (7): using the known vector magnetic field reconstructed in step (6) as the standard value, reproduce a series of known magnetic field states at the position of the SERF magnetometer (25) to be calibrated using a triaxial magnetic field coil (23). Read the output voltage V of the SERF magnetometer (25) to be calibrated under both magnetic field states. + and V - and the corresponding SERF baseline polarizability readings and Calculate the scale factor K and the zero bias δ. To improve calibration accuracy, measurements can be performed under multiple magnetic field conditions such as ±100pT and ±200pT. A weighted linear regression is performed with the magnetic field value as the independent variable and the output voltage as the dependent variable. The regression slope is K, and the intercept is δ.

[0075] In summary, this invention achieves self-calibrated absolute measurement of the triaxial vector magnetic field while maintaining the near-zero field conditions of the SERF atomic magnetometer through the coordinated operation of the helium-3 nuclear spin precession. It also completes the high-precision calibration of the scale coefficient and zero bias of the SERF magnetometer to be calibrated.

[0076] Contents not described in detail in this specification are existing technologies 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 the present invention, but does not limit the scope of protection of the present invention. Any equivalent substitutions, modifications, improvements, and / or simplifications of the above descriptions that do not depart from the essence of the present invention fall within the scope of protection of the present invention.

Claims

1. A three-axis vector near-zero extremely weak magnetic field absolute measurement device, characterized in that, It includes two gas chambers: a helium-3 atom gas chamber (1) and an alkali metal atom gas chamber (12) in the SERF atomic magnetometer. Both gas chambers are located within the same triaxial magnetic field coil (23), which is located within a permalloy magnetic shield (24). The negative z-axis side of the helium-3 atom gas chamber is connected to a helium-3 atom pump laser (7) via a beam expander (4), a helium atom beam reflector (5), and a helium atom beam circular polarization generator (6). The outer wall is attached with a radio frequency discharge electrode (2), which is connected to a radio frequency drive power supply (3). The triaxial magnetic field coil is connected to a pulse timing controller (21) through a low noise current source (22). Using the spin precession of the helium-3 nucleus as the magnetic field scale, the absolute measurement of the triaxial vector residual magnetic field (Bx, By, Bz) is achieved by applying an eight-state bias sequence of ±B0, ±2B0 and ±B1, ±2B1 along the z-axis and x-axis respectively, combined with the linear reading of the projection of the macroscopic magnetic moment of the helium-3 atom in the y-direction by the SERF atomic magnetometer.

2. The triaxial vector near-zero extremely weak magnetic field absolute measuring device according to claim 1, characterized in that, A SERF magnetometer (25) to be calibrated is also provided inside the triaxial magnetic field coil. It is used to receive the known vector magnetic field reconstructed by the triaxial magnetic field coil. The calibration coefficient K and zero bias δ ​​of the SERF magnetometer to be calibrated are determined by the reading result of the SERF magnetometer to be calibrated and the known vector magnetic field.

3. The triaxial vector near-zero extremely weak magnetic field absolute measuring device according to claim 1, characterized in that, The alkali metal atom gas chamber is located inside the ceramic oven (11). The negative y-axis side of the alkali metal atom gas chamber is connected to the alkali metal atom beam reflector (9) through the alkali metal atom circular polarization generator (10). The alkali metal atom beam reflector is connected to the alkali metal atom pump laser (8). The positive z-axis side of the alkali metal atom gas chamber is connected to the alkali metal atom detection laser (13) through the polarization beam splitter (14). The negative z-axis side of the alkali metal atom gas chamber is connected to the computer host (20) in sequence through the half-wave plate (15), the polarization differential polarization beam splitter (16), the photodetector (17), the differential circuit (18), and the transimpedance amplifier (19).

4. The triaxial vector near-zero extremely weak magnetic field absolute measuring device according to claim 1, characterized in that, Including the following formulas: in It is a bias magnetic field. It is the z-axis component of the vector residual magnetic field. It is a bias magnetic field. It is the x-axis component of the vector residual magnetic field. It is the y-axis component of the vector residual magnetic field. It is the nuclear spin gyromagnetic ratio of a helium-3 atom, Ω {z,+2} The helium-3 nuclear spin precession angular frequency corresponding to +2B0 is applied along the z-axis, Ω {z,-2} The helium-3 nuclear spin precession angular frequency corresponding to -2B0 is applied along the z-axis, Ω {z,+1} The helium-3 nuclear spin precession angular frequency corresponding to +B0 is applied along the z-axis, Ω {z,-1} The helium-3 nuclear spin precession angular frequency corresponding to -B0 is applied along the z-axis, Ω {x,+1} The helium-3 nuclear spin precession angular frequency corresponding to +B1 is applied to the x-axis, Ω {x,-1} The helium-3 nuclear spin precession angular frequency corresponding to -B1 is applied to the x-axis, Ω {x,+2} The helium-3 nuclear spin precession angular frequency corresponding to +2B1 is applied along the x-axis, Ω {x,-2} It is the spin precession angular frequency of the helium-3 nucleus corresponding to -2B1 applied along the x-axis.

5. The triaxial vector near-zero extremely weak magnetic field absolute measuring device according to claim 2, characterized in that, Including the following expressions: ， Where V + The output voltage, V, of the SERF magnetometer to be calibrated is given under the first known vector magnetic field state. - The output voltage of the SERF magnetometer to be calibrated is given under the known second vector magnetic field state. The baseline polarizability of SERF under the first known vector magnetic field state. It is the baseline polarizability of SERF under the known second vector magnetic field state.

6. A method for absolute measurement of near-zero weak magnetic fields using a three-axis vector vector, characterized in that, The three-axis vector near-zero extremely weak magnetic field absolute measurement device of any one of claims 1-5 is adopted.

7. The method for absolute measurement of near-zero weak magnetic fields using a three-axis vector according to claim 6, characterized in that, Includes the following steps: Step 1: Place the helium-3 atom gas chamber (1) and the alkali metal atom gas chamber (12) at the center of the triaxial magnetic field coil (23) inside the permalloy magnetic shield (24). Use non-magnetic electric heating technology to heat the ceramic oven (11) and the alkali metal atom gas chamber (12) therein to 150-200°C, so that the number density of alkali metal atoms reaches 10¹³-10¹⁰. 14 1 / cm³; turn on the alkali metal atom pump laser (8) and the alkali metal atom detection laser (13) to make the alkali metal atoms work in the SERF state; Step 2: Turn on the helium-3 atom pump laser (7) to output circularly polarized pump light with a wavelength of 1083nm. At the same time, through the radio frequency drive power supply (3) and the radio frequency discharge electrode (2), maintain the metastable state discharge of helium-3 in the helium-3 atom gas chamber (1) to perform metastable exchange light pump polarization on the helium-3 nuclear spin. Step 3: The π / 2 tilting pulse is applied by the RF drive power supply (3) synchronously triggered by the pulse timing controller (21) to tilt the helium-3 nucleus spin to the transverse plane. Then the tilting pulse is turned off. The helium-3 nucleus spin undergoes free induction decay precession at angular frequency Ω in the residual vector magnetic field. The precession signal is read by the SERF detection link composed of alkali metal atom gas cell (12), polarization beam splitter (14), half-wave plate (15), polarization differential polarization beam splitter (16), photodetector (17), differential circuit (18) and transimpedance amplifier (19). Step 4: The low-noise current source (22) is controlled by the pulse timing controller (21) to drive the triaxial magnetic field coil (23) to apply four bias magnetic fields of +B0, -B0, +2B0, and -2B0 in the z-axis direction in sequence, and the corresponding helium-3 nucleus spin precession angular frequency Ω is obtained in each bias state. {z,+1} Ω {z,-1} Ω {z,+2} Ω {z,-2} ; Step 5: Apply four bias states (+B1, -B1, +2B1, -2B1) sequentially along the x-axis, and obtain the corresponding helium-3 nuclear spin precession angular frequency Ω under each bias state. {x,+1} Ω {x,-1} Ω {x,+2} Ω {x,-2} ; Step 6: In the computer host (20), calculate the bias magnetic field amplitudes B0 and B1 and the residual magnetic field components Bz and Bx from the eight precession angular frequencies according to the self-calibration decoupling formula. {z,+1} Combine the decoupled Bz and Bx with the formula: The residual magnetic field component By is determined, thus obtaining the complete three-axis vector residual magnetic field (Bx, By, Bz), where γ He It is the nuclear spin gyromagnetic ratio of a helium-3 atom; Step 7: The known vector magnetic field reconstructed in Step 6 is used as a standard value and transmitted to the SERF magnetometer (25) to be calibrated placed next to it. The output voltage of the SERF magnetometer (25) to be calibrated is read under two or more known magnetic field conditions, and its calibration coefficient K and zero bias δ ​​are determined by weighted regression.

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