A method and device for simultaneously measuring DC and AC magnetic field intensities

Abstract

The present invention discloses a method and device for simultaneously measuring the intensities of direct current and alternating current magnetic fields, comprising the following steps: S1: placing a magnetically sensitive quantum state cold atom gas trapped in an optical trap in a magnetic field to be measured; S2: performing quantum state manipulation on the magnetically sensitive quantum state cold atom gas to complete Ramsey interference with an evolution time of 2T for the cold atom gas. During the process of performing Ramsey interference, a dynamical decoupling pulse sequence is also applied to the cold atom gas; S3: measuring the population difference of the cold atom gas that has completed Ramsey interference, changing the time T, repeating steps S1 and S2, obtaining the oscillation relationship of the population difference with respect to time T, extracting the oscillation frequency using Fourier transform, and then obtaining the intensities of the direct current component and the alternating current component of the magnetic field. The present invention utilizes the magnetically sensitive quantum state of the cold atom gas trapped in the optical trap and combines technologies such as Ramsey interference and dynamical decoupling to simultaneously measure the intensities of direct current and alternating current magnetic fields.

Description

Technical Field

[0001] The present invention relates to the field of quantum sensing, and in particular to a method and device for simultaneously measuring the intensities of direct current (DC) and alternating current (AC) magnetic fields. Background Art

[0002] Magnetic field is an important physical quantity characterizing the basic properties of matter, and sensitive detection of weak magnetic fields is a key issue in the field of precision measurement. A high-precision magnetometer is an essential key instrument for realizing geological exploration, deep-sea anti-submarine, material detection, in-vivo organ imaging, virus and cell detection, etc. By using the magnetically sensitive quantum state, the measurement of the DC magnetic field intensity can be realized according to the relationship between the phase accumulated in the traditional Ramsey interference process and the magnetic field to be measured. However, for the traditional Ramsey interference method, the AC magnetic field does not contribute to the phase accumulation, so the traditional Ramsey interference technology cannot effectively detect the AC magnetic field. Although there are currently methods using similar dynamical decoupling, applying a sequence of π pulses at the nodes of the AC magnetic field oscillation, so that the AC magnetic field can cause significant phase accumulation, but only the effective detection of the AC magnetic field intensity can be achieved.

[0003] As can be seen from the above, the current methods and devices for measuring magnetic fields can only achieve the separate measurement of DC magnetic fields or AC magnetic fields. Generally speaking, however, the actual magnetic field contains both DC and AC components at the same time. Therefore, how to simultaneously measure the intensities of DC and AC magnetic fields is an important issue in the field of quantum sensing. Summary of the Invention

[0004] In order to overcome the deficiencies of the prior art, the first object of the present invention is to provide a method for simultaneously measuring the intensities of DC and AC magnetic fields. By using the magnetically sensitive quantum state of cold atomic gas trapped in an optical trap and combining technologies such as Ramsey interference and dynamical decoupling, the intensities of DC and AC magnetic fields can be simultaneously measured. The second object of the present invention is to provide a device for simultaneously measuring the intensities of DC and AC magnetic fields, which is used to implement the method provided by the first object.

[0005] To solve the above problems, the technical solutions adopted by the present invention are as follows:

[0006] A method for simultaneously measuring the intensities of the DC component and the AC component of a magnetic field, characterized by comprising the following steps:

[0007] S1: Placing the cold atomic gas with a magnetically sensitive quantum state trapped in an optical trap in the magnetic field to be measured;

[0008] S2: Performing quantum state manipulation on the cold atomic gas with a magnetically sensitive quantum state to complete Ramsey interference with an evolution time of 2T for the cold atomic gas. During the process of performing Ramsey interference, a dynamical decoupling pulse sequence is also applied to the cold atomic gas;

[0009] S3: Measure the population difference of the cold atomic gas that has completed Ramsey interference. Change the time T, repeat the steps S1 and S2, obtain the oscillation relationship of the population difference with respect to the time T, extract the oscillation frequency using Fourier transform, and then obtain the intensities of the DC component and the AC component of the magnetic field.

[0010] As a preferred embodiment of the present invention, the quantum state manipulation is to perform quantum state manipulation on the cold atomic gas trapped in the optical trap by applying a laser field, a microwave field, a radio frequency field, etc.

[0011] As a preferred embodiment of the present invention, the cold atomic gas in the sensitive quantum state is respectively in the |a> state and the |b> state, placed in the magnetic field to be measured, and the frequency shifts caused by the magnetic field to be measured are γ1B and γ2B respectively, where γ1 and γ2 are the magnetic susceptibility coefficients of the |a> state and the |b> state, and B is the magnetic field strength.

[0012] As a preferred embodiment of the present invention, the step S1 is specifically: The step S1 is specifically: Place the alkali metal atoms in a vacuum with a vacuum degree of 10 -8 -10 -9 Pa order of magnitude. After laser cooling and evaporative cooling, the temperature of the atomic cluster is lower than the microkelvin order of magnitude, forming a cold atomic gas in a magnetic-sensitive quantum state and trapped in a red-detuned optical trap. The atoms are initially populated in the |a> state; the atoms are in the magnetic field to be measured, and the intensities B1 and B2 of the DC component and the AC component of this magnetic field are the quantities to be measured.

[0013] As a preferred embodiment of the present invention, the Ramsey interference includes three processes: beam splitting, phase accumulation, and beam combination. The beam splitting is controlled by a π / 2 pulse to manipulate the internal state of the atoms to achieve the preparation of the spin superposition state. The phase accumulation process consists of two evolutions with a time of T. In the first T time, the phase accumulated by the DC component of the magnetic field is φ dc = 2π(γ1 + γ2)B1T, and the phase accumulated by the AC component of the magnetic field is zero; in the second T time, the phase accumulated by the DC component of the magnetic field is zero, and the phase accumulated by the AC component of the magnetic field is φ ac = 4(γ1 + γ2)B2T; the beam combination is controlled by a π / 2 pulse to manipulate the internal state of the atoms to achieve the interference between the |a> state and the |b> state.

[0014] As a preferred embodiment of the present invention, the application of the dynamical decoupling pulse sequence is specifically that in the second T time, the π pulse sequence is applied at the nodes of the oscillation of the AC magnetic field to eliminate the influence of the DC component of the magnetic field and achieve the effective accumulation of the phase Φac brought by the AC component.

[0015] As a preferred embodiment of the present invention, the population difference measurement is to measure the difference in the number of atoms populated in the |a> state and the |b> state. An interference oscillation fringe signal is obtained through the population difference measurement, and a time-domain oscillation signal varying with time can be obtained by changing the phase accumulation time T.

[0016] As a preferred embodiment of the present invention, by performing a Fourier transform on the time-domain oscillation signal, two oscillation frequencies can be extracted, and from the relationships and the intensities of the DC component and the AC component of the magnetic field are obtained.

[0017] A device for simultaneously measuring the intensities of the DC component and the AC component of a magnetic field, comprising:

[0018] A preparation module: used for laser cooling and trapping of atoms, evaporative cooling, and preparation of magnetically sensitive states, to realize a magnetically sensitive quantum state cold atomic gas with a temperature below microkelvin in an optical trap;

[0019] A manipulation module: used for performing Ramsey interference, quantum state manipulation, and microwave control of π pulses on the magnetically sensitive quantum state cold atomic gas;

[0020] A CCD camera: used for measuring the number of atoms populated in the |a> state and the |b> state, and transmitting the measurement data to the manipulation module.

[0021] As a preferred embodiment of the present invention, the manipulation module includes:

[0022] A control and data acquisition system, including a computer, digital I / O, and analog I / O, where the computer is used to collect the measurement data of the CCD camera;

[0023] A microwave generator: used for generating a microwave signal, which is controlled by the analog I / O;

[0024] A microwave switch: used for controlling the on and off of the microwave signal, which is controlled by the digital I / O;

[0025] A microwave amplifier: used for amplifying the microwave signal generated by the microwave generating device;

[0026] A microwave antenna: used for transmitting the amplified microwave signal onto the atoms.

[0027] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0028] 1. Different from the general situation that quantum magnetometers can only achieve the separate measurement of DC or AC magnetic field intensity, the present invention utilizes the magnetically sensitive quantum state of cold atomic gas, combines Ramsey interference, quantum state manipulation and dynamical decoupling pulse sequence technology, and can simultaneously extract the intensities of the DC component and the AC component of the magnetic field only through the measurement of the population difference.

[0029] 2. Since the spatial size of cold atoms is on the order of micrometers and the uncertainty of their position is relatively small, by utilizing the above unique advantages of cold atoms, the present invention can achieve high-spatial-resolution magnetic field measurement.

[0030] The present invention will be further described in detail below in conjunction with the accompanying drawings and specific embodiments. Description of the Drawings

[0031] Figure 1 - is a flowchart of the method steps for simultaneously measuring the DC and AC magnetic field intensities of the present invention;

[0032] Figure 2 - is a schematic diagram of the principle of the method and device for simultaneously measuring the DC and AC magnetic field intensities of the present invention;

[0033] Figure 3 - is a schematic structural diagram of the device for simultaneously measuring the DC and AC magnetic field intensities of the present invention. Reference numerals in the drawings: 1. Preparation module; 2. Manipulation module; 3. CCD camera; 4. Control and data acquisition system; 5. Microwave generator; 6. Microwave switch; 7. Microwave amplifier; 8. Microwave antenna. Specific Embodiments

[0034] The method for simultaneously measuring the DC and AC magnetic field intensities provided by the present invention, as Figure 1 shown, includes the following steps:

[0035] S1: Place the magnetically sensitive quantum state cold atomic gas trapped in the optical trap in the magnetic field to be measured; further, the cold atomic gas trapped in the optical trap is a cold atomic gas that has undergone laser cooling and evaporative cooling in a vacuum with a vacuum degree of 10 -8 -10 -9 Pa and has a temperature lower than the microkelvin level or even reaches quantum degeneracy. As the atoms to be quantum manipulated, it has a magnetically sensitive quantum state that can be used for magnetic field measurement; further, as Figure 2 shown, the cold atomic gas is in the magnetically sensitive quantum states |a> and |b>, and the frequency shifts caused by the magnetic field are γ1B and γ2B respectively, where γ1 and γ2 are the magnetic sensitivity coefficients of the |a> and |b> states, and B is the magnetic field intensity to be measured, expressed as where B1 and B2 are the intensities of the DC component and the AC component of the magnetic field to be measured, ω B is the angular frequency of the AC magnetic field, Indicates the direction of the magnetic field to be measured. Further, the cold atomic gas trapped in the red-detuned optical trap is a rubidium-87 Bose condensate gas. The rubidium-87 atoms are in a vacuum with a vacuum degree of about 10 -9 Pa. After laser cooling and evaporative cooling, the temperature of the atomic cloud is lower than the microkelvin level, and a Bose condensate is formed in the red-detuned optical trap. The number of condensed atoms is greater than 10 4 atoms. The rubidium-87 atom has two ground states in the hyperfine energy level, and the energy level splitting is about 6.834 GHz, corresponding to F = 1 and F = 2, with 3 and 5 magnetic sub-levels respectively. The magnetic susceptibility coefficients are -γ1 = γ2 = γ = 0.7 MHz / G. In this embodiment, |a> = |F = 1, m F = -1> and |b> = |F = 2, m F = -1> are the quantum states for implementing magnetic field measurement, and the atoms are initially populated in the |a> state.

[0036] S2: Perform quantum state manipulation on the cold atomic gas with magnetic susceptibility quantum states to complete Ramsey interference with an evolution time of 2T for the cold atomic gas. During the process of Ramsey interference, it also includes applying a dynamical decoupling pulse sequence to the cold atomic gas; further, the quantum state manipulation is to perform quantum state manipulation on the cold atomic gas trapped in the optical trap by applying a laser field, a microwave field, a radio frequency field, etc. Specifically, pulses are applied to achieve spin superposition states, pulses are applied during the phase accumulation process to achieve unitary operations, and pulses are applied before the population difference measurement to achieve interference beam combination, etc. Further, Ramsey interference includes three processes: beam splitting, phase accumulation, and beam combination; beam splitting is controlled by a π / 2 pulse to manipulate the internal state of the atom to achieve the preparation of the spin superposition state. The phase accumulation process consists of two evolutions with a time of T. In the first T time, the phase accumulated by the DC component of the magnetic field is φ dc = 2π(γ1 + γ2)B1T, and the phase accumulated by the AC component of the magnetic field is zero; in the second T time, the phase accumulated by the DC component of the magnetic field is zero, and the phase accumulated by the AC component of the magnetic field is φ ac = 4(γ1 + γ2)B2T; beam combination is controlled by a π / 2 pulse to manipulate the internal state of the atom to achieve the interference of the |a> state and the |b> state. Further, applying the dynamical decoupling pulse sequence specifically means that in the second T time, the π pulse sequence is applied at the node of the oscillation of the AC magnetic field to eliminate the influence of the DC component of the magnetic field and achieve the effective accumulation of the phase Φ ac brought by the AC component.

[0037] S3: Measure the population difference of the cold atomic gas that has completed Ramsey interference. Change the time T, repeat steps S1 and S2, obtain the oscillation relationship of the population difference with respect to time T, extract the oscillation frequency using Fourier transform, and then obtain the intensities of the DC component and AC component of the magnetic field. Further, the population difference measurement is to measure the difference in the number of atoms populated in the |a> state and the |b> state, and this measurement can obtain the oscillation fringe signal of the interference. By changing the phase accumulation time T, a time-domain oscillation signal that changes with time can be obtained. Performing a Fourier transform on the time-domain oscillation signal can extract two main frequencies f1 and f2, specifically and and obtain the intensities of the DC component and AC component of the magnetic field from the relationship and .

[0038] A device for simultaneously measuring the intensities of the DC component and AC component of a magnetic field, as Figure 3 shown, includes:

[0039] Preparation module 1: For laser cooling and trapping of atoms, evaporative cooling, and preparation of magnetosensitive states, to achieve a cold atomic gas in a magnetosensitive quantum state with a temperature below microkelvin in an optical trap;

[0040] Manipulation module 2: For performing Ramsey interference, quantum state manipulation, and microwave control of π pulses on the cold atomic gas in the magnetosensitive quantum state;

[0041] CCD camera 3: The CCD camera 3 measures the number of atoms populated in the |a> state and the |b> state by absorption imaging in different time periods, thereby obtaining the population difference, and transmits the measurement data to the control and data acquisition system 4.

[0042] The preparation module 1, for laser cooling and trapping of atoms, evaporative cooling, and preparation of magnetosensitive states, to achieve a cold atomic gas in a magnetosensitive quantum state with a temperature below microkelvin in an optical trap, includes:

[0043] A vacuum device, for creating a vacuum environment;

[0044] A laser, for magneto-optical trapping of atoms;

[0045] A red-detuned laser, for evaporative cooling and optical trap confinement;

[0046] Helmholtz coils, for applying a DC bias Helmholtz magnetic field and an anti-Helmholtz magnetic field to achieve magneto-optical trapping;

[0047] An optical pumping device, for preparing cold atoms into magnetosensitive states.

[0048] In use, first, a laser and Helmholtz coils are utilized to capture atoms at a temperature of approximately 100 microkelvins through magneto-optical trapping. Then, the captured atoms are placed in a vacuum environment prepared by a vacuum device. The captured atoms are subjected to laser polarization and gradient cooling by a red-detuned laser, further reducing the atomic temperature to approximately 10 microkelvins. Subsequently, the atoms are prepared into a magnetically sensitive state by an optical pumping device and loaded into a red-detuned optical trap prepared by the red-detuned laser. Finally, evaporative cooling is performed by reducing the potential well depth of the optical trap to lower the atomic temperature below microkelvins, obtaining a magnetically sensitive quantum state cold atomic gas with a temperature below microkelvins in the optical trap.

[0049] The manipulation module 2 includes a control and data acquisition system 4, a microwave generator 5, a microwave switch 6, a microwave amplifier 7, and a microwave antenna 8. The control and data acquisition system 4 includes a computer, digital I / O, and analog I / O. Functions such as automatically generating digital signals, analog signals, collecting digital signals, and analog signals can be achieved through computer programs. The control and data acquisition system 4 controls the microwave switch 6 in time through digital I / O and controls the output power of the microwave source through analog I / O. The microwave generator 5 can generate a 6.834 GHz microwave signal, and its frequency, power, etc. can be controlled by the control and data acquisition system 4. The microwave switch 6 is a fast electronic switch controlled by the control and data acquisition system 4 and can be used to control the opening and closing of microwave signals, thereby generating a microwave pulse sequence. The microwave amplifier 7 is used to amplify microwave signals. The microwave emitted by the microwave antenna 8 irradiates the atoms to achieve interaction with the atoms, thereby manipulating quantum states, etc.

[0050] In use, it includes the following steps: Using the preparation module 1, rubidium-87 atoms are cooled by laser cooling and evaporative cooling to form a Bose condensate and loaded into a red-detuned optical trap, and the rubidium-87 atoms are in the initial state |a>; Using the manipulation module 2 to control the microwave and applying a π / 2 pulse to prepare the atoms into a superposition state of |a> and |b>; The atomic state undergoes free evolution for a time T to accumulate the phase brought by the DC component of the magnetic field; Using the manipulation module 2 to control the microwave and applying a π / 2 pulse to achieve a unitary operation; Applying a π pulse sequence during another free evolution time T, the π pulse is at the node of the alternating magnetic field to accumulate the phase brought by the AC component of the magnetic field; Using the manipulation module 2 to control the microwave and applying a π / 2 pulse to achieve the beam combination manipulation of Ramsey interference; Using the CCD camera 3 to measure the number of atoms populated in the |a> state and the |b> state through absorption imaging to obtain the population difference; Changing the time T and repeating the above process to obtain the oscillation relationship of the population difference with respect to the time T.

[0051] The above embodiments are only the preferred embodiments of the present invention and cannot be used to limit the scope of protection of the present invention. Any non-substantial changes and substitutions made by those skilled in the art based on the present invention fall within the scope of protection required by the present invention.

Claims

1. A method for simultaneously measuring the intensities of the DC component and the AC component of a magnetic field, characterized in that, Including the following steps: S1: Place the magnetically sensitive quantum state cold atom gas trapped in the optical trap in the magnetic field to be measured; S2: Perform quantum state manipulation on the magnetically sensitive quantum state cold atom gas to complete Ramsey interference with an evolution time of 2T for the cold atom gas. During the Ramsey interference process, it also includes applying a dynamical decoupling pulse sequence to the cold atom gas; S3: Measure the population difference of the cold atom gas that has completed Ramsey interference. Change the time T, repeat the steps S1 and S2, obtain the oscillation relationship of the population difference with respect to time T, extract the oscillation frequency using Fourier transform, and then obtain the intensities of the DC component and the AC component of the magnetic field.

2. The method for simultaneously measuring the intensities of the direct current component and the alternating current component of a magnetic field according to claim 1, characterized in that, The quantum state manipulation is to perform quantum state manipulation on the cold atom gas trapped in the optical trap by applying a laser field, a microwave field, or a radio frequency field.

3. The method for simultaneously measuring the intensities of the DC component and the AC component of a magnetic field according to claim 1, characterized in that The sensitive quantum state cold atom gases are respectively in state and state, placed in the magnetic field to be measured, and the frequency shifts caused by the magnetic field to be measured are respectively and , where and are state and state's magnetic susceptibility coefficients, is the magnetic field strength.

4. The method for simultaneously measuring the intensities of the DC component and the AC component of a magnetic field according to claim 3, characterized in that, The specific steps of S1 are as follows: Place alkali metal atoms in a vacuum with a vacuum degree of 10 -8 -10 -9 Pa magnitude. After laser cooling and evaporative cooling, the temperature of the atomic cluster is lower than the microkelvin magnitude, forming a magnetosensitive quantum state cold atomic gas and trapping it in a red-detuned optical trap; the atoms are in the magnetic field to be measured. The intensities of the DC component and the AC component of this magnetic field and are to be measured, is the angular frequency of the AC magnetic field, represents the direction of the magnetic field to be measured.

5. The method for simultaneously measuring the intensities of the direct current component and the alternating current component of a magnetic field according to claim 3, characterized in that, The Ramsey interference includes three processes: beam splitting, phase accumulation, and beam combination. The beam splitting is achieved by pulse-manipulating the internal state of atoms to prepare a spin superposition state. The phase accumulation process consists of two evolutions with a time of T. During the first T time, the phase accumulated by the DC component of the magnetic field is , and the phase accumulated by the AC component of the magnetic field is zero. During the second T time, the phase accumulated by the DC component of the magnetic field is zero, and the phase accumulated by the AC component of the magnetic field is ; The beam combination is achieved by pulse-manipulating the internal state of atoms to realize the interference between the state and the state.

6. The method for simultaneously measuring the intensities of the direct current component and the alternating current component of a magnetic field according to claim 5, characterized in that, The applied dynamic decoupling pulse sequence is specifically that within the second T time period, the pulse sequence is applied at the oscillation node of the alternating magnetic field to eliminate the influence of the DC component of the magnetic field and achieve the effective accumulation of the phase Φ brought by the AC component. ac ​ 7. The method for simultaneously measuring the intensities of the direct current component and the alternating current component of a magnetic field according to claim 6, characterized in that, The population difference measurement is to measure the difference in the number of atoms populated in state and state, obtain the oscillating fringe signal of the interference through the population difference measurement, and obtain the time-domain oscillating signal varying with time by changing the phase accumulation time T.

8. The method for simultaneously measuring the intensities of the direct current component and the alternating current component of a magnetic field according to claim 7, characterized in that, By performing a Fourier transform on the time-domain oscillation signal, two oscillation frequencies can be extracted, and the intensities of the DC component and the AC component of the magnetic field can be obtained from the relationships and .

9. A device for simultaneously measuring the intensities of the direct current component and the alternating current component of a magnetic field, characterized in that, This device is applied to a method for simultaneously measuring the intensities of the DC component and the AC component of a magnetic field as described in any one of claims 1 to 8. This device includes: A preparation module: used for laser cooling and trapping of atoms, evaporative cooling, and preparation of the magnetically sensitive state to achieve a magnetically sensitive quantum state cold atom gas with a temperature below microkelvin in the optical trap; Control module: used to perform Ramsey interference, quantum state control, and pulsed microwave control on the magnetosensitive quantum state cold atomic gas; CCD camera: used to measure the number of atoms populated in state and state, and transmit the measurement data to the control module.

10. The device for simultaneously measuring the intensities of the direct current component and the alternating current component of a magnetic field according to claim 9, characterized in that, The manipulation module includes: A control and data acquisition system, including a computer, digital I / O, and analog I / O. The computer is used to collect the measurement data of the CCD camera; A microwave generator, used to generate microwave signals, which is controlled by the analog I / O; A microwave switch, used to control the turning on and off of the microwave signal, which is controlled by the digital I / O; A microwave amplifier, used to amplify the microwave signal generated by the microwave generating device; A microwave antenna, used to emit the amplified microwave signal onto the atoms.

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